HomeMy WebLinkAboutCUP 04-13; BRESSI RANCH WELCOME CENTER; AS-GRADED REPORT OF MASS GRADING, AFFORDABLE HOUSING SITE; 2004-04-21AS-GRADED REPORT OF MASS GRADING,
AFFORDABLE HOUSING SITE, NORTHWEST
PORTION OF PLANNING AREA PA-15,
CARLSBAD, CALIFORNIA
Prepared for:
LENNAR COMMUNITIES
1525 Faraday Avenue, Suite 300
Carlsbad, California 92008
pXCF,WED
AUG 18 200
ENGINEERING
DEPARTMT.
Project No. 971009-014
April 21, 2004
• • • • • • cpPb'I(3
FrfIcMpL3
Leighton and Associates, Inc.
, l.!CN C,KOU' ;Cthtr'A1Y
April 21, 2004
Project No. 971009-014
To: Lennar Communities
1525 Faraday Avenue, Suite 300
Carlsbad, California 92008
Attention: Ms. Kristine Zortman
Subject: As-Graded Report of Mass Grading, Affordable Housing Site, Northwest Portion of
Planning Area PA-IS, Bressi Ranch, Carlsbad, California
In accordance with the request and authorization of representatives of Lennar Communities, we have
performed geotechnical services during the mass grading operations for the affordable housing site within a
portion of Planning Area PA-IS at the Bressi Ranch project (Carlsbad Tract No. 02-14), located in Carlsbad,
California. The accompanying report summarizes our geotechnical observations, field and laboratory test
results, and the geotechnical conditions encountered during the mass grading operations for the subject site.
In addition, the accompanying ieport presents our geotechnical conclusions and recommendations
concerning the fine and post grading and construction phases of site development.
The mass grading operations for the affordable housing site in Planning Area 15 of the Bressi Ranch project
were performed in general accordance with the project geotechnical reports (Appendix A), geotechnical
recommendations made during rough and fine grading, and the City of Carlsbad requirements. It is our
professional opinion that the subject site is suitable for its intended residential use provided the
recommendations included herein and in the project geotechnical reports are incorporated into the design
and construction of the residential structures and associated improvements. As of the date of this report, the
mass grading operations for the project are essentially complete.
If you have any questions regarding our report, please contact this office. We appreciate this opportunity to
be of service.
R 051
Respectfully submitted,
LEIGHTON AND ASSOCIA1UVI. No. 45283
William D. Olson, RCE 45283 CIVIV
Senior Project Engineer OF CA
Distribution: (4) Addressee
(6) Greystone Homes, Attention: Mr. Keith Randhahn
3934 Murphy Canyon Road, Suite B205 • San Diego, CA 92123-4425
858.292.8030 • Fax 856.292.0771 • www.leightongeo.com
K.
tQ2$cJ COITIFIED )J
ENGINEERING
andall K. Wagner T CE
Senior Associate OF AV%f
971009-014
TABLE OF CONTENTS
Section Paqe
1.0 INTRODUCTION 1
1.1 PROJECT DESCRIPTION 1
2.0 SUMMARY OF ROUGH AND FINE GRADING OPERATIONS 3
2.1 SITE PREPARATION AND REMOVALS 3 2.2 SUBDRAINS 3 2.3 CUT/FILL TRANSITION CONDITIONS 4 2.4 PLACEMENT OF OVERSIZED MATERIAL 4 2.5 CRITICALLY EXPANSIVE SOIL AT FINISH GRADE 4 2.6 FILL. PLACEMENT AND COMPACTION 4 2.7 FIELD DENSITY TESTING 5 2.8 LABORATORY TES TING 5 2.9 GRADED SLOPES 5
3.0 ENGINEERING GEOLOGIC SUMMARY 6
3.1 AS-GRADED GEOLOGIC CONDITIONS 6 3.2 GEOLOGIC UNITS 6 3.2.1 Topsoil (Unmappecfl ..............................................................................................................6
3.2.2 Afluvium/Colluvium. Undifferentiated (MaD Symbol-Oal/Qcol)6 3.2.3 Santiago Formation (Mao ymbol-Ts) ..................................................................................... 7 3.3 GEOLOGIC STRUCTURE .7 3.4 FAULTING AND SEISMICITY 7 3.5 GROUND WATER 8
4.0 CONCLUSIONS 9
4.1 GENERAL 9 4.2 SUMMARY OF CONCLUSIONS 9
5.0 RECOMMENDATIONS 11
5.1 EARTHWORK 11 5.1.1 Site Preparation 11 5.1.2 Mitigation of Cut/Fill Transition Conditions ..11 5.1.3 Mitigation of High to Very Highly Expansive Soils at Finish Grade ..................................12 5.1.4 Excavations 12 5.1.5 Fill Placement and Compaction.............................................................................................13
5.2 RESIDENTIAL FOUNDATION DESIGN CONSIDERATIONS 13 5.2.1 Moisture Conditioning 15 5.2.2 Seismic Design Parameters .16 5.2.3 Foundation Setback 17 5.2.4 Anticipated Settlement 17 5.3 LATERAL EARTH PRESSURES 18 5.4 FENCES AND FREESTANDING WALLS 19 5.5 CONCRETE DRIVEWAYS AND OTHER FLATWORK 20 5.6 CONCRETE 21 5.7 PREUMINARY PAVEMENT DESIGN 22
1.ighton
971009-014
5.8 SLOPE MAINTENANCE GUIDELINES 23
5.9 CONTROL OF SURFACE WATER AND DRAINAGE .24
5.10 LANDSCAPING AND POST-CONSTRUCTION .25
------------- 5.11 CONSTRUCTION OBSERVATION AND TESTING ..26
6.0 LIMITATIONS . 27
TABLES
TABLE 1 - POST-TENSIONED FOUNDATION DESIGN RECOMMENDATIONS - PAGE 14
TABLE 2 - PRESOAKING RECOMMENDATIONS BASED ON FINISH GRADE SOIL EXPANSION POTENTIAL -
PAGE 16
TABLE 3 - MINIMUM FOUNDATION SETBACK FROM SLOPE FACES- PAGE 17
TABLE 4 - LATERAL EARTH PRESSURES - PAGE 18
TABLE 5 - RECOMMENDATIONS FOR NONSTRUCTURAL CONCRETE FLATWORK ON VERY LOW TO MEDIUM EXPANSIVE SOILS
PAGE 21
TABLE 6 - PRELIMINARY PAVEMENT DESIGN - PAGE 22
FIGURE
FIGURE 1 - SITE LOCATION MAP - PAGE 2
PLATE
PLATE 1- AS-GRADED GEOTECHNICAL MAP - IN POCKET
APPENDICES
APPENDIX A - REFERENCES
APPENDIX B - SUMMARY OF FIELD DENSITY TESTS
APPENDIX C -LABORATORY TESTING PROCEDURES AND TEST RESULTS
APPENDIX D - GENERAL EARTHWORK AND GRADING SPECIFICATIONS FOR ROUGH GRADING
4
Leighton
I. S 971009-014
1.0 INTRODUCTION
In accordance with the request and authorization of representatives of Lennar Communities, we
have performed geotechnical observation and testing services during the mass grading operations
for the affordable housing site in the northwest, California (Figure 1). This as-graded report of mass
grading summarizes our geotechnical during the mass grading operations for the project. In
addition, this report provides conclusions and preliminary recommendations for the proposed
development of the site. As of this date portion of Planning Area PA-15, at the Bressi Ranch project
(Carlsbad Tract No. 02-14) located in observations, geologic mapping, field and laboratory test
results, and the geotechnical conditions encountered Carlsbad, the mass grading operations of the
site are essentially complete. However, the site is currently sheet-graded and will need to be fine
graded in order to construct the planned building pads, driveways, and parking areas. Another as-
graded report will need to be prepared upon completion of the fine grading operations documenting
the additional grading operations and provide addendum and/or additional geotechnical
recommendations relative to the proposed development.
The Mass Grading Plans for Bressi Ranch project, prepared by Project Design Consultants (PDS,
2003a) with the precise plans showing the proposed development overlaid onto the map, was
utilized as a base map to present the as-graded geotechnical conditions and approximate locations
of the field density tests. The-As-Graded Geotechnical Map (Plate 1) is presented in the pocket at
the rear of the text.
IN 1.1 Project Description
The Bressi Ranch development is located southeast of the intersection of El Camino Real
and Palomar Airport Road in the central portion of the City of Carlsbad, California
(Figure 1). The site consists of an irregular-shaped piece of property bordered on the north
by Palomar Airport Road, on the west by El Camino Real, on the southwest and south by
the La Costa - The Greens property, and by the Rancho Carrillo development and Melrose
Drive to the east.
The affordable housing site within Planning Area 15 of the Bressi Ranch project is located
in the northwest corner of Planning Area PA- 15. The site is bounded by Gateway Road to
the north, Town Garden Road to the south, Village Green Drive to the west, and the
remainder of Planning Area PA- 15 to the east.
The proposed development of Planning Area PA-15 will include 10 multi-unit affordable
housing buildings, driveways, parking areas, and associated open areas and minor slopes. It
is anticipated that the buildings will be one- to two-story residential structures that will be
constructed with slab-on-grade foundations and wood-frame and stucco construction. Other
site improvements will include retaining walls, underground utilities, concrete flatwork, and
landscaping.
4
-1-
Leighton
INNOVATION
- WAY
GAThWAY ROAD \\
:
TOWN GARDEN ROAD
N17'J j,
1•J i VJEREEN DRIVE
/3L Z>1 I GREENHAVEN
DRIVE
LiVL
AVIARA % PARKWAY
Project No. 971009-014
SITE LOCATION MAP Scale Not to scale
Engr./Geol. WDO/RKW
Planning Area PA-15
Affordable Housing I Bressi Ranch
Drafted By
Date
KAM . 04
0
April 2004
Carlsbad, California Leighton and Associates, Inc.
A L ...IN CJ ;r.ou'• CC) tIAFV FigUre NO.1
971009-014
2.0 SUMMARY OF ROUGH AND FINE GRADING OPERATIONS
The mass grading operations for the affordable housing site in the northwest portion of Bressi
Ranch. Planning Area PA-15 were performed between September 2003 and March 2004. The
grading operations were performed by Nelson and Belding while Leighton and Associates
performed the geotechnical observation and testing services. Our field technicians were on site full-
time during the grading operations while our field and project geologists were on site on a periodic
basis. Grading of the site included: 1) the removal of potentially compressible topsoil, colluvium,
alluvium, and weathered formational material; 2) preparation of areas to receive fill; 3) the
placement of subdrains in the canyon bottoms; 4) excavation of formational material; and 5)the
placement of compacted fill soils. Up to approximately 25 feet of cut was excavated and a
maximum of approximately 40 feet of fill was placed within the limits of Planning Area PA-IS.
The as-graded geotechnical conditions are presented on the As-Graded Geotechnical Map (Plate 1).
2.1 Site Preparation and Removals
Prior to grading, the areas of the proposed development were stripped of surface vegetation
and debris and these materials were disposed of away from the site. Removals of unsuitable
and potentially compressible soils (including topsoil, colluvium, alluvium, and weathered
formational material) were made to competent material. The removals of potentially
compressible material Were performed in accordance with the recommendations. of the
project geotechnical reports (Appendix A) and geotechnical recommendations made during
the course of grading.
After the removals were made, the removal areas flatter than 5:1 (horizontal to vertical)
were scarified a minimum of 12 inches, moisture-conditioned as needed to obtain a near-
optimum moisture content and compacted to a minimum 90 percent relative compaction, as
determined by American Society for Testing and Materials (ASTM) Test Method D1557.
The steeper natural hillsides were benched into competent material as fill was placed.
Representative bottom elevations in the removal areas are shown on the As-Graded
Geotechnical Map (Plate 1).
2.2 Subdrains
Canyon subdrains were placed under the observation of a representative of Leighton and
.
Associates during the mass grading operations for the Bressi Ranch project. After the
potentially compressible material in the canyons were removed to competent material or
when compacted fill was placed over competent material to obtain flow to a suitable outlet
location, a subdrain was installed along the canyon bottom. The canyon subdrains consist of
a 6-inch diameter perforated pipe surrounded by a minimum of 9-cubic feet (per linear foot)
of crushed 3/4-inch gravel wrapped in Mirafi 140N geofabric.
-3-
Leighton
971009-014
The location of the canyon subdrains placed during the mass grading operations for the
project were surveyed by the project civil engineer and the subdrain locations presented on
the As-Graded Geotechnical Map (Plate 1).
2.3 Cut/Fill Transition Conditions
The sheet-graded pad of Planning Area PA-15 consists of both cut and fills. Since the
location of the proposed residential structures on the sheet-graded pad were not known at
the time of the mass grading operations, the cut/fill transitions conditions were not
overexcavated. Cut/fill transitions should be mitigated during future fine grading operations
for the site as indicated in Section 5.1.2.
2.4 Placement of Oversized Material
Oversized rock (considered to be rocks or chunks of the cemented sandstone greater than 8
to 12 inches in maximum dimension) was placed within the limits of the site. To the best of
the grading contractor's ability and observations by our field. representatives, the oversized
rock was not placed within 10 feet of the finish grade elevation of the sheet-graded pad.
Oversized rock was placed in accordance with the project geotechnical recommendations.
2.5 Critically Expansive Soil at Finish Grade
Based on our geologic mapping during the mass-graded operations, we anticipate that high
to very high expansive soils are present in the southeast and eastern portion of the site.
Expansion potential testing of similar soils on other portions of the Bressi Ranch project has
indicated that the expansion potential may be as high as 160 or greater. Very high expansive
soils are defined as soils having an expansion index greater than 131. As mapped, the
critically expansive claystones and si!tstones are present in the southeastern and eastern
portion of the sheet-graded site at an approximate elevation of 397 to 403 feet msl. The
actual location and depth of the critically expansive soil should be evaluated during future
grading. Recommendations concerning these critically expansive soils are presented in
Section 5.1.3.
2.6 Fill Placement and Compaction
After the completion of the remedial grading removals, processing of the excavated areas,
and/or installation of the subdrains, native soil was placed as compacted fill. The maximum
fill depth within the limits of the site was on the order of 40 feet or less. The native soil was
generally spread in 4- to 8-inch loose lifts; moisture conditioned as needed to attain a near-
optimum moisture content, and compacted. Field density test results performed during the
grading operations indicated the fill soils were compacted to at least 90 percent of the
.4. S
4
Leighton
971009-014
maximum dry density in accordance with ASTM Test Method D1557.
Compaction of the fill soils was achieved by use of heavy-duty construction equipment
(including rubber-tire compactors and 637, 651, and 657 scrapers). Areas of fill in which field density tests indicated compactions less than the recommended relative compaction or
where the soils exhibited nonuniformity or had field moisture contents less than
approximately I to 2 percent below the laboratory optimum, moisture content, were
reworked. The reworked areas were recompacted, and re-tested until the recommended
minimum 90 percent relative compaction and near-optimum moisture content was
achieved.
S
2.7 Field Density Testing
I Field density testing and observations were performed using the Nuclear-Gauge Method
(ASTM Test Methods D2922 and D3017). The approximate test locations are shown on the
As-Graded Geotechnical Map (Plate 1). The results of the field density tests are summarized
I in Appendix B. The field density testing was performed in general accordance with the
applicable ASTM Standards, the current standard of care in the industry, and the precision
of the testing method itself. Variations in relative compaction should be expected from the
I results documented herein. It should be noted that the Summary of Field Density Tests
presented as Appendix B includes tests taken within portions of the streets that are not
adjacent to Planning Arei PA- 15, and consequently, are not presented on the Geotechnical
Map.
2.8 Laboratory Testing
Laboratory maximum dry density tests of representative on-site soils were performed in
general accordance with ASTM Test Method D1557. The test results are presented in
Appendix B. Expansion potential and soluble sulfate content testing of the finish grade soils
of the sheet-graded pad were not performed during the mass grading operations. However,
the expansion potential of the anticipated finish grade soils (i.e. soils within approximately S
feet of the existing ground surface) are assumed to be in the low to very high expansion
potential range. Once final grades are reached, representative finish grade soils should be
tested to determine the actual expansion potential of the soils.
2.9 Graded Slopes
Graded and natural slopes within the developed portion of the tract are considered grossly
and surficially stable from a gèotechnical standpoint. Manufactured cut and fill slopes
within the tract were surveyed by the civil engineer are understood to have been constructed
with slope inclinations of 2:1 (horizontal to vertical) or flatter.
-5- 4
Leighton
971009-014
3.0 ENGINEERING GEOLOGIC SUMMARY
3.1 As-Graded Geologic Conditions
The geologic or geotechnical conditions encountered during the mass grading of the site
were essentially as anticipated. A comprehensive summary of the geologic conditions
(including geologic units; geologic structure and faulting) is presented below. The as-graded
geologic conditions are presented on the As-Graded Geotechnical Map (Plate 1).
3.2 Geologic Units
The geologic units encountered during the mass grading operations consisted of topsoil,
colluvium, alluvium, and the Santiago Formation. Due to the potentially compressible
nature of the topsoil, colluvium, alluvium, and weathered formational material, these soils
were removed to competent material during the mass grading operations.
The approximate limits of the as-graded geologic units encountered during the grading
operations are presented on the As-Graded Geotechnical Map (Plate 1) and discussed
(youngest to oldest) below.
3.2.1 Topsoil (Unmapped)
A relatively thin veneer of topsoil was removed from the majority of the site. The
topsoil, as encountered, consisted predominantly of a brown, damp to moist, loose,
sandy clay and minor clayey to silty sand. The topsoil was generally massive,
porous, and contained scattered roots and organics. Topsoil removal thicknesses
were on the order of 1 to 4 feet thick. Duringthe grading operations, the topsoil was
observed to have been removed within the limits of grading.
3.2.2 Alluvium/Colluvium. Undifferentiated (Map Symbol-Qal/Qcol)
Alluvium and colluvium was encountered during the mass grading in the tributary
canyons and on the lower portion of the hillsides on the site. As encountered, the
alluvium and colluvium consisted of dark brown, moist, loose to stiff, clayey sand,
sandy clay, and silty sand. Where encountered, the alluvium and colluvium 'was
removed to competent material. Up to. approximately 16 feet of alluvium and
colluvium was removed during the mass grading operations.
-6- .4 .
Leighton
971009-014
"I 3.2.3 Formation iT(Map 1)
The Tertiary-aged Santiago Formation, as encountered during the mass grading
operations, consisted primarily of massively bedded sandstones and
claystones/siltstones. The sandstone generally consisted of orange-brown (iron-
oxide staining) to light brown, damp to moist, dense to very dense, silty very fine to
medium grained sandstone. The siltstones and claystones were generally olive-green
to gray (unweathered), damp to moist, stiff to hard, moderately weathered, and
occasionally fractured and moderately sheared. Several well-cemented fossiliferous
sandstone beds were encountered during the mass grading operations. Critically
expansive formational claystones and siltstones are present in the southeast and
eastern portion of sheet-graded pad at an approximate elevation of 397 to 403 feet
msl.
3.3 Geologic Structure
The general structure of the formational material appears to be near horizontal. Based on
I our geologic mapping during the mass grading operations, bedding within the Santiago
Formation generally exhibited somewhat variable bedding with strikes ranging from
northwest to northeast and dips typically 2 to 9 degrees to the southeast and northwest.
I Locally, cross bedding was observed with dips steeper than 15 degrees.
Jointing on-site was observed to be very variable, but predominantly trended subparallel to
the existing slopes. Jointing dips were found to be generally moderately to steeply dipping.
Jointing was mainly encountered in the upper portion of the bedrock becoming less
pronounced with depth.
Randomly oriented shears were encountered in the Santiago Formation claystone and
siltstone units. Numerous wide, diffuse zones of shearing, as well as more well-defined
zones, were encountered in the bedrock, and are thought to be the result of regional tectonic
shearing of the relatively stiff and unyielding siltstone and claystone.
3.4 Faultina and Seismici
Our discussion of faults on the site is prefaced with a discussion of California legislation
and state policies concerning the classification and land-use criteria associated with faults.
By definition of the California Mining and Geology Board, an active fault is a fault that has
had surface displacement within Holocene time (about the last 11,000 years). The State
Geologist has defined a potentially active fault as any fault considered to have been active
during Quaternary.time (last 1,600,000 years) but that has not been proven to be active or
inactive. This definition is used in delineating Fault-Rupture Hazard Zones as mandated by
- the Aiquist-Priolo Earthquake Fault Zoning Act of 1972 and as most recently revised in
4
Leighton
U S 971009-014
1997. The intent of this act is to assure that unwise urban development does not occur
across the traces of active faults. Based on our review of the Fault-Rupture Hazard Zones,
the site is not located within any Fault-Rupture Hazard Zone as created by the Aiquist- 5 Priolo Act (Hart, 1997).
San Diego, like the rest of southern California, is seismically active as a result of being
located near the active margin between the North American and Pacific tectonic plates. The
principal source of seismic activity is movement along the northwest-trending regional fault
zones such as the San Andreas, San Jacinto and Elsinore Faults Zones, as well as along less
active faults such as the Rose Canyon Fault Zone.
U As indicated in the Supplemental Geotechnical Report for the Bressi Ranch project
(Leighton, 2001), there are no known major or active faults on or in the immediate vicinity
of the site. Evidence of active faulting was not encountered during the mass grading
I operations. The nearest known active fault is the Rose Canyon Fault Zone, which is
considered a Type B Seismic Source based on the 1997 Uniform Building Code (UBC), is
located approximately 7.0 miles (11.2 kilometers) west of the site. Because of the lack of
I 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.
Liquefaction and dynamic settlement of soils can be caused by strong vibratory motion
due to earthquakes. Both research and historical data indicate that loose, saturated,
granular soils are susceptible to liquefaction and dynamic settlement. Liquefaction is
typified by a loss of shear strength in the affected soil layer, thereby causing the soil to act
as a viscous liquid. This effect may be manifested by excessive settlements and sand boils
at the ground surface. The fill and formational materials underlying the site are not
considered liquefiable due to their fine-grained nature, dense physical characteristics and
unsaturated condition.
3.5 Ground Water
Ground water or seepage was not encountered during the mass grading operations for
Planning Area PA- 15 (although ground water seepage conditions were noted in other areas
of the Bressi Ranch project). Subdrain systems. were installed along the bottom of the
tributary canyons within Planning Area PA-15 (as shown on the As-Graded Geotechnical
Map, Plate 1). Based on the site-specific as-graded geotechnical conditions and our
geotechnical analysis during site grading, the geotechnical consultant has analyzed
conditions that may result in ground water seepage and appropriate recommendations, if
necessary, have been made. 'However, unanticipated seepage or ground water conditions
may occur after the completion of grading and establishment of site irrigation and
landscaping. If these conditions should occur, steps to mitigate the seepage should be made
on a case-by-case basis.
-8-
---
Leighton
971009-014
4.0 CONCLUSIONS
4.1 General
The mass grading operations for the affordable housing site within Planning Area PA- 15 of
the Bressi Ranch project were performed in general accordance with the project
geotechnical reports (Appendix A), geotechnical recommendations made during grading,
and the City of Carlsbad requirements. It is our professional opinion that the subject site is
suitable for its intended residential use provided the recommendations included herein and
in the project geotechnical report are incorporated into the design and construction of the
residential structures and associated improvements. The following is a summary of our
conclusions concerning the rough and fine grading of the site.
4.2 Summary of Conclusions
Geotechnical conditions encountered during mass grading were generally as anticipated.
Site preparation and removals were geotechnically observed.
The geologic units encountered during the mass grading operations consisted of topsoil,
colluvium,.alluvium, and the Santiago Formation.
Unsuitable topsoil, colluvium, alluvium, and weathered formational material were
removed to competent material within the limits of grading.
. Subdrains were placed in the canyon bottoms prior to fill placement.
The cut/fill transition conditions present within the site were not mitigated during the
mass grading operations. Cut/fill transition conditions present within the limits of the
proposed buildings should be mitigated by the overexcavation of the cut portion of the
building pad or by special foundation design (as indicated in Section 5.1.2).
High to very high expansive formational soils are present at or near finish grade in the
southeast and eastern. portion of the site. Unless special foundation design
considerations are implemented, these expansive soils should be removed within the
limits of the proposed building or movement sensitive improvements and replaced with
soil having a lower expansive potential (as indicated in Section 5.1.3).
-9- .
4
__ Lghton
S 971009-014
Fill soils were derived from onsite soils. The maximum fill depth within the limits of
the site was on the order of 40 feet or less. Field density testing indicated that the fill
soils were placed and compacted to at least 90 percent relative compaction (based on
ASTM Test Method DI 557) and near-optimum moisture content in accordance with the
recommendations of Leighton and Associates and the requirements of the City of
Carlsbad. The results of the field density tests are summarized in Appendix B.
Ground water or seepage conditions were not encountered during the mass grading
operations.
No evidence of active or inactive faulting was encountered during the grading
ft1 operations within the limits of Planning Area PA- 15.
. Due to the dense nature of the onsite soils, it is our professional opinion that the
liquefaction hazard at the site is considered low.
The expansion potential of the finish grade soils of the sheet-graded pad was iLtested.
However, we anticipate the onsite soils posses low to very high expansion potentials.
Very highly expansive soils should not be placed as fill or left as cut within the limits of
J the proposed buildings or other movement sensitive improvements unless special
foundation design considerations are implemented. Once final grades are reached,
representative finish grade soils should be tested to determine the actual expansion
potential of the soils. •
•
The potential for soluble sulfate attack (on Type I/Il cement) of the finish grade soils
was not tested. We anticipate that the onsite soils may possess negligible to severe
soluble sulfate contents. Once final grades are reached, representative finish grade soils
should be tested to determine the actual potential for soluble sulfate attack of the soils.
It is our opinion that the slopes of the development possess a static factor of safety of at
least 1.5 to resist deep-seated failure (under normal irrigation/precipitation patterns),
provided the recommendations in the project geotechnical reports are incorporated into
the post-grading, construction and post-construction phases of site development.
10
••
S Leighton
971009-014
5.0 RECOMMENDATIONS
5.1 Earthwork
We anticipate that future earthwork at the site will consist of site preparation, fine grading,
utility trench excavation and backfill, retaining wall backfill, and street/driveway and
parking area pavement section preparation and compaction. We recommend that the
earthwork on site be performed in accordance with the following recommendations, the
General Earthwork and Grading Specifications for Rough Grading included in Appendix D,
and the City of Carlsbad grading requirements. In case of conflict, the following
recommendations shall supersede those in Appendix D. The contract between the developer
and earthwork contractor should be worded such that it is the responsibility of the contractor
to place the fill properly and in accordance with the recommendations of this report and the
specifications in Appendix D, notwithstanding the testing and observation of the
geotechnica! consultant.
5.1.1 Site PreDaration
During future grading, the areas to receive structural fill or engineered structures
should be cleared of surface obstructions, potentially compressible material (such as
desiccated fill soils or weathered formational material), and stripped of vegetation.
Vegetation and debris should be removed and properly disposed of off site. Holes
resulting from removal of buried obstructions that extend below finish site grades
should be replaced with suitable compated fill material. Areas to receive fill and/or
other surface improvements should be scarified to a minimum depth of 12 inches,
brought to moisture contents of at least 2-percent. over the optimum moisture
content, and recompacted to at least 90 percent relative compaction (based on
ASTM Test Method Dl 557).
If the length of time between the completion of grading añd.the construction of the
development is longer than six months, we recommend that the building pads be
evaluated by the geotechnical consultant and, if needed, the finish grade soils on the
building pads should be scarified a minimum of 12. inches, moisture-conditioned to
2-percent above the optimum moisture-content and recompacted to a minimum 90
percent relative compaction (based on ASTM Test Method Dl 557).
5.1.2 Mitigation of Cut/Fill Transition Conditions
In order to reduce the potential for differential settlement of the proposed buildings
in areas of cut/fill transitions, we recommend that the entire cut portion of the
building pad be overexcavated and replaced with properly compacted fill. The
-11-• . 4
Leighton
971009-014
cut/fill transition condition overexcavation should be made a minimum of 5 feet below the planned finish grade elevation and should extend laterally at least 10 feet
beyond the building perimeter or footprint. In order to minimize perched ground
water in the overexcavation, we recommend that the overexcavation bottom be
tilted a minimum of 2-percent toward the till side of the building pad.
Additional or revised recommendations may be warranted based on the
configuration and size of the proposed buildings. If the majority of the proposed
building is located on cut, the fill soils beneath the building may be removed and
recompacted to a minimum 95 percent relative compaction or a deep foundation
system founded completely on formational material may be utilized.
5.1.3 Mitigation of High to Very Highly Expansive Soils at Finish Grade
Due to the anticipated high to very high expansion potential of the claystones and
siltstones present at finish grade in the southeast and eastern portion of the site, we
recommend that either: 1) the high to very high expansive soil be removed to a
depth of at least 5 feet below the planned finish grade of the proposed buildings and
other movement sensitive improvements; or 2) the buildings and improvements be
designed for these critically expansive soils. If these critically expansive soils are
removed, the removal depth should be a minimum of 5 feet below the proposed
finish grade elevation or until lower expansive sandy soils are encountered. We also
recommend that the excavation bottom be tilted a minimum of 2-percent toward the
fill side of the building pad or toward the street/driveway in order to minimize
perched ground water conditions. The resulting. excavation should be replaced with
properly compacted fill possessing a lower expansion potential.
The actual location of the claystones and siltstones at or near finish grade at the site
should be evaluated during future grading. For planning purposes, the critically
expansive soils present in the southeast and eastern portion of the site are present at
an approximate elevation of 397 to 403 feet msl.
5.1.4 Excavations
Excavations of the on-site materials may generally be accomplished with
conventional heavy-duty earthwork equipment. It is not anticipated that blasting will
be required or that significant quantities of oversized rock (i.e. rock with maximum
dimensions greater than 8 inches) will be generated during future grading. However,
localized cemented zones within the cut areas and oversized rock placed within the
compacted fill may be encountered on the site that may require heavy ripping. If
oversized rock is encountered, it should be placed in accordance with the
recommendations. presented in Appendix D, hauled offsite, or placed in non-
-12-
Leighton
971009-014
structural or landscape areas.
Due to the relatively dense characteristics of the on-site soils, temporary excavations
such as utility trenches in the on-site soils should remain stable for the period
required to construct the utility, provided they are constructed and monitored in
accordance with OSHA requirements.
5.1.5 Fill Placement and Compaction
The on-site soils are generally suitable for use as compacted fill provided they are
free or organic material, debris, and rock fragments larger than 8 inches in
maximum dimension. We do not recommend that high or very high expansive soils
be utilized as fill for the building pads (unless special foundation design
considerations are implemented) or as retaining wall backfill.
In general, all fill soils should be brought to 2-percent over the optimum moisture
content and compacted in uniform lifts to at least 90 percent relative compaction
based on the laboratory maximum dry density (ASTM Test Method Dl 557). The optimum lift thickness required to produce a uniformly compacted fill will depend
on the type and. size of compaction equipment used. In general, fill should be placed
in lifts not exceeding 8 inches in compacted thickness. Placement and compaction
of fill should be performed in general accordance with Appendix D, the current City
of Carlsbad grading ordinances, sound construction practices, and the geotechnical
recommendations presented herein.
5.2 Residential Foundation Design Considerations
The proposed foundations and slabs of the proposed structures should be designed in
accordance with structural considerations and preliminary recommendations presented
herein. Since soils ranging from low to very high expansion potential are anticipated, as well
as building pads having a significant fill differential thickness, we recommend the use of
post-tensioned slab and foundation systems. Recommendations presented herein have been
made concerning the removal of high to very high expansive soils within the limits of the
proposed improvements. If removals are not feasible, special foundation designs assuming
high to very high expansion potentials will be required (as presented in Table 1).
We recommend that the post-tensioned slabs be designed in accordance with the following
design parameters presented on Table 1 and criteria of the current edition of the Uniform
Building Code/California Building Code. The post-tensioned foundations should be
designed in accordance with building pad specific expansion potential and anticipated long-
term differential settlement (if applicable) as indicated in Section 5.2.4.
-13- 4
Leighton
971009-014
Table 1
Post-Tensioned Foundation Design Recommendations
Design Criteria
___(0-50)
Expansion Index (UBC 18-2)
Very Low to
Low Medium
(51 -90)
High*
(91-130)
Very High*
(131 -200)
ge Moisture
riation, em
r Center Lift: 5.5 feet 5.5 feet 5.5 feet 5.5 feet
Edge Lift: 2.5 feet 3.0 feet 4.5 feet 5.5 feet
Differential Swell,
Ym
Center Lift: 1.0 inches 2.0 inches 3.0 inches 4.0 inches
Edge Lift: 0.4 inches 0.8 inches 1.2 inches 1.5 inches
Differential Settlement (Short-term): 1/2 inch
[Angular Distortion Value: 1/600
Allowable Bearing Capacity: 2,000 psf
Perimeter Footing Depth: 12 inches 18 inches 24 inches E30 inches
* As previously indicated, we have recommended that high to very high expansive soils be removed
within the limits of the proposed buildings. If the removals are not feasible, foundations utilizing
these parameters will be required.
Long-term differential settlement is anticipated to occur when the fill soils become wetted
by irrigation and/or precipitation years after the completion of construction. An angular
distortion value, which is the ratio of the estimated differential settlement to the horizontal
distance over which the settlement is likely to occur, will need to be provided for buildings
having significant differential fill thicknesses. The actual angular distortion value should be
provided after final building locations are known and the site is fine-graded. For planning
purposes, an angular distortion value of 1/600 should be assumed. The estimated angular
distortion value assumes 1) a relatively uniform fill settlement across the structure (unless
otherwise noted); and 2) the actual settlement will not likely vary more than one-quarter of
the stated angular distortion at any one point. The angular distortion values and differential
settlement estimates may also be further evaluated when the actual building footprint
location on the lots are known.
The post-tensioned foundations and slabs should be designed in accordance with structural
considerations. Continuous footings with a minimum width of 12 inches and a minimum
-14- 4
Leighton
971009-014
depth of 12, 18, 24 or 30 inches below adjacent grade (based on the expansion potential of
the slab subgrade soils as indicated on Table 1) may be designed for a maximum allowable
bearing pressure of 2,000 pounds per square foot if founded into competent formational
soils or properly compacted fill soils. The allowable bearing capacity may be increased by
one-third for short-term loading such as wind or seismic forces. Where the foundation is
within 4 feet (horizontally) of adjacent drainage swales, the adjacent footing should be
embedded a minimum depth of 12 inches below the swale flowline.
The post-tension slabs should be a minimum of 5 inches thick. The slabs should be
underlain by a minimum of 2 inches of clean sand (sand equivalent greater than 30) that is
in turn underlain by plastic sheeting (10-mil) and an additional 2 inches of clean sand. The
plastic sheeting should be sealed at all penetrations and laps. Moisture vapor transmission
may be additionally reduced by use of concrete additives. Moisture barriers (i.e. plastic
sheeting) can retard, but not eliminate moisture vapor movement from the underlying soils
up through the slabs. We recommend that the floor covering installer test the moisture vapor
flux rate prior to attempting applications of the flooring. "Breathable" floor coverings
should be considered if the vapor flux rates are high. A slipsheet or equivalent should be
utilized above the concrete slab if crack-sensitive floor coverings (such as ceramic tiles,
etc.) are to be placed directly on the concrete slab.
Our experience indicates that use of reinforcement in slabs and foundations will generally
reduce the potential for drying and shrinkage cracking. However, some cracking should be
expected as the concrete Cures. Minor cracking is considered normal; however, it is often
aggravated by a high water/cement ratio, high concrete temperature at the time of
placement, small nominal aggregate size, and rapid moisture loss due to hot, dry and/or
windy weather conditions during placement and curing. Cracking due to temperature and
moisture fluctuations can also be expected. The use of low slump concrete (not exceeding 4
to 5 inches at the time of placement) can reduce the potential for shrinkage cracking and the
action of tensioning the tendons can close small shrinkage cracks. In addition to the careful
control of water/cement ratios and slump of concrete, application of 50 percent of the
design post-tensioning load within three to four days of slab pour is found to be an effective
method of reducing the cracking potential.
The slab subgrade soils underlying the post-tensioned foundation systems should be
presoaked as indicated in Section 5.2.1 prior to placement of the moisture harrier and slab
concrete.
5.2.1 Moisture Conditioning
The slab subgrade soils underlying the post-tensioned foundation systems of the
proposed structures should be presoaked in accordance with the recommendations
presented in Table 2 prior to placement of the moisture barrier and slab concrete.
The subgrade soil moisture content should be checked by a representative of
4 -15-
Leighton
971009-014
Leighton and Associates prior to slab construction.
Presoaking or moisture conditioning may be achieved in a number of ways, but
based on our professional experience, we have found that minimizing the moisture
loss of pads that have been completed (by periodic wetting to keep the upper portion
of the pad from drying out) and/or berming the lot and flooding if for a short period
of time (days to a few weeks) are some of the more efficient ways to meet the
presoaking requirements. If flooding is performed, a couple of days to let the upper
portion of the pad dry out and form .a crust so equipment can be utilized should be
anticipated.
Table 2
Presaturation Recommendations Based on Finish Grade Soil Expansion Potential
Expansion Potential (per UBC 18-1-B)
Very Low Low Medium High Presaturation
Criteria
(0-20) (21-50) (51-90) (91-130)
Minimum
Presoaking Depth 6 12 18 24
(in inches)
Minimum Near optimum 1.2 times 1.2 times 1.3 times
Recommended moisture - optimum optimum optimum
Moisture Content moisture moisture moisture
5.2.2 Seismic Design Parameters
The site lies within Seismic Zone 4, as defined in the UBC, 1997 edition. The
nearest known active fault is the Rose Canyon Fault Zone, which is considered a
Type B Seismic Source (per 1997 UBC criteria), is located approximately 7.0 miles
(or 11.2 kilometers) west of the site. The closest Type A Seismic Source is the
Julian segment of the Elsinore Fault Zone, which is located approximately 23.5
miles (or 38 kilometers) east of the site.
The following data should be considered for the seismic analysis of the proposed
structures:
. Causative Fault: Rose Canyon Fault Zone
. Maximum Magnitude: 7.2
4 -16-
Leighton
971009-014
. Seismic Source Type: B
. Seismic Zone Factor: 0.40
. Soil Profile Type: Sc
. Near Source Factors: Na 1.0/Nv = 1 .0
5.2.3 Foundation Setback
We recommend a minimum horizontal setback distance from the face of slopes or
adjacent retaining walls for all structural foundations, footings, and other settlement-
sensitive structures as indicated on Table 3. This distance is measured from the
outside bottom edge of the footing, horizontally to the slope face and is based on the
slope height and type of soil. However, the foundation setback distance may be
revised by the geotechnical consultant on a case-by-case basis if the geotechnical
conditions are different than anticipated.
Table 3
- Minimum Foundation Setback from Slope Faces
Slope Height Minimum Recommended Foundation
Setback
Less than 5 feet 5 feet
5tol5feet 7feet
Please note that the soils within the structural setback area possess poor lateral
stability, and improvements (such as retaining walls, sidewalks, fences, pavements,
etc.) constructed within this setback area may be subject to lateral movement and/or
differential settlement. Potential distress to such improvements may be mitigated by
providing a deepened footing or a pier and grade beam foundation system to support
the improvement. The deepened footing should meet the setback as described
above.
5.2.4 Anticipated Settlement
Settlement is anticipated to occur at varying times over the life of the project. Short-
term settlement typically occurs upon application of the foundation loads and is
essentially completed within the construction period. Long-term
(hydroconsolidation) settlement typically occurs in deep fills upon additional water
infiltration into the fill soils (even in properly compacted fill soils and even with
-17- 4
Leighton
971009-014
subdrains provided). This settlement typically occurs over many years. Long-term
settlement values and the affects on the foundations should be evaluated after the
site is graded and the actual fill thicknesses beneath the proposed foundations
known. However, for preliminary planning purposes, total future settlement is
expected to be order of V2 to 1 inch and differential settlement is estimated to be on
the order of 1/2 inch in 25 feet (i.e. an angular distortion value of 1/600).
5.3 Lateral Earth Pressures
The recommended lateral pressures for the onsite very low to low expansive soil (expansion
index less than 50 per UBC Table 18-1-13) or medium expansive soil (expansion index
between 51 and 90 per UBC Table 18-I-13) and level or sloping backfill are presented on
Table 4. High to very high expansive soils (having an expansion potential greater than 91
per UBC Table 184-13) should not be used as backfill soils on the site.
Table 4
Lateral Earth Pressures
Conditions.
- Equivalent Fluid Weight (pcf)
Very Law to Low Expansive Soils Medium Expansive Soils
Expansion Index less than 50 Expansion. Index between 51 and 90
Level 2:1 Slope Level 2:1 Slope
Active 35 55 60 70
At-Rest 55 65 70 80
Passive 350 150 350 150
Embedded structural walls should be designed for lateral earth pressures exerted on them.
The magnitude of these pressures depends on the amount of deformation that the wall can
yield under load. If the wall can yield enough to mobilize the full shear strength of the soil,
it can be designed for "active" pressure. If the wall cannot yield under the applied load, the
shear strength of the soil cannot be mobilized and the earth pressure will be higher. Such
walls should be designed for "at rest" conditions. If a structure moves toward the soils, the
resulting resistance developed by the soil is the "passive" resistance. The above noted
passive resistance assumes an appropriate setback per Section 5.2.3.
For design purposes, the recommended equivalent fluid pressure for each case for walls
Leighton
971009-014
founded above the static ground water and backfihled with soils of very low to low
expansion potential or medium expansion potential is provided on Table 4. The equivalent
fluid pressure values assume free-draining conditions. If conditions other than those
assumed above are anticipated, the equivalent fluid pressures values should be provided on
an individual-case basis by the geotechnical engineer. The geotechnical and structural
engineer should evaluate surcharge-loading effects from the adjacent structures. All
retaining wall structures should be provided with appropriate drainage and appropriately
waterproofed. The outlet pipe should be sloped to drain to a suitable outlet. Typical wall
drainage design is illustrated in Appendix D.
For sliding resistance, the friction coefficient of 0.35 may be used at the concrete and soil
interface. In combining the total lateral resistance, the passive pressure or the frictional
resistance should be reduced by 50 percent. Wall footings should be designed in accordance
with structural considerations. The passive resistance value may be increased by one-third
when considering loads of short duration including wind or seismic loads. The horizontal
distance between foundation elements providing passive resistance should be minimum of
three times the depth of the elements to allow full development of these passive pressures.
The total depth of retained earth for the design of cantilever walls should be the vertical
distance below the ground surface measured at the wall face for stem design or measured at
the heel of the footing for overturning and sliding. All wall backcuts should be made in
accordance with the current OSHA requirements.
The granular and native backfill soils should be compacted to at least 90 percent relative
compaction (based on ASTM Test Method D1557). The granular fill should extend
horizontally to a minimum distance equal to one-half the wall height behind the walls. The
walls should be constructed and backfilled as soon as possible after backcut excavations.
Prolonged exposure of backcut slopes may result in some localized slope instability
Foundations for retaining walls in competent formational soils or properly compacted fill
should be embedded at least 18 inches below lowest adjacent grade. At this depth, an
allowable bearing capacity of 2,000 psf may be assumed.
5.4 Fences and Freestanding Walls
Footings for freestanding walls should be founded a minimum of 18 inches below lowest
adjacent grade (or 36 inches for walls founded on high to very high expansive soils). To
reduce the potential for unsightly cracks in freestanding walls, we recommend inclusion of
construction joints at a maximum of 15-foot intervals. This spacing may be altered in
accordance with the recommendations of the structural engineer, based on wall
reinforcement details.
Our experience on similar sites in older developments indicates that walls on shallow
foundations near the top-of-slopes tend to tilt excessively over time as a result of slope
-19- 4
Leighton
I 971009-014
I creep. If the effects of slope creep on top-of-slope walls are not deemed acceptable, one or
a combination of the options provided in the following paragraphs should be utilized in
the design of such structures, based on the desired level of mitigation of creep-related
I effects on them.
A relatively inexpensive option to address creep related problems in top-of-slope walls
and fences is to allow some degree of creep damage and design the structures so that
tilting or cracking will be less visually obvious, or such that they may be economically
repaired or replaced. If, however, a better degree of creep mitigation is desired, the walls I and fences may be provided with the deepened footings to met the foundation setback
criteria laid out in Figure 18-1-1 of the UBC, 1997 edition, or these structures may be
I constructed to accommodate potential movement.
Under certain circumstances, an effective solution to minimize the effects of creep on top-
of-slope walls and fences is to support these structures on a pier-and-grade-beam system.
The piers normally consist of minimum 12-inch diameter cast-in-place caissons spaced at a
maximum of 8 feet on center, and connected together by a minimum 12-inch-thick grade
I beam at a shallow depth. The piers are typically at least 10 feet deep for medium or high
expansive soil. The steel reinforcement for the system should be designed with
consideration of wall/fence type and loading. Walls or fences aligned essentially
I perpendicular to the top of the slope are normally supported on the pier-and-grade-beam
system for at least that part of the wall that is within 15 feet from the top-of-slope. Caisson
support is recommended br all top-of-slope walls where slopes are greater than 10 feet in
p height. And the slopes consist of high or very high expansive soils.
5.5 Concrete Driveways and Other Flatwork
Concrete driveways and nonstructural flatwork (such as walkways, swimming pool decks,
patio slabs, etc.) have a high potential for cracking or lifting due to change in soil volume.
To reduce the potential for excessive cracking, the concrete should be designed in
accordance with the minimum recommendations outlined in Table 5. The
recommendations presented in Table 5 assume very low to medium expansive soils. The
recommendations will reduce the potential for cracking and promote cracking along
construction joints, but will not eliminate all cracking or lifting. Thickening the concrete
and/or adding additional reinforcement will also help to reduce cosmetic distress. A
flexible seal should be provided between the garage and the driveway.
I . Recommendations concerning concrete flatwork on high to very high expansive subgrade
soils can be provided at a later date based on the actual expansion potential of the as-
graded soils. For preliminary planning purposes, recommendations concerning concrete
I placed on high to very high expansive soils may include overexcavation of the subgrade
soils and moisture conditioning it to a 6-percent over the optimum moisture content prior
to replacing the soil as compacted fill and/or providing an aggregate base layer beneath
.4 i -20-
Leighton
971009-014
the concrete flat.vork.
Table 5
Recommendations for Nônstructural Concrete Flatwork on Very Low to Medium Expansive Soils
City Sidewalk
Item Sidewalks Garage Driveways Patios! Entryways Curb and
Gutters
Minimum
Thickness
4 inches
(nominal)
5 inches (full) 5 inches (full) City Standard
Optimum moisture Optimum Optimum Presaturation content to 12 inches moisture content moisture content City Standard
to 12 inches to 12 inches
Minimum 6x6 #6 welded wire No. 3 at 24 inches No. 3 at 24 inches City Standard Reinforcement mesh at center of slab on centers on centers
Thickened Edge Not required Not required Not required City Standard
Saw cut of deep tool. Saw•cut every 6 Deep tool joint at
Crack Control joint to a minimum
of 1/3 the concrete feet, to 1/3 10' maximum City Standard
thickness concrete thickness spacing
Maximum Joint 10 feet or quarter
Spacing 5 feet cut whichever is 5 feet City Standard
closer
Aggregate Base Not required 2 inches Not required City Standard
5.6 Concrete
In-place concrete is subject to adverse conditions such as unsightly cracking, excessive
water vapor transmission, sulfate attack, efflorescence, and other adverse conditions.
Adherence to the following guidelines will help mitigate against the above adverse *hazards.
1) Exposure to sulfate-containing solutions:
The soluble sulfate content of the finish grade soils on the site are anticipated to
be in the negligible to severe range based on 1997 Uniform Building Code
criteria.
Comply with 1997 UBC Table 19-A-4; and
Maintain concrete water/cement ratio less than 0.5.
4
-21-
Leighton
971009-014
2) Drying shrinkage cracking:
Follow recommendations of AC! 302.117 for industrial/commercial structures,
or follow recommendations of AC! 332.R for residential construction, as
appropriate;
. Maintain concrete water/cement ratio less than 0.5.
Use minimum cement required to achieve desired strength;
Provide effective concrete curing for seven days after placing;
I . Design control joints into slab; and
Do not place concrete on hot, windy low-humidity days.
I
1•
I
5.7
I
I
I
I
I
3) Reduction of vapor transmission:
Maintain concrete water/cement ratio less than 0.5.
o Avoid construction punctures of vapor barriers;
Seal vapor barrier jOints;
Extend vapor barrier into footing/grade beam excavation (not covering bottom
of excavation);
Prevent excessive irrigation of landscaping; and
Use floor-covering adhesives that are not water-soluble.
Preliminary Pavement Design
The appropriate Asphalt Concrete (AC) and Class 2 aggregate base (AB) pavement section
will depend on the iype of subgrade soil, shear strength, traffic load, and planned pavement
life. Since an evaluation of the actual subgrade soils cannot be made at this time, we have
assumed an R-value of 12 and a Traffic Index (TI) of 6.0. The pavement section presented
on Table 6 is to be used for preliminary planning purposes only. Final pavement designs
should be completed in accordance with the City of Carlsbad design criteria after R-value
tests have been performed on the actual subgrade materials.
Table 6
Preliminary Pavement Section Design
Traffic Index Assumed R-Value Preliminary Pavement Section
6.0 12 4 inches AC over 12 inches Class 2
Aggregate Base
Asphalt Concrete and Class 2 aggregate base should conform to and be placed in
accordance with the latest revision of California Department of Transportation Standard
Specifications. Prior to placing the pavement section, the subgrade soils should have a
4 -22-
Leighton
971009-014
relative compaction of at least 95 percent to a minimum depth of 12 inches (based on
ASTM Test Method D1557). Aggregate Base should be compacted to a minimum of 95
percent relative compaction (based on ASTM Test Method D1557) prior to placement of
the AC.
If pavement areas are adjacent to heavily watered landscaping areas, we recommend some
measures of moisture control be taken to prevent the subgrade soils from becoming
saturated. It is recommended that the concrete curbing, separating the landscaping area from
I
.the pavement, extend below the aggregate base to help seal the ends of the sections where
heavy landscape watering may have access to the aggregate base. Concrete swales should be
designed if asphalt pavement is used for drainage of surface waters.
5.8 Slope Maintenance Guidelines
I It is the responsibility of the owner to maintain the slopes, including adequate planting,
proper irrigation and maintenance, and repair of faulty irrigation systems. To reduce the
I . potential for erosion and slumping of graded slopes, all slopes should be planted with
ground cover, shrubs, and plants that develop dense, deep root structures and require
minimal irrigation. Slope planting should be carried out as soon as practical upon
I completion of grading. Surface-water runoff and standing water at the top-of-slopes
should be avoided. Oversteepening of slopes should be avoided during construction
activities and landscaping. Maintenance of proper lot drainage, undertaking of property
improvements in accordance with sound engineçring practices, and proper maintenance
of vegetation, including regular slope irrigation, should be performed Slope irrigation
sprinklers should be adjusted to provide maximum uniform coverage with minimal of
water usage and overlap. Overwatering and consequent runoff and ground saturation
- should be avoided. If automatic sprinklers systems are installed, their use must be
adjusted to account for rainfall conditions.
I Trenches excavated on a slope face for any purpose should be properly backfilled and
compacted in order to obtain a minimum of 90 percent relative compaction, in accordance
I with ASTM Test Method D1557. Observation/testing and acceptance by the geotechnical
consultant during trench backfill is recommended. A rodent-control program should be
established and maintained. Prior to planting, recently graded slopes should be
I
temporarily protected against erosion resulting from rainfall, by the implementing slope
protection measures such as polymer covering, jute mesh, etc.
-23-
Leighton
971009-014
5.9 Control of Surface Water and Drainage
Surface drainage should be carefully. taken into consideration during precis
e
g
r
a
d
i
n
g
,
landscaping, and building construction. Positive drainage (e.g., roof gutters, down
s
p
o
u
t
s
,
area drain, etc.) should be provided to direct surface water away from structur
e
s
a
n
d
towards the Street or suitable drainage devices. Ponding of water adjacent to structu
r
e
s
should be avoided; roof gutters, downspouts, and area drains should be aligned
s
o
a
s
t
o
transport surface water to a minimum distance of 5 feet away from structures. The performance of structural foundations is dependent upon maintaining adequa
t
e
s
u
r
f
a
c
e
drainage away from structures.
Water should be transported off the site in approved drainage devices or unobs
t
r
u
c
t
e
d
swales. We recommend that the minimum flow gradient for the drainage be I-
p
e
r
c
e
n
t
f
o
r
area drains and paved drainage swales; and 2-percent for unpaved drainage swal
e
s
.
W
e
recommend that where structures will be located within 5 feet of a proposed drainage swale, the surface drainage adjacent to the structures be accomplished with a gradi
e
n
t
o
f
at least 3-1/2 percent away from the structure for a minimum horizontal distance
o
f
3
f
e
e
t
.
Drainage should be further maintained by a swale or drainage path at a gradient of at
l
e
a
s
t
1-percent for area drains and paved drainage swales and 2-percent for unpaved
d
r
a
i
n
a
g
e
swales to a suitable collection device (i.e. area drain, street gutter, etc.).
W
e
a
l
s
o
recommend that structural footings within 4 feet of the drainage swale ulo
w
l
i
n
e
b
e
deepened so that the bottom of the footing is at least 12 inches below the flow-
l
i
n
e
o
f
t
h
e
drainage swale. In places where the prospect of maintaining the minimum recomme
n
d
e
d
gradient for the drainage swales and the construction of additional area drai
n
s
i
s
n
o
t
feasible, provisions for specific recommendations may be necessary, outlining
t
h
e
importance of maintaining positive drainage.
The impact of heavy irrigation or inadequate runoff gradient can create perched
w
a
t
e
r
conditions, resulting in seepage or shallow groundwater conditions where previous
l
y
n
o
n
e
existed. Maintaining adequate surface drainage and controlled irrigation will sig
n
i
f
i
c
a
n
t
l
y
reduce the potential for nuisance-type moisture problems. To reduce differential e
a
r
t
h
movements (such as heaving and shrinkage due to the change in moisture con
t
e
n
t
o
f
foundation soils, which may cause distress to a structure or improvement), th
e
m
o
i
s
t
u
r
e
content of the soils surrounding the structure should be kept as relatively cons
t
a
n
t
a
s
possible.
All area drain inlets should be maintained and kept clear of debris in order to f
u
n
c
t
i
o
n
properly. In addition, yard landscaping should not cause any obstruction to the
y
a
r
d
drainage. Rerouting of yard drainage pattern and/or installation of area drains s
h
o
u
l
d
b
e
performed, if necessary. A qualified civil engineer or a landscape architect should
b
e
consulted prior to rerouting of drainage.
-24'-
Leighton
971009-014
5.10 Landscaping and Post-Construction
Landscaping and post-construction practices carried out by the owner(s) and their
representative bodies exert significant influences on the integrity of structures founded on
expansive soils. Improper landscaping and post-construction practices, which are beyo
n
d
the control of the geotechnical engineer, are frequently the primary cause of distress to these
structures. Recommendations for proper landscaping and post-construction practices are
provided in the following paragraphs within this section. Adhering to the
s
e
recommendations will help in minimizing distress due to expansive soils, and in ensuring
that such effects are limited to cosmetic damages, without compromising the overall
integrity of structures. The recommendations provided herein have been developed i
n
general accordance with the guidelines provided within the Post-Tensioning Institute's
(1996) recommendations for the design and construction of post-tensioned slabs-on-ground
.
Initial landscaping should be done on all sides adjacent to the foundation of a structure, and
adequate measures should be taken to ensure drainage of water away from the foundation. I
f
larger, shade providing trees are desired, such trees should be planted away from structur
e
s
(at a minimum distance equal to half the mature height of the tree) in order to prevent
penetration of the tree roots beneath the foundation of the structure.
Locating planters adjacent to buildings or structures should be avoided as much as possible.
If planters are utilized in these locations, they should be properly designed so as to prevent
fluctuations in the moisture content of subgrade soils. Planting areas at grade should be
provided with appropriate positive drainage. Wherever possible, exposed soil areas should
be above paved grades. Planters should not be depressed below adjacent paved grades
unless provisions for drainage, such as catch basins and drains, are made. Adequate
drainage gradients, devices, and curbing should be provided to prevent runoff from adjac
e
n
t
pavement or walks into planting areas.
Watering should be done in a uniform, systematic manner as equally as possible on all sides
of the foundation, to keep the soil moist. Irrigation methods should promote uniformity o
f
moisture in planters and beneath adjacent concrete flatwork. Overwatering a
n
d
underwatering of landscape areas must be avoided. Areas of soil that do no have ground
cover may require more moisture, as they are more susceptible to evaporation. Ponding or
trapping of water in localized areas adjacent to the foundations can cause differential
moisture levels in subsurface soils and should, therefore, not be allowed. Trees locate
d
within a distance of 20 feet of foundations would require more water in periods of extreme
drought, and in some cases, a root injection system may be required to maintain moisture
equilibrium. During extreme hot and dry periods, close observations should be carried out
around foundations to ensure that adequate watering is being undertaken to prevent soil
from separating or pulling back from the foundations.
4 -25-
Leighton
971009-014
5.11 Construction Observation and Testing
Construction observation and testing should be performed by the geotechnical consultant
during the remaining grading operations, future excavations and foundation or retaining wall
...construction.on.the graded portions of the site. Additionally, footing excavations should be
observed and moisture determination tests of subgrade soils should be performed by the
geotechnical consultant prior to the pouring of concrete. Foundation design plans should
also be reviewed by the geotechnical consultant prior to excavations.
I
•
-26-
Leighton
- . 971009-014
6.0 LIMiTATIONS
The presence of our field. representative at the site was intended to provide the owner with
professional advice, opinions, and recommendations based on observations of the.-con'tractor's...
work.. Although .the observations did, not reveal obvious deficiencies or deviations from project
specifications, we do not guarantee the contractor's work, nor do our services relieve the contractor
or his subcontractor's work, nor do our services relieve the contractor or his subcontractors Of their
responsibility if defects are subsequently discovered in their work. Our responsibilities did not
include any supervision or direction of the actual work procedures of the contractor, his personnel,
or subcontractors. The conclusions in this report are based on test results and observations of the
grading and earthwork procedures used and represent our engineering opinion as to the compliance
of the results with the project specifications.
.
- . .
Leighton