HomeMy WebLinkAboutPD 2020-0049; HOPE ELEMENTARY SCHOOL; GEOTECHNICAL AND GEOHAZARD INVESTIGATION; 2019-09-05REPORT
GEOTECHNICAL AND GEOHAZARD INVESTIGATION
Hope Elementary School Modernization
3010 Tamarack Avenue, Carlsbad, California
PREPARED FOR
Carlsbad Unified School District
6225 El Camino Real
Carlsbad, California 92009
PREPARED BY
NOVA Services, Inc.
4373 Viewridge Avenue, Suite B
San Diego, CA 92123
NOVA Project No. 2019157
September 05, 2019
G E O T E C H N I C A L ■ M A T E R I A L S ■ S P E C I A L I N S P E C T I O N S
S B E ■ S L B E ■ S C O O P
4373 Viewridge Avenue, Ste. B
San Diego, CA 92123
858.292.7575
Carlsbad Unified School District September 05, 2019
6225 El Camino Real NOVA Project 2019157
San Diego, CA 92104
Attention: Kelly Fleming
Subject: Report
Geotechnical and Geohazard Investigation
Hope Elementary School Modernization
3010 Tamarack Avenue, Carlsbad, California
Dear Ms. Fleming:
NOVA Services, Inc. (NOVA) is pleased to present herewith its report of the above-referenced
geotechnical and geohazard investigation. The work reported was completed by NOVA for Carlsbad
Unified School District in accordance with NOVA’s proposal dated July 5, 2019, as authorized on July
19, 2019.
NOVA appreciates the opportunity to be of service to CUSD. Should you have any questions regarding
this report or other matters, please do not hesitate to contact the undersigned at (858) 292-7575.
Sincerely,
NOVA Services, Inc.
______________________ __________________________
Melissa Stayner, C.E.G. Bryan Miller-Hicks, C.E.G.
Senior Engineering Geologist Senior Engineering Geologist
_________________________ _________________________
John F. O’Brien, P. E., G. E. Wail Mokhtar
Principal Geotechnical Engineer Senior Project Manager
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
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REPORT
GEOTECHNICAL AND GEOHAZARD INVESTIGATION
Hope Elementary School Modernization
3010 Tamarack Avenue, Carlsbad, California
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TABLE OF CONTENTS
1.0 INTRODUCTION............................................................................................................. 1
1.1 Terms of Reference .................................................................................................................. 1
1.2 Objectives, Scope and Limitations of This Work .................................................................... 1
1.3 Organization of This Report .................................................................................................... 3
2.0 PROJECT INFORMATION ........................................................................................... 4
2.1 Location ................................................................................................................................... 4
2.2 Site Description ....................................................................................................................... 4
2.3 Historic Development of the Site and Site Area ...................................................................... 5
2.4 Planned Modernization ............................................................................................................ 8
3.0 SUBSURFACE EXPLORATION AND LABORATORY TESTING ....................... 10
3.1 Overview................................................................................................................................ 10
3.2 Geologic Borings ................................................................................................................... 11
3.3 Engineering Borings .............................................................................................................. 14
3.4 Geotechnical Laboratory Testing........................................................................................... 16
4.0 SITE CONDITIONS ....................................................................................................... 19
4.1 Geologic Setting .................................................................................................................... 19
4.2 Surface and Subsurface .......................................................................................................... 21
5.0 REVIEW OF GEOLOGIC, SOIL AND SITING HAZARDS ................................... 24
5.1 Overview................................................................................................................................ 24
5.2 Geologic Hazards ................................................................................................................... 24
5.3 Soil Hazards ........................................................................................................................... 28
5.4 Siting Hazards ........................................................................................................................ 30
6.0 EARTHWORK AND FOUNDATIONS ....................................................................... 32
6.1 Overview................................................................................................................................ 32
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
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6.2 Seismic Design Parameters .................................................................................................... 32
6.3 Corrosivity and Sulfates ........................................................................................................ 33
6.4 Earthwork .............................................................................................................................. 35
6.5 Shallow Foundations ............................................................................................................. 38
6.6 Capillary Break and Underslab Vapor Retarder .................................................................... 40
6.7 Retaining Walls ..................................................................................................................... 41
6.8 Temporary Slopes .................................................................................................................. 43
6.9 Drilled Pile Foundations for Solar Panels .............................................................................. 43
7.0 PAVEMENTS ................................................................................................................. 45
7.1 General ................................................................................................................................... 45
7.2 Control of Moisture and Preventive Maintenance ................................................................. 45
7.3 Setback from Slopes .............................................................................................................. 45
7.4 Subgrade Preparation ............................................................................................................. 46
7.5 Flexible Pavements ................................................................................................................ 46
7.6 Rigid Pavements .................................................................................................................... 47
8.0 REFERENCES ................................................................................................................ 48
8.1 Site Specific ........................................................................................................................... 48
8.2 Design .................................................................................................................................... 48
8.3 Geologic and Site Setting ...................................................................................................... 49
List of Plates
Plate 1 Subsurface Exploration Map
Plate 2 Cross Sections
List of Appendices
Appendix A Use of the Geotechnical Report
Appendix B Geologic Borings
Appendix C Geotechnical Borings
Appendix D Records of Laboratory Testing
Appendix E Landslide Hazard Assessment
Appendix F Slope Stability Analysis
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Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
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List of Tables
Table 3-1. Abstract of the Geological Borings Abstract of the Large Diameter Borings and Coring
Table 3-2. Abstract of the Engineering Borings
Table 3-3. Abstract of the Soil Gradation Testing
Table 3-4. Abstract of the Direct Shear Testing
Table 3-5. Abstract of the Unconfined Compression Testing
Table 6-1. Seismic Design Parameters, ASCE 7-10
Table 6-2. Summary of Corrosivity Testing of the Near Surface Soil
Table 6-3. Soil Resistivity and Corrosion Potential
Table 6-4. Exposure Categories and Requirements for Water-Soluble Sulfates
Table 6-5. Wall Lateral Loads from Soil
Table 6-6. Estimated Range of Loads to Solar Canopy Piers
Table 6-7. Soil Properties for Pier Design
Table 7-1. Preliminary Pavement Sections, R = 65
Table 7-2. Requirements for Concrete Pavements
List of Figures
Figure 1-1. Vicinity Map
Figure 2-1. Site Location and Limits
Figure 2-2. 1938 Aerial View of the Site Area
Figure 2-3 1947 Aerial View of the Site Area
Figure 2-4. 1980 Aerial View of the Site Area
Figure 2-5. 1990 Aerial View of the Site Area
Figure 2-6. Hope Elementary School Modernization Plan
Figure 3-1. Location of the Geologic And Geotechnical Borings
Figure 3-2. Large Diameter Drilling Operations, July 26, 2019
Figure 3-3. Massive Sandstone Observed during Downhole Logging
Figure 3-4. Removing Recovered Rock Core, August 9, 2019
Figure 3-5. Drilling Operations, Geotechnical Borings, July 23, 2019
Figure 4-1. Geologic Mapping of the Site Vicinity
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List of Figures (continued)
Figure 4-2. Surface Conditions, Looking South Across the Grass Play Field, July 2019
Figure 4-3. Surface Conditions, Looking North Across the Playground, July 2019
Figure 4-4. Unit 1 Fill
Figure 4-5. Unit 2 Sandstone in the Near Subsurface
Figure 5-1. Faulting in the Site Vicinity
Figure 5-2. Landslide Susceptibility Mapping of the Site Area
Figure 5-3. CGS Landslide Inventory Map Indicating Questionable Landslide
Figure 5-4. Flood Mapping of the Site Vicinity
Figure 6-1. Ground Supported Slab With Thickened Edge
Figure 6-2. Conceptual Design for Wall Drainage
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Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
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1.0 INTRODUCTION
1.1 Terms of Reference
This report presents the findings of a geotechnical and geohazard investigation for a variety of
improvements planned for the existing Hope Elementary School. The report addresses planning for
modernization of Hope Elementary by the addition of various buildings, classrooms, an amphitheater,
playground facilities, a kitchen, and upgrades to the school’s energy system.
The school is administered by Carlsbad Unified School District (CUSD). The work reported herein was
completed by NOVA for CUSD in accordance with NOVA’s proposal to CUSD dated July 5, 2019, as
authorized on July 19, 2019.
Hope Elementary School is located at 3010 Tamarack Avenue in Carlsbad, California (hereinafter, also
referenced as ‘the site’ or ‘the school’). Figure 1-1 depicts the vicinity of the school.
Figure 1-1. Vicinity Map
1.2 Objectives, Scope and Limitations of This Work
1.2.1 Objectives
The objectives of the work reported herein are twofold, as described below.
1. Geohazards. Assessment of geologic and geotechnical hazards associated with Hope Elementary
School, completing this assessment in a manner that meets the requirements of Checklist for
Review of Engineering Geology and Seismology Reports for California Public Schools,
Hospitals, and Essential Service Buildings (California Geological Survey, Note 48, October
2013, hereinafter ‘CGS 2013’).
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2. Geotechnical. Development of subsurface information sufficient to provide recommendations for
earthwork and foundation-related design and construction.
1.2.2 Scope
To accomplish the above objectives, NOVA undertook the task-based scope of work described below.
1. Task 1, Background Review. Reviewed background data, including geotechnical reports, fault
investigation reports and maps, topographic maps, geologic reports and publications, current and
historical aerial photographs and stereo aerial photograph sets, and geologic data. Preliminary
architectural, civil and structural documentation was also reviewed.
2. Task 2, Subsurface Exploration. Subsurface exploration included the subtasks listed below.
o Subtask 2-1, Geologic Reconnaissance. Prior to undertaking any invasive work, a
Certified Engineering Geologist and staff geologist conducted a detailed geologic site
reconnaissance, intending observation of any evident surface features that may be used to
identify the presence of a dormant or active landslide.
o Subtask 2-2, Coordination and Permitting. NOVA retained specialty subcontractors to
conduct geologic-focused borings, engineering borings, and rock coring. Prior to
initiating the subsurface exploration, NOVA coordinated with CUSD regarding access
and scheduling for field work. Borings were permitted in accordance with County of San
Diego Department of Environmental Health (DEH) requirements.
o Subtask 2-3, Large Diameter Geologic Borings. Three large diameter (30-inch) geologic
borings were drilled using bucket auger techniques. The borings were of sufficient
diameter to allow entry by appropriately experienced Certified Engineering Geologists,
providing direct observation and logging/mapping of geologic materials and any geologic
structure exposed on the walls of the borings.
o Subtask 2-4, Geologic Core Boring. Groundwater and very dense formational materials
limited the depths of the Subtask 2-3 borings. In order to obtain additional information at
greater depth, a continuously cored boring was extended at the location of the first large
diameter boring. A NOVA Certified Engineering Geologist directed continuous coring
from 60 feet below ground surface (bgs) to 120 feet bgs at that location, recovering 2.5”
diameter core using HQ coring tools.
o Subtask 2-5, Geotechnical Borings. Eleven (11) geotechnical borings were extended to
depths of 7 to 37 feet bgs. The borings were completed under the direction of a NOVA
geologist who logged the borings and directed sampling using ASTM methods. Samples
recovered by this work were delivered to NOVA’s geotechnical laboratory.
o Subtask 2-6, Closure. Upon completion, each boring was backfilled to the ground
surface in conformance with DEH requirements. The area around each boring was, to the
degree practical, restored to its approximate condition prior to drilling.
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3. Task 3, Laboratory Testing. Laboratory testing of bulk, disturbed and relatively undisturbed
samples was completed. Testing addressed both strength and index soil and rock characteristics,
as well as the potential for corrosivity and sulfate attack to embedded metals and concrete.
4. Task 4, Geologic and Geotechnical Evaluations. The findings of Tasks 1-3 were utilized to
support geohazard and geotechnical related evaluations.
5. Task 5, Reporting. Submittal of this report completes NOVA’s scope of work.
1.2.3 Limitations
Geotechnical engineering and the related geological sciences are characterized by uncertainty. The
recommendations provided in this report have been developed by NOVA using judgment and opinion and
based upon the limited information available from the borings. NOVA can finalize its recommendations
only by observing actual subsurface conditions revealed during construction. NOVA cannot assume
responsibility or liability for the report's recommendations if NOVA does not perform construction
observation.
This report does not address any environmental assessment or investigation for the presence or absence of
hazardous or toxic materials in the soil, groundwater, or surface water within or beyond the site.
Appendix A provides additional discussion regarding limitations and use of this report.
1.3 Organization of This Report
The remainder of this report is organized as described below.
• Section 2 reviews the presently available project information.
• Section 3 describes the field investigation and laboratory testing.
• Section 4 describes geologic and subsurface conditions.
• Section 5 reviews geologic and geotechnical hazards common to development of civil works in
this region, considering each for its potential to affect the proposed new construction during its
expected useful life.
• Section 6 provides recommendations for earthwork and foundations, including drilled pier
foundations for solar panels.
• Section 7 provides recommendations for pavement design.
• Section 8 lists the principal references used in the development of this report.
Figures and tables that amplify the discussions in the text are embedded therein. Larger scale plates that
show the locations of subsurface exploration, and generalized subsurface conditions, are provided
immediately following the text of the report.
The report is supported by six appendices. Appendix A provides guidance regarding the use and
limitations of this report. Appendix B presents logs of the geologic borings. Appendix C presents logs of
the geotechnical borings. Appendix D provides records of the geotechnical laboratory testing. Appendix
E provides detail regarding the assessment of geological landslide hazards. Appendix F provides records
and discussion of the numerical modeling of slope stability.
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2.0 PROJECT INFORMATION
2.1 Location
Hope Elementary School is located at 3010 Tamarack Avenue in the Calavera Hills area of Carlsbad,
California. The approximately 12.5-acre campus is bounded to the north, west and south by descending
natural slopes. A residential development is located across Tamarack Avenue to the east.
Figure 2-1 depicts the site location and limits.
Figure 2-1. Site Location and Limits
2.2 Site Description
The site is currently occupied by Hope Elementary School, consisting of a number of school buildings,
asphalt playgrounds, a parking area, and a grass playfield.
Insight to current site conditions may be obtained by review of older civil drawings associated with
development of the groundform that is now occupied by Hope Elementary School (reference, Grading
and Drainage Plan—Preliminary Phase, Calavera Hills Elementary Site, Carlsbad Unified School
District, Davis Duhaime Associates, Architects, Job No. A85-87, March 1986, hereinafter ‘DDA 1986’).
DDA 1986 indicates that the campus is located atop a large cut-and-fill pad with a finished elevation of
+224.5 feet mean sea level (msl). Pre-grading topography consisted of a ridge with descending slopes
and drainages on the north, south, and west of the site, with elevations that ranged from +255 feet msl at
the top of the hill, near the eastern boundary of the campus, descending to about +120 feet msl to the
bottom of the western drainage. Development required fills and cuts that ranged to about 30 feet.
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2.3 Historic Development of the Site and Site Area
2.3.1 Overview
NOVA reviewed aerial photographs dating to 1938 for correlations with observations of NOVA’s
geologic reconnaissance and as a basis for understanding historic civil development on the site. This
review indicates that no structures were developed on the site prior to its initial development as an
elementary school in approximately 1985. The primary land use was agricultural for the site in the
general area prior to 1985. Since 1985, the primary land use within the general site area has been
residential.
Supplemental discussion and more detailed information regarding the following subsections are included
in Appendix E.
2.3.2 Development 1938 - 1985
Figure 2-2 provides a 1938 aerial view of the property, depicting many of the surface and geomorphic
features still visible today. Evident in this view is the site of the school situated on top of the edge of a
broad, rounded, east-west trending ridge. At the toe of the natural slopes supporting the site, there is a
stream flowing from the southeast, along the southern boundary drainage, and bending to flow northward
along the western site boundary.
Figure 2-2. 1938 Aerial View of the Site Area
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Figure 2-3 provides a 1947 aerial view. The photograph provides important insight to surface features in
the area of the school; specifically, an area near the toe of the southwest slope, where the slope is
significantly steeper than the surrounding slopes, giving it a scarp-like appearance. The 1947 photo
depicts development of a water impoundment at the location of that feature. Evident in this aerial view is
the existence of a small lake created by damming of the stream, most likely for agricultural use. It
appears that material for construction of a small earth dam was derived from excavation into the face of
the slope descending west from the ridge top.
Figure 2-3. 1947 Aerial View of the Site Area
Review of aerial photography indicates that the site area use did not change materially from 1947 through
1980. Figure 2-4 (following page) presents the site configuration in 1980 with the school limits.
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Figure 2-4. 1980 Aerial View of the Site Area
2.3.3 Development 1985 - Present
Prior to development of the school in 1985, topographical relief across the limits of the site was about 120
feet, with ground surface elevations ranging from about +255 feet msl to +135 feet msl.
Development of the site in 1985 included substantial grading operations to create the current, relatively
level groundform. Grade modifications included cuts and fills up to approximately 30 feet. In particular,
new fill slopes ranging to a maximum height of approximately 45 feet were constructed along the south,
west, and north property lines atop moderately steeply sloping terrain.
Figure 2-5 (following page) depicts the site in 1990, four years after the completion of earthwork and
construction of what was then known as Calavera Hills Elementary School. Within the general site area,
major residential development and the supporting infrastructure occurred over the ten-year period of
1980-1990, as is evident by comparison of Figure 2-5 with Figure 2-4.
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Figure 2-5. 1990 Aerial View of the Site Area
2.4 Planned Modernization
2.4.1 General
NOVA’s understanding of the planned development is based upon review of an architectural graphic
developed by Rachlin Partners (references, Site Plan, Hope Elementary School Modernization, Rachlin
Partners, undated, received by NOVA in June 2019, and DSA Progress Drawings, Hope Elementary
School New Classroom Building/Modernization, Rachlin Partners, dated August 1 2019; hereinafter, ‘RP
2019a and 2019b’).
Review of RP 2019a indicates that CUSD proposes a number of improvements and additions to the
existing school facilities. Improvements include modernization of various buildings, classrooms, and
toilet facilities. Additions include new classroom buildings, kitchen, playground and equipment, lunch
shelter and an amphitheater. The school’s energy system will be upgraded with the addition of solar
panels and batteries. Figure 2-8 (following page) reproduces RP 2019a. RP 2019b is used in conjunction
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with the existing topographic map of the site and surrounding slopes that was provided to NOVA by
Rachlin Partners in July, 2019 as the base map for the geotechnical and geohazard investigation (Plate 1).
Figure 2-8. Hope Elementary School Modernization Plan
(source: RP 2019a)
2.4.2 Structural
It is expected that the ground level floor slabs of new buildings will be set between El +225 feet msl and
+226 feet msl, very near existing site grades.
The new buildings will be single story, lightweight wood framed buildings with a maximum column load
of 50 kips. Building A , which is existing, but is planned for expansion, contains some steel beams and
columns. The existing buildings have 12” wide x 18” deep thickened edge foundations. New structures
will utilize similar foundations.
2.4.3 Earthwork
Review of RP 2019b indicates that modernization of Hope Elementary School will largely adapt new
buildings and facilities to the current groundform. To this end, it is expected that earthwork will be
relatively limited, comprised of grading to provide uniform fill conditions below proposed building pads,
suitable drainage and new utilities.
The exception to this generality is that development of the amphitheater may include either cuts or fills to
establish seating elevations.
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3.0 SUBSURFACE EXPLORATION AND LABORATORY TESTING
3.1 Overview
NOVA’s field exploration included subsurface exploration methods with separate geologic and
geotechnical objectives.
• Three large diameter (30”) geologic borings (LD-1, LD-2 and LD-3) were drilled to depths of 40
to 78 feet on July 26 through July 30, 2019.
• A single geologic core boring (C-1) was drilled on August 8-9, 2019 to a depth of 120 feet. The
core boring was drilled through large-diameter boring LD-1 which was initially advanced to 40
feet bgs and was abandoned without downhole logging due to unstable and unsafe conditions, i.e.
perched irrigation water resulting in the overlying fill caving into the boring.
• Eleven (11) geotechnical borings ( B-1 through B-11) were drilled on July 23-24, 2019.
Figure 3-1 presents a plan view of the site indicating the location of the geologic and geotechnical
borings. Plate 1, provided immediately following the text of this report, shows the location of this work
in larger scale.
Figure 3-1. Locations of the Geologic and Geotechnical Borings
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All of the subsurface exploration was completed under the direction of NOVA geologists by specialty
drilling subcontractors. Samples recovered from the geotechnical borings were returned to NOVA’s
laboratory for inspection and laboratory testing. The remainder of this section describes the subsurface
exploration and laboratory testing.
3.2 Geologic Borings
3.2.1 General
Three large diameter borings were drilled to depths of 40, 60 and 78 feet below ground surface (bgs) on
July 26 through July 30, 2019. Two of the borings (LD-2 and LD-3) were downhole logged by NOVA
geologists. Figure 3-2 (following page) depicts drilling operations.
NOVA attempted to complete and log a large diameter boring at location LD-1 (see Figure 3-1).
However, this boring was terminated at a depth of 40 feet due to caving of the borehole. Because
downhole conditions were deemed unsafe, this boring was not logged by a geologist. In order to obtain
deep subsurface information at this location, rock coring (designated as C-1 on Figure 3-1) was utilized to
extend this boring to the planned depth using continuous coring from 60 feet bgs to 120 feet bgs on
August 8 and 9, 2019.
The large diameter borings and the core boring locations are presented on Figure 3-1. Table 3-1 provides
an abstract of the large diameter engineering borings and coring. Large diameter borings are designated
as “LD” and the coring hole is designated with a “C”.
Table 3-1. Abstract of the Large Diameter Borings and Coring
Boring
Ref
Approximate
Ground
Surface
Elevation (ft,
msl)
Total Depth
Below Ground
Surface (feet)
Elevation at
Completion
(feet, msl)
Depth to
Formation
(feet)
Depth to
Seepage
(feet)
Seepage
Elevation
(feet, msl)
LD-2 +223.0 78 +145 4 69 +154
LD-3 +226 60 +166 1 60 +166
C-1(LD-1) +219.0 120 +99 25 n/r n/r
Notes:
1. C-1 completed at the location of LD-1 by HQ coring from 60 feet depth to 120 feet depth
2. Groundwater level could not be recorded in C-1 (‘n/r’) due to the coring method.
3.2.2 Large Diameter Borings
Two large diameter borings (LD-2 and LD-3) were drilled to depths of 78 feet and 60 feet bgs
respectively on July 26th through July 30th, 2019 using an EZ Bore bucket auger drill rig. A 30-inch
bucket auger advanced the boring in the near the subsurface; however, a large-diameter solid-stem auger
was needed below approximately 15 feet in order to drill through the indurated sandstone. The borings
were terminated at the depths listed on Table 3-1 due to water seepage and difficulty in drilling through
very dense formational materials. These borings were downhole logged by NOVA Certified Engineering
Geologists.
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Figure 3-2. Large Diameter Boring Drilling Operations, July 26, 2019
Large-diameter borings were completed because these types of borings provide distinct advantages over
other methods of subsurface exploration in the assessment of landslides. Large diameter borings that are
physically inspected and logged by experienced geologists allow the mapping of three-dimensional
structural features in situ. In particular, large diameter borings allow the observation and identification of
relevant stratigraphy and geologic structure including any shear zones along which landslide movement
has occurred or can occur.
Downhole logging of the exposed sidewalls of NOVA’s large diameter borings indicated massive
sandstone with poorly defined bedding. NOVA’s geologists identified no evidence of slide planes, shear
zones, pervasive open or infilled vertical fractures, or disrupted bedding and deformed structures which
would characterize a slide mass. Figure 3-3 (following page) provides a representative view of the
massive sandstone observed during downhole logging.
On completion, all large diameter borings were closed by backfilling the boring with alternating layers of
bentonite and soils excavated during drilling, per San Diego County Department of Environmental Health
(DEH) requirements.
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Figure 3-3. Massive Sandstone Observed during Downhole Logging
3.2.3 Core Boring
As discussed above, the large diameter boring planned for location LD-1 was terminated at a depth of 40
feet below ground surface. Conditions in the borehole were deemed unsafe for downhole logging, due to
loose fill material caving into the boring at approximately 15 to 25 feet below ground surface. There was
a zone of clayey fill between 15 and 25 feet that was found to be wet. Since this area is in the grass
playfield, this moisture is assumed to be irrigation water infiltrating through the upper sandy fill and
perching on the clay fill in this zone. When the rig stopped to drive a sample at 40 feet, drillers and the
geologist heard large sections of fill caving into the hole, and the driller would not allow the geologist to
enter. The boring was then backfilled.
The backfilled portion of LD-1 and the underlying formation was subsequently redrilled by hollow stem
augering techniques to a depth of 60 feet bgs. From 60 feet to 120 feet, the formational materials were
continuously cored. The boring at this location is therefore designated as LD-1/C-1.
NOVA’s Certified Engineering Geologist directed the coring and maintained a log of the subsurface
materials that were encountered. Coring was conducted by drilling through subsurface materials using
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HQ sized coring tools with a diamond-tipped coring bit and high-pressure water injection to cut and
extract 2.5-inch diameter rock core. Figure 3-4 depicts operations to remove recovered rock core from
the coring tube on August 9, 2019.
Figure 3-4. Removing Recovered Rock Core, August 9, 2019
Generally, core recovery was good to excellent except in the depth intervals 73 feet to 80 feet and 85 feet
to 90 feet bgs, where little to no recovery of subsurface materials was attained. The intervals of low
recovery may indicate zones of friable, less well cemented sandstone with low cohesiveness, or possibly
softer claystone lenses. Within these depth intervals, materials were washed away by high pressure water
injected during coring operations. Alternatively, it is possible that recovered cores may have fallen out of
the core barrels due to damage such as fracturing similar to that observed near the bottoms of LD-2 and
LD-3.
On completion, the rock core boring was closed by backfilling the hole with cuttings and bentonite. Logs
of the geologic borings, including the rock core boring, are included in Appendix B.
3.3 Engineering Borings
3.3.1 General
Eleven (11) hollow-stem auger borings were drilled to depths between 7 to 37 feet below ground surface
(bgs) on July 23 and 24, 2019 at the locations shown on Figure 3-1. The borings were drilled under the
surveillance of a NOVA geologist. Samples recovered from the borings were delivered to NOVA’s
materials laboratory for analysis.
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The borings were advanced by a truck-mounted drilling rig utilizing hollow-stem auger drilling
equipment at locations determined in the field by the geologist. Table 3-2 (following page) provides an
abstract of the borings. Figure 3-5 (following page) depicts drilling operations.
Table 3-2. Abstract of the Engineering Borings
Boring
Reference
Approximate
Ground Surface
Elevation (feet, msl)
Total Depth
Below Ground
Surface (feet)
Elevation at
Completion
(feet, msl)
Depth to
Formation
(feet)
B-1 +226 21 +205 4.0
B-2 +226 21 +205 1.5
B-3 +225.5 16 +209.5 1.0
B-4 +225 21 +204 8.0
B-5 +221 26 +195 15.0
B-6 +225.5 16 +209.5 2.0
B-7 +226.5 20.5 +206 1.0
B-8 +226 7 +219 2.0
B-9 +227 20.5 +206.5 4.0
B-10 +227 15.5 +211.5 3.0
B-11 +226.5 37 +189.5 6.0
Note: ‘n/e’ indicates ‘groundwater not encountered’
Figure 3-5. Drilling Operations, Geotechnical Borings, July 23, 2019
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3.3.2 Logging and Sampling
The NOVA geologist directed sampling and maintained a log of the subsurface materials that were
encountered. Both disturbed and relatively undisturbed samples were recovered from the borings.
Sampling of soils is described below.
1. The Modified California sampler (‘ring sampler’, after ASTM D3550) was driven using a 140-
pound hammer falling for 30 inches with a total penetration of 18 inches, recording blow counts
for each 6 inches of penetration.
2. The Standard Penetration Test sampler (‘SPT’, after ASTM D1586) was driven in the same
manner as the ring sampler, recording blow counts in the same fashion. SPT blow counts for the
final 12 inches of penetration comprise the SPT ‘N’ value, an index of soil consistency.
3. Bulk samples were recovered from the upper 5 feet of the subsurface, providing composite
samples for testing of soil moisture and density relationships and corrosivity.
Logs of the engineering borings are provided in Appendix C. The group symbols for each soil type are
indicated in parentheses following the soil descriptions on the logs. The stratification lines designating the
interfaces between earth materials on the boring logs and profiles are approximate; in-situ, the transitions
may be gradual.
3.3.3 Closure
On completion, the borings were backfilled with soil cuttings. The area of each boring was cleaned and
left as close as practical to its original condition.
3.4 Geotechnical Laboratory Testing
3.4.1 General
Soil samples were returned to the laboratory where a geotechnical engineer reviewed the field logs and
classified each soil sample on the basis of texture and plasticity in accordance with the Unified Soil
Classification System (‘USCS,’ ASTM D 2487).
Representative soil samples were selected and tested in NOVA’s materials laboratory to check visual
classifications and to determine pertinent engineering properties. The laboratory testing program included
index and strength testing on selected soil samples. Testing was performed in general accordance with
ASTM standards.
Records of the laboratory testing are presented in Appendix D.
3.4.2 Moisture Density Characteristics
A single test after ASTM D 1557 (the ‘modified Proctor) was undertaken to determine the moisture
density relationship of the near surface soil. This testing provides an indication of the behavior of the soil
as a construction material.
Testing of two bulk samples of the fill that mantles the site indicates an optimum dry density (γd max) of
about γd max = 126.5 pounds per cubic foot at an optimum moisture content (w)of about w = 8.5%,
characteristic of the dominant relatively cleaner sandy fill.
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3.4.3 Soil Gradation
The visual classifications were further evaluated by performing grain size testing. Gradation testing was
performed after ASTM D422. Table 3-3 (following page) provides a summary of this testing.
Table 3-3. Abstract of the Soil Gradation Testing
Sample Reference Percent Finer
than the U.S.
No 200 Sieve
Classification
after
ASTM D2488 Boring Depth
(feet)
B-1 1-3 14 SP-SM
B-1 5-8 29 SM
B-5 1-5 20 SM
B-5 10-15 42 SC
B-7 1-5 25 SM
B-10 1-3 27 SC
B-11 1-3 25 SM-SC
B-11 4-6 25 SC
B-11 15 28 SM
B-11 30 29 SM
LD-2 30-35 28 SM
LD-2 55-60 13 SP-SM
Note: ‘Percent finer’ is percent by weight passing the U.S.
# 200 sieve (0.074 mm), after ASTM D6913.
3.4.4 Plasticity
As is noted in Section 3.4.1 a geotechnical engineer reviewed the field logs and classified each soil
sample on the basis of texture and plasticity in accordance with the USCS. Plasticity testing was
undertaken to support classification of finer grained soil.
As is discussed in more detail in Section 4, the site is mantled by engineered fill that ranges from a few
feet to approximately 30 feet in thickness. The bulk of this fill is relatively clean sand (i.e., sand with
limited amounts of silt and clay-sized particles). However, as may be seen by review of the boring logs,
the base of the fill includes some areas of soil that are clayey.
The original geotechnical report for the development of the school recommended that the clayey fill
materials be kept a minimum of three feet below the structures (Baca, 1985), which is consistent with
findings from Borings LD-1, B-5, and B-4, in which the clayey fill is located directly above the bedrock
contact, and the sandy engineered fill is placed above it.
Plasticity testing (Atterberg Limits after ASTM D4318) of the clayey fraction of the fill indicated a
Plasticity Indexes (PI) of PI = 12 and PI = 24, identifying these soils as a low plasticity clay (‘CL’, after
ASTM D2487).
3.4.5 Direct Shear
Representative samples from the bedrock were tested in direct shear after ASTM D3080. Table 3-4
abstracts the indications of this testing.
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Table 3-4. Abstract of the Direct Shear Testing
Sample Reference Apparent
Cohesion
(c, psf)
Angle of Internal
Friction
(ϕ, degrees) Boring Depth
(feet)
B-11 20 310 46
LD-1 40 224 48
LD-3 60 251 18
3.4.6 Unconfined Compression
Four samples of the sandstone recovered from the core boring (C-1) were tested in unconfined
compression. The intent of this testing was to bound the spectrum of rock strengths by testing two cores
representative of apparently ‘weaker’, less well cemented zones of sandstone and two cores representative
of apparently stronger zones. Table 3-5 abstracts the indications of this testing.
Table 3-5. Abstract of the Unconfined Compression Testing
Sample Reference Unconfined
Compressive
Strength
(lb/in2)
Unconfined
Compressive
Strength
(tons/ft2) Boring Depth
(feet)
C-1 65.5 480 34.6
C-1
82 140 10.2
C-1 95 490 35.3
C-1 113 50 3.6
3.4.7 R-Value
The Resistance Value (R-value) test is a material stiffness test, demonstrating a material’s resistance to
deformation as a function of the ratio of transmitted lateral pressure to applied vertical pressure.
The purpose of this test is to determine the suitability of prospective subgrade soils and road aggregates
for use in the pavement sections of roadways. The test is used by Caltrans for pavement design, replacing
the California Bearing Ratio (CBR) test. A saturated cylindrical soil sample is placed in a Hveem
Stabilometer device and then compressed. The stabilometer measures the horizontal pressure that is
produced while the specimen is under compression.
A sample representative of soils from the fill was tested after ASTM D2844 to indicate an R-value of 75.
This value is characteristic of R-values for the sandy soil such as the fill that mantles the site.
3.4.8 Corrosion Potential
Electrical resistivity, chloride content, and pH level are all indicators of the soil’s tendency to corrode
ferrous metals. High concentrations of water-soluble sulfates can react with and damage concrete.
Representative samples of the soils were tested to evaluate the potential for soils to corrode embedded
metals or concrete. The testing showed the soils are not a threat to embedded metals or concrete. The
results of the testing are tabulated and discussed in Section 6.3.
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4.0 SITE CONDITIONS
4.1 Geologic Setting
4.1.1 Regional
The project area is located in the coastal portion of the Peninsular Range geomorphic province. This
geomorphic province encompasses an area that extends approximately 900 miles from the Transverse
Ranges and the Los Angeles Basin south to the southern tip of Baja California. The province varies in
width from approximately 30 to 100 miles.
This area of the Province has undergone several episodes of marine inundation and subsequent marine
regression (coastline changes) throughout the last 54 million years. These events have resulted in the
deposition of a thick sequence of marine and nonmarine sedimentary rocks on the basement igneous rocks
of the Southern California Batholith and metamorphic rocks. The coastal portion of the Peninsular Range
geomorphic province increases in elevation from west to east across marine terrace surfaces uplifted
during Pleistocene time, such that the area geology is controlled by both alluvial and marine influences.
Gradual emergence of the region from the sea occurred in Pleistocene time, and numerous wave-cut
platforms, most of which were covered by relatively thin marine and nonmarine terrace deposits, formed
as the sea receded from the land. Accelerated fluvial erosion during periods of heavy rainfall, along with
the lowering of base sea level during Quaternary times, resulted in the rolling hills, mesas and deeply
incised canyons which characterize the landforms in western San Diego County.
4.1.2 Site Specific
Geologic units encountered by the subsurface investigation included engineered fill (Qaf) and the
underlying sandstones of the Eocene-Age Santiago Formation (Tsa).
NOVA was not able to locate the as-graded geotechnical report documenting the placement of engineered
fill on the site. Fill encountered in the borings is of consistent quality, of medium dense to dense
consistency, suggesting controlled placement. The fill is a predominantly silty sand extending from the
ground surface to depths up to approximately 30 feet within the site limits. The majority of areas of the
proposed new buildings and improvements include four feet of fill or less with only three exceptions
along the western portion of the site (B-4, B-5, and LD-1).
The Santiago Formation has been subdivided into “members” with differing stratigraphic characteristics,
mineral composition, color, texture, and fossil assemblages by geologists. Wilson (1972) designated the
members as “A, B and C” (with geologic map symbols Tsa, Tsb, and Tsc for the purpose of his study).
NOVA’s research indicates that the site is underlain by Member B. This research is discussed in more
depth in Appendix E.
According to Wilson (1972), Member B rocks originated as marine, near shore shelf and beach sediments
deposited in high to moderately low energy environments. They are characterized by predominantly fine-
grained sandstones with thin, relatively continuous shaley beds and conglomerate layers, and foreset type
crossbedding. Weber (1982) describes Tsb rocks as massive pale gray to white quartz-feldspar sandstone
with a relatively small proportion of clayey sandstone and siltstone. Claystone is very uncommon. The
rocks logged by NOVA geologists in the borings correlate well with these descriptions. Especially
characteristic are the thin fine-grained beds and foreset crossbeds observed in cores extracted from C-1,
and the overall lack of observed claystone beds relative to the other members of the formation.
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Regionally, the Santiago Formation is associated with some landsliding. If claystone interbeds and lenses
of claystone are laterally extensive and are ‘adversely dipping’ (i.e., dipping out of slope), there is
potential for bedding shear planes or slide planes to form along these materials. Published geologic
mapping of the site area indicates two landslides in the site vicinity: a large block glide with northern
movement, and a smaller feature, topographically higher and aerially smaller. The site is located on the
smaller feature, which has been mapped as “questionable” on some geologic references (Weber 1982 and
CGS Landslide Inventory), and not mapped as a landslide by others (Tan 1995). Questionable landslides
are designated within the Landslide Inventory as having a 50% probability of existence.
Logging of the large diameter borings and the continuous coring showed no indication of adverse
structure characteristic of a landslide. A more detailed discussion of NOVA’s assessment of the site for
evidence for the existence of modern or ancient landslide deposits underlying Hope Elementary School is
presented in Appendix E.
Figure 4-1 reproduces geologic mapping of the site area, indicating landslide deposits mapped underlying
the site.
Figure 4-1. Geologic Mapping of the Site Vicinity
(Source: 2007 Oceanside 30 x 60 Geologic Map)
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4.2 Surface and Subsurface
4.2.1 Surface
Figure 4-2 and Figure 4-3 provide representative views of surface conditions. As may be seen by review
of these figures, the ground surface in the area planned for new structures is relatively level, covered by
light grasses and asphalt playground surfacing.
Figure 4-2. Surface Conditions, Looking South Across the Grass Play Field, July 2019
Figure 4-3. Surface Conditions, Looking North Across the Playground, July 2019
4.2.2 Subsurface
The sequence of soils and rock disclosed by the borings may be generalized to occur as described below.
1. Unit 1, Fill. A layer of fill of varying thickness mantles the site. The fill ranges from
approximately 1 to 25 feet in thickness, where encountered in the borings. In the areas of planned
construction, the deepest fill encountered was 8 feet below proposed Building H, and 15 feet
below the proposed amphitheater. The fill is comprised of predominantly ‘cleaner’ fine to
medium grained sands (i.e., sands with limited amounts of silt and clay -sized particles) of
medium dense to dense consistency. In areas of fills thicker than 5 feet, the lower portions of the
fill encountered was found to be clayey, ranging from clayey sands to low plasticity clays.
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Though the fill is known to be engineered, NOVA has not been able to locate the grading reports
that document grading of the site. Most of the fill appears to have been derived by processing
sandstones from the Santiago Formation.
As encountered in the borings, the fill was found to be relatively uniform in quality. Figure 4-4
depicts this soil.
Figure 4-4. Unit 1 Fill
2. Unit 2, Santiago Formation. Beneath the fill, the borings encountered sandstones of the Tertiary-
aged Santiago Formation (Tsa). These formational sedimentary rocks were observed to consist of
white to light gray silty sandstone of dense to very dense consistency. The sandstone extends to
below the depths explored in the borings. The dense consistency of the sandstones is
characterized by SPT blow counts (‘N’, after ASTM D 1586), commonly in excess of practical
refusal of the sampling device at N > 100. Unconfined compression testing of this unit showed
strengths ranging from 50 pounds per square inch (psi) to 490 psi. The hollow stem auger drilling
tools were refused on two borings.
Figure 4-5 (following page) depicts a representative near-surface sample of this unit.
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4.2.3 Groundwater
Groundwater seepage was encountered in the geologic borings at depths of 60 to 78 feet below existing
ground surface. No seepage was observed at the surface of the underlying natural slopes at or below this
elevation during the site reconnaissance. Groundwater will not affect the planned construction.
Figure 4-5. Unit 2 Sandstone in the Near Subsurface
4.2.4 Surface Water
No surface water was evident on the site at the time of NOVA’s work. There were no evident signs of
recent problems with surface water (i.e., no evidence of staining, erosion, seeps, springs, etc.) within the
limits of the school or on slopes at the periphery of the school property, as observed by geologic
reconnaissance.
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5.0 REVIEW OF GEOLOGIC, SOIL AND SITING HAZARDS
5.1 Overview
This section provides review of geologic and soil-related hazards common to this region of California,
considering each for its potential to affect the planned development. Such review is required by CGS
2013.
The principal hazard identified by this review is that the site is at risk for moderate-to-severe ground
shaking in response to a large-magnitude earthquake during the lifetime of the new and existing school
structures. This circumstance is common to all civil works in this area of California. The strong ground
motion will not precipitate soil liquefaction or related seismic phenomena. The fill embankments that rim
the site will be stable in a seismic event. There is no risk of landsliding.
The following subsections address NOVA’s review of potential geologic, soil and siting hazards.
5.2 Geologic Hazards
5.2.1 Strong Ground Motion
The seismicity of the site was evaluated utilizing a web-based seismic design tool provided by the
American Society of Civil Engineers (ASCE). This evaluation shows the site may be subjected to a
seismic event associated that, with adjustment for site class effects, yields a risk-based Peak Ground
Acceleration (PGAM) of PGAM ~ 0.41 g. This ground surface acceleration is associated with a seismic
event with a 2% probability of exceedance in 50 years.
5.2.2 Fault Rupture
No surface evidence of faulting was observed during NOVA’s geologic reconnaissance of the site. There
are no known active faults underlying the property. The nearest mapped active faults are offshore, within
the Oceanside section of the Newport-Inglewood-Rose Canyon fault zone, about 6.4 miles to the west.
The site is not located within a designated earthquake fault hazard zone. Therefore, an onsite
investigation of fault rupture hazard according to the Alquist-Priolo Earthquake Fault Zoning Act does
not apply.
Figure 5-1 (following page) maps faulting in the site vicinity, from which it can be seen that there are no
active or potentially active faults in the site vicinity.
Because of the lack of known active faults on the site, the potential for surface rupture at the site is
considered low. Shallow ground rupture due to shaking from distant seismic events is not considered a
significant hazard, although it is a possibility at any site.
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Figure 5-1. Faulting in the Site Vicinity
5.2.3 Landslide
Terminology
As used herein, ‘landslide’ describes downslope displacement of a mass of rock, soil, and/or
debris by sliding, flowing, or falling. Deep-seated mass earth movements can be greater than 10
feet thick and larger than 300 feet across. These landslides typically include cohesive block
glides and disrupted slumps that are formed by translation or rotation of the slope materials along
one or more slip surfaces. Other types of mass displacements include similarly larger-scale, but
more narrowly confined modes of mass wasting such as rock topples, ‘mud flows’ and ‘debris
flows’.
The causes of classic landslides of all types start with a preexisting condition- characteristically, a
plane of weak soil or rock- inherent within the rock or soil mass. Thereafter, movement may be
precipitated by earthquakes, wet weather, and changes to the structure or loading conditions on a
slope (e.g., by erosion, cutting, filling, release of water from broken pipes, etc.). Rainfall and
earthquakes are is the most common triggers for landslide events. In the San Diego region
landsliding has been also been precipitated by large-scale earthwork, destabilizing slopes by the
cutting and/or filling on existing adverse geologic structure.
Scope of Assessment
Given that the Hope Elementary campus is shown to be mapped on a landslide, or landslides
depending on the geologic reference, an in-depth assessment of this hazard was performed.
NOVA (i) conducted a geologic reconnaissance of the site; (ii) reviewed geologic mapping and
historic aerial photography for indications of landslide geomorphology; (iii) completed large
diameter geologic borings with downhole inspection by Certified Engineering Geologists; and
(iv) completed continuous core sampling to depths below which large diameter borings could not
be completed due to safety concerns. This scope of review indicated no evidence of active or
dormant landsliding.
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As is evident by review of the foregoing, the scope of this assessment was wide-ranging.
Appendix E to this report provides additional detail regarding the assessment and findings.
Geologic Mapping
Assessments of relative landslide hazard for an area can be assisted by review of mapping that
depicts both historic landslides and landslide prone geology/topography. Figure 5-2 reproduces
such mapping for the site area. 1 The mapping indicates that a portion of the site is in an area
judged ‘most susceptible’ to landsliding, while the bulk of the site, including the school structures
is located in an area with less landslide risk.
Figure 5-2. Landslide Susceptibility Mapping of the Site Area (Tan, 1995)
The California Geological Survey (CGS) Landslide Inventory indicates that Hope Elementary School is
underlain by a slide area (see Figure 5-3, following page). CGS notes that the slide area is “dormant”,
with no specific age, and is “questionable.” This classification means that there is only a 50% confidence
factor, and that the feature could be explained by other processes. Mapping of this questionable slide
dates back to maps produced by the CGS (formerly, California Division of Mines and Geology) in 1982
1 reference, Landslide Hazards in the Northern Part of the San Diego Metropolitan Area, San Diego County,
Relative landslide susceptibility, Oceanside and San Luis Rey Quadrangles, Tan and Giffen, 1995
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(Weber 1982). It is important to note that these landslide features were mapped from aerial photo
interpretation and limited field checking, and thus, do not claim to be definitive.
Figure 5-3. CGS Landslide Inventory Map Indicating the
School is Sited on a Questionable Landslide
Geologic Borings
Large diameter borings were downhole logged by two NOVA Certified Engineering Geologists,
and one rock core to 120 feet in depth was also performed in an effort to characterize the
subsurface geology below the site, and identify any shear surfaces, or zones of disrupted material
characteristic of an ancient, deep-seated block glide.
The indurated, massive, silty sandstone was extremely difficult and slow to drill. Bedding was
difficult to distinguish within the borings, but near-horizontal where observed. Where lenses
within the massive material were visible, they were generally gradational, and not continuous
around the hole, making reliable measurements of bedding attitude extremely difficult.
During the downhole logging and rock coring operations, NOVA did not observe any of the
features characteristic of landslide slip surfaces or disturbed material indicative of landslide
debris. Claystone layers were distinctly absent from the Santiago Formation which underlies the
Hope Elementary Campus, making the probability of deep-seated failures in this unit significantly
more unlikely.
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Conclusions
Based upon the foregoing and the in-depth discussion in Appendix E, it is the judgment of NOVA
that the area of the existing Hope Elementary School and the locations planned for structures
associated with the modernization are not at risk due to the presence of an active or dormant
landslide.
As is discussed above, attempts to complete a large diameter boring and subsequent completion
of a continuously cored boring near the northern end property limits may have disclosed weaker
zones of the Santiago Formation in the depth intervals 73 feet to 80 feet and 85 feet to 90 feet
bgs, where little to no recovery of subsurface materials was attained by coring. While NOVA
believes the intervals of low recovery indicate zones of less indurated sandstone, the inability to
recover rock in this area could also possibly be interpreted as discovery of softer claystone lenses
or layers, although that appears to be unlikely based on NOVA’s research of geologic mapping
literature. Even accepting that there are softer sandstone or claystone zones, NOVA does not
consider the site to be at risk for reactivation of mass soil movement, as described below.
1. There has been no movement of this area in at least the past 80 years. There is no surficial
evidence found by geologic reconnaissance or review of historic aerial photography of the
development of landslide geomorphology during this period.
2. There is no clear subsurface evidence of landslide deposits. The Santiago Formation
sandstone above and below the poor recovery zones in the coring borehole shows no indication of
pervasive fracturing, shear zones, and deformed and disturbed bedding, which would characterize
a landslide deposit.
3. Member B of the Santiago Formation is recognized to be composed primarily of indurated
sandstone with a distinct lack of claystone beds. The probability of landsliding in Eocene rocks is
known to increase with increased amounts of claystone beds. This section of the formation is not
geologically prone to landslides.
5.3 Soil Hazards
5.3.1 Embankment Stability
As used herein, ‘embankment stability’ is intended to mean the safety of localized natural or man-made
embankments against failure. Unlike landslides described above, embankment stability can include
smaller-scale slope failures such as erosion-related washouts and more subtle, less evident processes such
as soil creep.
The south and west periphery of the site is bounded by 30-foot to 100-foot tall descending embankments.
Visual reconnaissance of the embankments completed as a part of the geologic reconnaissance indicated
no surface evidence of embankment instability, as would be indicated by areas of sloughing, bulging at
the toe of the slope, excessive erosion, or tension cracking within or at the crest of the slope.
The judgment that embankments surrounding the school are safe against deeper seated global instability is
based primarily upon quantitative (both deterministic and probabilistic) evaluations of the global stability
of embankments in these areas. These evaluations utilized the soil properties developed by the
subsurface exploration and laboratory testing. Analyses were performed using the SLIDE v5.0
(Rocscience, Inc.) computer program to calculate the factors of safety against slope failure using limit
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equilibrium procedures and assuming two-dimensional, plane strain conditions. Spencer's procedure was
selected for the stability analysis. Like all limit equilibrium methods of slope stability analysis, the factor
of safety (FS) calculated by the Spencer procedure uses:
FS = shear strength of the soil (resisting force)
shear stress required for equilibrium (driving force)
Details regarding the slope stability analyses are provided in Appendix F. These analyses show that the
factor of safety against deeper seated global instability for the static case is FS > 1.5. Pseudostatic
analyses intended to emulate the seismic case, applied a static horizontal inertial force (intended to
represent the destabilizing effects of the earthquake) to the potential sliding mass. The horizontal inertial
force is expressed as the product of a seismic coefficient (k) and the weight (W) of the potential sliding
mass. Seismic stability was modeled with a seismic coefficient of kh = 0.15, showing FS ≥ 1.15 for the
seismic case.
Though presently stable, the long-term stability of embankments that bound the site will be dependent on
care given these embankments over the longer term. For example, inattention to vegetation maintenance
and related surface erosion could diminish stability. Similarly, excavations at the toe of slopes could
diminish stability. It is thus NOVA’s judgment that structures set back a minimum of 20 feet or more
from the crest of embankments in their current condition are not at risk for damage due to embankment
instability. Longer-term stability will be dependent upon the maintenance-related care given to these
embankments.
5.3.2 Seismic
Liquefaction
‘Liquefaction’ refers to the loss of soil strength during a seismic event. The phenomenon is
observed in areas that include geologically ‘younger’ soils (i.e., soils of Holocene age), shallow
water table (less than about 60 feet depth), and cohesionless (i.e., sandy and silty) soils of looser
consistency. The seismic ground motions increase soil water pressures, decreasing grain-to-grain
contact among the soil particles, which causes the soils to lose strength.
Resistance of a soil mass to liquefaction increases with increasing density, plasticity (associated
with clay-sized particles), geologic age, cementation, and stress history. The cemented and
geologically ‘older’ Santiago Formation sandstones have no potential for liquefaction.
Seismically Induced Settlement
Apart from liquefaction, a strong seismic event can induce settlement within loose to moderately
dense, unsaturated granular soils. Neither cohesionless sandy fill of Unit 1 nor the Unit 2
sandstone will be prone to seismic settlement.
Lateral Spreading
Lateral spreading is a phenomenon in which large blocks of intact, non-liquefied soil move
downslope on a liquefied soil layer. Lateral spreading is often a regional event. For lateral
spreading to occur, a liquefiable soil zone must be laterally continuous and unconstrained, free to
move along sloping ground. Due to the absence of a potential for liquefaction, there is no
potential for lateral spreading.
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5.3.3 Expansive Soil
Expansive soils are clays characterized by their ability to undergo significant volume changes (shrinking
or swelling) due to variations in moisture content¸ the magnitude of which is related to both clay content
and plasticity index. These volume changes can be damaging to structures. Nationally, the value of
property damage caused by expansive soils is exceeded only by that caused by termites.
As is discussed in Section 3 and in Section 4, the dominant fraction of the fill at the site is a relatively
clean sand with no expansion potential. Locally, the fill was shown to include zones of clayey soil at
depth. Testing to assess the plasticity of the soils showed very low liquid limit (LL = 27) and low
plasticity index (PI = 12) such that the clay fraction of the Unit 1 soil may be expected to possess ‘Low’
expansion potential.
5.3.4 Hydro-Collapsible Soils
Hydro-collapsible soils are common in the arid climates of the western United States in specific
depositional environments- principally, in areas of young alluvial fans, debris flow sediments, and loess
(wind-blown sediment) deposits. These soils are characterized by low in situ density, low moisture
contents, and relatively high unwetted strength. The soil grains of hydro-collapsible soils were initially
deposited in a loose state (i.e., high initial ‘void ratio‘) and thereafter lightly bonded by water sensitive
binding agents (e.g., clay particles, low-grade cementation, etc.). While relatively strong in a dry state,
the introduction of water into these soils causes the binding agents to fail. Destruction of the
bonds/binding causes relatively rapid densification and volume loss (collapse) of the soil. This change is
manifested at the ground surface as subsidence or settlement. Ground settlements from the wetting can be
damaging to structures and civil works. Human activities that can facilitate soil collapse include
irrigation, water impoundment, changes to the natural drainage, disposal of wastewater, etc.
The consistency and geologic age of the Unit 1 fill and Unit 2 sandstones are such that these units are not
potentially hydro-collapsible.
5.3.5 Difficult Excavation
The sandstone bedrock (see Section 4) may contain cemented concretionary zones that will present
excavation difficulties. Planning for earthwork should anticipate the localized need to employ hoe rams,
rippers or similar devices to excavate formational materials near existing pad grades.
5.3.6 Corrosive Soils
Chemical testing of the near surface soils indicates the soils contain low concentrations of soluble sulfates
and chlorides, but also have low resistivity measurements suggesting these soils will not be corrosive to
embedded concrete, but may be potentially moderately to severely corrosive to buried metals based on
resistivity. Section 6 addresses this consideration in more detail.
5.4 Siting Hazards
5.4.1 Effect on Adjacent Properties
Development of the proposed school will not affect the structural integrity of adjacent properties or
existing public improvements and street right-of-ways located adjacent to the site if the recommendations
of this report are incorporated into project design.
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5.4.2 Flood
The site is not located within a FEMA-designated flood zone. FEMA shows the site area to be designated
“Zone X,” an ‘area of minimal flood hazard.’ Figure 5-4 reproduces flood mapping by FEMA of the
area of Hope Elementary School.
Figure 5-4. Flood Mapping of the Site Vicinity
5.4.3 Tsunami
Tsunami describes a series of fast moving, long period ocean waves caused by earthquakes or volcanic
eruptions. The altitude and distance of the school from the ocean preclude this threat.
5.4.4 Seiche
Seiches are standing waves that develop in an enclosed or partially enclosed body of water such as lakes
or reservoirs. Harbors or inlets can also develop seiches. Most commonly caused by strong winds and
rapid atmospheric pressure changes, seiches can be effected by seismic events and tsunamis. The school
is not located near a body of water that could generate a seiche that could affect the site.
5.4.5 Inundation
The school is not located within a mapped zone of inundation by release of surface waters from a failed
dam or similar structure.
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6.0 EARTHWORK AND FOUNDATIONS
6.1 Overview
6.1.1 Review of Site Hazards
Section 5 provides review of geologic, soils and siting hazards that commonly affect civil works in this
area and that are required for review by CGS 2013. Based upon the review described in Section 5, the
principal hazard to development of the structures associated with modernization of Hope Elementary
School is the risk for moderate-to-severe ground shaking in response to a large-magnitude earthquake
during the lifetime of the new structures. While there is no risk of liquefaction or related seismic
phenomena, strong ground motion could affect the site. This circumstance is common to all civil works
in this area of California. Section 6.2 addresses seismic design parameters.
6.1.2 Site Suitability
This site is suitable for support the proposed construction provided the recommendations of this report are
incorporated into project design. Development of the proposed school will not affect the structural
integrity of adjacent properties or existing public improvements and street right-of-ways located adjacent
to the site if recommendations are followed.
6.1.3 Review and Surveillance
The following subsections provide geotechnical recommendations for the planned development as it is
now understood. It is intended that these recommendations provide sufficient geotechnical information to
develop the project in general accordance with 2016 California Building Code (CBC) requirements.
NOVA should be given the opportunity to review the grading plan, foundation plan, and geotechnical-
related specifications as they become available to confirm that the recommendations presented in this
report have been incorporated into the plans prepared for the project. All earthwork related to site and
foundation preparation should be completed under the observation of NOVA.
6.2 Seismic Design Parameters
6.2.1 Soil Class C
The site-specific data used to determine the Site Class typically includes borings drilled to refusal
materials to determine Standard Penetration resistances (N-values). The depth of soil information
available for this site is limited to 70 feet. However, it is well-known that weak rock and rock-like
materials extend to great depth, such that the site is classified as Site Class C per ASCE 7 (Table 20.3-1).
6.2.2 Risk Category
Per commentary in the ASCE 7-16, the Risk Categories in Table 1.5-1 of ASCE 7-10 are used to relate
the criteria for maximum environmental loads or distortions specified in the ASCE 7 to the consequence
of the loads being exceeded for the structure and its occupants. Structures under this category include
buildings where persons have limited mobility or ability to escape to a safe haven in the event of failure
such as grade schools, prisons, and small healthcare facilities. The structures at Hope Elementary School
should be considered in Risk Category III.
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6.2.3 Seismic Design Parameters
Table 6-1 provides seismic design parameters for the site in accordance with 2016 CBC. Note that these
parameters assume the school structures are Risk Category III.
Table 6-1. Seismic Design Parameters, ASCE 7-10
Parameter Value
Site Soil Class C
Risk Category III
Site Latitude (decimal degrees) 33.170811°N
Site Longitude (decimal degrees) -117.304991°W
Site Coefficient, Fa 1
Site Coefficient, Fv 1.384
Mapped Short Period Spectral Acceleration, SS 1.077 g
Mapped One-Second Period Spectral Acceleration, S1 0.416 g
Short Period Spectral Acceleration Adjusted For Site Class, SMS 1.077 g
One-Second Period Spectral Acceleration Adjusted For Site Class, SM1 0.576 g
Design Short Period Spectral Acceleration, SDS 0.718 g
Design One-Second Period Spectral Acceleration, SD1 0.384 g
Source: ASCE 7-10 Hazard Tool, found at https://asce7hazardtool.online/
6.3 Corrosivity and Sulfates
6.3.1 General
Electrical resistivity, chloride content, and pH level are all indicators of the soil’s tendency to corrode
ferrous metals. Levels of water-soluble sulfates are used as an index of the potential for sulfate attack to
concrete. These chemical tests were performed on representative samples of the near surface soils.
Records of this testing are provided in Appendix D.
The results of the testing are tabulated on Table 6-2.
Table 6-2. Summary of Corrosivity Testing of the Near Surface Soil
Parameter Units Boring 5
1’ to 7’
Boring 7
1’ to 7’
Boring 10
1’ to 7’
pH standard unit 9.1 8.7 8.9
Resistivity Ω-cm 2,200 1,600 1,400
Water Soluble Chloride ppm 21 32 64
Water Soluble Sulfate ppm 51 27 63
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6.3.2 Metals
Caltrans considers a soil to be corrosive if one or more of the conditions listed below exist for
representative soil and/or water samples taken at the site.
• Chloride concentration is 500 parts per million (ppm) or greater.
• Sulfate concentration is 2,000 ppm (0.2%) or greater
• The pH is 5.5 or less.
Based on the Caltrans criteria, the on-site soils would not be considered corrosive to buried metals.
Appendix D provides records of the chemical testing that include estimates of the life expectancy of
buried metal culverts of varying gauge.
Soil resistivity may be used to express the corrosivity of soil only in unsaturated soils. Corrosion of
buried metal is an electrochemical process in which the amount of metal loss due to corrosion is directly
proportional to the flow of DC electrical current from the metal into the soil. As the resistivity of the soil
decreases, the corrosivity generally increases. A common qualitative correlation (cited in Romanoff 1989,
NACE 2007) between soil resistivity and corrosivity to ferrous metals is tabulated below.
Table 6-3. Soil Resistivity and Corrosion Potential
Minimum Soil
Resistivity
Qualitative Corrosion
Potential
0 to 2,000 Severe
2,000 to 10,000 Moderate
10,000 to 30,000 Mild
Over 30,000 Not Likely
Despite the relatively benign environment for corrosivity indicated by pH and water-soluble chlorides, the
resistivity testing suggests that design should consider that the soils may be corrosive to embedded
ferrous metals.
Typical recommendations for mitigation of such corrosion potential in embedded ferrous metals include:
• a high-quality protective coating such as an 18-mil plastic tape, extruded polyethylene, coal tar
enamel, or Portland cement mortar;
• electrical isolation from above grade ferrous metals and other dissimilar metals by means of
dielectric fittings in utilities and exposed metal structures breaking grade; and,
• steel and wire reinforcement within concrete having contact with the site soils should have at
least 2 inches of concrete cover.
If extremely sensitive ferrous metals are expected to be placed in contact with the site soils, it may be
desirable to consult a corrosion specialist regarding choosing the construction materials and/or protection
design for the objects of concern.
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6.3.3 Sulfates and Concrete
The three soil samples tested in this evaluation indicated a maximum water-soluble sulfate (SO4) content
of 64 parts per million (‘ppm,’ 0.006% by weight). The American Concrete Institute (ACI) 318-08
considers soil with this concentration of SO4 to not be a risk of sulfate attack to embedded concrete (i.e.,
Exposure Class ‘S0’). Table 6-4 reproduces the ACI guidance.
Table 6-4. Exposure Categories and Requirements for Water-Soluble Sulfates
Exposure
Category Class Water-Soluble Sulfate
(SO4) In Soil
(percent by weight)
Cement Type
(ASTM C150)
Max. Water-
Cement Ratio
Min. f’c
(psi)
Not Applicable S0 SO4 < 0.10 - - -
Moderate S1 0.10 ≤ SO4 < 0.20 II 0.50 4,000
Severe S2 0.20 ≤ SO4 ≤ 2.00 V 0.45 4,500
Very severe S3 SO4 > 2.0 V + pozzolan 0.45 4,500
Adapted from: ACI 318-08, Building Code Requirements for Structural Concrete
6.3.4 Limitations
Testing to determine chemical parameters that indicate a potential for soils to be corrosive to construction
materials are traditionally completed by the Geotechnical Engineer, comparing testing results with a
variety of indices regarding corrosion potential.
Like most geotechnical consultants, NOVA does not practice in the field of corrosion protection, since
this is not specifically a geotechnical issue. Should you require more information, a specialty corrosion
consultant should be retained to address these issues.
6.4 Earthwork
6.4.1 Standards for Earthwork
Earthwork should be performed in accordance with Section 300 of the most recent approved edition of the
“Standard Specifications for Public Works Construction” and “Regional Supplement Amendments.”
6.4.2 Site Preparation
Establish Erosion and Sedimentation Control
Construction-related erosion and sedimentation must be controlled in accordance with Best
Management Practices and City of San Diego requirements. These controls should be established
at the outset of site disturbance.
Clearing and Grubbing
Before proceeding with construction, all structures, pavements, vegetation, root systems, topsoil,
refuse and other deleterious non-soil materials should be stripped from construction areas.
Underground utilities within the footprint of the proposed structures should be grouted in place or
removed. Clearing, include the removal of any abandoned utilities, should be extended a
minimum of 5 feet beyond the building and pavement limits. Excavations should be backfilled
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with engineered fill placed at a minimum of 90% relative compaction and 2% over optimum
moisture content
Stripped materials consisting of vegetation and organic materials should be hauled away from the
site, or used in landscaping non-structural areas.
6.4.3 Select Fill
Material Requirements
Any fill or backfill for structures should be ‘Select Fill’, a mineral soil free of organics and
regulated constituents with the characteristics listed below:
▪ at least 40 percent by weight finer than ¼-inches in size,
▪ maximum particle size of 3 inches;
▪ classified as SW, SM, SP-SM, SC, GW, GM or GC after ASTM D 2487; and
▪ expansion index (EI) less than 50 (i.e., EI < 50, after ASTM D 4829).
On-site fill and bedrock soils are expected to conform to the above criteria or can be processed to
be in conformance.
Placement Requirements
All Select Fill should be compacted to a minimum of 90 percent relative compaction after ASTM
D1557 (the ‘modified Proctor’) following moisture conditioning to at least 2% above the
optimum moisture content.
Effective densification of the cohesionless (i.e., ‘sandy’) Select Fill, will require the use of
vibratory compaction methods, using equipment designed for this application.
Select Fill should be placed in loose lifts no thicker than the ability of the vibratory compaction
equipment to thoroughly densify the lift. For most self-propelled construction equipment, this
will limit loose lifts to on the order of 10-inches or less, applying a minimum of four passes in the
forward direction.
Lift thickness for hand-operated vibratory equipment (tampers, walked behind compactors, etc.)
will be limited to on the order of 4 inches or less.
6.4.4 Excavation Characteristics
General
The upper few feet of the Santiago Formation will generally be readily excavated by earthwork
equipment usual for construction of this nature. However, locally well-cemented zones could
drive a need for special excavation techniques, such as single shank ripping with a D-9 bulldozer
or the use of hoe rams to fracture the Unit 2 sandstone prior to removal.
Contracting for Earthwork
Questions regarding ‘What is rock?’ often drive cost overruns and related post-construction
claims/litigation in projects with large amounts of earthwork.
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When planning for earthwork for this project, the client/developer may consider establishing
criteria for ‘rock’ prior to initiating excavation. For example, “rock” may be defined based upon
the nature of the work and the equipment that is driven by subsurface conditions. Example
descriptors are cited below.
1. Common Excavation. Excavation of all materials that can be loosened using heavy ripping
equipment then excavated, dumped or loaded by or onto hauling equipment by equipment
using attachments appropriate to the material type, character, and nature of the materials.
‘Heavy ripping’ might be defined by the use of a single tooth ripper drawn by a crawler
tractor having a drawbar pull rated at not less than 56,000 pounds (for example, a Caterpillar
D8, or equivalent). A loader might be a front-end loader with a minimum bucket breakout
force of 25,600 pounds (for example, a Caterpillar 977 or equivalent).
2. Mass Rock Excavation. Excavation of all hard or cemented materials that require blasting or
the use of ripping and excavating equipment larger than that defined for Common
Excavation. The excavation and removal of isolated boulders or rock fragments larger than 1
cubic yard encountered in materials otherwise conforming to the definition of common
excavation should be classified as rock excavation.
3. Trench Rock Excavation. Any material which cannot be excavated with a backhoe having a
bucket curling force rated at not less than 33,000 pounds (for example, a Caterpillar 225B, or
equivalent) and a maximum 24-inch bucket with teeth suitable for rock excavation.
The specifications should require that the Contractor furnish the above specified or similar
equipment to demonstrate the nature of rock materials. The Geotechnical Engineer-of-Record
(GEOR) should be the sole judge of the nature of materials determined to be rock excavation.
When rock excavation has been determined by the GEOR, it should be classified as ‘Mass Rock
Excavation’ or ‘Trench Rock Excavation’. The classification should be made and recorded by the
GEOR and approved by the Owner’s Representative.
• Mass Rock Excavation is defined as rock that must be removed to establish pavement
subgrade, pad grade, pad over excavation, footing subgrade, or footing over excavation.
• Trench Rock Excavation is defined as rock that must be removed beneath the above-noted
subgrades or building pads to install underground pipelines.
The Contractor should be paid for Mass Rock Excavation and Trench Rock Excavation at the unit
price stated in the Contract or at a price agreed to by the Owner and the Contractor based on fair
market equipment and labor rates. Quantities of these should be field measured and noted on
applicable approved plans and agreed to by the Contractor, GEOR, and Owner’s Representative.
6.4.5 Foundation Preparation
To minimize differential settlement within the structures, address unsuitable soils, and support the
buildings on the same uniform soil, NOVA recommends a four-foot over excavation be performed below
each of the proposed building structures. This over excavation should extend a minimum of three feet
outside the building footprint. This will create a minimum of a 24-inch blanket of engineered fill below
the thickened edge of ground supported slabs.
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Prior to backfill of the over excavation with engineered fill, the bottom of the excavation should be
scarified, moisture conditioned to a minimum of 2% above optimum moisture, and compacted to a
minimum of 90% relative compaction.
6.4.6 Trenching and Backfilling for Utilities
Excavation for utility trenches must be performed in conformance with OSHA regulations contained in 29
CFR Part 1926.
Utility trench excavations have the potential to degrade the properties of the adjacent soils. Utility trench
walls that are allowed to move laterally will reduce the bearing capacity and increase settlement of
adjacent footings and overlying slabs.
For utility trenches it is as important as the original subgrade preparation or the engineered fill placed to
support a foundation or slab. Backfill for utility trenches must be placed to meet the criteria specifications
for the Select Fill and for embedment near foundations.
Unless otherwise specified, the backfill for the utility trenches should be placed in 4 to 6-inch loose lifts
and compacted to a minimum of 90 percent relative compaction after ASTM D 1557 (the ‘modified
Proctor’) at soil moisture +2 percent of the optimum moisture content. Up to 4 inches of bedding material
placed directly under the pipes or conduits placed in the utility trench can be compacted to 90 percent
relative compaction with respect to the Modified Proctor.
Compaction testing should be performed for every 20 cubic yards of backfill placed or each lift within 30
linear feet of trench, whichever is less.
Backfill of utility trenches should not be placed with water standing in the trench. If granular material is
used for the backfill, the material should have a gradation that will filter protect the backfill material from
the adjacent soils. If this gradation is not available, a geosynthetic non-woven filter fabric should be used
to reduce the potential for the migration of fines into the backfill material.
6.4.7 Flatwork
Prior to casting exterior flatwork, the upper one foot of subgrade soils should be removed and replaced
with Select Fill in conformance with Section 6.4.3. Concrete slabs for pedestrian traffic or landscaping
should be at least four (4) inches thick. Slabs should be provided with weakened plane joints. Joints
should be placed in accordance with the American Concrete Institute (ACI) guidelines. The project
architect should select the final joint patterns.
6.5 Shallow Foundations
6.5.1 General
Ground supported slabs bearing on properly compacted fill may be designed using a modulus of subgrade
reaction (k) of 120 pounds per cubic inch (i.e., k = 120 pci).
The actual slab thickness and reinforcement should be designed by the Structural Engineer. NOVA
recommends the slab be a minimum 5 inches thick, reinforced by at least #3 bars placed at 16 inches on
center each way within the middle third of the slabs by supporting the steel on chairs or concrete blocks
("dobies").
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The existing buildings are developed with ground supported slabs and 12” wide x 18” deep thickened
edge foundations that support wall and roof loads. New foundations may be of the same or similar design.
Figure 6-1 depicts these foundations conceptually.
Figure 6-1. Ground Supported Slab With Thickened Edge
6.5.2 Isolated and Continuous Foundations
Spread or continuous footings can be used to support the new structures following earthwork to prepare
foundations. Foundations for the new buildings should bear on a minimum of 2 feet of engineered fill per
Section 6.4.5.
• Minimum Dimensions and Embedment. Continuous footings should be at least 12 inches wide
and have a minimum embedment of 18 inches below lowest adjacent grade.
Isolated square or rectangular footings should be a minimum of 36 inches wide. Foundations for
the buildings should be embedded at least 24 inches below surrounding grade. It is recommended
that all foundation elements, including any grade beams, be reinforced top and bottom. The
actual reinforcement should be designed by the Structural Engineer.
• Contact Stress. Continuous and isolated footings constructed as described in the preceding
sections may be designed using an allowable (net) contact stress of 2,500 pounds per square foot
(psf). This allowable bearing value applies to combined dead and sustained live loads (DL + LL).
The allowable bearing pressure may be increased by 500 psf for every 12-inch increase in
embedment depth, up to a maximum of 3500 psf. This bearing value may be increased by one-
third for transient loads such as wind and seismic.
• Lateral Resistance. Resistance to lateral loads will be provided by a combination of (i) friction
between the fill soils and foundation interface; and, (ii) passive pressure acting against the
vertical portion of the footings. Passive pressure may be calculated at 200 psf per foot of depth.
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A frictional coefficient of 0.35 may be used. No reduction is necessary when combining
frictional and passive resistance.
• Settlement. Structures supported on shallow foundations as recommended above will settle less
than 0.5 inch, with about 80% of this settlement occurring during the construction period.
The differential settlement between adjacent, unevenly loaded areas will be on the order of 0.5
inch over a horizontal distance of 40 feet.
• Footing Construction and Inspection. Foundation excavations should be cleaned of loose
material and observed by a qualified Geotechnical Engineer or Engineering Geologist prior to
placing steel or concrete to verify soil conditions exposed at the base of the excavations.
6.5.3 Foundation Setbacks
Based on the slope stability calculations performed by NOVA, foundations for structures should be set
back a minimum of 20 feet from the crest of embankments to limit the risk for damage in the event of
embankment instability resulting from a major seismic event. This requirement is not intended to
supersede CBC-mandated requirements regarding setbacks for foundations at the top of descending
embankments. According to CBC guidelines, setbacks in the areas of the new buildings, range from 20
feet in the southern portion of the site, to 35 feet along the western edge of the site, based on the total
height of the supporting slope. Plate 1 (provided immediately following the text of this report) provides
setback measurements along the top of slope based on the variability of the slope height.
6.6 Capillary Break and Underslab Vapor Retarder
6.6.1 Capillary Break
NOVA recommends that the requirements for a capillary break (‘sand layer’) beneath building slabs be
determined in accordance with ACI Publication 302 “Guide for Concrete Floor and Slab Construction.”
A “capillary break” may consist of a 4-inch thick layer of compacted, well-graded sand should be placed
below the floor slab. This porous fill should be clean coarse sand or sound, durable gravel with not more
than 5 percent coarser than the 1-inch sieve or more than 10 percent finer than the No. 4 sieve, such as
AASHTO Coarse Aggregate No. 57.
6.6.2 Vapor Retarder
Soil moisture vapor that penetrates ground-supported concrete slabs can result in damage to moisture-
sensitive floors, some floor sealers, or sensitive equipment in direct contact with the floor. It is not the
responsibility of the geotechnical consultant to provide recommendations for vapor retarders to address
this concern. This responsibility usually falls to the Architect. Decisions regarding the appropriate vapor
retarder are principally driven by the nature of the building space above the slab, floor coverings,
anticipated penetrations, concerns for mold or soil gas, and a variety of other environmental, aesthetic and
materials factors known only to the Architect.
A variety of specialty polyethylene (polyolefin)-based vapor retarding products are available to
retard moisture transmission into and through concrete slabs. This remainder of this section
provides an overview of design and installation guidance, and considers the use of vapor
retarders in the building construction in the San Diego area.
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A minimum 15-mil polyethylene membrane, or similarly-rated vapor barrier, should be placed
over the porous fill to preclude floor dampness. Membranes set below floor slabs should be
rugged enough to withstand construction. NOVA recommends that a minimum 15 mil low
permeance vapor membrane be used. For example, Carlisle-CCW produces the Blackline 400®
underslab, vapor and air barrier, a 15-mil low-density polyethylene (LDPE) rated at 0.012 perms
after ASTM E 96.
The person responsible for design of the vapor barrier should consult with product vendors to ensure
selection of the vapor retarder that best meets the project requirements. For example, concrete slabs with
particularly sensitive floor coverings may require lower permeance or other performance-related factors
are specified by the ASTM E1745 class rating.
The performance of vapor retarders is particularly sensitive to the quality of installation. Installation
should be performed in accordance with the vendor’s recommendations under full-time surveillance.
6.7 Retaining Walls
6.7.1 General
Only conceptual design information is available at this time. Review of this information does not indicate
retaining walls of any scale will be employed. However, NOVA expects that final design may employ
some smaller retaining walls (i.e., less than 5 feet height) in some areas; for example, in stormwater
detention areas.
The following subsections provide guidance for design of cantilevered retaining walls, should planning
change and such retaining structures be employed.
6.7.2 Lateral Pressures
Lateral earth pressures for wall design are provided on Table 6-5 (following page) as equivalent fluid
weights, in psf/foot of wall height or pounds per cubic foot (pcf). These values do not contain a factor of
safety. Walls less than 8 feet height can neglect the lateral seismic pressure.
6.7.3 Resistance to Lateral Loads
Lateral loads to wall foundations will be resisted by a combination of frictional and passive resistance as
described below.
• Frictional Resistance. A coefficient of friction of 0.35 between the soil and base of the footing.
• Passive Resistance. Passive soil pressure against the face of footings or shear keys will
accumulate at an equivalent fluid weight of 200 pounds per cubic foot (pcf). Ignore the upper 12
inches of material in areas not protected by floor slabs or pavement.
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Table 6-5. Wall Lateral Loads from Soil
Loading Condition
Equivalent Fluid Density (pcf) for
Approved Backfill Notes A, B
Level Backfill 2:1 Backfill
Sloping Upwards
Active (wall movement allowed) 35 60
“At Rest” (no wall movement) 65 100
‘Passive” (wall movement toward the soils) 260 220
Note A: assumes site-sourced soil with EI < 50 after ASTM D4546.
Note B: assumes wall includes appropriate drainage.
6.7.4 Foundation Uplift
A soil unit weight of 125 pcf may be assumed for calculating the weight of soil over the wall footing.
6.7.5 Drainage
The above recommendations assume a wall drainage panel or a properly compacted granular free-
draining backfill material. If wall drainage cannot be insured, design for wall pressures should include
allowance for hydrostatic buildup. Figure 6-2 presents the conceptual design for permanent wall
drainage.
Figure 6-2. Conceptual Design for Wall Drainage
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6.8 Temporary Slopes
Temporary slopes may be required for excavations during grading. All temporary excavations should
comply with local safety ordinances. The safety of all excavations is solely the responsibility of the
Contractor and should be evaluated during construction as the excavation progresses.
Based on the data interpreted from the borings, the design of temporary slopes may assume Unit 1 fill
conforms to California Occupational Safety and Health Administration (Cal/OSHA) Soil Type C.
6.9 Drilled Pile Foundations for Solar Panels
6.9.1 General
No detail is available regarding planning for placement of solar panels. The intent of this subsection is to
provide feasibility-level information regarding development of foundations for solar panels, should such
panels be supported on drilled piles (‘drilled piers’). Because of the significant effect of wind in design of
solar panel foundations, drilled piers are often an economic foundation alternative.
For the purpose of this subsection, NOVA has assumed that axial loads (compression and tension) solar
panel loads will be relatively light, dominated by wind loading. Table 6-6 provides an estimate of the
range of loads to individual piers.
Table 6-6. Estimated Range of Loads to Solar Canopy Piers
Pier Loading Estimated Range
Compression 15 k to 25 k
Tension (Uplift) 15 k to 30 k
Base Moment 80 k∙ft to 130 k∙ft
Base Shear 5 k to 10 k
6.9.2 Soil Characteristics
Table 6-7 provides soil properties for pier design based upon the indications of the borings.
Table 6-7. Soil Properties for Pier Design
Soil
Unit Description Depth
(feet)
Unit
Weight
(γ, lb/in3)
Friction
(ϕ, degrees)
Cohesion
(c, psf)
Lateral
Modulus
(k, lb/in3)
1 Fill 0 120 34 0 100
2 Santiago Fm. 2 115 37 200 400
6.9.3 Axial Capacities
The allowable compressive capacity of a 12-foot long, 30-inch diameter drilled pier will be about 120
kips, The allowable uplift capacity of a 12-foot long, 30-inch diameter drilled pier will be about 20 kips,
with this this allowable capacity rising to about 28 kips at a depth of 15 feet.
The estimates of axial capacity include factor of safety (F) on compression of F = 2.5 on both tip and side
resistance. Allowable uplift capacity includes F = 2.2 for side resistance and F = 1 for pile weight of
0.736 k/ft. Settlement of individual piles will be on the order of 0.1 inch and will be primarily elastic.
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
_____________________________________________________________________________________________
44
6.9.4 Lateral Response
Assuming pier design that approximates ‘free head’ conditions (i.e., the top of the individual pile is
allowed to both translate and rotate), a 30-inch diameter, 15-foot long drilled pile will have a lateral
capacity of about 10 kips at about 0.4 inch top deflection.
6.9.5 Construction
Drilled piles should be installed under the observation of the GEOR.
The design of the concrete piers may be based on concrete with a 30-day compressive strength of 2,500
psi. Excavations can remain open for 48 hours before concrete is poured. However, these excavations
must be inspected immediately prior to the pour and cleaned of any loose soil that may have fallen into
the excavation. At least one set of four ASTM C 31 cylinder specimens be cast per pile, in order to verify
achievement of the design compressive strength.
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
_____________________________________________________________________________________________
45
7.0 PAVEMENTS
7.1 General
Details regarding planning for design of pavements are not yet available. Review of RP 2019a
(reproduced in this report as Figure 2-8) indicates that design may include development of additional
parking along Tamarack Avenue, as well as a trash enclosure near Building A.
NOVA expects that most pavements will be asphalt surfaced. However, pavements periodically receiving
heavier loads (for example, at the trash enclosure) may require concrete pavements. The following
subsections provide recommendations for development of both asphalt and concrete-surface pavements.
7.2 Control of Moisture and Preventive Maintenance
7.2.1 Control of Moisture
Moisture must be controlled around and beneath pavements. Moreover, where standing water develops
either on the pavement surface or within the base course- softening of the subgrade and other problems
related to the deterioration of the pavement can be expected. Furthermore, good drainage should minimize
the risk of the subgrade materials becoming saturated and weakened over a long period of time.
The following recommendations should be considered to limit the amount of excess moisture, which can
reach the subgrade soils:
•maintain surface gradients at a minimum 2% grade away from the pavements;
•compact utility trenches for landscaped areas to the same criteria as the pavement subgrade;
•seal all landscaped areas in or adjacent to pavements to minimize or prevent moisture migration to
subgrade soils;
•planters should not be located next to pavements (otherwise, subdrains should be used to drain the
planter to appropriate outlets);
•place compacted backfill against the exterior side of curb and gutter; and,
•concrete curbs bordering landscaped areas should have a deepened edge to provide a cutoff for
moisture flow beneath pavements (generally, the edge of the curb can be extended an additional
twelve inches below the base of the curb).
7.2.2 Planning for Preventive Maintenance
Preventative maintenance should be planned and provided for. Preventative maintenance activities are
intended to slow the rate of pavement deterioration and to preserve the pavement investment.
Preventative maintenance consists of both localized maintenance (e.g. crack sealing and patching) and
global maintenance (e.g. surface sealing).
Preventative maintenance is usually the first priority when implementing a planned pavement
maintenance program and provides the highest return on investment in the pavements.
7.3 Setback from Slopes
Pavements should be set back a minimum of 25 feet from the crest of any slope.
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
_____________________________________________________________________________________________
46
7.4 Subgrade Preparation
7.4.1 Rough Grading
Grading should be as described in Section 6.4, removing and replacing the Unit 1 fill to a minimum depth
of two feet below the base course level, or excavating to the level of the Unit 2 Santiago Formation.
Any subgrade soils disturbed by excavation should be moisture conditioned and re-densified. Soil should
be moisture conditioned to at least 2% above the optimum moisture content and replaced to at least 95%
relative compaction after ASTM D 1557 (the ‘modified Proctor’).
Replacement filling should be done in lifts (i) not to exceed 10-inches thickness; or, (ii) the ability of the
compaction equipment employed to densified through a complete lift, whichever is less. Fill will be
characteristically sandy, such that densification should be completed using vibratory equipment.
7.4.2 Proof-Rolling
After the completion of compaction/densification, or excavation to the design subgrade level, areas to
receive pavements should be proof-rolled. A loaded dump truck or similar should be used to aid in
identifying localized soft or unsuitable material. Any soft or unsuitable materials encountered during this
proof-rolling should be removed, replaced with an approved backfill, and compacted.
7.4.3 Surveillance
The preparation of roadway and parking area subgrades should be observed on a full-time basis by a
representative of Geotechnical Engineer of Record (GEOR) to confirm that any unsuitable materials have
been removed and that the subgrade is suitable for support of the proposed driveways and parking areas.
7.5 Flexible Pavements
The structural design of flexible pavement depends primarily on anticipated traffic conditions, subgrade
soils, and construction materials. Table 7-1 provides preliminary flexible pavement sections for design
purposes using an R-value of 31. An R-value of 31 was determined by laboratory testing of the sandy
Unit 1 fill. An R-value test of the subgrade soils can be performed after the grading operations are
complete in order to provide a final pavement section.
Table 7-1. Preliminary Pavement Sections, R = 31
Area Traffic
Index
Asphalt
Thickness
(inches)
Base Course
Thickness
(inches)
Passenger Car Driveways 5.0 4 6
Driveways/Fire lane 6.0 4 6.5
Note: 1) The above sections assume a min 4 over 6 section in accordance with
City of Carlsbad min pavement section requirements; 2) the subgrade should be
prepared per Section 7.4, with aggregate base placed at a minimum of 95%
relative compaction.
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
_____________________________________________________________________________________________
47
7.6 Rigid Pavements
7.6.1 General
Concrete pavement sections should be developed in the same manner as undertaken for all other slabs and
pavements: removal of the upper two feet of the Unit 1 fill and replacement of that soil in an engineered
manner as described in Section 7.4.
Concrete pavement sections consisting of 7 inches of Portland cement concrete over a base course of 6
inches and a properly prepared subgrade support a wide range of traffic indices.
Where rigid pavements are used, the concrete should be obtained from an approved mix design with the
minimum properties of Table 7-2.
Table 7-2. Recommended Concrete Requirements
Property Recommended Requirement
Compressive Strength @ 28 days 3,250 psi minimum
Strength Requirements ASTM C94
Minimum Cement Content 5.5 sacks/cu. yd.
Cement Type Type I Portland
Concrete Aggregate ASTM C33 and Caltrans Section
703
Aggregate Size 1-inch maximum
Maximum Water Content 0.50 lb/lb of cement
Maximum Allowable Slump 4 inches
7.6.2 Jointing and Reinforcement
Longitudinal and transverse joints should be provided as needed in concrete pavements for
expansion/contraction and isolation. Sawed joints should be cut within 24-hours of concrete placement,
and should be a minimum of 25% of slab thickness plus 1/4 inch. All joints should be sealed to prevent
entry of foreign material and doweled where necessary for load transfer.
Load transfer devices, such as dowels or keys are recommended at joints in the paving to reduce possible
offsets. Where dowels cannot be used at joints accessible to wheel loads, pavement thickness should be
increased by 25 percent at the joints and tapered to regular thickness in 5 feet.
Report of Geotechnical and Geohazard Investigation September 05, 2019
Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157
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48
8.0 REFERENCES
8.1 Site Specific
8.1.1 Architectural
Rachlin Partners, undated, Site Plan, Hope Elementary School Modernization, received by NOVA June
2019.
Rachlin Partners, 2019, Carlsbad Unified School District, Hope Elementary School New Classroom
Buildings and Modernization, DSA Progress Drawings, August 1, 2019.
8.1.2 Civil and Geotechnical
Baca Associates, 1985. Report, Geotechnical Investigation, Calavera Hills Elementary School, Tamarack
Avenue, Carlsbad, California, Project No. A-0231-F, December 31, 1985.
Davis Duhaime Associates, Architects, 1986, Grading and Drainage Plan - Preliminary Phase, Calavera
Hills Elementary Site, Carlsbad Unified School District, Job No. A85-87, March 1986.
Excel Engineering, 2019, Hope Elementary School Existing Topography Exhibit, Plot Date June 14,
2019, Scale 1:40.
Geocon Inc., 2012, Geotechnical Investigation—Preliminary Phase, Quarry Creek II,
Carlsbad/Oceanside, California, Project No. 07135-42-03, May 2012.
Geocon Inc., 2015, Geotechnical Investigation—Update, Quarry Creek, Carlsbad/Oceanside, California,
Geocon Incorporated, Project No. 07135-42-05, February 2015.
8.2 Design
American Concrete Institute, 2002, Building Code Requirements for Structural Concrete (ACI 318-02)
and Commentary (ACI 318R-02).
American Concrete Institute, 2015, Guide to Concrete Floor and Slab Construction, ACI Publication
302.1R-15.
American Concrete Institute, 2016, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring
Materials (ACI 302.2R-06)
American Society of Civil Engineers, Minimum Design Load for Buildings and Other Structures,
ASCE7-10.
American Society of Civil Engineers, ASCE 7 Hazard Tool, found at: https://asce7hazardtool.online/.
California Code of Regulations, Title 24, 2016 California Building Standards Code.
California Geological Survey, Checklist for Review of Engineering Geology and Seismology Reports for
California Public Schools, Hospitals, and Essential Service Buildings, Note 48, October 2013.
California Department of Transportation (Caltrans), 2003, Corrosion Guidelines, Version 1.0, found at
http://www.dot.ca.gov/hq/esc/ttsb/corrosion/pdf/2012-11-19-Corrosion-Guidelines.pdf.
Report of Geotechnical and Geohazard Investigation September 05, 2019
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Christian. J. T. , Ladd, C. C., and Baecher, G. B. 1994, Reliability Applied To Slope Stability Analysis,
Journal of Geotechnical Engineering, ASCE. Vol. 120:12. P 2180-2207.
Duncan, J. M. 2000, Factors Of Safety And Reliability In Geotechnical Engineering, Journal Of
Geotechnical and Geoenvironmental Engineering, ASCE. Vol. 126:4. P 307-316.
Hart, E.W., and Bryant, W.A., 2007, Fault-Rupture Hazard Zones in California, Alquist-Priolo
Earthquake Fault Zoning Act with Index to Earthquake Fault Zone Maps: California Geological Survey,
Special Publication 42.
NACE International, 2007, Standard Practice, Control of External Corrosion on Underground or
Submerged Metallic Piping Systems, SPO169-2007.
OSHA Technical Manual, Excavations: Hazard Recognition in Trenching and Shoring, OSHA
Instruction TED 01-00-015, Section V, Chapter 2. Found at: https://www.osha.gov/dts/osta/otm/otm_v/
otm_v_2.html#1.
Romanoff, Melvin. Underground Corrosion, NBS Circular 579. Reprinted by NACE, Houston, 1989.
Standard Specifications for Public Works Construction (Green Book), Public Works Standards, Inc.
Terzaghi, Karl, Evaluating Coefficients of Subgrade Reaction, Geotechnique, Vol 5, 1955, pp 297-326.
Seed, H. B., 1979. Considerations In The Earthquake-Resistant Design Of Earth And Rockfill Dams,
Geotechnique 29, 215–263.
Southern California Earthquake Center (SCEC), 1999, Recommended Procedures for Implementation of
DMG Special Publication 117 - Guidelines for Analyzing and Mitigating Liquefaction in California,
Martin, G.R. and Lew, M. eds.
U.S. Army Corps of Engineers, 2003, Slope Stability, Engineering Manual 1110-2-1902, 31 Oct 2003.
U.S. Army Corps of Engineers, May 1999, Risk-Based Analysis In Geotechnical Engineering For
Support Of Planning Studies. Engineering Technical Letter No. 1110-2-556, 28 May 1999.
8.3 Geologic and Site Setting
8.3.1 General
California Division of Mines and Geology (CDMG), 2008, Guidelines for Evaluating and Mitigating
Seismic Hazards in California, Special Publication 117A.
California Geological Survey (CGS), Information Warehouse: Landslide Inventory:
https://maps.conservation.ca.gov/cgs/lsi/, accessed August 2019.
Historic Aerials website, 2019, www.historicaerials.com: accessed in August.
Jennings, C. W. and Bryant, W. A., 2010, Fault Activity Map of California, California Geological
Survey, Geologic Data Map No. 6.
Kennedy, M.P. and Tan, S.S., 2007 Geologic Map of the Oceanside 30’ x 60’ Quadrangle, California,
Scale 1:100,000.
Norris, R. M. and Webb, R. W., 1990, Geology of California, Second Edition: John Wiley & Sons, Inc.
SANGIS, 2009, Liquefaction County of San Diego Hazard Mitigation Planning Map.
Report of Geotechnical and Geohazard Investigation September 05, 2019
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Tan and Giffen, 1995, Landslide Hazards in the Northern Part of the San Diego Metropolitan Area, San
Diego County, Relative Landslide Susceptibility,, Oceanside and San Luis Rey Quadrangles, Landslide
Hazard Identification Map No. 35, Open-File Report 95-04.California Geologic Survey, 1995.
United States Geological Survey and California Geological Survey, 2011, Quaternary Fault and Fold
database for the United States, http://earthquake.usgs.gov/regional/qfaults/.
United States Geological Survey (USGS), 1949, Topographic Map of San Luis Rey Quadrangle,
California, 7.5-Minute Series: Scale 1:24,000.
Weber Jr, Harold F., 1982, Recent Slope Failures, Ancient Landslides, and Related Geology of the North-
Central Costal Area, San Diego County, California: California Department of Conservation, Division of
Mines and Geology, DMG Open-File Report 82-12, 1982.
Wilson, K.L., 1972, Eocene and Related Geology of a Portion of the San Luis Rey and Encinitas
Quadrangles, San Diego County, California; Unpublished Master’s Thesis, University of California,
Riverside.
8.3.2 Stereo-Paired Aerial Photo Log
DATE FLIGHT # - NEG # SCALE
8-12-98 C 123-4 – 24 & 25 1”=2000’
10-3—93 C 98-4 – 133 & 134 1”=2000’
1-14-88 SD-2 – 8 & 9 1”=2000’
4-9-80 FC-SD-7 – 19 & 20 1”=3500’
12-13-78 210-15B – 24 & 25 (E)
2-17-79 210-14B – 19, 20, & 21 (W)
9-6-73 128B-6 – 3, 4, & 5
4-16-72 107-5 – 6 & 7 1”=3500’
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APPENDIX D
LABORATORY ANALYTICAL RESULTS
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APPENDIX E
GEOLOGIC HAZARD ASSESSMENT
APPENDIX E LANDSLIDE HAZARD ASSESSMENT HOPE ELEMENTARY SCHOOL CARLSBAD, CALIFORNIA
1. Introduction
General
The campus of Hope Elementary School has been mapped on two landslides by multiple geologic source
since the early 1980s. These landslides have been designated as ancient low-angle block glides. The total length of the landslides mapped are approximately 0.8 miles from the top of the slope to the furthest reach of the western toe, approximately 0.3 miles wide near the top of the feature, and is estimated to be up to
320 acres in area.
The history and sources of this mapping were studied by NOVA as part of its assessment of the landslide hazard for Hope Elementary School. The field investigation focused on attempts to physically identify a
slide plane or other indications of slope instability. This appendix provides a summary of the investigation specific to the landslide evaluation at the site.
Terminology
As used herein, ‘landslide’ describes downslope displacement of a mass of rock, soil, and/or debris by sliding, flowing, or falling. Such mass earth movements are greater than about 10 feet thick and larger than 300 feet across. Landslides typically include cohesive block glides and disrupted slumps that are
formed by translation or rotation of the slope materials along one or more slip surfaces. These mass displacements can also include similarly larger-scale, but more narrowly confined modes of mass wasting such as rock topples, ‘mud flows’ and ‘debris flows’.
The causes of classic landslides start with a preexisting condition- characteristically, a plane of weak soil or rock- inherent within the rock or soil mass. Thereafter, movement may be precipitated by earthquakes, wet weather, and changes to the structure or loading conditions on a slope (e.g., by erosion, cutting, filling, release of water from broken pipes, etc.). Rainfall and earthquakes are the most common triggers for landslide events. In the San Diego region, landsliding has also been precipitated by larger-scale earthwork, by destabilizing slopes by the cutting and/or filling on existing adverse geologic structure.
Report of Geotechnical and Geohazard Investigation, Appendix E September 05, 2019 Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157 _____________________________________________________________________________________________
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2. Review of Published Mapping
Weber 19821
In 1982, due to a rapidly increasing population and increased amount of construction, the California
Division of Mines and Geology released a report documenting slope failures and ancient landslides within the north-central area of San Diego County, identifying areas of instability and potential instability for planning and development purposes. The goal was to decrease or eliminate damage to property by
landsliding (Weber 1982).
Weber maps “probable landslide” 17a and “questionable landslide” 17b to underlie the Hope Elementary site, and identifies this landslide complex as a:
“north-facing feature… about one-half mile long and between 160 and 320 acres in area; it may be an ancient, very low angle block glide, as the dip of the bedrock in this area is northwesterly to northerly.”
Figure E-1 depicts the location of the site relative to these features. According to this map, the Hope Elementary Campus is mapped entirely upon the questionable portion of the landslide, 17b.
Figure E-1. Location of Hope Elementary Campus Relative to Landslides Mapped by Weber 1982
1 Weber Jr, Harold F., 1982, Recent Slope Failures, Ancient Landslides, and Related Geology of the North-Central
Costal Area, San Diego County, California: California Department of Conservation, Division of Mines and Geology, DMG Open-File Report 82-12, 1982.
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Weber indicates that the upper portion, 17b, may have been the source area for the lower part of the
landslide. “The feature is relatively deeply eroded which suggests it is very ancient; however, steep,
relatively uneroded slopes at its toe suggest fairly recent movement.” The bedrock unit is mapped as Tsa (the oldest member of the Santiago Formation). Figure E-1 depicts the limits of these features in context with the geologic mapping of the area.
Tan 19952
In the early 1990s, the California Division of Mines and Geology carried out the Landslide Hazard Identification Project. The evaluation of the landslide hazards was performed in accordance with the Landslide Hazard Identification Act of 1993. The San Luis Rey 7.5 Minute Quadrangle was evaluated in 1995. This project resulted in the northern edge of the Hope Elementary campus being mapped on a landslide. Figure E-2 depicts the study area overlain on the Landslide Susceptibility Map and Landslide Distribution Map. The large slide is mapped to extend northward, terminating at the Buena Vista Creek.
Figure E-2. Site Location Relative to the 1995 Landslide Susceptibility
and Landslide Distribution Map
2 Tan and Giffen, 1995, Landslide Hazards in the Northern Part of the San Diego Metropolitan Area, San Diego
County, Relative Landslide Susceptibility, Oceanside and San Luis Rey Quadrangles, California Geologic Survey, 1995.
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According to the report attached to the Landslide Hazard Identification Map, conclusions drawn relative to existence of landslides were determined either by direct observation in the field or analysis of aerial photographs. The mapped slides identified during this project were judged by the reviewers to present geomorphology characteristic of cohesive block glides.
The area mapped to the north of the site was designated by the authors as Most Susceptible to landsliding, noting “these areas are characterized by unstable slopes… and slopes where there is evidence of
downslope creep of surface materials… Slopes within Area 4 should be considered naturally unstable.”
While the configuration of the landslide shown on this map generally agrees with the limits of the “probable landslide” presented by Weber 1982, the upper “questionable landslide” (Qls 17b) is not
indicated on this hazard map, nor is it defined as an area considered “Most Susceptible” to landslides.
Kennedy and Tan 20073
The 2007 Oceanside 30 x 60 Geologic Map by Kennedy and Tan present the area underlying the Hope
campus with the general landslide configuration shown in Weber 1982, and cite Weber’s 1982 report. On this map, both the probable and questionable landslides appear. Figure E-3 presents the site vicinity overlaid on the geologic map.
Figure E-3. Site Vicinity Relative to the 2007 Oceanside 30 x 60 Geologic Map
3 Kennedy, M.P. and Tan, S.S., 2007 Geologic Map of the Oceanside 30’ x 60’ Quadrangle, California,
Scale 1:100,000.
Report of Geotechnical and Geohazard Investigation, Appendix E September 05, 2019 Hope Elementary School Modernization, Carlsbad, California NOVA Project No. 2019157 _____________________________________________________________________________________________
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Landslide Inventory Map4
This same landslide complex is identified as two landslides on the California Geologic Survey Landslide
Inventory Maps. The larger lower feature to the north is defined as a probable dormant slide with 75% confidence it is a landslide. The upper feature is mapped as a questionable dormant slide with “50% confidence it is a landslide… cannot be sure it is a landslide without detailed site investigation.” As
Figure E-4 indicates, the entirety of the Hope Elementary campus is located on this feature.
Figure E-4. Site Vicinity Presented on Landslide Inventory Map.
Landslide Assessments from Recent Construction
General
Besides Hope Elementary School, there are three existing residential developments within the limits of the probable and questionable landslides shown above in Figure E-4: one located directly east of Hope Elementary School, built into the existing hillside, one to the north of the Hope school, and one more recent development (‘Quarry Creek’) in the north eastern area on top of the toe of the probable slide.
Hope Elementary School and the developments immediately to the east and north of Hope were developed during the 1980s. No records of geotechnical investigations or as-graded geotechnical reports were available for review for the two residential developments. However, NOVA was able to review the
4 California Geologic Survey Landslide Inventory Maps, found at https://maps.conservation.ca.gov/cgs/lsi/
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geotechnical investigation for the development of the subject school site, then called Calavera Hills
Elementary School (Baca 19855), as well as the geotechnical investigation and as-graded reports for the recently developed Quarry Creek development to the far north (Geocon 20156).
Calavera Hills Elementary School
Landsliding is not addressed in detail within the report, which states only that “…geologic conditions for
the planned slopes are not prone to seismically induced mass landsliding.” The report mentions that the bedrock encountered on the site is slightly cemented sand, very massive in structure, with one bedding plane measured on the east-facing Tamarack Avenue slope-cut as dipping to the northeast at a gentle inclination (N50W/13NE).
Quarry Creek Residential Development
As part of the geotechnical investigation for the Quarry Creek development, five 30-inch large diameter borings were advanced and logged to a depth of 56 feet below ground surface. No slide planes were discovered within these borings. The aforementioned mapping of the landslide by Tan 1995, is addressed and discounted within this report, saying that observations of intact outcrops and the current subsurface
information confirm that the landslide does not exist (Geocon 2015).
Also of note, is the fact that Geocon 2015 maps the area that is the toe of the probable landslide as Quaternary terrace deposits, lithologically very different from the Santiago Formation. These deposits were encountered to the total depth explored in the borings. No Tertiary-aged Santiago Formation was encountered either above or below these deposits during the investigation.
Geology of the Santiago Formation
As previously discussed, the subject site is underlain by the Eocene-aged Santiago Formation. Detailed studies of the Santiago Formation were reviewed as part of this investigation. In 1972 an in-depth geologic investigation of the Eocene rocks of a portion of the San Luis Rey and Encinitas quadrangles was conducted (Wilson, 19727). According to this study, the Santiago Formation is approximately 2700
vertical feet in total thickness, and can be broken out into three members, A, B and C. The lowermost, and least aerially extensive is Member A. Member A is composed of green mudstones and sandy mudstone with interbedded greenish-gray sandstone and minor light blue tuffaceous sandstone and hard
concretionary siltstone lenses. According to Wilson 1972, this member is mapped at the surface only on the east side of the San Luis Rey 7.5 Minute quadrangle, the quadrangle containing the study site.
5 Baca Associates, 1985. Report, Geotechnical Investigation, Calavera Hills Elementary School,
Tamarack Avenue, Carlsbad, California, Project No. A-0231-F, December 31, 1985.
6 Geocon Inc., 2015, Geotechnical Investigation—Update, Quarry Creek, Carlsbad/Oceanside,
California, Geocon Incorporated, Project No. 07135-42-05, February 2015.
7 Wilson, K.L., 1972, Eocene and Related Geology of a Portion of the San Luis Rey and Encinitas Quadrangles, San Diego County, California; Unpublished Master’s Thesis, University of California,
Riverside.
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Member B overlies Member A and is the most aerially extensive within the San Luis Rey quadrangle,
extending from the northeast to the sea cliffs of Encinitas. This sub-unit generally consists of massively bedded, moderately-well indurated pale gray to white quartz-feldspar sandstone along with a smaller portion of clayey sandstone and clayey siltstone. Claystone is very uncommon in this unit. Beds generally dip westward with dips rarely exceeding 5 or 10 degrees.
Member C is the youngest and is mapped west of the other two members. It is described as interbedded massive sandstone, clayey sandstone, clayey siltstone, and claystone which dip gently westward from 5
to10 degrees. This member lithologically resembles the A Member more than the B Member.
Weber 1982 maps the area surrounding the Hope Elementary Campus as the A Member to the east and the B Member to the west. Wilson 1972 maps this area as being underlain by the B Member with the contact with the C Member directly west of the western drainage. Figure E-5 presents the mapping of Wilson 1972 within the study area. It is worth noting that consistent with Geocon 2015, the feature that was mapped by Weber 1982 as the toe of the probable landslide, was mapped by Wilson as a separate
Quaternary geologic unit (Qu).
Based on the lithologic description of the Members presented in Wilson 1972 and Weber 1982, NOVA concurs with Wilson 1972, that this site is underlain by Member B of the Santiago Formation.
Figure E-5. Wilson 1972 Mapping of Site Vicinity
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Historical Aerial Photo and Historic Topographic Map Review
General
Historical aerial photo and historical topographic map review was performed for the purpose of
establishing site history as well as examining the photos and topography for evidence of landslide geomorphology below the Hope Elementary campus and surrounding areas.
Photos reviewed include photos and topographic maps from the Historic Aerials website
(www.historicaerials.com) from 1938 to the present, and photo stereo pairs of the site between the years 1972 and 1998.
Site Use
The earliest available aerial photo of the site is from 1938. Figure E-6 presents the 1938 photo with the approximate limits of the Hope campus.
Figure E-6. 1938 Aerial Photo
As may be seen by review of Figure E-6, the site is supported by relatively steep natural slopes on the south and west, with shallower slopes to the north. There is a stream flowing from the southeast, along the southern boundary drainage, and bending to flow northward along the western site boundary. On top
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of the slopes, there is a roughly east-west trending ridge. Where the steep sided slopes become more
shallow part way up the hill, the area appears to have been grubbed of the native bushes and grasses. It appears that the site may have been used for agriculture, with water running off the north side of the site, down into the northern drainage. The next ridge to the south appears to be used in the same manner, and the area to the west of the drainage appears also to be used as agricultural land.
Figure E-7 presents the 1947 configuration with the subject site limits.
Figure E-7. 1947 Aerial Photo
The 1947 aerial photo shows that the stream that previously ran along the drainages south and west of the site was dammed, with the ponded water visible in this photo. NOVA assumes this dam was used for
agricultural purposes. It appears that the material used for building the dam came from cutting a steep slope into the bedrock on the southwest corner of the slope below the site, leaving behind a feature that resembles a scarp, that is still visible, but not landslide-related.
Figure E-8 (following page) presents the site, taken from the 1949 San Luis Rey topographic map, with the dam identified.
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Figure E-8. 1949 San Luis Rey Topographic Map
Aerial photography indicates that this general area was used for agricultural purposes until its development in the early to mid-1980s. Figure E-9 (following page) presents the 1980 configuration, with the pond still present.
Geomorphic Landslide Expression in Photos
Landslide geomorphology generally includes but is not limited to headwall scarps, ground cracks, rounded toes, well-defined benches, closed depressions, springs and irregular or hummocky topography (Tan 1995). While the literature reviewed does not discuss the specific geomorphic features observed in the study area that lead to the conclusion that this area is a large block glide, the toe of the mapped slide does appear in aerial photos to be hummocky and rounded, and the top area of the probable landslide could be interpreted by aerial photo to be a bench, with the head scarp being the northern descending slope of the landform that Hope Elementary is built on.
It is the opinion of NOVA that these geomorphic characteristics may be explained by other geologic processes, and upon completion of the landslide investigation, now appear to be unlikely to be features of a large block glide.
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Figure E-9. Aerial Photograph of the 1980 Site Configuration
Field Investigation
Geologic Reconnaissance
A NOVA Certified Engineering Geologist explored the Hope Elementary campus and supporting slopes and drainages on foot for indication of physical expressions of landslides, such as tension cracks, scarps, and seeps, but none were found.
There are storm drains from below Tamarack Avenue that outlet into the drainage bottoms in the northern and southern drainages. The bottoms of these drainages are overgrown with vegetation, and difficult to navigate on foot. Figure E-10 (following page) depicts the storm drain that empties into the northern
drainage.
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Figure E-10. View of the Storm Drain from Below Tamarack
that Empties into the Northern Drainage
The engineered drainage features for the site were observed during site reconnaissance. The structures were found to be in good condition, and clear of debris that could affect site drainage. Figures E-11 and E-12 (following page) show existing conditions within the V-ditches providing drainage to the slopes.
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Figure E-11. Northern Slope V-ditch below Hope Elementary School
Figure E-12. Southern Slope V-ditch below Hope Elementary School
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Geologic Subsurface Investigation
As is discussed in more detail in the text of this report, as supported by documentation included with
Appendix B, Appendix C and Appendix D, NOVA completed a subsurface investigation at the Hope Elementary campus with a twofold intent.
1. Geohazards. Assessment of geologic and geotechnical hazards associated with Hope Elementary School, with specific concern for the potential that the school could be affected by active or dormant landslides. 2. Geotechnical. Development of subsurface information sufficient to provide recommendations for earthwork and foundation-related design and construction. The assessment of the geohazards was directed toward a landslide and slope stability evaluation with the scope of subsurface exploration described below
• Two 30-inch diameter geologic borings were drilled and downhole logged to 60 and 78 feet in depth.
• A single geologic core boring was drilled to a depth of 120 feet using HQ sized coring tools to extract 2.5-inch diameter rock core. The core boring was drilled through a large-diameter boring (LD-1) which was abandoned without downhole logging due to unsafe conditions.
• Eleven (11) geotechnical borings (B-1 through B-11) were drilled with primarily geotechnical intent, though also providing geologic information.
Findings
The geologic reconnaissance did not observe any landslide geomorphology or related ground conditions that would suggest the presence of active landsliding. The subsurface investigation did not disclose the presence of landslide deposits or pervasive rock fracturing, shear zones, or deformed and disturbed
bedding, which would characterize a landslide deposit.
Assessment of the Landslide Hazard
The school is underlain by Member B of the Eocene-aged Santiago Formation to the total depth explored
during downhole logging and rock coring. This member was not observed to contain claystone layers. Weber 1982 notes that the probability of landslides in Eocene rocks is greater in slopes having a higher portion of clay-bearing layers. No such layers are evident within the Santiago Formation at the school,
diminishing the potential that the Santiago Formation below the school is slide prone.
The rock observed by the geologic and geotechnical borings was generally found to be indurated, increasing its total strength and stability. Characterized by generally high strength and limited fracturing,
this unit does not have the general characteristics of a slide-prone unit. Geologic reconnaissance and careful review of historic aerial photography confirms a lack of decisive geomorphic expression of landsliding.
A landslide investigation performed by Geocon in 2015 on a property within the limits of the same landslide complex studied in this investigation, concluded that the landslide does not exist.
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In consideration of the foregoing, NOVA judges that this site is not located on an active or dormant
landslide and is suitable for its intended use.
APPENDIX F
SLOPE STABILITY ANALYSES
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APPENDIX F
GLOBAL SLOPE STABILITY ANALYSES HOPE ELEMENTARY SCHOOL CARLSBAD, CALIFORNIA
METHOD OF STABILITY ANALYSES
General
Slope stability analyses were performed using the SLIDE v5.0 (Rocscience, Inc.) computer program to calculate the factors of safety against slope failure using limit equilibrium procedures and assuming two-dimensional, plane strain conditions. SLIDE completes 2D stability calculations in rocks or soils offering the user the choice of procedures of varying rigor. The less rigorous alternatives available with SLIDE are: Bishop simplified, Corps of Engineers, Janbu simplified/corrected, Lowe-Karafiath and Ordinary/Fellenius. The more rigorous choices include the Spencer and Morgenstern-Price procedures.
The program allows the user to complete alternative evaluations of embankment safety, as abstracted below.
• Deterministic. Analyses calculate the lowest factor of safety for given soil parameters and slopes.
• Probabilistic. Analyses vary sensitive input parameters such as soil strength (e.g., cohesion and friction angle) to determine the probability of failure, an alternative representation of safety. Both deterministic and probabilistic stability analyses were undertaken. The probabilistic analyses of embankment stability undertaken using SLIDE include two statistical parameters with the output.
• Reliability Index (RI). RI is a statistical parameter that represents the number of standard deviations which separate the mean FS, from the critical FS (i.e., FS = 1).
• Probability of Failure (PF). Probabilistic analyses include evaluations of the stability of the embankment at reduced soil strengths. PF is the probability slope evaluations with statistically reduced soil strengths will yield instances where FS < 1.
Spencer’s Procedure
Spencer's procedure was used for this work. The differences between the many alternative procedures of limit equilibrium analyses are largely due to varying hypotheses regarding the location and direction of internal forces within the sliding soil mass. The assumption inherent in all limit equilibrium procedures is
that the soil is at limit equilibrium with a constant FS along the entire slip surface.
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Limit equilibrium analysis procedures currently in use do not model progressive failure mechanisms, which can occur in materials of widely dissimilar stress-strain characteristics. This consideration is of
limited consequence for the denser, cemented soils analyzed in this instance.
Spencer's procedure was selected for this analysis because of its relative rigor in solution of equations of equilibrium for both moments and forces. Duncan (1992) recommends the use of Spencer’s procedure, assessing it to generally be within 12 percent of that computed by other analyses of similar capability and within 6 percent of what may fairly be considered the correct answer. Like all limit equilibrium methods of slope stability analysis, the factor of safety (FS) calculated by the Spencer procedure uses:
FS = shear strength of the soil (resisting force)
shear stress required for equilibrium (driving force)
NUMERICAL MODEL
Slope Geometry
Plates F-1 through F-4, provided following the text of this report depict the slope profile and stratigraphy utilized for the global stability analyses. As may be seen by review of these graphics, the embankments
support a mantle of Unit 1 fill. The subsurface below the fill is Unit 2 Santiago Formation.
Groundwater
Stability analyses assumed the groundwater surface to be well below the toes of the slopes considered by the analyses. Effective stress analyses were performed assuming drained strength parameters.
Strength of the Soil Units
Deterministic stability analyses were undertaken using the effective stress parameters listed in Table 1.
Table 1. Effective Stress Strength Parameters Used in the Deterministic Analyses
Unit Reference
Effective Stress Parameters
Unit Wt
(γ, pcf)
Friction
(ø’, deg)
Cohesion
(c’, psf)
1 Qf- Fill 125 35 0
2 Tsa- Santiago Formation 115 35 200
The strength of the Unit 1 fill was assumed based on the indications of in situ testing associated with the geotechnical borings. The strength of the Unit 2 sandstones were conservatively assumed based on
NOVA’s experience with these units in other projects in the Carlsbad area, as well as the indications of direct shear and unconfined compression testing of relatively undisturbed samples and of core recovered from drilling, respectively.
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Strength Parameters for Probabilistic Analyses
Probabilistic analyses were undertaken to allow for the certainty of variations from the strength values of
Table 1. It is highly unlikely that each soil/rock unit is properly characterized by a single strength. As is evident by review of Table 2, it is more likely that each soil/rock unit includes a range of strengths.
Table 2. Values of Coefficient of Variation (COV) 1,2
Parameter Coefficient of Variation
Unit weight (γ) 3 – 7 %
Effective stress friction angle (ø’) 2 – 13 %
Cohesion (c) 13 – 40 %
Note 1. COV is a statistical measure of the ‘dispersion’ of data, defined as the ratio of the standard deviation to the mean. Note 2. Table 2 is adapted from Duncan 2000.1 It is well known that both cohesion and angle of friction can vary substantially for a given soil unit over relatively short horizontal distances and depths. Table 2 provides an indication of this variability, from which it can be seen that while soil unit weight and angle of friction can be determined with confidence, soil cohesion can vary substantially.
When used in probabilistic analyses, SLIDE v5.0 varies a model parameter identified as a ‘variable’ about a mean value within a defined range. The probabilistic analyses conducted for this review considered slope geometry to be fixed, and varied the friction and cohesion about a mean value, computing the safety factor for the global minimum slip surface. The range of strength parameters used in the probabilistic analyses is tabulated on Table 3.
Table 3. Effective Stress Soil Strength Parameters Used in Probabilistic Analyses
Soil Unit Description
Low Parameter High Parameter
Friction
(ø’)
Cohesion
(c’)
Unit
Wt (γ)
Friction
(ø’)
Cohesion
(c’)
Unit
Wt (γ)
1 Qf- Fill 32 0 125 38 0 125
2 Tsa- Santiago Formation 20 100 120 26 400 120
1 Duncan, J. M. 2000, Factors Of Safety And Reliability In Geotechnical Engineering, Journal Of Geotechnical and Geoenvironmental Engineering, ASCE. Vol. 126:4. P 307-316. See also: Christian. J. T. , Ladd, C. C., and Baecher, G. B. 1994, Reliability Applied To Slope Stability Analysis, Journal of Geotechnical Engineering, ASCE. Vol. 120:12. P 2180-2207.
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Pseudo-Static Analyses
Analyses of seismic slope stability problems using limit equilibrium methods model the inertia forces due
to earthquake shaking by a constant horizontal force (equal to the weight of the potential sliding mass multiplied by a coefficient). These analyses are commonly referred to as “pseudostatic analyses.”
In pseudostatic slope stability analyses such as that described in this memo, FS is computed using a limit
equilibrium method in which a static horizontal inertial force that is intended to represent the destabilizing effects of the earthquake is applied to the potential sliding mass. The horizontal inertial force is expressed as the product of a seismic coefficient (k) and the weight (W) of the potential sliding mass. The
pseudostatic analysis also requires the use of appropriate material dynamic strengths. If the FS approaches unity, then the embankment is considered unsafe. Such analyses are relatively simple to perform, appropriate for applications such as this site, where the materials involved will not undergo a significant
loss of strength during earthquake shaking.
The U.S. Army Corps of Engineers (USACE) manual for seismic design of new dams requires use of a seismic coefficient of 0.1 to 0.15 in high to very high seismic areas, in conjunction with a minimum factor of safety (FS) of FS = 1.0. Seed (1979) drew the general conclusion that for embankments composed of materials which show no significant loss of strength as a result of cyclic loading,
"…. it is only necessary to perform a pseudo-static analysis for a seismic coefficient of 0.1 for magnitude 6.5 earthquakes or 0.15 for magnitude 8.25 earthquakes and obtain a factor of safety of the order of 1.15 to ensure that displacements will be acceptably
small". 2
The embankments are located in a higher risk seismic area, with an expected PGA of about 0.4 g in a M = 7 event. Accordingly, seismic stability was modeled with a seismic coefficient of kh = 0.15.
Target Stability
Static The standards for such embankments should be in general conformance with the standards for such analyses provided by the US Army Corps of Engineers (USACE 2003). USACE recommends long term static slope stability for circumstances such as the embankments addressed herein target FS = 1.5 USACE 2003 recommends that long term static embankment stability for permanent developments such
as that at this site target FSstatic ≥ 1.5. 3
Seismic USACE and the industry allows more judgment in the assessment of allowable factor of safety for seismic slope stability (i.e., FSseismic). As a matter of practice, FERC (Federal Energy Regulatory Commission) and the NRCS (Natural Resources Conservation Service), both federal regulators of large
2 Seed, H. B., 1979. Considerations In The Earthquake-Resistant Design Of Earth And Rockfill Dams,
Geotechnique 29, 215–263.
3 U.S. Army Corps of Engineers, Slope Stability, Engineering Manual 1110-2-1902, 31 Oct 2003. See also U.S. Army Corps of Engineers, 1999, Risk-Based Analysis In Geotechnical Engineering For Support Of Planning Studies. Engineering Technical Letter No. 1110-2-556, 28 May 1999.
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embankment dams, seek seismic stability in the range FSseismic > 1 to FSseismic > 1.1 for slopes evaluated using the pseudostatic methods described herein.
Stability Results, Section A-A’
Static The global stability of Section A-A’ was analyzed as representative of the worst case of stability for
embankments descending from the north of the school grounds, in the direction of the ‘landslide’ identified in the documentation reviewed for this report.
Figure 1 depicts the static analysis, indicating that the slope meets the static stability criteria described
above, with FSstatic = 1.5. As may be seen by review of Figure 1, the analysis indicates that the deterministic failure plane occurs with a rotation through the toe of the approximately 30 feet of fill above the Santiago Formation. Figure 1 is provided in larger scale as Plate 1 in Attachment 1 to this appendix.
Figure 1. Static Embankment Stability, Section A-A’, FS = 2.2
Analysis of Section A-A’ included 1,000 separate evaluations of stability. Random variations of the
strength of the separate soil units within the ranges indicated on Table 2 yields a view of a safe slope, with no instance in which FS < 1. Defined as Probability of Failure (PF), the analyses indicate PF = 0%.
Seismic Figure 2 depicts the results of the seismic analysis of Section A-A’, indicating FSseismic ~ 1.25. This FS meets the stability criteria discussed above, suggesting the slope will be stable in a M ~ 7 seismic event. The deterministic failure plane occurs as rotation through the toe of the fill embankment that sits atop the Santiago Formation. Probabilistic analyses indicate PF – 0 for seismic stability (i.e., 0 surfaces failed in 1,000 surfaces tested within the slope limits).
Figure 2 is provided in larger scale as Plate 2 in Attachment 1 to this appendix.
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Figure 2. Seismic Embankment Stability, Section A-A’, F = 1.45, kh = 0.15
Stability Results, Section C-C’
Static The global stability of Section C-C’ was analyzed as representative of the worst case of stability for
embankments in the area of the existing and planned school buildings.
Figure 3 depicts the outcome of the static analysis for Section C-C’, indicating that global stability exceeds the static stability criteria described herein, with FSstatic > 1.5. As may be seen by review of Figure 3, the analysis indicates that the deterministic failure plane occurs with a rotation through the toe of the Unit 1 fill embankment that sits atop the Santiago Fm. The failure plane daylights about 13 feet inside the crest of the slope.
Figure 3 is provided in larger scale as Plate 3 in Attachment 1 to this appendix.
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Figure 3. Static Embankment Stability, Section C-C’, FS = 1.6
Analysis included 1,000 separate evaluations of stability. Random variations of the strength of the separate soil units within the ranges indicated on Table 2 yields a view of a safe slope, with no instances in which FS < 1. Thus, the analyses indicate PF ~ 0.
Seismic Figure 4 depicts the results of the seismic analysis for Section C-C’, indicating FSseismic ~ 1.15. This FS
meets the stability criteria discussed herein, suggesting that this slope will be stable in a major (i.e., M ~ 7) seismic event. The deterministic failure plane is similar to the static case: rotation through the toe of the Unit 1 fill embankment that is developed atop the Santiago Fm. These analyses indicate PF ~ 11% for
seismic stability (i.e., 106 surfaces failed in 1,000 surfaces tested within the slope limits).
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Figure 4 is provided in larger scale as Plate 4 in Attachment 1 to this appendix.
Figure 4. Seismic Embankment Stability, Section C-C’, F = 1.15, kh = 0.15
DISCUSSION
The Site Embankments Are Stable The quantitative results provided in this analyses show that the slopes that descend from the Hope Elementary School campus are stable. With a view for the potential effects of uncertainty in the
subsurface information, probabilistic evaluations were used to vary soil strength parameters sufficiently to provide a fair estimate of the global stability of the embankments for a variety of conditions. The indications of the assessment of global stability of the embankments are summarized below.
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1. Static Stability. This modeling shows that all of the embankments are stable against a deep-seated rotational failure. However, the embankment could be at risk for other modes of
failure controlled by factors not amenable to this modeling (most significantly, erosional processes). Similarly, relatively localized failures could also occur (for example, shallow-seated sliding in the Unit 1 fill if the embankments is not maintained. 2. Seismic Stability. All of the slopes will be safe against global rotational failure in a larger-scale (M ~ 7) seismic event.
Structure Setbacks Analysis of the worst case of embankment stability (Section C-C’) within the area of the school structures indicates that the worst-case failure plane extends to about 18 feet inside the crest of the embankment. Though still safe at this point, this dimension cautions that school structures should be set back a minimum of 20 feet from the crest of the embankment.
The Importance of Slope Maintenance Regular maintenance is essential to the continued stability of all the embankments. While the stability of the embankments against deeper seated slope failure is high, localized surficial sloughing related to
erosion may occur. Such instabilities may be managed by implementation of routine maintenance of the embankments.
The greatest threat to embankment stability is a loss of control of surface drainage. Reconnaissance of the
embankments indicates that the present design of the embankments provides for collection and direction of surface water to collection devices located away from the embankments. The function of this design should be checked during wet weather to confirm that control of surface water is occurring as planned. As necessary, berms, curbs, gutters, swales or other devices may need to be added to prevent an excessive amount of concentrated runoff from draining over the crest of the embankments and creating erosion problems. The embankments should be inspected on a regular basis, observing signs of surface erosion, loss of vegetative/ground cover, sloughing, etc. Loss of ground at the toe of any embankment can affect stability. Repairs should be made as appropriate.
Limitations of this Assessment The assessment of global embankment stability addressed by this evaluation is limited to a quantitative evaluation of the balance of slope stressors and soil resisting strength within the narrow physical limits
(both known and assumed) described by this report.
As used herein, ‘embankment stability’ is intended to mean the safety of localized natural or man-made embankments against failure. Unlike landslides described
above, embankment stability includes smaller-scale slope failures such as erosion-related washouts and more subtle, as well as less evident processes such as slope ‘creep.’
As with all analyses of this genre, this assessment of embankment stability in a specific, limited site area should be understood to be distinct from an assessment of ‘landslide’ risk. The risk of an embankment failure is distinct from the risk of a ‘landslide.’ NOVA considers a landslide to be a phenomenon as
described below.
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‘Landslide’ describes downslope displacement of a mass of rock, soil, and/or debris by sliding, flowing, or falling. Such mass earth movements are greater than about 10
feet thick and larger than 300 feet across. Landslides typically include cohesive block glides and disrupted slumps that are formed by translation or rotation of the slope materials along one or more slip surfaces. These mass displacements can also include
similarly larger-scale, but more narrowly confined modes of mass wasting such as ‘mud flows’ and ‘debris flows’. The causes of classic landslides start with a preexisting condition- characteristically, a plane of weak soil or rock- inherent within
the rock or soil mass. Thereafter, movement may be precipitated by earthquakes, wet weather, and changes to the structure or loading conditions on a slope (e.g., by erosion, cutting, filling, release of water from broken pipes, etc.).
Attachments:
Attachment 1: Plates 1-4 Attachment 2: Output of the Global Stability Analyses
ATTACHMENT 1 PLATES F-1 THROUGH F-4
ATTACHMENT 2 OUTPUT OF THE GLOBAL STABILITY ANALYSES
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Slide Analysis Information
Section A-A’ Static
Document Name
File Name: Hope ES Stability Section AA.sli
Project Settings
Project Title: Hope ES Section A-A'
Failure Direction: Right to Left
Units of Measurement: Imperial Units
Pore Fluid Unit Weight: 62.4 lb/ft3
Groundwater Method: Water Surfaces
Data Output: Standard
Calculate Excess Pore Pressure: Off
Allow Ru with Water Surfaces or Grids: Off
Random Numbers: Pseudo-random Seed
Random Number Seed: 10116
Random Number Generation Method: Park and Miller v.3
Analysis Methods
Analysis Methods used:
Spencer
Number of slices: 25
Tolerance: 0.005
Maximum number of iterations: 50
Surface Options
Surface Type: Circular
Search Method: Grid Search
Radius increment: 10
Composite Surfaces: Disabled
Reverse Curvature: Create Tension Crack
Minimum Elevation: Not Defined
Minimum Depth: Not Defined
Material Properties
Material: Engineered Fill (Af)
Strength Type: Mohr-Coulomb
Unit Weight: 125 lb/ft3
Cohesion: 0 psf
Friction Angle: 35 degrees
Water Surface: None
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Material: Santiago Fm (Tsa)
Strength Type: Mohr-Coulomb
Unit Weight: 115 lb/ft3
Cohesion: 300 psf
Friction Angle: 23 degrees
Water Surface: Water Table
Custom Hu value: 1
Probabilistic Analysis Input
Project Settings
Sensitivity Analysis: Off
Probabilistic Analysis: On
Sampling Method: Monte-Carlo
Number of Samples: 1000
Analysis Type: Global Minimum
Material: Engineered Fill (Af)
Property: Cohesion
Distribution: Normal
Minimum: 0 (relative minimum: 0)
Mean: 0
Maximum: 0 (relative maximum: 0)
Standard Deviation: 0
Material: Engineered Fill (Af)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 32)
Mean: 35
Maximum: 73 (relative maximum: 38)
Standard Deviation: 3
Material: Santiago Fm (Tsa)
Property: Cohesion
Distribution: Normal
Minimum: 100 (relative minimum: 200)
Mean: 300
Maximum: 700 (relative maximum: 400)
Standard Deviation: 100
Material: Santiago Fm (Tsa)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 20)
Mean: 23
Maximum: 49 (relative maximum: 26)
Standard Deviation: 3
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List of All Coordinates
Material Boundary
271 185
368 195
662 209
External Boundary
185 100
662 100
662 209
662 225
566 223
470 221
411 220
376 219
350 215
340 210
319 200
295 195
271 185
219 175
185 165
Water Table
185 149
368 182
662 182
Search Grid
139 247
437 247
437 544
139 544
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Slide Analysis Information
Section A-A’ Seismic
Document Name
File Name: Hope ES Stability Section AA.sli
Project Settings
Project Title: Hope ES Section A-A'
Failure Direction: Right to Left
Units of Measurement: Imperial Units
Pore Fluid Unit Weight: 62.4 lb/ft3
Groundwater Method: Water Surfaces
Data Output: Standard
Calculate Excess Pore Pressure: Off
Allow Ru with Water Surfaces or Grids: Off
Random Numbers: Pseudo-random Seed
Random Number Seed: 10116
Random Number Generation Method: Park and Miller v.3
Analysis Methods
Analysis Methods used:
Spencer
Number of slices: 25
Tolerance: 0.005
Maximum number of iterations: 50
Surface Options
Surface Type: Circular
Search Method: Grid Search
Radius increment: 10
Composite Surfaces: Disabled
Reverse Curvature: Create Tension Crack
Minimum Elevation: Not Defined
Minimum Depth: Not Defined
Loading
Seismic Load Coefficient (Horizontal): 0.15
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Material Properties
Material: Engineered Fill (Af)
Strength Type: Mohr-Coulomb
Unit Weight: 125 lb/ft3
Cohesion: 0 psf
Friction Angle: 35 degrees
Water Surface: None
Material: Santiago Fm (Tsa)
Strength Type: Mohr-Coulomb
Unit Weight: 115 lb/ft3
Cohesion: 300 psf
Friction Angle: 23 degrees
Water Surface: Water Table
Custom Hu value: 1
Global Minimums
Method: spencer
FS: 1.435790
Center: 243.624, 470.017
Radius: 286.076
Left Slip Surface Endpoint: 271.797, 185.332
Right Slip Surface Endpoint: 381.112, 219.146
Resisting Moment=1.5456e+007 lb-ft
Driving Moment=1.07648e+007 lb-ft
Resisting Horizontal Force=51414.8 lb
Driving Horizontal Force=35809.4 lb
Probabilistic Analysis Input
Project Settings
Sensitivity Analysis: Off
Probabilistic Analysis: On
Sampling Method: Monte-Carlo
Number of Samples: 1000
Analysis Type: Global Minimum
Material: Engineered Fill (Af)
Property: Cohesion
Distribution: Normal
Minimum: 0 (relative minimum: 0)
Mean: 0
Maximum: 0 (relative maximum: 0)
Standard Deviation: 0
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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7
Material: Engineered Fill (Af)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 32)
Mean: 35
Maximum: 73 (relative maximum: 38)
Standard Deviation: 3
Material: Santiago Fm (Tsa)
Property: Cohesion
Distribution: Normal
Minimum: 100 (relative minimum: 200)
Mean: 300
Maximum: 700 (relative maximum: 400)
Standard Deviation: 100
Material: Santiago Fm (Tsa)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 20)
Mean: 23
Maximum: 49 (relative maximum: 26)
Standard Deviation: 3
Probabilistic Analysis Results (Global Minimum)
Method: spencer
Factor of Safety, mean: 1.448633
Factor of Safety, standard deviation: 0.163980
Factor of Safety, minimum: 1.001770
Factor of Safety, maximum: 2.010650
Probability of Failure: 0.000% (= 0 failed surfaces / 1000 valid surfaces)
Reliability index: 2.73591 (assuming normal distribution)
Reliability index: 3.22818 (assuming lognormal distribution)
* best fit = Gamma
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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List of All Coordinates
Search Grid
139.440 246.766
437.107 246.766
437.107 544.434
139.440 544.434
Material Boundary
271.000 185.000
367.742 194.623
662.000 208.600
External Boundary
185.000 100.000
662.000 100.000
662.000 208.600
662.000 225.000
566.000 223.000
470.000 221.000
411.000 220.000
376.000 219.000
350.000 215.000
340.000 210.000
319.000 200.000
295.000 195.000
271.000 185.000
219.000 175.000
185.000 165.000
Water Table
185.000 148.996
368.154 181.881
662.000 181.881
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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9
Slide Analysis Information
Section C-C’ Static
Document Name
File Name: Hope ES Stability Section CC.sli
Project Settings
Project Title: Hope Elementary School Sec C-C'
Failure Direction: Right to Left
Units of Measurement: Imperial Units
Pore Fluid Unit Weight: 62.4 lb/ft3
Groundwater Method: Water Surfaces
Data Output: Standard
Calculate Excess Pore Pressure: Off
Allow Ru with Water Surfaces or Grids: Off
Random Numbers: Pseudo-random Seed
Random Number Seed: 10116
Random Number Generation Method: Park and Miller v.3
Analysis Methods
Analysis Methods used:
Spencer
Number of slices: 25
Tolerance: 0.005
Maximum number of iterations: 50
Surface Options
Surface Type: Circular
Search Method: Grid Search
Radius increment: 10
Composite Surfaces: Disabled
Reverse Curvature: Create Tension Crack
Minimum Elevation: Not Defined
Minimum Depth: Not Defined
Material Properties
Material: Engineered Fill (Af)
Strength Type: Mohr-Coulomb
Unit Weight: 125 lb/ft3
Cohesion: 0 psf
Friction Angle: 35 degrees
Water Surface: None
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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Material: Santiago Fm (Tsa)
Strength Type: Mohr-Coulomb
Unit Weight: 120 lb/ft3
Cohesion: 300 psf
Friction Angle: 23 degrees
Water Surface: Water Table
Custom Hu value: 1
Global Minimums
Method: spencer
FS: 1.570430
Center: 317.229, 342.456
Radius: 140.399
Left Slip Surface Endpoint: 347.610, 205.383
Right Slip Surface Endpoint: 397.116, 227.000
Resisting Moment=2.90138e+006 lb-ft
Driving Moment=1.84751e+006 lb-ft
Resisting Horizontal Force=18825.4 lb
Driving Horizontal Force=11987.4 lb
Valid / Invalid Surfaces
Method: spencer
Number of Valid Surfaces: 2453
Number of Invalid Surfaces: 2398
Error Codes:
Error Code -1000 reported for 2398 surfaces
Error Codes
The following errors were encountered during the computation:
-1000 = No valid slip surfaces are generated
at a grid center. Unable to draw a surface.
Probabilistic Analysis Input
Project Settings
Sensitivity Analysis: Off
Probabilistic Analysis: On
Sampling Method: Monte-Carlo
Number of Samples: 1000
Analysis Type: Global Minimum
Material: Engineered Fill (Af)
Property: Cohesion
Distribution: Normal
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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11
Minimum: 0 (relative minimum: 0)
Mean: 0
Maximum: 0 (relative maximum: 0)
Standard Deviation: 0
Material: Engineered Fill (Af)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 32)
Mean: 35
Maximum: 73 (relative maximum: 38)
Standard Deviation: 3
Material: Santiago Fm (Tsa)
Property: Cohesion
Distribution: Normal
Minimum: 100 (relative minimum: 200)
Mean: 300
Maximum: 700 (relative maximum: 400)
Standard Deviation: 100
Material: Santiago Fm (Tsa)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 20)
Mean: 23
Maximum: 49 (relative maximum: 26)
Standard Deviation: 3
Probabilistic Analysis Results (Global Minimum)
Method: spencer
Factor of Safety, mean: 1.581670
Factor of Safety, standard deviation: 0.179038
Factor of Safety, minimum: 1.093770
Factor of Safety, maximum: 2.195300
Probability of Failure: 0.000% (= 0 failed surfaces / 1000 valid surfaces)
Reliability index: 3.24886 (assuming normal distribution)
Reliability index: 4.00686 (assuming lognormal distribution)
* best fit = Gamma
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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12
List of All Coordinates
Search Grid
280.382 250.337
403.208 250.337
403.208 373.163
280.382 373.163
Material Boundary
347.000 205.000
356.556 205.168
385.318 209.668
471.910 222.726
External Boundary
210.000 120.000
471.910 120.000
471.910 222.726
471.910 227.000
382.000 227.000
347.000 205.000
329.000 200.000
299.000 180.000
266.000 160.000
255.000 165.000
210.000 165.000
Water Table
210.000 148.080
304.632 148.080
471.910 161.906
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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13
Slide Analysis Information
Section C-C’ Seismic
Document Name
File Name: Hope ES Stability Section CC.sli
Project Settings
Project Title: Hope Elementary School Sec C-C'
Failure Direction: Right to Left
Units of Measurement: Imperial Units
Pore Fluid Unit Weight: 62.4 lb/ft3
Groundwater Method: Water Surfaces
Data Output: Standard
Calculate Excess Pore Pressure: Off
Allow Ru with Water Surfaces or Grids: Off
Random Numbers: Pseudo-random Seed
Random Number Seed: 10116
Random Number Generation Method: Park and Miller v.3
Analysis Methods
Analysis Methods used:
Spencer
Number of slices: 25
Tolerance: 0.005
Maximum number of iterations: 50
Surface Options
Surface Type: Circular
Search Method: Grid Search
Radius increment: 10
Composite Surfaces: Disabled
Reverse Curvature: Create Tension Crack
Minimum Elevation: Not Defined
Minimum Depth: Not Defined
Loading
Seismic Load Coefficient (Horizontal): 0.15
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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14
Material Properties
Material: Engineered Fill (Af)
Strength Type: Mohr-Coulomb
Unit Weight: 125 lb/ft3
Cohesion: 0 psf
Friction Angle: 35 degrees
Water Surface: None
Material: Santiago Fm (Tsa)
Strength Type: Mohr-Coulomb
Unit Weight: 120 lb/ft3
Cohesion: 300 psf
Friction Angle: 23 degrees
Water Surface: Water Table
Custom Hu value: 1
Global Minimums
Method: spencer
FS: 1.152220
Center: 311.088, 367.022
Radius: 165.585
Left Slip Surface Endpoint: 347.937, 205.589
Right Slip Surface Endpoint: 399.476, 227.000
Resisting Moment=3.44421e+006 lb-ft
Driving Moment=2.98919e+006 lb-ft
Resisting Horizontal Force=19189.8 lb
Driving Horizontal Force=16654.6 lb
Probabilistic Analysis Input
Project Settings
Sensitivity Analysis: Off
Probabilistic Analysis: On
Sampling Method: Monte-Carlo
Number of Samples: 1000
Analysis Type: Global Minimum
Material: Engineered Fill (Af)
Property: Cohesion
Distribution: Normal
Minimum: 0 (relative minimum: 0)
Mean: 0
Maximum: 0 (relative maximum: 0)
Standard Deviation: 0
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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15
Material: Engineered Fill (Af)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 32)
Mean: 35
Maximum: 73 (relative maximum: 38)
Standard Deviation: 3
Material: Santiago Fm (Tsa)
Property: Cohesion
Distribution: Normal
Minimum: 100 (relative minimum: 200)
Mean: 300
Maximum: 700 (relative maximum: 400)
Standard Deviation: 100
Material: Santiago Fm (Tsa)
Property: Phi
Distribution: Normal
Minimum: 3 (relative minimum: 20)
Mean: 23
Maximum: 49 (relative maximum: 26)
Standard Deviation: 3
Probabilistic Analysis Results (Global Minimum)
Method: spencer
Factor of Safety, mean: 1.161790
Factor of Safety, standard deviation: 0.131496
Factor of Safety, minimum: 0.803205
Factor of Safety, maximum: 1.614540
Probability of Failure: 10.600% (= 106 failed surfaces / 1000 valid surfaces)
Reliability index: 1.23038 (assuming normal distribution)
Reliability index: 1.27276 (assuming lognormal distribution)
* best fit = Gamma
List of All Coordinates
Search Grid
280.382 250.337
403.208 250.337
403.208 373.163
280.382 373.163
Material Boundary
347.000 205.000
356.556 205.168
385.318 209.668
471.910 222.726
Report of Geotechnical Investigation, Appendix F September 05, 2019 Hope Elementary School Modernization, Carlsbad, CA NOVA Project 2019157
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16
External Boundary
210.000 120.000
471.910 120.000
471.910 222.726
471.910 227.000
382.000 227.000
347.000 205.000
329.000 200.000
299.000 180.000
266.000 160.000
255.000 165.000
210.000 165.000
Water Table
210.000 148.080
304.632 148.080
471.910 161.906