HomeMy WebLinkAboutCT 15-05; QUARRY CREEK PLANNING AREA R3; GEOTECHNICAL INVESTIGATION; 2015-02-24UPDATE
GEOTECHNICAL INVESTIGATION
QUARRY CREEK
CARLSBAD/OCEANSIDE,, CALIFORNIA
PREPARED FOR
CORNERSTONE COMMUNITIES
SAN DIEGO, CALIFORNIA
FEBRUARY 24, 2015
PROJECT NO. 07135··42-05
GEOCON
INCORPORATED
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Project No. 07135-42-05
February 24, 2015
Cornerstone Communities
4365 Executive Drive, Suite 600
San Diego, California 92121
Attention:
Subject:
Mr. Jack Robson
UPDATE GEOTECHNICAL INVESTIGATION
QUARRY CREEK
CARLSBAD/OCEANSIDE, CALIFORNIA
Dear Mr. Robson:
In accordance with your authorization, we are pleased to submit the results of our update
geotechnical investigation for the proposed Quarry Creek project. Conclusions and recommendations
presented herein are based on review of available published geotechnical reports and literature,
observations during previous grading performed on the property for reclamation, previous subsurface
geotechnical exploration and site reconnaissance of existing conditions.
The eastern half of the property has been used for mining and crushing rock to produce commercial
aggregates. As the result, the eastern half of the site is underlain by compacted fill, previously placed
fill, undocumented fill, sedimentary, volcanic, and intrusive bedrock. Reclamation grading has
occurred on this portion of the site. The western half of the site is in an ungraded natural condition.
The accompanying report presents fmdings from our studies relative to geotechnical engineering
aspects of developing the property. The site is considered suitable for the proposed improvements
provided the recommendations of this report are followed.
Should you have questions regarding this report, or if we may be of further service, please contact
the undersigned at your convenience.
Very truly yours,
GEOCON INCORPORATED
Ali Sadr
,CEG 1778
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6960 Fl:mders Driv-a a Son Diego, Colffornio 9212.!.297 4 II Tek:phone 8.58.558.6900 II Fcx &58.553.6159
TABLE OF CONTENTS
1. PURPOSE AND SCOPE ...................................................................................................................... 1
2. PREVIOUS SITE USAGE AND GRADING ....................................................................................... 2
3. SITE AND PROJECT DESCRIPTION ................................................................................................ 3
4. SOIL AND GEOLOGIC CONDITIONS .............................................................................................. 3
4.1 Compacted Fill (Qcf) .................................................................................................................. 4
4.2 Undocumented Fill (Qudf) ......................................................................................................... 4
4.3 Previously Placed Compacted Fill (Qpcf) .................................................................................. 4
4.4 Previously Placed Fill (Qpf) ....................................................................................................... 5
4.5 Topsoil (Unmapped) ................................................................................................................... 5
4.6 Surficial Landslide Debris (Qlsf) ............................................................................................... 5
4.7 Alluvium (Qal) ........................................................................................................................... 5
4.8 Colluvium (Qc) ............................................................................................................................ 6
4.9 Terrace Deposits (Qt) .................................................................................................................. 6
4.10 Santiago Formation (Ts) ............................................................................................................. 6
4.11 Salto Intrusive (Jspi) ........................................ , .......................................................................... 7
5. GROUNDWATER ............................................................................................................................... 7
6. GEOLOGIC HAZARDS ....................................................................................................................... 7
6.1 Faulting and Seismicity .............................................................................................................. 7
6.2 Liquefaction ................................................................................................................................ 9
6.3 Flow Slide Potential ................................................................................................................. 10
6.4 Landslides ................................................................................................................................. 10
7. CONCLUSIONS AND RECOMMENDATIONS .............................................................................. 11
7.1 General ..................................................................................................................................... 11
7.2 Excavation and Soil Characteristics ......................................................................................... 11
7.3 Subdrains .................................................................................................................................. 12
7.4 Grading ..................................................................................................................................... 13
7.5 Surcharge Fill ........................................................................................................................... 16
7.6 Settlement Monitoring .............................................................................................................. 16
7.7 Slope Stability .......................................................................................................................... 16
7.8 Seismic Design Criteria ............................................................................................................ 18
7.9 Foundation and Concrete Slab-On-Grade Recommendations ................................................... 19
7.10 Retaining Wall Recommendations ........................................................................................... 24
7.11 Detention Basin and Bios wale Recommendations .................................................................... 26
7.12 Site Drainage and Moisture Protection .................................................................................... 27
LIMITATIONS AND UNIFORMITY OF CONDITIONS
TABLE OF CONTENTS (Concluded)
MAPS AND ILLUSTRATIONS
Figure 1, Vicinity Map
Figure 2 and 3, Geologic Map
Figure 4, Geologic Cross Sections AA' through DD'
Figure 5, Typical Canyon Subdrain Detail
Figure 6, Typical Subdrain Cut-Off Wall Detail
Figure 7, Typical Subdrain Outlet Headwall Detail
Figure 8, Constmction Detail for Lateral Extent of Removal
Figure 9, Settlement Monument
Figures 10 -14, Slope Stability Analysis
Figure 15, Typical buttress Fill Detail
Figure 16, Typical Retaining Wall Drain Detail
APPENDIX A
FIELD INVESTIGATION
Figures A-1-A-5, Logs of Large Diameter Borings
Figures A-6-A-27, Logs of Trenches
Figures A-28 -A-33, Logs of Small Diameter Borings
Figures A-34-A-39, Logs of Trenches (Geocon 9-10-09)
APPENDIXB
LABORATORY TESTING
Table B-I, Summary of Laboratory Maximum Dry Density and Optimum Moisture Content
Table B-II, Summary of Laboratory Direct Shear Test Results
Table B-Ill, Summary of Laboratory Expansion Index Test Results
Table B-IV, Summary of Laboratory Expansion Index Test Results
Performed During Reclamation Grading
Table B-V, Summary of Laboratory Water-Soluble Sulfate Test Results
Table B-VI, Summary ofLaboratory Water-Soluble Sulfate Test Results
Performed During Reclamation Grading
APPENDIXC
SLOPE STABILITY ANALYSIS
Table C-I, Summary of Soil Properties used for Slope Stability Analyses
Table C-II, Summary of Slope Stability Analyses
Figure C-1, Cut Slope
Figures C-2-C-3, Buttress Fill Slope
APPENDIXD
FLOW ANALYSIS
Figure D-1, Stability Analysis for Flow Slide Potential
Figure D-2, Stability Analysis to Determine Yield Coefficient
APPENDIX E
RECOMMENDED GRADrNG SPECIFICATIONS
LIST OF REFERENCES
UPDATE GEOTECHNICAL INVESTIGATION
1. PURPOSE AND SCOPE
This report presents the results of an update geotechnical investigation for the proposed Quarry
Creek development. The purpose of the geoteclmical investigation is to evaluate surface and
subsurface soil conditions and general site geology, and to identify geotechnical cons1raints that may
impact development of the property. In addition, the purpose of this report is to provide foundation
design criteria, concrete flatwork recommendations, retaining wall recommendations, excavation and
remedial grading considerations that can be utilized in developing project budgets. The scope of this
investigation also included a review of readily available published and unpublished geologic
literature, aerial photographs and the following documents previously prepared for the property:
1. Preliminary Geotechnical Investigation, Quarry Creek IL Carlsbad/Oceanside, California,
prepared by Geocon Incorporated, dated February 24, 2015 (Project No. 07135-42-03).
2. Foundation Report, Quarry Creek Bridge, Carlsbad, California, prepared by Geocon
Incorporated, dated August 21, 2014 (Project No. 07135-42-04A).
3. Mass Grading Plans for Quany Creek, prepared by Project Design Consultants, undated.
4. Final Report of Testing and Observation Services During Site Grading, Quarry Creek,
Carlsbad, California, prepared by Geocon Incorporated, dated April 4, 2013 (Project
No. 07135-42-02).
5. Final Report of Testing and Observation Services During Site Grading, Quarry Creek,
Oceanside, California, prepared by Geocon Incorporated, dated March 11, 2013 (Project
No. 07135-42-02).
6. Update Geotechnical Investigation, Amended Reclamation Plan, Quarry Creek Refined
Alternative 3, Carlsbad, California, prepared by Geocon Incorporated, dated September 10,
2009 (Project No. 07135-42-01).
7. Limited Geotechnical Investigation to Evaluate Hardrock Constraints for Quarry Creek,
Carlsbad, California, prepared by Geocon Incorporated, dated April 9, 2004 (Project
No. 07135-42-01B.
8. EIR -Level Soil and Geologic Reconnaissance Quarry Creek II Carlsbad/Oceanside,
California, prepared by Geocon Incorporated, dated October 20, 20ll(Project No. 07135-
42-0IB).
The site is located south of State Route 78 and west of College Boulevard in Carlsbad, California
(see Vicinity Map, Figure 1).
The eastern portion of the site has been graded as part of the reclamation process. Geocon
Incorporated performed compaction testing and observation services during the grading operations.
Project No. 07135-42-05 - 1 -February24, 2015
References 4 and 5 are the as-graded reports prepared for reclamation grading. As-graded conditions
are shown on the Geologic Map (Figures 2 and 3).
Previous field investigations were conducted in 2012 (Reference 1) and 2004 (Reference 7). These
investigations included large-diameter borings, small-diameter boring, and exploratory trenches.
Pertinent information, including boring and trench logs and other details of the field investigations
are presented in Appendix A.
We tested selected soil samples obtained during the previous field investigations to evaluate physical
properties for engineering analyses and to assist in providing recommendations for site grading. Details
of the laboratory tests and a summary of the test results are presented in Appendix B and on the boring
logs in Appendix A. The Geologic Map, Figures 2 and 3 also present the locations of the exploratory
excavations. The base map used to generate Figures 2 and 3 is a CAD file of Reference 3.
Other reports reviewed as part of this study are summarized on the List of References at the end of
this report.
2. PREVIOUS SITE USAGE AND GRADING
The Quarry Creek property has undergone many years of mining, crushing, and screening to produce
commercial aggregate products. The majority of previous mining activity occurred in the eastern and
southern portions of the site. Mining waste products were placed in canyon or pit areas to reclaim
quarry excavations. This resulted in placement of mostly undocumented fill in depressions, as well as
some compacted fill. A former concrete batch plant and base-coarse crushing and screening plant
operated by Hanson Aggregates occupied the central portion of the property. Other portions of the
property were previously used for storage purposes, which include stockpiles of concrete and asphalt
rubble, bioremediation stockpiles, and other materials.
Reclamation grading of the previously mined area commenced in July 2011 and was completed in
December 2012. During reclamation grading, undocumented fills were removed and replaced as
compacted fill. Alluvium, within the drainage area, was removed to approximately 3 feet above
groundwater elevation and replaced with compacted fill. Drop structures, levees, and rock revetment
slopes were constmcted along and in Buena Vista Creek drainage. Reclamation grading has resulted
in removal of undocumented fill and replacement with compacted fill on the south side of Buena
Vista Creek and majority of the areas north of the creek. Reclamation grading resulted in large sheet-
graded pads on the eastem half of the property on both the north and south sides of Buena Vista
Creek. The western portion of the property has remained in an ungraded condition.
Project No. 07135-42-05 -2-February 24, 2015
3. SITE AND PROJECT DESCRIPTION
The overall site slopes northward, southward, and westward, following the east-west natural drainage
of Buena Vista Creek valley and its tributaries. The original valley-slope topography has been
lowered by quarry operations and then regarded to the current sheet grade elevations during
reclamation grading. Mining of rock in the n01theast quadrant has created near-vertical rock slopes.
The cut has exposed fractured rock, which is very strong and considered stable in its temporary steep
condition. Recommendations for a permanent slope condition are provided in the slope stability
section of this report.
Slopes on the south side of the valley have been graded to permanent 2:1 (horizontal: vertical) cut
slopes with benches, bench-drains and brow-ditches. On the north side of the property, reclamation
grading has resulted in 2: 1 cut slopes. Elevations in the eastern half of the property vary from
approximately 80 feet Mean Sea Level (MSL) to above 300 feet MSL in open-space areas. Sheet
graded pad elevations vary from approximately 105 to 115 feet MSL. On the western ungraded
portion of the site, existing site elevations vary from approximately 80 feet MSL to 160 feet MSL.
Review of the grading plan for Quarry Creek indicates regrading in the eastern half of the property will
generally consist of cuts up to approximately 35 feet and fills of 15 feet, respectively. Within the
ungraded western portion, cuts and fills up to 40 feet and 30 feet, respectively will occur to create large
sheet-graded pads. Development will also include the construction of a bridge across the creek,
roadways, and utilities. A report specific to the bridge has been provided separately (see Reference 2).
Final plans for development have not yet been completed, however, we understand plans are to
construct 3-story multi-family buildings, 2-story single family attached units, affordable housing
units, and a community facility. Plans include street improvements, utilities, and several
neighborhood parks. Several water quality basins are planned within the development.
The site description and proposed development are based on a site reconnaissance and review of the
available plans. If development plans differ significantly from those described herein, we should be
contacted for review and possible revisions to this report.
4. SOIL AND GEOLOGIC CONDITIONS
Eight surficial soil deposits and four geologic formations were encountered and/or mapped on the
property. Surficial soil deposits include undocumented fill, compacted fill, previously placed fill,
topsoil (unmapped), surficial landslide debris, alluvium, and colluvium. Formational units include:
Quaternary-age Terrace Deposits; Tertiary-age Santiago Formation; and Jurassic-age Saito Intrusive
rock. Mapped limits of the geologic units are shown on the Geologic Maps (Figures 2 and 3).
Project No. 07135-42-05 -3 -February 24, 2015
Geologic Cross Sections are presented on Figure 4. The surficial soil types and geologic units are
described below.
4.1 Compacted Fill (Qcf)
Compacted fill placed during reclamation grading exists across the eastern half of the property.
Observation and compaction testing of the fill has been performed by Geocon Incorporated. Report
documenting compaction tests and as-graded conditions were prepared in 2013 (see References 4 and
5). The fill is predominately comprised of silty to clayey sand with varying amounts of rock
fragments, soil rock fills, and windrows of oversize rock and concrete. A 10-foot hold-down for
oversize rock was provided during reclamation grading. However, in the southwest portion of the
reclamation grading area, the hold down was raised to 7 feet below finish grade. Compacted fill is
considered suitable for support of additional fill and structural loads.
4.2 Undocumented Fill (Qudf)
Undocumented fill exists in the northeast portion of the property beyond the reclamation grading
limit and within the existing access road from Haymar Drive. The majority of this undocumented fill
will likely be removed to achieve sheet grades based on proposed cuts shown on the project grading
plans. However, we expect some remedial grading will be needed below proposed cut elevations to
completely remove undocumented fill. A small amount of undocumented fill also exists just west of
the graded reclamation parcels near the central portion of the project. Undocumented fill is
unsuitable in its present condition, and will require removal and recompaction to support additional
fill or structural improvements. Oversize materials encountered during remedial grading may require
breaking down and special placement procedures in deeper fill areas.
In the southwest portion of Lot 2, a limited amount of undocumented fill was left in-place during
reclamation grading due to the presence of groundwater. Based on our observations during
reclamation grading and potholes performed, we expect less than 3 to 5 feet of fill was left below
groundwater in some areas. We do not expect the presence of the undocumented fill will impact
future development.
4.3 Previously Placed Compacted Fill (Qpcf)
Limited areas in the northeast and southeast portions of the property are underlain by previously
placed compacted fill (see Geologic Map). According to a report by Ninyo and Moore (dated
August 31, 2000), most of the approximately 10 feet of documented fill in the bottom of the northern
pit area had been placed between approximately 1988 and 2000. The report describes the fill
Project No. 07135-42-05 -4-February 24,2015
as ... interlayered, medium dense to dense, clayey and silty sand, clayey gravel and stiff sandy clay.
Portions of the compacted fill were buried beneath stockpiles of oversize shot-rock that was removed
during reclamation grading. The upper approximately 3 to 5 feet of previously placed compacted fill
was removed dming reclamation grading and recompacted.
Previously placed compacted fill associated with the development of the eastern quan-y (Wal-Mart
shopping center) encroaches into the southeastern p01tion of the property. These materials were
partially removed and recompacted during reclamation grading operations. Based on observations
during reclamation grading, the fill appears to be relatively dense with adequate moistme content and
considered suitable for support of structural improvements.
4.4 Previously Placed Fill (Qpf)
Previously placed fill exists near Haymar Drive and Highway 78 along the northern property
boundary. The approximate limit of the previously placed fill is shown on Figure 2 (Geologic Map).
These soils should not impact future development of the property.
4.5 Topsoil (Unmapped)
Portions of the western side of the site are irregularly blanketed by 1 to 3 feet of topsoil consisting of
loose, porous, dark brown, silty to clayey, fine sand. Topsoil is compressible and expansive, and will
require removal and recompaction within areas of planned development. Expansive clays should be
placed in deeper excavations during grading.
4.6 Surficial Landslide Debris (Qisf)
Several suspicious surficial landslides are mapped within the western portion of the site, along the
south banks of the Buena Vista Creek basin. Due to the limited access to these areas, subsurface
investigation was not practical at this time. Their existence will be verified when access is available
or during the grading operations. Trench T -5 was excavated at one of these areas and showed
approximately 5 feet of sandy clay material overlying bedrock formation. The surficial landslide
debris, if they exist, are considered unsuitable for receiving fill or structures and require removal.
4.7 Alluvium (Qal)
Alluvial deposits are present within the major east-west drainage of Buena Vista Creek, as well as in
the northeastern and southwestern tributary canyons that converge with Buena Vista Creek in the
central portion of the site. The alluvial soils generally consist of loose, porous dark gray to dark
brown, very clayey, fine to medium sand, and clayey sand and silt with occasional layers of slightly
silty sand. Areas of deepest alluvium are located in the central portion of the site adjacent to the
Project No. 07135-42-05 -5-February 24, 2015
original channel of Buena Vista Creek and its tributaries. The alluvium is compressible and not
suitable for support of additional fill and/or structural loads and will require partial (dependent upon
groundwater depths) to complete removal. Remedial grading of the alluvium along the north and
south sides of the main Buena Vista Creek drainage has occurred during the reclamation grading.
Alluvium is expected to be encountered along the toe of the south facing fill slope at the west end of
the property.
4.8 Colluvium (Qc)
Colluvial deposits were encountered in the southwest portion of the site mostly along the sides of the
draining tributary canyons. Colluvium is comprised of approximately 4 to 6 feet of loose dark brown,
very clayey to silty, fine sand. Due to the loose unconsolidated condition of the colluvium, removal
and recompaction will be required to provide suitable support for placement of compacted fill or
structural improvements.
4.9 Terrace Deposits (Qt)
Extensive and thick river terrace deposits consisting of medium-dense to dense, light reddish-brown
to olive-brown, gravelly, silty to clayey, medium to coarse sand to cohesionless sand with occasional
layers of silty clay are present in the western and southwest portions of the site. Except near
depositional contacts (or unconformities) with older formations, this unit is typically massive to
horizontally bedded, relatively dense and exhibits low compressibility characteristics. Terrace
Deposits are most prevalent in the southwestern portion of the site. The sandy zones are suitable for
support of fill and/ or structural loads in their present condition. The clayey zones, however, possess
low shear strength and high expansion potential.
Our large diameter boring LB-2 performed in the proposed cut slope located at the southwest
boundary, adjacent to Simsbury Court, encountered materials consists of interbedded silty sand,
cohessionless sand and clay layers which are occasionally partially remolded. 'vVe recommend a
stability buttress be constructed along this cut slope to provide adequate slope stability.
4.10 Santiago Formation (Ts)
The Eocene-aged Santiago Fonnation, consisting of dense, massive bedded light brown to greenish-
gray sandstones and thin interbedded siltstones is present in the nmth-central and south-central
portions of the site. The Santiago Fonnation is generally granular and possesses suitable
geotechnical characteristics in either an undisturbed and/or properly compacted condition. However,
the occurrence of clayey siltstones and claystone layers in this unit may generate moderate to highly
Project No. 07135-42-05 -6 -February 24, 2015
expansive materials, or localized expansive zones at grade. Where practical, clayey zones of the
Santiago Formation should be placed at least 3 feet below proposed subg:rade elevations.
4.11 Saito Intrusive (Jspi)
The Jurassic-aged Saito Intrusive consists of a steeply jointed, dark gray, very strong tonalite to
gabbro rock considered to be older than the Peninsular Range Batholith and more closely related to
the formation of the Santiago Peak Volcanics (Larsen, 1948). This granitoid bedrock unit is present
in the northeast and southeast corners of the property and is the predominant geologic unit that has
been mined for aggregate on the property. Typically, this bedrock unit outcrops along the eastern or
southeastern boundary of the site, or is covered by fill in the central portions of the site. Exploratory
excavations encountered mostly buried intrusive rock that exhibited a variable weathering pattern
ranging from intensely weathered and fractured material near contacts with the overlying
sedimentary rocks, to fresh, extremely strong crystalline rock within quarried areas.
5. GROUNDWATER
Groundwater was encountered in the major lower elevation drainage areas of Buena Vista Creek and
its tributaries at elevations between 70 to 80 feet MSL. Depth of groundwater is subject to
fluctuation from natural seasonal variations. The relationship between alluvial removals and the
position of groundwater table and time of year remedial grading is performed are discussed in the
Conclusions and Recommendations section of this report.
6. GEOLOGIC HAZARDS
6.1 Faulting and Seismicity
Review of geologic literature, previous geotechnical reports for the property, and observations during
our current field investigation indicates no active faults traverse the property. One fault was observed
in Saito Intrusive rock across the quarry slope in the northeast comer of the property. Ho-.,vever, an
exploratmy trench excavated through the Tertiary Santiago Formation across the fault confirmed the
fault did not displace the Eocene-age sedimentary unit. As such, the fault is considered inactive and
not a constraint to the property.
According to the results of the computer program EZ-FRISK (Version 7.62), 8 knovvn active faults
are located within a search radius of 50 miles from the property. The nearest known active fault is
the Newp011-Inglewood-Rose Canyon Fault Zone, located approximately 6 miles east ofthe site and
is the dominant source of potential ground motion. Earthquakes that might occur on the Newport-
Inglewood-Rose Canyon Fault Zone or other faults within the southem Califomia and northern Baja
Project No. 07135-42-05 -7-February 24,2015
California area are potential generators of significant ground motion at the site. The estimated
deterministic maximum earthquake magnitude and peak ground acceleration for the Newport
Inglewood -Rose Canyon Fault are 7.5 and 0.34 g, respectively.
We used Boore-Atkinson (2008) NGA USGS2008, Campbell-Bozorgnia (2008) NGA USGS 2008,
and Chiou-Youngs (2008) NGA acceleration-attenuation relationships in the calculation of the peak
ground accelerations (PGA). Table 6.1.1 lists the estimated maximum earthquake magnitudes and
PGA's for the most dominant faults for the site location calculated for Site Class D as defined by
Table 1613A.5.3 ofthe 2010 CBC.
TABLE 6.1.1
DETERMINISTIC SPECTRA SITE PARAMETERS
Maximum Peak Ground Acceleration
Distance Earthquake Fault Name from Site Boore-Campbell-Chiou-
(miles) Magnitude Atkinson Bozorgnia Youngs (Mw) 2008 (g) 2008 (g) 2008 (g)
NewpOii-Inglewood-Rose Canyon 6 7.50 0.30 0.26 0.34
Elsinore 21 7.85 0.21 0.15 0.19
Coronado Bank ?" _.J 7.40 0.18 0.12 0.14
Palos Verdes Connected 23 7.70 0.19 0.13 0.16
San Joaquin Hills Thmst 35 7.10 0.18 0.10 0.09
Earthquake Valley 42 6.80 0.13 0.09 0.11
San Jacinto 45 7.88 0.13 0.08 0.10
Chino 47 6.80 0.08 0.05 0.05
We used the computer program EZ-FRISK to perform a probabilistic seismic hazard analysis. The
computer program EZ-FRISK operates under the assumption that the occurrence rate of earthquakes
on each mapped Quatemary fault is proportional to the fault slip rate. The program accounts for
earthquake magnitude as a function of fault rupture length. Site acceleration estimates are made
using the earthquake magnitude and distance from the site to the rupture zone. The program also
accounts for uncertainty in each of following: ( 1) earthquake magnitude, (2) rupture length for a
given magnitude, (3) location of the rupture zone, (4) maximum possible magnitude of a given
earthquake, and (5) acceleration at the site from a given earthquake along each fault. By calculating
the expected accelerations from considered earthquake sources, the program calculates the total
average annual expected number of occurrences of site acceleration greater than a specified value.
We utilized acceleration-attenuation relationships suggested by Boore-Atkinson (2008) NGA
USGS2008, Campbell-Bozorgnia (2008) NGA USGS 2008, and Chiou-Youngs (2008) in the
Project No. 07135-42-05 - 8 -February 24, 2015
analysis. Table 6.1.2 presents the site-specific probabilistic seismic hazard parameters including
acceleration-attenuation relationships and the probability of exceedence for Site Class D.
TABLE 6.1.2
PROBABILISTIC SEISMIC HAZARD PARAMETERS
Peak Ground Acceleration
Probability of Exceedence Boore-Atkinson, Camp bell-Bozo rgnia, Chiou-Youngs,
2008 (g) 2008 (g) 2008 (g)
2% in a 50 Year Period 0.52 0.42 0.47
5% in a 50 Year Period 0.39 0.32 0.35
10% in a 50 Year Period 0.31 0.25 0.27
The California Geologic Survey (CGS) provides a computer program that calculates the ground
motion for a 10 percent of probability of exceedence in 50 years based on the average value of
several attenuation relationships. Table 6.1.3 presents the calculated results from the Probabilistic
Seismic Hazards Mapping Ground Motion Page from the CGS website.
TABLE 6.1.3
PROBABILISTIC SITE PARAMETERS FOR SELECTED FAULTS
CALIFORNIA GEOLOGIC SURVEY
Calculated Acceleration (g) Calculated Acceleration (g) Calculated Acceleration (g)
Firm Rock Soft Rock Alluvium
0.27 0.29 0.33
While listing peak accelerations is useful for comparison of potential effects of fault activity in a
region, other considerations are important in seismic design, including the frequency and duration of
motion and the soil conditions underlying the site. Seismic design of the stmctures should be
evaluated in accordance with the California Building Code (CBC) guidelines.
6.2 Liquefaction
Liquefaction analyses were performed during Geocon's 2009 report for reclamation grading. Results
of the analyses indicate alluvial deposits below· the groundwater should not liquefy for the design
level acceleration. However, design accelerations for under current building codes have significantly
increased over the last several years. Under current design accelerations, portions of the alluvium
below groundwater could experience liquefaction. Liquefaction, should it occur, is expected to be
limited to the area within the existing Buena Vista Creek drainage, and in the tributary drainage
Project No. 07135-42-05 - 9 -February 24,2015
southwest of the site. With respect to the alluvium left in place within Lot 2, the alluvium is likely
less than 10 feet thick, has been surcharged with approximately 15 to 20 feet of fill, will receive an
additional 5 feet of fill to achieve proposed sheet grades, and will be surcharged with an additional 5
feet of fill during regrading (see Section 7.5 of this report). In our opinion, liquefaction, if it were to
occur in this area would not cause surface manifestations (i.e., sand boils). Settlement, should it
occur is expected to be relatively uniform and less than l-inch total. We estimate differential
settlement as a result of liquefaction to be Y2-inch or less.
6.3 Flow Slide Potential
We analyzed flow slide potential for liquefaction conditions along the channel bank. We performed a
slope stability analysis using residual shear strength parameters for the potentially liquefiable soils.
Residual shear strengths were determined using information provided in Recommended Procedures
for Implementation of DMG Special Publication 117 Guidelines for Analyzing and Mitigating
Liquefaction in California. It is our opinion that there is a low potential for flow slide impacts to
structures at the locations shown on Figure 2. The results of the stability analysis are shown
graphically in Appendix D (see Figures D-1 and D-2).
6.4 Landslides
Review of 1995 published landslide maps of the California Geological Survey (formerly the Division
, --" of Mines and Geology) and a previous geotechnical report by Ninyo and Moore (August 23, 2000),
suggested the presence of suspected landslide deposits in the southwest quadrant of the site.
However, observations of intact outcrops and current subsurface investigation confmned that the
landslide does not exist. Several suspicious surficial landslides are mapped along the south bank of
the creek. These areas were not accessible for subsurface investigation. These potential landslides,
even if confirmed, should not have significant impact on the proposed development.
Project No. 07135-42-05 -10-February 24,2015
7. CONCLUSIONS AND RECOMMENDATIONS
7.1 General
7.1.1 From a geotechnical engineering standpoint, it is our opinion that the site is suitable for the
proposed development, provided the recommendations presented herein are implemented
in design and construction of the project.
7 .1.2 Soil conditions identified during this study that may impact development include
compressible surficial soils (undocumented fill, alluvium, colluvium, surficial landslide
debris and topsoil) that will require remedial grading. Undocumented fill may contain
large rock fragments that require special placement procedures.
7.1.3 The property is approximately 7 miles from the Newport Inglewood/Rose Canyon Fault. It
is our opinion active and potentially active faults do not extend across or trend toward the
site. Risks associated with seismic activity consist of the potential for strong seismic
shaking. Building setbacks will not be required for the planned development due to
faulting.
7.1.4 Several potential surficial landslides are mapped along the north-facing slope within the
western portion of the site. Due to the limited access, these areas could not be reached for
subsurface investigation. If encountered during the grading operations, total removal of the
slide debris within the grading area is recommended.
7.1.5 Subsurface conditions observed may be extrapolated to reflect general soil/geologic
conditions; however, some variations in subsurface conditions between trench and boring
locations should be anticipated.
7.2 Excavation and Soil Characteristics
7.2.1 Excavation of the Terrace Deposits, Santiago Formation and weathered portion of the
Saito Intrusive is expected to require a heavy to very heavy effort to excavate. Less
weathered and fresh Salto Intmsive bedrock may require blasting or specialized rock
breaking techniques to efficiently excavate and handle. Very heavy effort with possible
refusal is expected for excavations into the volcanic and intwsive rocks. Oversize material
may be generated which would require special handling or exportation from the site.
7.2.2 The soil encountered in the field investigation is considered to be "expansive" (expansion
index greater than 20) as defined by 2013 California Building Code (CBC) Section
1803.5.3. Table 7.2 presents soil classifications based on the expansion index. Based on
Project No. 07135-42-05 -11 -February 24, 2015
the results of our laboratory testing, presented in Appendix B, we expect the on-site
mate1ials will possess a "very low" to "very high" expansion potential (Expansion Index of
20 and greater).
TABLE 7.2
EXPANSION CLASSIFICATION BASED ON EXPANSION INDEX
ASTM 4829 AND 2013 CBC
Expansion Index (El) Expansion Classification 2013 CBC
ASTM 4829 Expansion Classification
0-20 Very Low Non-Expansive
21-50 Low
51-90 Medium
91-130 High
Expansive
Greater Than 130 Very High
7.2 .3 We performed laboratory tests on samples of the site materials to evaluate the percentage
of water-soluble sulfate content. Results from the laboratory water-soluble sulfate content
tests are presented in Appendix B and indicate that the on-site materials at the locations
tested possess "negligible" sulfate exposure to concrete stmctures as defined by 2013 CBC
Section 1904 and ACI 318-08 Sections 4.2 and 4.3. We recommend the requirements set
·forth by 2013 CBC Section 1904 and ACI 318 be followed when determining the type of
concrete to be used. The presence of water-soluble sulfates is not a visually discernible
characteristic; therefore, other soil samples from the site could yield different
concentrations. Additionally, over time landscaping activities (i.e., addition of fertilizers
and other soil nutrients) may affect the concentration.
7.2.4 Geocon Incorporated does not practice in the field of corrosion engineering. Therefore,
further evaluation by a conosion engineer may be performed if improvements that could
be susceptible to corrosion are planned.
7.3 Subdrains
7.3.1 Canyon subdrains are recommended to mitigate the potential for adverse impacts
associated with observc:cl and potential seepage conditions and to collect perched water
that migrates along the contact between natural ground and fill surfaces. Figure 5 presents
a typical canyon subdrain detail. Recommended subdrain locations are depicted on the
Geologic Map, Figures 2 and 3.
Project No. 07135-42-05 -12-Febmary 24, 2015
7.3.2 The final 20-foot segment of a subdrain should consist of non-perforated drainpipe. At the
non-perforated/perforated interface, a seepage cutoff wall should be constructed on the
downslope side of the junction in accordance with Figure 6. Subdrains that discharge into
a natural drainage course or open space area should be provided with a permanent
headwall structure in accordance with Figure 7.
7.3.3 Final grading plans should show the location of the proposed subdrains. Upon completion
of remedial excavations and subdrain installation, the project civil engineer should survey
the drain locations and prepare an "as-built" map depicting the existing conditions. The
final outlet and connection locations should be determined during grading. The grading
contractor may consider videoing the subdrains shortly after burial to check proper
installation and to check that the pipe has not been crushed. As a minimum, we
recommend the subdrain for the buttress fill be videoed. The contractor is responsible for
the performance of the drains.
7.4 Grading
7.4.1 All grading should be performed in accordance with the Recommended Grading
Specifications contained in Appendix E. Where the recommendations of Appendix E
conflict with this section of the report, the recommendations of this section take
precedence.
7.4.2 Prior to commencing grading, a preconstruction conference should be held at the site with
the owner or developer, grading contractor, civil engineer, and geotechnical engineer in
attendance. Special soil handling and/or the grading plans can be discussed at that time.
7.4.3 Grading should be performed in conjunction with the observation and compaction testing
services of Geocon Incorporated. Fill soil should be observed on a full-time basis during
placement and tested to assess in-place dry density and moisture content.
7.4.4 Site preparation should begin with removal of all deleterious material and vegetation. The
depth of removal should be such that material exposed in cut areas or soil to be used for
fill is relatively free of organic matter. Deleterious material generated during stripping
and/or site demolition should be exported from the site.
7.4.5 Undocumented fill, topsoil, co11uvium, alluvium and landslide debris within areas of
planned grading should be removed to firm natural ground and properly compacted prior to
placing additional fill and/or structural loads. The actual extent of unsuitable soil removals
should be determined in the field by the soil engineer and/or engineering geologist. Overly
Project No. 07135-42-05 -13-February 24, 2015
wet surficial materials, where encountered, will require drying and/or mixing with drier
soils to facilitate proper compaction.
7.4.6 Alluvium should be removed down to competent formational bedrock or to within
approximately 3 feet of the groundwater table, whichever occurs first. During excavation
of the alluvium, test pits should be periodically excavated to determine groundwater
depths. Dewatering and special equipment such as swamp cats, excavators, and top loading
operations may be required to excavate the alluvium. Removals at the toe of slopes along
the drainage channel at the southwest comer of the property should extend out at a 1:1
plane from the toe to the bottom of the removal. A typical lateral extent of removal is
shown on Figure 8.
7 .4. 7 Graded areas may expose volcanic/intrusive rock at finish grade. The presence of hard
rock may impact future development. We recommend hard rock be undercut to a depth of
at least 5 feet below finish grade in building pads and 2 feet below utilities and a soil cap
replaced.
7.4.8 After removal of unsuitable material as recommended above, the base of overexcavations
and natural ground surfaces (including previous compacted fill soil) to receive additional
fill should be scarified approximately 12 inches, moisture conditioned, and compacted.
7.4.9 The site should then be brought to final subgrade elevations with structural fill compacted
in layers. In general, soils native to the site are suitable for re-use as fill if free from
vegetation, debris and other deleterious material. Layers of fill should be no thicker than
will allow for adequate bonding and compaction. All fill, backfill, and scarified ground
surfaces should be compacted to a dry density of at least 90 percent of maximum dry
density near to slightly above optimum moisture content, as determined in accordance with
ASTM Test Procedure D 1557. Fill areas with in-place density test results indicating
moisture contents less than optimum will require additional moisture conditioning prior to
placing additional fill.
7.4.10
7.4.11
To reduce the potential for differential settlement, it is recommended that the cut portion
of building pads with cut-fill transitions be undercut at least 3 feet and replaced. where
practical, vvith "very low" to '·medium'' expansive compacted fill soils.
Cut pads exposing concretions, cemented material, or expansive soil should be undercut at
least 3 feet and replaced with properly compacted "very low" to "medium" expansive soil
to facilitate excavation offoundations and shallow utilities.
Project No. 07135-42-05 -14-February 24,2015
7.4.12 Undercuts ( overexcavations) performed on pads with cut-fill transitions, cemented
sandstone, hard rock or expansive soil materials at grade should be undercut at a gradient
of 1 percent toward the street or toward the deepest fill area to provide drainage for
moisture migration along the contact between the native soil and compacted fill.
7.4.13 The on-site soil is suitable to be used as fill if relatively free of debris and organic
material. The depth of removal should be such that dense natural ground is exposed at the
base of the overexcavation.
7 .4.14 Grading should be performed such that highly expansive soils are placed in the deeper fill
areas and outside of slope zones. Materials within 3 feet of fmish grade on lots and the
upper 12 inches of subgrade within streets, where practical, should consist of very low to
medium expansive soils (soil with an Expansion Index less than 90).
7.4.15
7.4.16
7.4.17
Cut and fill slopes should be constructed at an inclination of 2:1 (horizontal to vertical) or
flatter. An approximately 15-foot-high 1.5:1 cut slope in the Saito Intrusive is planned in
the southeast portion of the property and an approximately 30-foot-high 1.5:1 cut slope in-
Santiago Formation is planned at the northeast portion. These 1.5: 1 slopes are acceptable
provided they are free of adverse bedding. It is recommended that the slope excavations be
observed during grading by a representative of Geocon Incorporated to check that soil and
geologic conditions do not differ significantly from those anticipated and adverse bedding
does not exist.
All fill slopes should be constmcted of granular material and compacted out to the face of
the finish slope. If complete removals cannot be performed outside the toe of slopes due to
environmentally sensitive areas and/or groundwater conditions, Geocon Incorporated
should be consulted to provide recommendations.
Excavations in cemented zones of formational units will likely generate oversize rock
chunks. Oversized materials can be placed in fill areas in accordance with the
recommendations contained within the Recommended Grading Specifications in
Appendix D. Oversize materials (rocks or hard lumps in excess of 12 inches in least
dimension) should be kept Jt least 10 feet below proposed finish grade within building
pads and at least 2 feet below the deepest utility \Vithin street right-of-\vays. Modifications
to the hold down depths can be made at the owner's desecration.
Project No. 07135-42-05 -15-February 24, 2015
7.5 Surcharge Fill
7.5.1 In Lot 2 where undocumented fill and alluvium were left in-place below groundwater, we
recommend a surcharge fill height of at least 5 above fmish pad grade be placed. The
approximate area of recommended surcharge is shown on Figure 2. The top of the
surcharge fill should extend out to a horizontal distance of at least 10 feet beyond the
limits where alluvium was left in-place. We recommend the bottom 1-foot ofthe surcharge
fill be placed and compacted as structural fill. This will result in compacted fill at finish
grade once settlement of the underlying alluvium occurs.
7.6 Settlement Monitoring
7 .6.1 Settlement monitoring is recommended where surcharge fill is placed. Once rough pad
grade is attained, we recommend surface monuments be installed to measure settlement.
The locations and number of monuments should be determined by Geocon once building
pad locations are known. A typical surface settlement monument detail is presented as
Figure 9.
7.6.2 Surface settlement monuments should be read by the project surveyor every week for the
first 4 weeks, and then every two weeks until measured settlement is within tolerable limits
such that additional settlement will not impact site improvements. Based on our experience
with similar soil conditions, we estimate 2 to 4 months of monitoring would be necessary
to demonstrate that primary consolidation is essentially complete.
7. 7 Slope Stability
7. 7.1 Slope stability analyses, utilizing average drained direct shear strength parameters, indicate
proposed fill slopes constructed with on-site granular materials and cut slopes within
formational material should have calculated factors of safety of at least 1.5 under static
conditions with respect to both deep-seated failure and shallow sloughing conditions.
Results of the analyses are presented on Figures 10 through 14. Additionally, an
inclination of 1.5 to 1 (horizontal to vertical) is acceptable for slopes excavated into the
Saito Intrusive and Santiago Formation provided no adverse jointing, fractures, or bedding
exist. All cut slopes should be observed by a geologist to assess if adverse bedding,
jointing, or fractures e:.;ist.
7.7.2 There are steep to near vertical cut slopes within the northeast portion of the site. These
slopes were excavated during mining operations. The stability of these slopes should be
evaluated when ultimate grading and use of this area is known. Various options such as lay
back, stability fill or soil nailing/rock bolting may be considered at the time. Alternatively,
Project No. 07135-42-05 -16 .. February 24, 2015
slope set-backs and rock retention fences can be utilized. For preliminary planning
purposes, we recommend a set-back for site improvements of 20 feet from the toe of slope
in conjunction with the construction of a deblis ditch and rock retention fence.
Additionally, we recommend loose rock and accumulated soil on the slope face and at the
top of the slope be removed. The constmction of the deblis ditch and rock retention fence
should occur during fine grading of the pad once pad usage and improvement locations
have been determined.
7.7.3 The proposed 45 feet high cut slope located at the southwest boundary, adjacent to
Simsbury Court, will be excavated into Terrace Deposits. Our large diameter boring LB-2
encountered matelials consisting of interbedded silty sand, cohessionless sand and clay
layers which were occasionally partially remolded. Our analysis indicates that a buttress
fill will be required to provide adequate slope stability. Based on our analysis, the stability
buttress will need to have a minimum width of 30 feet to provide adequate stability. A
typical buttress fill detail is provided in Figure 15. A discussion our stability analysis is
presented in Appendix C.
7.7.4 Excavations including buttresses, shear keys, and stability fills should be observed during
grading by an engineering geologist to evaluate whether soil and geologic conditions do
not differ significantly from those expected. Buttress shear key and associated subdrains
should be surveyed during construction and depicted on the final as-built grading plans.
7.7.5 We performed the slope stability analyses based on the interpretation of geologic
conditions encountered during our field investigation. In certain areas, the geologic
conditions such as the localized or continuous features of the bedding plane shears may
need to be further defined by additional field exploration based on our review of the
40-scale grading plans.
7.7.6 The outer 15 feet (or a distance equal to the height of the slope, whichever is less) of fill
slopes should be composed of properly compacted granular "soil" fill to reduce the
potential for surficial sloughing.
7. 7. 7 Fill slopes should be overbuilt at least 3 feci and cut back to the design finish grades.
Altematively, fill slopes can be compacted by backrolling with a loaded sheepsfl)ot roller
or tracked walked by sufficiently by a D-8 dozer or equivalent, at vertical intervals not to
exceed 4 feet. Slope should be unifonnly compacted to a dry density of at least 90 percent
of the laboratory maximum dry density to the face of the finished slope.
Project No. 07135-42-05 -17-February 24, 2015
7.7.8 All slopes should be landscaped with drought-tolerant vegetation having variable root
depths and requiring minimal landscape irrigation. In addition, all slopes should be drained
and properly maintained to reduce erosion. Slope planting should generally consist of
drought tolerant plants having a variable root depth. Slope watering should be kept to a
minimum to just support the plant growth.
7.8 Seismic Design Criteria
7.8.1 We used the computer program U.S. Seismic Design Maps, provided by the USGS.
Table 7.8.1 summarizes site-specific design criteria obtained from the 2013 California
Building Code (CBC; Based on the 2012 International Building Code [IBC] and ASCE 7-
10), Chapter 16 Structural Design, Section 1613 Earthquake Loads. The short spectral
response uses a period of 0.2 second. The building structures and improvements should be
designed using a Site Class C. We evaluated the Site Class based on the discussion in
Section 1613.3.2 of the 2013 CBC and Table 20.3-1 of ASCE 7-10. The values presented
in Table 7.8.1 are for the risk-targeted maximum considered earthquake (MCER)-
TABLE 7.8.1
2013 CBC SEISMIC DESIGN PARAMETERS
Parameter Value 2013 CBC Reference
Site Class D Section 1613.3.2
MCER Ground Motion Spectral Response 1.067g Figure 1613.3.1(1) Acceleration-Class B (short), Ss
MCER Ground Motion Spectral Response 0.413g Figure 1613.3.1(2) Acceleration-Class B (1 sec), S1
Site Coefficient, FA 1.073 Table 1613.3.3(1)
Site Coefficient, Fv 1.587 Table 1613.3.3(2)
Site Class Modified MCER 1.145g Section 1613.3.3 (Eqn 16-37) Spectral Response Acceleration (short), S\IS
Site Class Modifieu MCER 0.656g Section 1613.3.3 (Eqnl6-38) Spectral Response Acceleration (l sec), S\11
5% Damped Design 0.763g Section 1613.3.4 (Eqn 16-39) Spectral Response Acceleration (short), Sos
5% Damped Design 0.437g Section 1613.3.4 (Eqn 16-40) Spcctr~l Response Acceler~tiotl ( 1 scci. S;,: I
7.8.2 Table 7.8.2 presents additional seismic design parameters for projects located in Seismic
Design Categories of D through F in accordance with ASCE 7-10 for the mapped
maximum considered geometric mean (MCEo).
Project No. 07135-42-05 -18-February 24,2015
TABLE 7.8.2
2013 CBC SEISMIC DESIGN PARAMETERS
Parameter Value ASCE 7-10 Reference
Mapped MCEa Peak Ground Acceleration, PGA 0.407g Figure 22-7
Site Coefficient, FPGA 1.093 Table 11.8-1
Site Class Modified MCEa 0.445g Section 11.8.3 (Eqn 11.8-1) Peak Ground Acceleration, PGAM
7.8.3 Conformance to the criteria in Tables 7.8.1 and 7.8.2 for seismic design does not constitute
any kind of guarantee or assurance that significant structural damage or ground failure will
not occur if a large earthquake occurs. The primary goal of seismic design is to protect life,
not to avoid all damage, since such design may be economically prohibitive.
7.9 Foundation and Concrete Slab-On-Grade Recommendations
7.9.1 The foundation recommendations that follow are for one-to three-story residential
structures and are separated into categories dependent on the thickness and geometry of
the underlying fill soils as well as the expansion index of the prevailing sub grade soils of a
particular building pad (or lot). Categories for each building pad or lot will be provided
after the completion of grading once fill thickness is known and expansion index testing
has been performed on finish grade soils.
Foundation
Category
I
II
III
TABLE 7.9.1
FOUNDATION CATEGORY CRITERIA
Maximum Fill Differential Fill
Thickness, T (feet) Thickness, D (feet)
T<20 --
20<T<50 IO<D<20
T:::so D>20
Expansion
Index (EI)
EIS50
50<EI<90
90<EI<130
7.9.2 Table 7.9.2 presents minimum foundation ~nd interior concrete slab design criteria for
conventional foundation system,;;.
Project No. 07135-42-05 -19-February 24, 2015
TABLE 7.9.2
CONVENTIONAL FOUNDATION RECOMMENDATIONS BY CATEGORY
Foundation Minimum Footing Continuous Footing Interior Slab
Category Embedment Depth Reinforcement Reinforcement (inches)
I 12 Two No. 4 bars, 6x6-1 0/10 welded wire
one top and one bottom mesh at slab mid-point
II 18 Four No. 4 bars, No.3 bars at 24 inches
two top and two bottom on center, both directions
III 24 Four No. 5 bars, No.3 bars at 18 inches
two top and two bottom on center, both directions
7.9.3 The embedment depths presented in Table 7.9.2 should be measured from the lowest
adjacent pad grade for both interior and exterior footings. The conventional foundations
should have a minimum width of 12 inches and 24 inches for continuous and isolated
footings, respectively.
7.9.4 The concrete slab-on-grade should be a minimum of 4 inches thick for Foundation
Categories I and II and 5 inches thick for Fotmdation Category III.
7.9.5 Slabs that may receive moisture-sensitive floor coverings or may be used to store moisture-
sensitive materials should be underlain by a vapor retarder. The vapor retarder design should
be consistent with the guidelines presented in the American Concrete Institute's (ACI) Guide
for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials (ACI 302.2R-06). In
addition, the membrane should be installed in accordance with manufacturer's
recommendations and ASTM requirements, and in a manner that prevents puncture. The
project architect or developer should specify the vapor retarder based on the type of floor
covering that will be installed and if the structure will possess a humidity controlled
environment.
7.9.6 The project foundation engmeer, architect, and/or developer should determine the
thickness of bedding sand below the slab. In general, 3 to 4 inches of sand bedding is
typically used. Geocon should be contacted to provide recommendations if the bedding
sand is thicker than 6 inches.
7.9.7 The foundation design engineer should provide appropriate concrete mix design criteria and
curing measures to assure proper curing of the slab by reducing the potential for rapid
moisture loss and subsequent cracking and/or slab curl. The foundation design engineer
should specify the concrete mix design and proper curing methods on the foundation plan. It
Project No. 07135-42-05 -20-February 24, 2015
is critical that the foundation contractor understands and follows the recommendations
presented on the foundation plan.
7.9.8 As an alternative to the conventional foundation recommendations, consideration should be
given to the use of post -tensioned concrete slab and foundation systems for the support of the
proposed structures. The 2013 CBC has updated the design requirements for post-tensioned
foundation systems. The post-tensioned systems should be designed by a stmctural engineer
experienced in post-tensioned slab design and design criteria of the Post-Tensioning Institute
(PTI), Third Edition, as required by the 2013 CBC (Section 1805.8). Although this procedure
was developed for expansive soil conditions, we understand it can also be used to reduce the
potential for foundation distress due to differential fill settlement. The post-tensioned design
should incorporate the geotechnical para~eters presented in Taple 7.9.3 for the particular
Foundation Category designated. The parameters presented in Table 7.9.3 are based on the
guidelines presented in the PTI, Third Edition design manual.
TABLE 7.9.3
POST-TENSIONED FOUNDATION SYSTEM DESIGN PARAMETERS
Post-Tensioning Institute (PTI) Foundation Cat1~gory
Third Edition Design Parameters I II III
Thomthwaite Index -20 -20 -20
Equilibrium Suction 3.9 3.9 3.9
Edge Lift Moisture Variation Distance, eM (feet) 5.3 5.1 4.9
Edge Lift, YM (inches) 0.61 1.10 1.58
Center Lift Moisture Variation Distance, eM (feet) 9.0 9.0 9.0
Center Lift, YM (inches) 0.30 0.47 0.66
7.9.9 If the structural engineer proposes a post-tensioned foundation design method other than
the 2013 CBC:
• The criteria presented in Table 7.9.3 arc still applicable.
,. Interior stiffener beams shouid bt used for Foundcttion Categories ll and IlJ.
• The width of the perimeter foundations should be at least 12 inches.
• The perimeter footing embedment depths should be at least 12 inches, 18 inches
and 24 inches for foundation categories T, IT, and III, respectively. The embedment
depths should be measured from the lowest adjacent pad grade.
Project No. 07135-42-05 -21 -February 24, 2015
7.9.10
7.9.11
The foundations for the post-tensioned slabs should be embedded in accordance with the
recommendations of the stmctural engineer. If a post-tensioned mat foundation system is
planned, the slab should possess a thickened edge with a minimum width of 12 inches and
extend at least 6 inches below the clean sand or cmshed rock layer.
Our experience indicates post-tensioned slabs are susceptible to excessive edge lift,
regardless of the underlying soil conditions. Placing reinforcing steel at the bottom of the
perimeter footings and the interior stiffener beams may mitigate this potential. Current PTI
design procedures primarily address the potential center lift of slabs but, because of the
placement of the reinforcing tendons in the top of the slab, the resulting eccentricity after
tensioning reduces the ability of the system to mitigate edge lift. The structural engineer
should design the foundation system to reduce the potential of edge lift occurring for the
proposed stmctures.
7.9.12 During the constmction of the post-tension foundation system, the concrete should be
placed monolithically. Under no circumstances should cold joints form between the
footings/grade beams and the slab during the constmction of the post-tension foundation
system.
7.9.13
7.9.14
7.9.15
Category I, II, or III foundations may be designed for an allowable soil bearing pressure of
2,000 pounds per square foot (psf) (dead plus live load). This bearing pressure may be
increased by one-third for transient loads due to wind or seismic forces. The estimated
maximum total and differential settlement for the planned stmchlres due to foundation
loads is 1-inch and Y2-inch, respectively. Differential settlement is estimated to occur over
a span of 40 feet.
We expect primary settlement of existing fills is essentially complete. Ho\vever, we
estimate that additional settlemen't as a result of hydro-consolidation to be approximately
0.2 to 0.3 percent of the total fill thickness. \Ve expect hydro-consolidation to occur over a
20 year or more duration. We estimate a total fill settlement as a result of hydro-
consolidation to be 1-inch or less in areas where compacted fill exists. A more refined
estimate of total and differential fill thickness can be made once building locations on
Foundations will need to be designed to accommodate estimated total and differential fill
settlement from both building loading and hydroconsolidation. In addition, building pads
on Lot 2 where alluvium was left in-place should incorporate the estimated liquefaction
settlement.
Project No. 07135-42-05 -22-February 24,2015
7.9.16
7.9.17
7.9.18
7.9.19
Isolated footings, including PT foundation systems where footings are not reinforced with
PT cables, should have the minimum embedment depth and width recommended for
conventional foundations (see Section 7.9.1 through 7.9.3) for a particular foundation
categmy. The use of isolated footings, which are located beyond the perimeter of the
building and support structural elements connected to the building, are not recommended
for Category III. Where this condition cannot be avoided, the isolated footings should be
connected to the building foundation system with grade beams.
For Foundation Category III, consideration should be given to using interior stiffening
beams and connecting isolated footings and/or increasing the slab thickness. In addition,
consideration should be given to connecting patio slabs, which exceed five feet in width, to
the building foundation to reduce the potential for future separation to occur.
Special sub grade presaturation is not deemed necessary prior to placing concrete; however,
the exposed foundation-and slab-subgrade soil should be moisture conditioned, as
necessary, to maintain a moist condition as would be appropriate in any such concrete
placement.
Where buildings or other improvements are planned near the top of a slope steeper
than 3:1 (horizontal:vertical), special foundations and/or design considerations are
recommended due to the tendency for lateral soil movement to occur.
• For fill slopes less than 20 feet high or cut slopes regardless of height, footings
should be deepened such that the bottom outside edge of the footing is at
least 7 feet horizontally from the face of the slope.
• For fill slopes greater than 20 feet high, foundations should be extended to a depth
where the minimum horizontal distance is equal to H/3 (where H equals the
vertical distance from the top of the fill slope to the base of the fill soil) with a
minimum of 7 feet but need not exceed 40 feet. The horizontal distance is
measured from the outer, deepest edge of the footing to the t~1cc of the slvpe. A
post-tensioned slab and foundation system or mat foundation system can be used
to help reduce potential foundation distress associated with slope creep and lateral
fill extension. Specific design parameters or recommendations for either of these
al temati ves can be provided once the bui !ding location aud fil.l s1 op;:: gc>Jlrlctry
h~lV=2 b . .:;en det~n11inc~t.
• If swimming pools are planned, Geocon Incorporated should be contacted for a
review of specific site conditions.
• Swimming pools located within 7 feet of the top of cut or fill slopes are not
recommended. Where such a condition cannot be avoided, the portion of the
swimming pool wall within 7 feet of the slope face be designed assuming that the
Project No. 07135-42-05 -23-February 24, 2015
7.9.20
7.9.21
7.9.22
7.10
7.10.1
adjacent soil provides no lateral support. This recommendation applies to fill
slopes up to 30 feet in height, and cut slopes regardless of height. For swimming
pools located near the top of fill slopes greater than 30 feet in height, additional
recommendations may be required and Geocon Incorporated should be contacted
for a review of specific site conditions.
• Although other improvements that are relatively rigid or brittle, such as concrete
flatwork or masonry walls, may expelience some distress if located near the top of
a slope, it is generally not economical to mitigate this potential. It may be possible,
however, to incorporate design measures that would permit some lateral soil
movement without causing extensive distress. Geocon Incorporated should be
consulted for specific recommendations.
The exterior flatwork recommendations provided herein assumes that the near surface soils
are very low to low expansive (EI S 50). Exterior slabs not subjected to vehicular traffic
should be a minimum of four inches thick and reinforced with 6 x 6-6/6 welded wire mesh.
The mesh should be placed in the middle of the slab. Proper mesh positioning is critical to
future performance of the slabs. The contractor should take extra measures to provide
proper mesh placement. Prior to construction of slabs, the upper 12 inches of subgrade
soils should be moisture conditioned at or slightly above optimum moisture content and
compacted to at least 90 percent of the laboratory maximum dry density per ASTM 1557.
The recommendations of this report are intended to reduce the potential for cracking of
slabs due to expansive soil (if present), differential settlement of existing soil or soil with
varying thicknesses. However, even with the incorporation of the recommendations
presented herein, foundations, shicco walls, and slabs-on-grade placed on such conditions
may still exhibit some cracking due to soil movement and/or shrinkage. The occurrence of
concrete shrinkage cracks is independent of the supporting soil characteristics. The
occurrence may be reduced and/or controlled by: (1) limiting the slump of the concrete,
(2) proper concrete placement and curing, and by (3) the placement of crack control joints
at periodic intervals, in particular, -v\·here re-entrant slab corners occur.
Geocon Incorporated should be consulted to provide additional design parameters as
required by the structural engineer.
Retaining Wall Recommendations
Retaining walls that are allowed to rotate more than 0.001 H (where H equals the height of
the retaining portion of the wall) at the top of the wall and having a level backfill surface
should be designed for an active soil pressure equivalent to the pressure exerted by a fluid
density of 35 pcf. Where the backfill will be inclined at 2:1 (horizontal:vertical), an active
Project No. 07135-42-05 -24-February 24,2015
7.10.2
7.10.3
7.10.4
7.10.5
7.10.6
soil pressure of 50 pcf is recommended. Expansive soils should not be used as backfill
material behind retaining walls. All soil placed for retaining wall backfill should have an
Expansion Index less than 50.
Soil contemplated for use as retaining wall backfill, including impmi materials, should be
identified in the field prior to backfill. At that time Geocon Incorporated should obtain
samples for laboratory testing to evaluate its suitability. Modified lateral emih pressures
may be necessary if the backfill soil does not meet the required expansion index or shear
strength. City or regional standard wall designs, if used, are based on a specific active
lateral earth pressure and/or soil friction angle. In this regard, on-site soil to be used as
backfill may or may not meet the values for standard wall designs. Geocon Incorporated
should be consulted to assess the suitability of the on-site soil for use as wall backfill if
standard wall designs will be used.
Unrestrained walls will move laterally when backfilled and loading is applied. The amount
of lateral deflection is dependent on the wall height, the type of soil used for backfill, and
loads acting on the wall. The wall designer should provide appropriate lat1eral deflection
quantities for planned retaining walls stmctures, if applicable. These lateral values should
be considered when planning types of improvements above retaining wall structures.
Where walls are restrained from movement at the top, an additional uniform pressure of
8H psf should be added to the active soil pressure where the wall possesses a height of
8 feet or less and 12H where the wall is greater than 8 feet. For retaining walls subject to
vehicular loads within a horizontal distance equal to two-thirds the wall height, a surcharge
equivalent to 2 feet of fill soil should be added (unit weight 130 pcf).
Retaining walls should be provided with a drainage system adequate to prevent the buildup
of hydrostatic forces and should be vvaterproofed as required by the project architect. The
use of drainage openings through the base of the wall (weep holes) is not recommended
where the seepage could be a nuisance or otherwise adversely affect the property adjacent
to the base of the wall. The above recommendations assume a properly compacted granular
(EI of less than 50) free-draining backfill materia! wirh no hyurostatic forces or imposed
surcharge load. Figure 16 presents ·a typicol r~tJining \Va1l dr~1nage d·~tJ.iL Tf conditions
different than those described are expected, or if specific drainage details are desired,
Geocon Incorporated should be contacted for additional recommendations.
The structural engineer should determine the seismic design category for the project in
accordance with Section 1613 of the CBC. If the project possesses a seismic design
Project No. 07135-42-05 -25-February 24, 2015
category of D, E, or F, retaining walls that support more than 6 feet of backfill should be
designed with seismic lateral pressure in accordance with Section 18.3.5.12 of the 2013
CBC. The seismic load is dependent on the retained height where H is the height of the
wall, in feet, and the calculated loads result in pounds per square foot (psf) exerted at the
base of the wall and zero at the top of the wall. A seismic load of 21 H should be used for
design. We used the peak ground acceleration adjusted for Site Class effects, PGAM, of
0.445g calculated from ASCE 7-10 Section 11.8.3 and applied a pseudo-static coefficient
of0.33.
7.10.7 In general, wall foundations having a minimum depth and width of one foot may be
designed for an allowable soil bearing pressure of 2,000 psf, provided the soil within 3 feet
below the base of the wall consists of compacted fill with an Expansion Index of less
than 90. The allowable soil bearing pressure can be increased by 300 psf and 500 psf for
each additional foot of foundation width and depth, respectively, up to a maximum
allowable soil bearing of 4,000 psf. The proximity of the foundation to the top of a slope
steeper than 3: 1 could impact the allowable soil bearing pressure. Therefore, Geocon
Incorporated should be consulted where such a condition is anticipated.
7.10.8 Resistance to lateral loads will be provided by friction along the base of the wall
foundation or by passive earth pressure against the side of the footing. Allowable
coefficients of friction of 0.35 are recommended for footings in compacted fill. Passive
earth pressure may be taken as 150 pcf for walls founded on a 2:1 slope, and 300 pcf for
horizontal ground in front of the wall. The allowable passive pressure assumes a horizontal
surface extending at least 5 feet or three times the surface generating the passive pressure,
whichever is greater. The upper 12 inches of material in areas not protected by floor slabs
or pavement should not be included in design for passive resistance.
7.1 0. 9 .The recommendations presented above arc generally applicabk to the c.ksig:n of rigid
concrete or masonry retaining 'Xa11s hm·ing a maximum height of S feet. In the c\-cnt that
v::alls higher than 8 feet arc planned, Gcocon Incorporatcu should be consult.:::d for
additional recommendations.
7_11
7.11.1
Detention 8a;;;ln ::1nd Bioswale Recommendations
Any detention basins, bioswales and bio-remediation areas should be designed by the
project civil engineer and reviewed by Geocon Incorporated. Typically, bioswales consist
of a surface layer of vegetation underlain by clean sand. A subdrain should be provided
beneath the sand layer. Prior to discharging into the stonn drain pipe, a seepage cutoff wall
should be constructed at the interface between the subdrain and storm drain pipe. The
Project No. 07135-42-05 -26-February24, 2015
concrete cut-off wall should extend at least 6-inches beyond the perimeter of the gravel-
packed subdrain system.
7.11.2 Distress may be caused to planned improvements and properties located hydrologically
downstream or adjacent to these devices. The distress depends on the amount of water to
be detained, its residence time, soil permeability, and other factors. We have not
performed a hydrogeology study at the site. Downstream and adjacent properties may be
subjected to seeps, springs, slope instability, raised groundwater, movement of foundations
and slabs, or other impacts as a result of water infiltration. Due to site soil and geologic
conditions, permanent bioswales and bio-remediation areas should be lined with an
impermeable barrier, such as a thick visqueen, to prevent water infiltration in to the
underlying compacted fill.
7.11.3
7.12
7.12.1
7.12.2
7.12.3
7.12.4
The landscape architect should be consulted to provide the appropriate plant
recommendations. If drought resistant plants are not used, irrigation may be required.
Site Drainage and Moisture Protection
Adequate site drainage is critical to reduce the potential for differential soil movement,
erosion and subsurface seepage. Under no circumstances should water be allowed to pond
adjacent to footings. The site should be graded and maintained such that surface drainage
is directed away from stmctures in accordance with 2013 CBC 1804.3 or other applicable
standards. In addition, surface drainage should be directed away from the top of slopes into
swales or other controlled drainage devices. Roof and pavement drainage should be
directed into conduits that carry nmoff away from the proposed structure.
In the case of basement walls or building walls retaining landscaping areas, a water-
proofing system should be used on the \vall and joints. and a Miradrain drainage panel (or
similar) should be placed over the waterproofing. The project architect or civil engineer
should provide detailed specifications on the plans for all v.:at,:;rproofing and drainage.
Underground utilities should be leak free. Utility and irrigation lines should be checked
periodically for leaks, and detected leaks should be repaired promptly. Detrimental soil
movement could occur if water is allowed to infiltrate the soil for prolonged periods of
time.
Adequate drainage provisions are imperative. Under no circumstances should water be
allowed to pond adj.acent to footings. The building pads should be properly finish graded
after the buildings and other improvements are in place so that drainage water is directed
Project No. 07135-42-05 -27-February24, 2015
away from foundations, pavements, concrete slabs, and slope tops to controlled drainage
devices.
Project No. 07135-42-05 -28-February 24, 2015
LIMITATIONS AND UNIFORMITY OF CONDITIONS
1. The firm that performed the geotechnical investigation for the project should be retained to
provide testing and observation services during construction to provide continuity of
geotechnical interpretation and to check that the recommendations presented for
geotechnical aspects of site development are incorporated during site grading, construction
of improvements, and excavation of foundations. If another geotechnical firm is selected to
perform the testing and observation services during construction operations, that firm should
prepare a letter indicating their intent to assume the responsibilities of project geotechnical
engineer of record. A copy of the letter should be provided to the regulatory agency for their
records. In addition, that firm should provide revised recommendations concerning the
geotechnical aspects of the proposed development, or a written acknowledgement of their
concurrence with the recommendations presented in our report. They should also perform
additional analyses deemed necessary to assume the role of Geotechnical Engineer of
Record.
2. The recommendations of this report pertain only to the site investigated and are based upon
the assumption that the soil conditions do not deviate from those disclosed in the
investigation. If any variations or undesirable conditions are encountered during
constmction, or if the proposed constmction will differ from that anticipated h1erein, Geocon
Incorporated should be notified so that supplemental recommendations can be given. The
evaluation or identification of the potential presence of hazardous or corrosive materials was
not part of the scope of services provided by Geocon Incorporated.
3. This report is issued with the understanding that it is the responsibility of the owner or his
representative to ensure that the information and recommendations contained herein are
brought to the attention of the architect and engineer for the project and incorporated into the
plans, and the necessary steps are taken to see that the contractor and subcontractors carry
out such recommendations in the field.
4. The findings of this report are valid as of the present date. However, changes in the
conditions of a property can occur with the passage of time, whether they be due to natural
processes or the \vorks of n1~111 on this or adjacen~ properti:2s. 1n J.ddition. changt.>~ in
applicable or appropriate standards may occur, whether they result from legislation or the
broadening of knowledge. Accordingly, the findings of this report may be inva] ida ted \vholly
or partially by changes outside our control. Therefore, this report is subject to review and
should not be relied upon after a period of three years.
Project No. 07135-42-05 Febmary 24, 2015