HomeMy WebLinkAboutApn G - GeotechnicalAPPENDIX G
Geotechnical Report
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
REVISED GEOTECHNICAL INVESTIGATION
VILLAGE 8 WEST
OTAY RANCH
CHULA VISTA, CA
For:
OTAY LAND COMPANY
October 22, 2010
P/W 1009-05
Report No. 1009-05-B-2
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
TABLE OF CONTENTS
1.0 INTRODUCTION .................................................................................................................... 2
1.1 Background and Purpose ............................................................................................. 2
1.2 Scope of Study ............................................................................................................. 2
1.3 Project Location and Description ................................................................................. 3
1.4 Land Use ...................................................................................................................... 3
1.5 Proposed Development ................................................................................................ 4
1.6 Previous Geotechnical Studies ..................................................................................... 4
2.0 GEOLOGIC SETTING ............................................................................................................ 5
2.1 Regional. ...................................................................................................................... 5
2.2 Stratigraphy .................................................................................................................. 5
2.2.1 Santiago Peak Volcanics (Map Symbol Jsp) ...................................................... 5
2.2.2 Unnamed Fanglomerate Deposits (Map Symbol Tfg) ........................................ 6
2.2.3 Otay Formation (Map Symbol 00) ..... ................................................... 6
2.2.4 Terrace Deposits (Map Symbol Qt) ..... .............................................................. 7
2.2.5 Alluvium (Map Symbol Qal) ............... : ............................................................ 7
2.2.6 Topsoil ................................................. .............................................................. 8
2.2.7 Artificial Fills (Map Symbol af) .......... .............................................................. 8
2.3 Geologic Structure/Tectonic Setting ............. .............................................................. 8
2.3.1 Tectonic Framework ............................ .............................................................. 8
2.3.2 Regional Faulting ................................. .............................................................. 8
2.3.3 Site Geologic Structure ........................ .............................................................. 9
2.4 Groundwater ................................................. .............................................................. 9
2.5 Mass Wasting ................................................ ............................................................ 10
2.6 Seismic Hazards ............................................ ............................................................ 10
2.6.1 Surface Fault Rupture .......................... ............................................................ 10
TABLE 2-1 .................................................... ............................................................ 11
2.6.2 Ground Motion ..................................... ............................................................ 11
TABLE 2-2 .................................................... ............................................................ 12
2.6.3 Liquefaction ......................................... ............................................................ 13
2.6.4 Seismically-Induced Landsliding ......... ............................................................ 13
2.6.5 Seiches and Tsunamis .......................... ............................................................ 14
3.0 ENGINEERING ANALYSES .................................. ............................................................ 14
3.1 Material Properties ........................................ ............................................................ 14
3.1.1 Excavation Characteristics ................... ............................................................ 14
3.1.2 Compressibility .................................... ............................................................ 15
3.1.3 Expansion Potential .............................. ............................................................ 15
3.1.4 Earthwork Adjustments ........................ ............................................................ 16
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TABLE 3-1 ................................................................................................................. 16
3.1.5 Chemical Analyses ............................................................................................ 16
3.2 Slope Stability ............................................................................................................ 17
4.0 GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS ................................. 17
4.1 Site Preparation and Removals ........ ......................................................................... 17
TABLE 4-1 ....................................... ......................................................................... 18
4.2 Slope Stability Remediation ............ ......................................................................... 19
4.2.1 Cut Slopes ............................... ......................................................................... 19
4.2.2 Fill Slopes ................................ ......................................................................... 19
4.2.3 Terrace Drains ......................... ......................................................................... 20
4.2.4 Natural Slopes ......................... : ....................................................................... 20
4.3 Overexcavation of Building Pads and Streets ............................................................ 20
4.4 Subsurface Drainage .................................................................................................. 21
4.5 Construction Staking and Survey ............................................................................... 22
4.6 Settlement Monitoring ............................................................................................... 22
4.7 Earthwork Considerations .......................................................................................... 23
4.7.1 Compaction Standards ....................................................................................... 23
4.7.2 Documentation of Removals and Drains .......................................................... 23
4.7.3 Treatment of Removal Bottoms ........................................................................ 23
4.7.4 Fill Placement .................................................................................................... 23
4.7.5 Benching ........................................................................................................... 24
4.7.6 Mixing ............................................................................................................... 24
4.7.7 Fill Slope Construction ...................................................................................... 24
4.7.8 Oversized Materials ........................................................................................... 25
4.7.8.1 Rock Blankets .................................................................................... 25
4.7.8.2 Rock Windrows ................................................................................. 26
4.7.8.3 Individual Rock Burial ....................................................................... 27
4.7.8.4 Rock Disposal Logistics .................................................................... 27
4.8 Haul Roads ................................................................................................................. 27
4.9 Import Materials ......................................................................................................... 27
5.0 DESIGN RECOMMENDATIONS ........................................................................................ 28
5.1 Structural Design - Residential .................................................................................. 28
5 .1.1 Foundation Design ........................................................................................... 29
5.l.2 Post- Tensioned Slab Foundation System Design Recommendations ............... 29
5.1.3 Conventional Slab Recommendations .............................................................. 30
5.l.4 Total & Differential Settlement.. ....................................................................... 30
5.1.5 Deepened Footings and Structural Setbacks .................................................... 31
FIGURE 1 .................................................................................................................. 31
5.1.6 Backyard Improvements ................................................................................... 32
5.1.7 Miscellaneous Foundation Recommendations .................................................. 32
5.2 Retaining Wall Design ............................................................................................... 32
5.2.1 Earth Pressure Coefficients - Select Backfill.. .................................................. 32
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5.2.2 Other Design Considerations ............................................................................33
5.2.3 Waterproofing and Drainage System ................................................................ 33
FIGURE 2 .................................................................................................................. 34
5.3 Other Design and Construction Recommendations ................................................... 34
5.3.1 Site Drainage ..................................................................................................... 34
5.3.2 Concrete Flatwork and Lot Improvements ....................................................... 34
5.3.3 Utility Trench Excavation ................................................................................. 35
5.3.4 Utility Trench Backfill ...................................................................................... 35
5.4 Preliminary Pavement Design .......................................................................................... 36
TABLES-I .............................................................................................................................. 36
6.0 FUTURE STUDY NEEDS .................................................................................................... 37
7.0 LIMITATIONS ...................................................................................................................... 37
APPENDIX A - REFERENCES
APPENDIX B - SUBSURFACE INVESTIGATION
APPENDIX C - LABORATORY TESTING
APPENDIX D - SLOPE STABILITY CALCULATIONS
APPENDIX E - EARTHWORK SPECIFICATIONS AND GRADING DETAILS
APPENDIX F - HOMEOWNERS MAINTENANCE GUIDELINES
POCKET ENCLOSURES:
SHEETS 3 through 5 and 7
PLATE 1
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
529 West 4th Street, Suite B
Escondido, California 92025
Telephone: (619) 850-3980 Fax: (714) 409-3287
ORANGE AND L.A. COUNTIES INLAND EMPIRE SAN DIEGO AND IMPERIAL COUNTIES
(714) 786-5661 (619) 708-1649 (619) 850-3980
OTAY LAND COMPANY October 22, 2010
1903 Wright Place, Suite 220 P/W 1009-05
Carlsbad, CA 92008-1420 Report No. 1009-05-B-2
Attention: Mr. Curt Noland
Vice President
Subject: Geotechnical Investigation, Otay Ranch,
Village 8 West, Chula Vista, California
References: Appendix A
Gentlemen:
Presented herein is Advanced Geotechnical Solutions, Inc. (AGS) revised Geotechnical Investigation for
Otay Ranch, Village 8 West, Chula Vista, California. AGS has been retained by Otay Land Company to
complete the geotechnical services supporting the EIR and Tentative Tract submittal and approval process
for this project. Signatories on this report have previous experience on this project and have accepted the
findings and recommendations from the reports prepared on the project by Pacific Soils Engineering, Inc.
(PSE). Based upon this previous work, AGS will become the Geotechnical Engineer and Engineering
Geologist of record for the project.
The purpose of this revision to the Geotechnical Investigation is to address outstanding review comments
on the May 26, 2010 Geotechnical Investigation report prepared by PSE. An EIR level investigation was
prepared by PSE (2006) based upon subsurface work compiled in 2004 (PSE). That information and data
is included herein. In response to outstanding review comments, the May 26, 2010 document was
published by PSE. Additional review comments have been provided addressing that submittal and this
document has been produced to respond to those comments via a revised version of the May 26, 2010
report.
Presented in this transmittal is explanatory text and geotechnical maps and cross sections utilizing the
most current Tentative Tract Map (dated August 11, 2010) prepared by Hale Engineering (Sheets 3
through 5, & 7 of 7).
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
AGS appreciates the opportunity to provide you with geotechnical consulting services on this project. If
you have questions concerning this report, please do not hesitate to contact the undersigned at (619) 708-
1649
Respectfully submitted, a Advanced Geotechnical Solutions, Inc.
___________________________________ __________________________________
JEFFREY A. CHANEY, Vice President PAUL DE RISI, Vice President
RCE 46544/RGE 2314, Reg. Exp. 6-30-11 CEG 2536, Reg. Exp. 5-31-11
Distribution: (8) Addressee (1) Hale Engineering, Attn: John Hayes (pdf)
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1.0 INTRODUCTION
1.1 Background and Purpose
The purpose of this report is to provide a "Tentative Tract Level" (TTM) geotechnical study of
Portions of Otay Ranch, Village 8 West, Chula Vista, California that may be utilized to support
the EIR submittal. This report has been revised to geotechnically address the most current TTM
design prepared by Hale Engineering and to integrate responses to outstanding City of Chula
review comments. This report has been prepared in a manner consistent with City of Chula Vista
geotechnical report guidelines and the current standard of practice. Geotechnical conclusions and
recommendations are presented herein and items addressed include: 1.) Unsuitable soil removals;
2.) Cut, fill and natural slope stability and remedial grading, where necessary; 3.)
Buttress/Stabilization fill requirements; 4.) Cut/fill pad over excavation criteria; 5.) Remedial and
design grading recommendations; 6.) Handling and disposal of oversize hard earth materials and
6.) Foundations design recommendations based upon anticipated as graded soil conditions.
1.2 Scope of Study
This study is aimed at providing geotechnical/geologic conclusions and recommendations for
development of TTM for residential uses, attendant streets, parks, school and open space areas.
The scope of this study included the following tasks:
Review of readily available maps, literature and aerial photographs (Appendix A).
Review of site geologic mapping conducted by PSE and refinement by AGS for this report.
Compilation of previous subsurface data from PSE (2004) including seventy-three (73)
backhoe test pits and sixteen (16) bucket auger borings that were also included in the
subsequent 2006 EIR study (Appendix B). This data has been plotted onto the current
Tentative Tract map for Village 8 West (Sheets 3 through 5, and 7 of 7) prepared by Hale
Engineering (dated August 11, 2010).
Compilation of previous laboratory testing conducted by PSE (Appendix C).
Preparation of geologic cross sections (Plate 1)
Stability analysis of both the highest cut and fill slopes (Appendix D)
Data analyses in relation to the site specific proposed improvement.
Analysis of the excavation characteristics (i.e. rippability) of onsite bedrock materials.
Discussion of pertinent geologic and geotechnical topics.
Preparation of this report and accompanying exhibits.
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1.3 Project Location and Description
The subject tentative map is part of the larger Otay Ranch Master Planned Community.
Specifically, this report covers portions of Otay Ranch, Village 8 West. The 320-acre property is
rectangular-shaped, and is located along the southern portions of the Otay Mesa, north of the
Otay River (Figure 1). The site consists of two (2) bedrock controlled, topographic regimes. The
north and east portions are underlain by Otay Formation and consist of gently rolling terrain that
is punctuated by south flowing "V"-shaped drainages. Most drainages are broad and relatively
shallow; however, locally along incised flow-lines, gradients on the order of 1.5: 1 (horizontal to
vertical) or steeper exist. Low-relief river terraces superjacent to the Otay River occupy the
southernmost portion of this terrain. The southwest portion of the parcel reflects of more rugged
terrain underlain by Santiago Peak Volcanics. Surface outcrops and large in-place exposed
boulders are common, reflecting the bedrocks resistant character.
Approximately 37.3 acres of slopes with gradients greater than 25% are present within the project
limits. This area is referred to as Rock Mountain and relief over the volcanic area within project
limits is over 300 feet (Sheet 4). The southerly extension of the offsite 30 foot access road for the
proposed sewer, storm drain, and water mains extends approximately 4,000 feet traversing
through relatively level Terrace deposits before it drops into the alluviated Otay River drainage.
1.4 Land Use
Current and past use includes light agriculture such as dry farming and pasture. A 19.6-acre City
of San Diego reservoir occupies the central part of the site (Sheet 3). Reservoir-associated large-
diameter aqueducts forming the Coronado Wye traverse the site at the approximate locations
shown on Sheets 3, 4, 5 and 7. The aqueducts were conducted by cut and cover methods and in
tunnels whose locations are within the easements shown on the accompanying plates. A rock
quarry is located southwest of the project.
1.5 Proposed Development
It is AGS's understanding that the Otay Ranch, Village 8 West will be developed to include
mixed use of residential, commercial, recreational and institutional components. The approximate
320-acre site is scheduled for 2050 living units with 21 acres allocated for a school site and 19.8
acres for three parks. Additionally, a sewer access road is proposed at the south project boundary
and ties into the existing sewer that parallels the Otay Valley (Sheet 7). Dedicated open space
totals 22.8 acres and public streets cover 28.6 acres. It is estimated that 4,676,000 cubic yard of
excavation will be required to grade the project. Import or export of materials in not planned for
the project.
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The grading concept shown on Sheet 3 through 5 depicts 20 large pads located on the north and
east portions of the property. Grade splits of several feet to as much as 30 feet separate the pads.
The single-family neighborhoods flank these larger pads to the south and southwest. Cuts as deep
as 150± feet are planned in the southwest portion of the site while fills up to about 50 feet deep
are shown in the south portion of the site. The cut slopes onsite are designed at ratios of 2: 1
(horizontal to vertical) to vertical heights of 135 feet. Fill slopes are designed at ratios of 2:1
(horizontal to vertical) up to vertical heights of 70 feet. It is our understanding that the existing
City of San Diego Reservoir will remain in-place and that the large-diameter aqueducts will be
relocated.
1.6 Previous Geotechnical Studies
Pacific Soils Engineering, Inc. (2003) has completed a feasibility-level geotechnical report for
Parcels "A", "B" and "C". Neblitt and Associates, Inc. (2003) undertook geologic mapping along
a waterline trench excavation in Parcel "B". An EIR level report was prepared in June of 2006
utilizing data generated from a subsurface investigation (conducted in 2004). A revised
Geotechnical Investigation was prepared by PSE in May of 2010. AGS has reviewed the PSE
and NA reports and have incorporated those findings and revisions into this study.
2.0 GEOLOGIC SETTING
2.1 Regional
Otay Mesa in general and Village 8 West in particular are part of a broad, relatively undeformed,
uplifted highland encompassing much of western and southern San Diego County. Elements of
the Elsinore and Laguna Salada Faults bound the highlands in the east and the Rose Canyon and
associated offshore fault to the west. Otay Mesa consists of Mesozoic metamorphic, volcanic and
igneous rocks on which marine and non-marine Cretaceous through Holocene sediments have
been deposited (Kennedy and Tan, 1977). These deposits have been only mildly deformed and
are easily recognized as widespread, near-horizontal, sedimentary beds forming the broad
tablelands and rolling hills of Otay Mesa.
2.2 Stratigraphy
The local stratigraphy reflects the regional, near-horizontal to gently southwest dipping Oligocene
Otay Formation, and a Tertiary un-named fanglomerate. These mapped units non-conformably
overlie volcanic and metavolcanic rocks of the Mesozoic Santiago Peak Volcanics. In turn,
various Pleistocene and Holocene non-marine sediments mantle those formations, particularly in
the south part of the site. Approximate geologic contacts are shown on Sheets 3 through 5 and
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sheet 7 with subsurface relationships depicted on the geologic cross sections (Plate 1).
2.2.1 Santiago Peak Volcanics (Map Symbol Jsp)
The Santiago Peak Volcanics crop out in the southwest part of the property and represent the east
flank of Rock Mountain. The contact between the Santiago Peak Volcanics and the overlying
younger geologic units represents a significant geologic hiatus. This contact is irregular and
reflects a relatively high relief Mesozoic landscape. Subsequent erosion has exhumed portions of
this ancient landscape, creating modern topographic highs including Rock Mountain.
In addition, buried, near-surface Santiago Peak Volcanics were observed in the subsurface
investigation generally along the west property line (BA-12, TP-43 and TP-46) of Sheet 3 and
cross section A-A".
The Santiago Peak Volcanics are generally dense and mildly metamorphosed volcanic rocks.
Large in-place surface boulders occur on natural slope areas in Neighborhoods P and N (Sheet 4).
Composition of the volcanic rocks varies from basalt to rhyolite but is predominantly dacite and
andesite (Kennedy and Tan, 1977). Typically the meta-volcanics display crude to moderate
bedding and foliation. Fracturing is poorly to moderately well developed. In general, outside of
boulder areas, a weathered halo of only a few feet thick exists. Below this the rock is very dense
and will require blasting to excavate. Blasting operations have been widespread in the nearby
quarry, where the formation has been mined for aggregate.
2.2.2 Unnamed Fanglomerate Deposits (Map Symbol Tfg)
A Tertiary fanglomerate initially mapped by Kennedy and Tan (1977) crops out in the lower-
elevation slopes above the Otay River (Sheets 4, 5 and 7). This mapping unit either locally
pinches out or is covered by river terrace deposits south of the property. Angular metamorphic
boulders typify the clasts within the unit. The matrix is rubified, dense and moderately to well
cemented. Crude horizontal to sub-horizontal stratification is identifiable in some outcrops.
Rubification, cementation and presence of subrounded "meta-breccia" cobbles distinguish this
formation from overlying Pleistocene terrace deposits.
2.2.3 Otay Formation (Map Symbol 0o)
The Oligocene Otay Formation underlies most of the study area. Brown to light gray
sandstone/gritstone typifies the formation. It is generally poorly to moderately indurated and is
locally cross-bedded. Infrequent to common gray bentonite beds occur throughout the section.
Typically, these beds are one to several feet thick and have relatively sharp contact with the
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interbedded sandstones. The clay beds are expansive and exhibit low shear strengths when
wetted. Harder and more resistant "gritstone" sub-units are common within the Otay Formation
and can range from a few feet to tens of feet thick. Breccia sub-units consisting of rounded to
angular cobbles to boulder-sized clasts can also be found within the Otay Formation near the
contact of the Santiago Peak Volcanics. These beds are likely equivalent to the Tertiary
Fanglomerate (Kennedy and Tan, 1977). The Breccia subunits encountered in the subsurface
explorations onsite consisted of gravel to cobble-sized clasts. The Otay Formation is less resistant
than the Santiago Peak Volcanics and unnamed Fanglomerate and thus forms subdued, rolling
topography exemplifying Otay Mesa. Its steepest slopes occur where young consequent
tributaries to the Otay River are actively eroding headward and downward.
2.2.4 Terrace Deposits (Map Symbol Qt)
Veneers of Pleistocene cobbley to bouldery, well oxidized, dense sands have been mapped on
surfaces 90 to 170 feet above the modern Otay River channel. These deposits are depicted as
terrace deposits in Neighborhoods U and V (Sheet 5) and along the majority of the sewer access
road alignment (sheet 7). These deposits vary from a few tens of feet thick to only a veneer of lag
gravel composed of residual dense cobbles and boulders.
2.2.5 Alluvium (Map Symbol Qal)
Alluvium occupies the onsite drainages. The alluvium observed is porous, expansive, and exhibits
low in-situ density. Typical these sediments vary from but a few to ten (10) feet in thickness with
local variations.
2.2.6 Topsoil
A mantle of residual "topsoil" is present over much of the rolling hills underlain by Otay
Formation. The soils are typically one (1) to five (5) feet thick, low density, organic-rich and
porous. Generally, the areas underlain by Santiago Peak Volcanics have thinner soils and are
locally absent as evidenced by the frequent occurrence of surface boulders.
2.2.7 Artificial Fills (Map Symbol af)
Significant deposits of artificial fill are associated with the reservoir and also exist over the
aqueducts crossing the site. Sheets 2 through 5, and 7 depict the locations of the major, most
recognizable artificial fill. Small prisms of fill that have not been mapped are primarily associated
with unimproved Jeep trails.
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2.3 Geologic Structure/Tectonic Setting
2.3.1 Tectonic Framework
The Otay Mesa is part of the Santa Ana sub-block of the Peninsular Ranges that extends from
Baja California in the south and that terminates in the north against the Transverse Ranges east of
Los Angeles (Jennings, 1994). The Santa Ana sub-block is a highland bounded by the Elsinore
Fault Zone on the east and by the Rose Canyon Fault Zone on the west. Many areas, especially in
the San Diego region, have remained relatively un-deformed. The Otay Mesa is such an example
as indicated by the unfolded, nearly horizontal section of sedimentary rocks.
2.3.2 Regional Faulting
Regional faults in southernmost California typically trend northwest (Figure 2) and display major
right lateral slip with common smaller scale vertical displacements (Jennings, 1994). Significant
faults of this system displaying Holocene offset are the San Andreas, Elsinore, San Jacinto,
Coronado Bank, Newport-Inglewood and Rose Canyon Faults. Of these, the Rose Canyon Fault
is the closest, being approximately twelve (12) miles (19.3 km) west.
The closest mapped fault is the La Nacion (Kennedy and Tan, 1977) about two (2) miles to the
west. Those authors show the fault separating Tertiary sedimentary rocks in the west from
Mesozoic rocks in the east. A nearby subsurface investigation by PSE along that fault
demonstrated that it is not active as evidenced by unbroken Pleistocene sediments overlying the
fault. Jennings (1994) mapped a "pre-Quaternary" fault in and paralleling the Otay River. The
"matching" of Mission Valley Formation outcroppings west of the study sites on either side of the
Otay River (Kennedy and Tan, 1977) argue for limited displacement along this postulated fault, if
it even exists.
2.3.3 Site Geologic Structure
The Santiago Peak Volcanics typically exhibit poorly to moderately well developed fracturing
and display crude to moderate bedding/foliations. As-exposed in the nearby quarry, joints and
foliation/bedding dip steeply.
The Tertiary fanglomerate and Otay Formation are only slightly fractured and represent an
essentially horizontal section of rocks. Local dips of less than 5 degrees to the south or southwest
have been recognized in the region (Kennedy and Tan, 1977) although local undulations are
possible. These dips may reflect broad up-warping of the region or, alternatively, they may
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represent initial dips. Faults were not mapped within onsite outcrops or subsurface explorations.
2.4 Groundwater
Active springs or surface seeps were not observed during our geologic field mapping or
subsurface investigation. It is possible that seasonal groundwater associated with precipitation
intermittently occurs in onsite drainages. Owing to the depth of cut, it is possible that seasonal
nuisance water trapped along joints or beds may be encountered during grading especially in the
Santiago Peak Volcanics. Minor seeps or wet areas were observed in Borings BA-9, BA-11 and
BA-12.
2.5 Non-Seismic Geologic Hazards
2.5.1 Mass Wasting
The Otay Formation is susceptible to erosion and slumping. Surficial slumps and bedrock
landslides were observed within the Otay Formation west of the project but not onsite. These
features often are associated with the La Nacion fault and/or bentonite beds exposed by erosion
and baseline down cutting.
No indications of mass wasting was observed nor mapped within the Santiago Peak Volcanics.
The absence of mass wasting and the existence of relatively stable, steep natural slopes is
common is this erosion resistant unit.
Within the project significant landslides were not identified during site reconnaissance and
subsurface investigation.
2.5.2 Flooding
According to FEMA, the site is not within a FEMA identified flood hazard.
2.5.3 Subsidence and Ground Fissuring
The Santiago Formation, Otay Formation and the Fanglomerate/Terrace deposits are not
susceptible to subsidence and or ground fissuring. The surficial units on site (alluvium,
undocumented fill and topsoil’s) can be susceptible to minor amounts of subsidence and ground
fissuring.
2.6 Seismic Hazards
The project is located in the tectonically active southern California, and will likely experience
some effects from future earthquakes. Alquist-Priolo Fault Hazard Zones have not been imposed
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within the subject property. The State of California Seismic Hazards Mapping program
identifying areas of potential liquefaction and earthquake induced landsliding has not addressed
the Otay Quadrangle as of this writing.
The type or severity of seismic hazards affecting the site is chiefly dependent upon the distance to
and direction from causative faults, the intensity and duration of the seismic events, and the onsite
soil characteristics. The seismic hazard may be primary, such as surface rupture and/or ground
shaking, or secondary, such as liquefaction or landsliding. The following is a brief seismic
hazards assessment for the project
2.6.1 Surface Fault Rupture
Active, potentially active and inactive faults are not known to exist at the site. According to the
literature, the nearest known active fault is the Rose Canyon Fault, a Type "B" fault (UBC, 1997),
located approximately twelve (12) miles (19 km) west. Accordingly, the potential for fault
surface rupture within the project is not significant. A listing of active faults within about 100-
kilometers (62 miles) of the site is presented in Table 2-1.
Table 2-1
Distance to Known Active Faults
Fault Name Distance Maximum Moment
Magnitude (Mmax)* (mi) (Km)
Rose Canyon 12 19 6.9
Coronado Bank 28 28 7.4
Elsinore-Julian 43 69 7.1
Elsinore-Coyote Mountain 45 73 6.8
Earthquake Valley 46 75 6.5
Newport-Inglewood (Offshore) 47 75 6.5
Elsinore-Temecula 54 88 6.9
San Jacinto-Coyote Creek 63 105 6.8
San Jacinto-Borrego 63 105 6.6
Laguna Salada 66 106 7.0
* Petersen and others (1996) and Blake (FRISKSP, ver.4.0
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2.6.2 Seismicity
Although no known active faults exist within the project limits, the site will experience ground
motion and associated effects from earthquakes generated along regional active faults such as
those listed in Table 2-1. Figures 2 and 3 are generalized maps showing the active faults
considered in the assessment of ground motion. Figures 4 and 5 show regional historical
earthquakes between 1800 to 1999.
To estimate potential ground shaking, PSE performed a probabilistic seismic hazard analysis
(PSHA) that is consistent with the commonly accepted procedures outlined in Petersen and others
(1996) and the UBC (ICBO, 1997). PSE used FRISKSP, developed from United States Geologic
Survey software (FRISK) by Blake (2000); to derive hypothetical probabilistic peak horizontal
accelerations using three commonly employed attenuation relationships of Boore and others
(1997), Campbell (1997,1999 Revised) and Sadigh and others (1997). Subsoil types SB (shear
wave velocity 1070 m/s) and SD (shear wave velocity 250 m/s) were used by FRISKSP. Based on
limited information, soil type SB is representative of the Santiago Peak Volcanics and SD was
preliminarily judged typical of all other soil/rock types. For a complete discussion of the software
and probabilistic methods, the reader is referred to Blake (2000)
Table 2-2 presents the horizontal ground accelerations and the average of same, representing the
UBC-consistent 10 percent probability of exceedance in 50-years (475-year return period) as
calculated by FRISKSP. Figures 6 through 11 show the probability of exceedance curves for each
attenuation relationship, using both subsoil types. Whereas the 10 percent probability of
exceedance in 50 years is the Design Basis Earthquake generally applied to "normal" structures
such as housing, the Upper Bound Earthquake (10 % chance of exceedance in 100 years) is
required for design of critical structures such as schools and emergency facilities. The Upper
Bound numbers can be deduced by inspection of Figures 6 through 11.
Table 2-2
Horizontal Ground Accelerations
Investigators Volcanics Other Geologic Units (onsite)
Boore and others 1997 0.17g 0.25g
Campbell, 1997 0.17g 0.21g
Sadigh and others, 1997 0.2g 0.2g
Average 0.18g 0.22g
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It should be noted that these hypothetical numbers are based on recent standards of practice (for
example, Martin and Lew, 1999, Petersen, 1996) and thus differ from numbers derived from past
standards of practice.
In sum, these results are based on many unavoidable geological and statistical uncertainties, yet
are consistent with current standard of practice. As engineering seismology evolves and as more
fault-specific geological data are gathered, more certainty and different methodologies may also
evolve.
2.6.3 Liquefaction
Seismic agitation of relatively loose saturated sands and silty sands can result in a buildup of pore
pressure. If the pore pressure exceeds the overburden stresses, a temporary quick condition
known as liquefaction may occur. The effects of liquefaction at a site can manifest in several
ways, and may include: 1) ground oscillations; 2) loss of bearing; 3) lateral spread; 4) dynamic
settlement; and 5) flow failure.
Within the vast majority of the project, the potential for liquefaction in both the pre- and post-
development condition is very low due to the lack of liquefaction susceptible earth materials and
the dense nature of the onsite geologic units.
2.6.4 Seismically-Induced Landsliding
No landslides, including shallow surficial failures, have been mapped onsite. This indicates the
lack of any seismically activated land sliding during past historic and ancient seismic events.
Seismically induced land sliding is not considerate a significant concern in both the pre- and post-
development site condition.
2.6.5 Seiches and Tsunamis
Seismically induced hazards such as tsunamis and earthquake-induced flooding are not
considered significant hazards. If at full capacity, it is possible that during a strong seismic event
with a long duration of shaking, minor localized overtopping of the City of San Diego reservoir
could occur. It appears that adequate freeboard exists for nearly the entire reservoir with the
possible exception at the low point where the paved access road meets the reservoir. At this
location, any overtopping would be directed onto the proposed paved access road and down to
Street " storm drain improvements. Given the likelihood, volume of the reservoir, and area of
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potential overtopping, it is likely that this event would not be a significant hazard.
3.0 ENGINEERING ANALYSES
Presented herein is a discussion of the geotechnical properties of the various soil types and earth materials
that have been developed from our site-specific analyses of the design as shown on Sheets 2 through 5
and 7, PSE’s subsurface investigation, and the referenced reports.
3.1 Material Properties
3.1.1 Excavation Characteristics
The excavation characteristics of the onsite materials are highly variable. The meta-volcanic rock,
which occurs in the southwest portion of the site, will require blasting at or near the natural
ground surface for efficient excavation in order to achieve design grade as well as the undercuts
to accommodate footings, utilities and other subsurface improvements. It is likely that the
blasting and excavation operations will generate oversized rock fragments requiring specialized
techniques.
The other onsite materials, including Otay Formation, Tertiary Fanglomerate, terrace deposits,
topsoil and artificial fill can be excavated with conventional techniques (scrapers) assisted by
minor to moderate ripping. Oversize material can be expected in the Fanglomerate. Heavy ripping
and possible local blasting should be anticipated in some localized areas of the Otay Formation.
In most cases this would be due to highly cemented sandstone, gritstone and/or the presence of
cobbles, boulders and concretions. Excavations into this formation can possibly generate oversize
material that may require special handling and placement within selected fill areas (See Section
4.7.8). Discussion of undercut recommendations is contained in Section 4.3 of this report.
3.1.2 Compressibility
Onsite materials that are significantly compressible include slope wash, topsoil and the
undocumented artificial fill, and the highly weathered portions of older alluvium, terrace, Tertiary
Fanglomerate, Otay Formation and metavolcanic rock. Mitigation measures addressing
potentially compressible soils are presented in Section 4.1
3.1.3 Expansion Potential
It is anticipated that the expansion potential of the majority of onsite materials will vary from
"low" to "very high". The majority of earth materials to be used in grading will possess an
expansion index in this range with some of the bentonite clays possibly in the "Very high"
category (UBC Table 18-1). Review of the grading design and boring information preliminarily
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indicates a paucity of thick and common bentonite sub-units with the Otay section. Mitigation for
this condition is further discussed in sections 5.1.
3.1.4 Earthwork Adjustments
In consideration of the proposed mass grading to develop the project as currently proposed on the
Preliminary Tentative Tract maps the following average earthwork adjustment factors presented
in Table 3-1 have been formulated for use in the earthwork design of the project.
.
Table 3-1
Earthwork Adjustments
Geologic Unit (map symbol) Adjustment Factor
Undocumented Artificial Fill (afu) 6%-10% Shrink
Alluvium (Qal) 8%-12% Shrink
Terrace/Fanglomerate (Qt/Tfg) 0%- 2% Bulk
Otay Formation (Oo) 0%- 5% Bulk
Santiago Peak Volcanics (Jsp) 15%- 25% Bulk
3.1.5 Chemical Analyses
The results of chemical/resistivity tests, presented in Appendix C (Plates C-l through C-13)
indicate that the soluble sulfate potential for the majority of the site can be classified as
"negligible" in accordance with ACI 318-05 per the 2007 CBC. Test results for chloride and pH
indicate a potential for corrosive attack on metal portions of the proposed structures. We
recommend that a corrosion engineer be consulted for additional testing and design/construction
recommendations. Testing should be conducted on the near-surface soils after grading completion
Mitigation for this condition is further discussed in section 5.1.
3.2 Slope Stability
Typically, the Santiago Peak Volcanics are grossly stable in cut and natural slopes owing to their
induration. Local wedge-type failures are not likely, but should be assessed by geologic mapping
during grading. The Otay Formation in many places possesses only moderate to weak shear
strengths, particularly where bentonite beds are present. The wetting of the bentonite can lead to
further reduction in shear strengths both along and across bedding. Review of the subsurface data
and proposed design indicates that bentonitic claystone beds could possibly daylight on the face
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of proposed cut slopes. The Fanglomerate and the Terrace deposits are anticipated to be grossly
and surficially stable in cut and natural slopes owing to their lack of defined bedding and the
granular nature of these deposits.
Slope stability analyses were performed using the Modified Bishop Method for circular and non-
circular failure surfaces. Stability calculations were compiled using STEDwin in conjunction with
GSTABL7 computer code. The results of our analyses is presented on Plates D-l through D-4 in
Appendix D. Additionally, a surficial slope stability analysis was conducted on Plate D-5.
4.0 GEOTECHNICAL CONCLUSIONS AND RECOMMENDATIONS
Development of the subject property as proposed is considered feasible, from a geotechnical standpoint,
provided that the conclusions and recommendations presented herein are incorporated into the design and
construction of the project. Presented below are issues identified by this study or previous studies as
possibly impacting site development. They are further summarized and described in Sections 2.0 and 3.0.
Recommendations to mitigate these issues and geotechnical recommendations for use in planning and
design are presented in the following sections of this report.
4.1 Site Preparation and Removals
Grading should be accomplished under the observation and testing of the project soils engineer
and engineering geologist or their authorized representative in accordance with the
recommendations contained herein, the current Municipal Code of the City of Chula Vista, and
PSE's Earthwork Specifications (Appendix E). Existing vegetation, trash, debris and other
deleterious materials should be removed and wasted from the site prior to removal of unsuitable
soils and placement of compacted fill. Artificial fill, topsoil, alluvium, highly weathered terrace
deposits and highly weathered Otay Formation, Tertiary Fanglomerate and Santiago Peak
Volcanics should be removed in areas planned to receive fill or where exposed at final grade. The
resulting undercuts should be replaced with engineered fill. Estimated depths of removals based
upon the geologic unit are presented in Table 4-1, it should be noted that local variations can be
expected requiring an increase in the depth of removal for unsuitable and weathered deposits. The
extent of removals can best be determined in the field during grading when observation and
evaluation can be performed by the soil engineer and/or engineering geologist. Removals should
expose competent Terrace deposits, Otay Formation, Tertiary Fanglomerate or Santiago Peak
Volcanics and be observed and mapped by the engineering geologist prior to fill placement. In
general, soils removed during remedial grading will be suitable for reuse in compacted fills
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provided they are properly moisture conditioned and do not contain deleterious materials.
The northwest-trending tunnels that currently contain the large-diameter aqueduct should be
properly mitigated prior to improvement construction. Mitigation methods include excavation to
expose the tunnel and then infill with engineered, compacted fill or infill with lightweight
sand/cement grout. A combination of these methodologies may be employed depending upon the
depth of the tunnel below the design ground surface.
Table 4-1
Estimated Depth of Removal
Geologic Unit (map symbol) Estimated Removal Depth
Undocumented Artificial Fill (afu) 3-15 feet
Topsoil (No Map Symbol) 2-5 feet
Alluvium (Qal) 4-10 feet
Terrace Deposits (Qt) 1-4 feet
Otay Formation (Oo) 3-6 feet
Tertiary Fanglomerate (Tfg) 1-3 feet
Santiago Peak Volcanics (Jsp) 1-3 feet
4.2 Slope Stability Remediation
4.2.1 Cut Slopes
Cut slopes have been designed at slope ratios of 2: 1 (horizontal to vertical) to heights of one
hundred thirty five feet. Stability calculations are presented on Plates D-1 through D- 2. The
engineering geologist should observe all cut slopes during grading. Based on our analysis of the
subsurface data and review of the proposed design, it is possible that flat lying bentonitic
claystone beds may be exposed on finished slope faces. Additional evaluation for this condition
should be conducted once 40-scale plans become available. At that time more detailed
recommendations can be presented. A skin fill slope and a fill over cut slope-requiring
stabilization fills have been identified on Sheet 4.
Upon completion of 40-scale grading plans, these areas should be evaluated for the appropriate
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remedial measure. At that time a more detailed analysis can be provided that can be based on
detailed slope dimensions and allow for accurate keyway determinations. Ultimately, all final
remedial cut slope recommendations should be based upon conditions exposed during grading to
confirm that assumptions developed during this and future grading plan reviews are consistent
with those observed in the field.
All buttress and stabilization fills will require backdrain systems as shown on Detail-3 (Appendix
E). Upper tier backdrains are recommended in stabilization and buttress fills where geologic
contacts and/or claystones daylight in the backcut or where backcut heights exceed thirty (30)
feet.
4.2.2 Fill Slopes
Fill slopes are designed at ratios of 2: 1 (horizontal to vertical) or flatter up to heights
approaching seventy (70) feet. Fill slopes, when properly constructed with onsite materials, are
expected to be grossly stable as designed. Stability calculations are presented on Plates D-3
through D- 5.
4.2.3 Natural Slopes
Within the project limits, natural slopes are either very low relief or present below propose
developed areas consisting of relatively stable Santiago Peak Volcanic terrain. An example is at
the south end of the project south of Neighborhoods P and V. Accordingly, natural slopes are
considered relatively stable.
4.3 Overexcavation of Building Pads and Streets
4.3.1 Building Pads
It is recommended that overexcavation of "cut" lots in hard rock, well cemented sandstones
and/or cemented breccia (Tertiary Fanglomerate) be performed. The cut and any shallow fill
portions of these lots should be overexcavated a minimum of three (3) feet and replaced to design
grade with select compacted fill. The undercut should be excavated such that a gradient of at least
one percent be maintained toward the front of the pad. Replacement fill should be eight inch
minus in particle size and compacted to project specifications. Preliminarily, anticipated rock
undercuts are indicated by a "circled R" (see legend, Sheet 3). Final determination of pad
undercut for hard rock at grade should be determined when more detailed plans are available. It is
possible that relatively thin (i.e. less than one (1) foot) bentonitic claystone beds may be exposed
at pad grade. Review of the data and design indicate that most of the recognized beds will not be
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exposed at pad grades. This includes the single family lots and larger pads areas. Accordingly, no
undercuts to mitigate the effects of claystone beds are formulated at this time; however should
such beds occur in the near-surface, undercutting to depths of 5 to 10 feet and replacement with
compacted fill may be warranted. When grading plans are available, a thorough evaluation should
be performed for this condition.
Where design or remedial grading activities create a cut/fill transition in areas of the Otay or other
formations that do not require blasting, excavation of the cut or shallow fill portion should be
performed such that at least three (3) feet of compacted fill exits over the pad. The undercut
overexcavation should maintain a minimum one (1) percent gradient to the front of the lot. In
addition, where steep cut/fill transitions are created, additional overexcavation and flattening of
the transitions are recommended.
Undercuts for the larger sheet graded pads should be deferred until actual product types and
finished grades are determined.
4.3.2 Streets
Street undercuts in hard rock areas should be based on depth of utilities within "public right of
way". The depth of undercut for streets should be at least one (1) feet below the deepest utility.
Final determination of undercut depths should be dependent upon review of more detailed plans
once they become available.
4.4 Subsurface Drainage
Six- (6) and eight- (8) inch canyon subdrains are recommended onsite in canyon areas that will
receive compacted fill. The drains should be placed along the lowest alignment of canyon
removals. Final locations of subdrains should be assessed during preliminary investigations and
plan analyses. Preliminary subdrain locations are depicted on Plates 3 through 5 and 7. Final
determination of drain locations will be made in the field, based on exposed conditions. All drains
should be constructed in accordance with the details shown on Detail 1 and 2 (Appendix E).
In some instances post-grading irrigation practices and rainfall patterns can create seepage in cut
and fill slopes. This seepage is more prevalent in cut slopes excavated in Santiago Peak Volcanics
or fill slopes constructed out of shot rock. Where nuisance seepage is observed, drains are
typically installed to collect this water and outlet it into suitable surface or subsurface drainage
devices. These drains, if required, should be installed on a case by case basis per the geotechnical
consultant’s recommendations.
The infiltration of standing water into all BMP's could potentially be detrimental to improvements
such as slopes, foundations, utility trenches, retaining walls and pavement sections. Geotechnical
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review of grading plans should be performed when available to determine which storm drain
infiltration devices may require mitigation such as collecting and discharging accumulated
subsurface water away from improvements.
4.5 Construction Staking and Survey
Removal bottoms, keyways, sub drains and backdrains should be surveyed by the civil engineer
after observation by the geotechnical engineer/engineering geologist and prior to the placement of
fill. Toe stakes should be provided by the civil engineer in order to verify required key
dimensions and locations. Survey support will also be required to monitor settlement as discussed
in Section 4.6.
4.6 Settlement Monitoring
Fills are subject to post-grading settlement it is recommended that all fills greater than fifty (50)
feet in thickness be monitored prior to release for construction. The monitoring can be
accomplished by installation of surface monuments as detailed on Detail 12 (Appendix E).
Tentative settlement monument locations should be determined once more detailed 40-scale plans
become available.
Surface monuments should be surveyed every two (2) weeks for two (2) months and monthly
thereafter until data warrants release of the area for utility or residential construction. It is likely
that infrastructure development can be initiated in advance of completion of the primary
settlement process, depending upon the sensitivity of improvements to the anticipated settlement.
4.7 Earthwork Considerations
4.7.1 Compaction Standards
Fill and processed natural ground shall be compacted to a minimum relative compaction of 90
percent as determined by ASTM Test Method: D 1557. All fill to be placed below fifty (50) feet
from ultimate grade and/or below subdrains should be compacted to at least 93 percent of
maximum density. Care should be taken that the ultimate grade be considered when determining
the compaction requirements for disposal fill and "super pad" areas. Compaction shall be
achieved at slightly above the optimum moisture content, and as generally discussed in the
attached Earthwork Specifications (Appendix E).
4.7.2 Documentation of Removals and Drains
Removal bottoms, canyon subdrains, fill keys, backcuts, backdrains and their outlets should be
observed by the engineering geologist and/or geotechnical engineer and documented by the civil
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engineer prior to fill placement.
4.7.3 Treatment of Removal Bottoms
At the completion of removals, the exposed bottom should be scarified to a depth of
approximately 8 to 12 inches, moisture conditioned to above optimum conditions, and compacted
in-place to the standards set forth in this report.
4.7.4 Fill Placement
After removals, scarification, and compaction 'of in-place materials are completed, additional fill
may be placed. Fill should be placed in thin lifts [eight- (8) inch bulk], moisture conditioned to
slightly above the optimum moisture content, mixed, compacted, and tested as grading
progresses until final grades are attained.
4.7.5 Benching
Where the natural slope is steeper than 5-horizontal to 1-vertical, and where designed by the
project geotechnical engineer or geologist, compacted fill material should be keyed and benched
into competent bedrock or firm natural soil.
4.7.6 Mixing
In order to provide thorough moisture conditioning and proper compaction, processing (mixing)
of materials is necessary. Mixing should be accomplished prior to, and as part of the compaction
of each fill lift.
4.7.7 Fill Slope Construction
Fill slopes shall be overfilled to an extent determined by the contractor, but not less than two (2)
feet measured perpendicular to the slope face, so that when trimmed back to the compacted core,
the required compaction is achieved.
Compaction of each fill lift should extend out to the temporary slope face. Backrolling during
mass filling at intervals not exceeding four (4) feet in height is recommended unless more
extensive overfill is undertaken.
As an alternative to overfilling, fill slopes may be built to the finish slope face in accordance with
the following recommendations:
Compaction of each fill lift shall extend to the face of the slopes.
Backrolling during mass grading shall be undertaken at intervals not exceeding four (4)
feet in height. Backrolling at more frequent intervals may be required.
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Care should be taken to avoid spillage of loose materials down the face of the slopes
during grading.
At completion of mass filling, the slope surface shall be watered, shaped and compacted
first with a sheepsfoot roller, then with a grid roller operated from a side boom Cat, or
equivalent, such that compaction to project standards is achieved to the slope face.
Proper seeding and planting of the slopes should follow as soon as practical, to inhibit erosion
and deterioration of the slope surfaces. Proper moisture control will enhance the long-term
stability of the finished slope surface.
4.7.8 Oversized Materials
Oversized rock material [i.e., rock fragments greater than eight (8) inches] will be produced
during the excavation of the design cuts and undercuts. Provided that the procedure is acceptable
to the developer and governing agency, this rock may be incorporated into the compacted fill
section to within three (3) feet of finish grade within residential areas and to two (2) foot below
the deepest utility in street and house utility connection areas. Maximum rock size in the upper
portion of the hold-down zone is restricted to eight (8) inches. Disclosure of the above rock hold-
down zone should be made to prospective homebuyers explaining that excavations to
accommodate swimming pools, spas, and other appurtenances will likely encounter oversize rock
[i.e., rocks greater than eight (8) inches] below three (3) feet. Rock disposal details are presented
on Detail-10, Appendix E. Rocks in excess of eight (8) inches in maximum dimension may be
placed within the deeper fills, provided rock fills are handled in a manner described below. In
order to separate oversized materials from the rock hold-down zones, the use of a rock rake may
be necessary
4.7.8.1 Rock Blankets
Rock blankets consisting of a mixture of gravel, sand and rock to a maximum dimension
of two (2) feet may be constructed. The rocks should be placed on prepared grade, mixed
with sand and gravel, watered and worked forward with bulldozers and pneumatic
compaction equipment such that the resulting fill is comprised of a mixture of the various
particle sizes, contains no significant voids, and forms a dense, compact, fill matrix.
Rock blankets may be extended to the slope face provided the following additional
conditions are met: 1) no rocks greater than twelve (12) inches in diameter are allowed
within six (6) horizontal feet of the slope face; 2) 50 percent (by volume) of the material
is three-quarter- (3/4) inch minus; and 3) backrolling of the slope face is conducted at
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four- (4) foot verticals and satisfies project compaction specifications (See Section 5.9.7).
4.7.8.2 Rock Windrows
Rocks to maximum dimension of four (4) feet may be placed in windrows in deeper fill
areas in accordance with the details on Detail 10 (Appendix E). The base of the windrow
should be excavated an equipment-width into the compacted fill core with rocks placed in
single file within the excavation. Sands and gravels should be added and thoroughly
flooded and tracked until voids are filled. Windrows should be separated horizontally by
at least fifteen (15) feet of compacted fill, be staggered vertically, and separated by at
least four (4) vertical feet of compacted fill. Windrows should not be placed within ten
(10) feet of finish grade, within two (2) vertical feet of the lowest buried utility conduit in
structural fills, or within fifteen (15) feet of the finish slope surface unless specifically
approved by the developer, geotechnical consultant, and governing agency.
4.7.8.3 Individual Rock Burial
Rocks in excess of four (4) feet, but no greater than eight (8) feet may be buried in the
compacted fill mass on an individual basis. Rocks of this size may be buried separately
within the compacted fill by excavating a trench and covering the rock with sand/gravel,
and compacting the fines surrounding the rock. Distances from slope face, utilities, and
building pad areas (i.e., hold-down depth) should be the same as windrows.
4.7.8.4 Rock Disposal Logistics
The grading contractor should consider the amount of available rock disposal volume
afforded by the design when excavation techniques and grading logistics are formulated.
Rock disposal techniques should be discussed and approved by the geotechnical
consultant and developer prior to implementation.
4.8 Haul Roads
Haul roads, ramp fills, and tailing areas should be removed prior to placement of fill.
4.9 Import Materials
Import materials, if required, should have similar engineering characteristics as the onsite soils
and should be approved by the soil engineer at the source prior to importation to the site.
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5.0 DESIGN RECOMMENDATIONS
5.1 Structural Design – Residential
It’s our understanding that the site will be graded and lots will be ultimately sold to merchant builders;
thus precise building products, loading conditions, and locations are not currently available. It is expected
that for typical one to three story residential products and loading conditions (1 to 3 ksf for spread and
continuous footings), conventional shallow slab-on-grade foundations will be utilized in areas with low
expansive and shallow fill areas (<50 feet) and where the as-grade differential fill depth meets h/3 criteria
(where h is the maximum depth of fill). Post-tensioned slab/foundations may also be used for all of the
residential lots. Typically post-tensioned slab/foundations will be used for lots which exhibit expansion
potentials ranging from “moderate” to “very high”, for lots in a areas where the fill depth exceed fifty
(50) feet in depth, and where the as-grade differential fill depth exceeds h/3 criteria (where h is the
maximum depth of fill).
Upon the completion of rough grading, finish grade samples should be collected and tested to develop
specific recommendations as they relate to final foundation design recommendations for individual lots.
These test results and corresponding design recommendations should be presented in a Final Rough
Grading Report.
It is anticipated that the as-graded near-surface soils could from "low" to "very high" in expansion
potential when tested in accordance with UBC Table 18-1- B. For preliminary budgeting purposes and
procedures can be anticipated, subject to confirmation of assumptions and as-graded conditions.
5.1.1 Foundation Design
Residential structures can be supported on conventional shallow foundations and slab-on-grade or
post-tensioned slab/foundations systems, as discussed above. The design of foundation systems
should be based on as-graded conditions as determined after grading completion. The following
values may be used in preliminary foundation design:
Allowable Bearing: 2000 psf.
Lateral Bearing: 250 psf. per foot of depth to a maximum of 2000 psf. for level
conditions. Reduced values may be appropriate for descending
slope conditions.
Sliding Coefficient: 0.35
The above values may be increased as allowed by Code to resist transient loads such as wind or
seismic. Building code and structural design considerations may govern. Depth and reinforcement
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requirements and should be evaluated by a qualified engineer.
5.1.2 Post-Tensioned Slab Foundation System Design Recommendations
Preliminary geotechnical engineering design and construction parameters for post-tensioned slab
foundations are as follows:
Post-tensioned slabs should incorporate a perimeter-thickened edge to reduce the potential for
moisture infiltration, seasonal moisture fluctuation and associated differential movement
around the slab perimeter. Deeper embedment will be required for “very high” expansion
conditions, if existent. The minimum depth of the thickened edge could vary from 12-inches
for “low” expansion to 24-inches for “high” expansion potential.
Design and construction of the post-tensioned foundations should be undertaken by firms
experienced in the field. It is the responsibility of the foundation design engineer to select the
design methodology and properly design the foundation system for the onsite soils
conditions. The slab designer should provide deflection potential to the project
architect/structural engineer for incorporation into the design of the structure.
The project foundation design engineer should use the Post-Tensioning Institute (PTI)
foundation design procedures as described in UBC, based upon appropriate soil design
parameters relating to edge moisture variation and differential swell provided by the
geotechnical consultant at the completion of rough grading operations.
A vapor/moisture barrier is recommended below all moisture sensitive areas.
5.1.3 Conventional Slab Recommendations
Conventional foundations and slabs-on grade can be considered for very low and low expansion
conditions on shallow fill areas (<50 feet). Final foundation design should be provided by the
project geotechnical engineer.
5.1.4 Total & Differential Settlement
In addition to the potential effects of expansive soils, the proposed residential structures should be
designed in anticipation of total and differential settlements. The following lot categories are
presented based upon anticipated settlement, fill thickness and expansion potential
Category I
“Very low to low” expansion potential and fill depths less than 50 feet. Minimum
fill depth meets h/3 criteria where h is the maximum fill thickness.
Total = 3/4 inch
Differential = 3/8 inch in 20 feet
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Category II
“Medium” expansion potential and/or fill depths less than 50 feet. Minimum fill
depth meets h/5 criteria where h is the maximum fill thickness.
Total = 3/4 inch
Differential = 1/2 inch in 20 feet
Category III
High expansion potential and/or fill depths greater than 50 feet.
Total = 1 inch
Differential = 1/2 inch in 20 feet
5.1.5 Deepened Footings and Structural Setbacks
It is generally recognized that improvements constructed in proximity to natural slopes or
properly-constructed, manufactured slopes can, over a period of time, be affected by natural
processes including gravity forces, weathering of surficial soils, and long-term (secondary)
settlement. Most building codes, including the Uniform Building Code (UBC), require that
structures be set back or footings deepened, where subject to the influence of these natural
processes.
For the subject site, where foundations for residential structures are to exist in proximity to
slopes, the footings should be embedded to satisfy the requirements presented in Figure 1.
FIGURE 1
H
TOP OF SLOPE
FACE OF FOOTING
TOE OF SLOPE
FACE OF
STRUCTURE H/3 BUT NEED NOT EXCEED 40 FT. MAX.
H/2 BUT NEED NOT
EXCEED 15 FT. MAX.
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
5.1.6 Miscellaneous Foundation Recommendations
Soils from the footing excavations should not be placed in slab-on-grade areas unless properly
compacted and tested. The excavations should be cleaned of all loose/sloughed materials and be
neatly trimmed at the time of concrete placement.
5.2 Retaining Wall Design
For preliminary design of retaining walls the values presented in section 5.1.1 and the following
recommendations can be utilized:
5.2.1 Earth Pressure Coefficients - Select Backfill
Level Backfill 2:1 Sloping Backfill
Ka = 0.31 Ka = 0.47
Kp = 3.25 Kp (+) = 8.61
Ko = 0.47 Kp(+) = 1.24 (Ascending Slope)
Ko(-) = 0.85 (Descending Slope)
Equivalent fluid pressure can be calculated utilizing a soil unit weight (y) of 130 pcf.
Restrained retaining walls should be designed for "at-rest" conditions, utilizing Ko'
5.2.2 Other Design Considerations
Passive earth pressure coefficients used in the design of retaining walls should consider
descending slopes as presented in section 5.2.1 as Kp () when retaining walls are positioned at the
top of slopes and/or on the face of slopes. Retaining wall design should consider the additional
surcharge loads for superjacent slopes and/or footings, where appropriate.
5.2.3 Waterproofing and Drainage System
Retaining walls should be waterproofed to minimize water staining. The walls should be
backfilled with free draining material (SE>20) to within twelve (12) inches of grade extending
horizontally one-half (1/2) the wall height compacted to project specifications. Native soils shall
be utilized in the upper eighteen (18) inches. Drainage systems including, as a minimum, a four-
(4) inch diameter perforated drain line surrounded by four (4) cubic feet per lineal foot of three-
quarters- (3/4) inch to one (1) inch crushed rock wrapped with a suitable filter fabric, should be
provided to cantilever and restrained retaining walls to relieve hydrostatic pressure (see Figure 2).
October 22, 2010 Page 27
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
FIGURE 2
5.3 Other Design and Construction Recommendations
5.3.1 Site Drainage
Positive drainage away from structures should be provided and maintained.
5.3.2 Concrete Flatwork and Lot Improvements
In an effort to minimize shrinkage cracking, concrete flatwork should be constructed of
uniformly cured, low-slump concrete and should contain sufficient control/contraction joints
(typically spaced at 8 to 10 feet, maximum).
Additional provisions need to be incorporated into the design and construction of all
improvements exterior to the proposed structures (pools, spas, walls, patios, walkways,
planters, etc.) to account for the hillside nature of the project, as well as being designed to
October 22, 2010 Page 28
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
account for potential expansive soil conditions. Design considerations on any given lot may
need to include provisions for differential bearing materials (bedrock vs. compacted fill),
ascending/descending slope conditions, bedrock structure, perched (irrigation) water, special
surcharge loading conditions, potential expansive soil pressure, and differential
settlement/heave.
All exterior improvements should be designed and constructed by qualified professionals
using appropriate design methodologies that account for the onsite soils and geologic
conditions. The aforementioned considerations should be used when designing, constructing,
and evaluating long-term performance of the exterior improvements on the lots.
The homeowners should be advised of their maintenance responsibilities as well as
geotechnical issues that could affect design and construction of future homeowner
improvements. The information presented in Appendix F should be considered for inclusion
in homeowner packages in order to inform the homeowner of issues relative to drainage,
expansive soils, landscaping, irrigation, sulfate exposure, and slope maintenance.
5.3.3 Utility Trench Excavation
All utility trenches should be shored or laid back in accordance with applicable OSHA standards.
Excavations in bedrock areas should be made in consideration of underlying geologic structure.
The project geotechnical consultant should be consulted on these issues during construction.
5.3.4 Utility Trench Backfill
Mainline and lateral utility trench backfill should be compacted to at least 90 percent of
maximum dry density as determined by ASTM D- 1557. Onsite soils will not be suitable for use
as bedding material but will be suitable for use in backfill, provided oversized materials are
removed. No surcharge loads should be imposed above excavations. This includes spoil piles,
lumber, concrete trucks, or other construction materials and equipment. Drainage above
excavations should be directed away from the banks. Care should be taken to avoid saturation of
the soils.
Compaction should be accomplished by mechanical means. Jetting of native soils will not be
acceptable. Under-slab trenches should also be compacted to project specifications. If native soils
are used, mechanical compaction is recommended. If select granular backfill (SE> 30) is used,
compaction by flooding will be acceptable. The soil engineer should be notified for inspection
prior to placement of the membrane and slab reinforcement.
5.4 Preliminary Pavement Design
Final pavement design should be made based upon sampling and testing of post-grading
conditions. For preliminary design and estimating purposes the pavement structural sections
presented in Table 5-1 can be used for the range of likely traffic indices. The structural sections
October 22, 2010 Page 29
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
are based upon an assumed R - Value of 30.
Table 5-1
Preliminary Pavement Sections
Traffic Index (TI) Asphaltic Concrete (AC)
Inches
Aggregate Base (AB)
Inches
5.0 3 6
6.0 4 9
7.0 4 10
8.0 5 13
Sub grade soils should be compacted to at least 95 percent of maximum density as determined by
ASTM D-1557. Aggregate base materials should be compacted to at least 95 percent of
maximum density as determined by California Test 216. Final determination of pavement
sections will be provided by the City of Chula Vista based upon sampling and testing of the
subgrade soils.
6.0 FUTURE STUDY NEEDS
This report represents a "Tentative Tract" map level review of Otay Land Company- Village 8 West. As
the project design progresses, additional site specific geologic and geotechnical issues will need to be
considered in the ultimate design and construction of the project. Consequently, future geotechnical
reviews are necessary. These reviews may include reviews of:
Rough grading plans.
Precise grading plans.
Foundation plans.
Retaining wall plans.
These plans should be forwarded to the project geotechnical engineer/geologist for evaluation and
comment, as necessary.
October 22, 2010 Page 30
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
7.0 LIMITATIONS
This report is based on the project as described and the information obtained from referenced reports.
The findings are based on the review of the field and laboratory data provided combined with an
interpolation and extrapolation of conditions between and beyond the reviewed exploratory excavations.
The results reflect an interpretation of the direct evidence obtained. Services performed by AGS have
been conducted in a manner consistent with that level of care and skill ordinarily exercised by members
of the profession currently practicing in the same locality under similar conditions. No other
representation, either expressed or implied, and no warranty or guarantee is included or intended.
The recommendations presented in this report are based on the assumption that an appropriate level of
field review will be provided by geotechnical engineers and engineering geologists who are familiar with
the design and site geologic conditions. That field review shall be sufficient to confirm that geotechnical
and geologic conditions exposed during grading are consistent with the geologic representations and
corresponding recommendations presented in this report. AGS should be notified of any pertinent
changes in the project plans or if subsurface conditions are found to vary from those described herein.
Such changes or variations may require a re-evaluation of the recommendations contained in this report.
The data, opinions, and recommendations of this report are applicable to the specific design of this project
as discussed in this report. They have no applicability to any other project or to any other location, and
any and all subsequent users accept any and all liability resulting from any use or reuse of the data,
opinions, and recommendations without the prior written consent of AGS.
AGS has no responsibility for construction means, methods, techniques, sequences, or procedures, or for
safety precautions or programs in connection with the construction, for the acts or omissions of the
CONTRACTOR, or any other person performing any of the construction, or for the failure of any of them
to carry out the construction in accordance with the final design drawings and specifications.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX A
REFERENCES
October 22, 2010 Page A-1
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
REFERENCES
Anderson, lC., Rockwell, T.K., and Agnew, D.C., 1989, Past and possible future earthquakes of
significance to the San Diego region: Earthquake Spectra, vol. 5, no. 2, p. 299-335.
Blake, T.F., 2000, FRISKSP: A computer program for the probabilistic estimation of peak
acceleration and uniform hazard spectra using 3-D faults as earthquake sources, v.4.00:
Thomas F. Blake, Newbury Park, California, 199 p.
Boore, D.M., Joyner, W.B., and Fumal, T.E., 1997, Equations for estimating horizontal
response spectra and peak acceleration from western North American earthquakes: A
summary of recent work: Seis. Res. Let. v. 68, n. 1, p. 128-153.
California Department of Conservation, Division of Mines and Geology, 1998, Maps of
known active fault near source zones in California and adjacent portions of Nevada,
published by ICBO February 1998.
California Division of Mines and Geology, 1999, Fault rupture hazard zones in California:
Special Publication 42.
California Division of Mines and Geology, 1997, Guidelines for evaluating and mitigating
seismic hazards in California: Department of Conservation, special publication 117, 74 p.
California Division of Mines and Geology, 1993, The Rose Canyon Fault Zone, southern
California: Department of Conservation, Open File Report 93-02, 45 pp.
California Division of Mines and Geology, 1962, Geologic Map of California, San Diego-El
Centro Sheet, California Department of Conservation, Scale 1 :250,000.
Campbell, K.W., 1997, Empirical near-source attenuation relationships for horizontal and
vertical components of peak ground acceleration, peak ground velocity, and pseudo-
absolute acceleration response spectra: Seis. Res. Let. v. 68, n. 1, p. 154-179.
Hart, E. W., 1994, Fault-rupture hazard zones in California: California Division of Mines and
Geology, special publication 42, 1992 revised edition, 34 p.
Jennings, C. W., 1994, Fault activity map of California and adjacent areas: California
Division of Mines and Geology, California geologic map data series, map no. 6, scale
1:750,000.
Jennings, C. W., 1994, An explanatory text to accompany the fault activity map of California
and adjacent areas: California Division of Mines and Geology, 92 p.
Kennedy, M. P., and Tan, S.S., 1977, Geology of National City, Imperial Beach and Otay
Mesa Quadrangles, southern San Diego metropolitan area, California: California
Division of Mines and Geology, Map Sheet 29.
October 22, 2010 Page A-2
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
REFERENCES continued
Kennedy, M.P., 1975, Character and Recency of Faulting, San Diego Metropolitan Area,
California: California Division of Mines and Geology, Special Report 123, 33p.
Legg, M. R., 1989, Faulting and seismotectonics ofthe inner continental borderland west of San
Diego: in Roquemore, G. (ed.), Proceedings workshop on the Seismic Risk in the San Diego
Region: Special Focus on the Rose Canyon Fault System, p. 50-70.
Martin, G.R. and Lew, M., 1999, Recommended procedures for implementing of DMG special
publication 117, Guidelines for analyzing and mitigating liquefaction hazards in
California: Southern California Earthquake Center, University of Southern California,
March, 1999.
Neblitt and Associates, Inc., 2003, Results of Geologic Field Mapping, Portions of Otay
Ranch Parcel B, Along 48" Diameter Water Line Trench Excavation, City of Chula
Vista, County of San Diego, California dated October 16, 2003 (Proj. No. 328001-03) .
Pacific Soils Engineering, Inc 2010, Revised Geotechnical Investigation for Village 8 West,
Otay Ranch, Chula Vista, California, dated May 26, 2010 (Work Order 400948B4). ..•
Pacific Soils Engineering, Inc 2006, Geotechnical Investigation for EIR Purposes, Parcel "B"
Otay Ranch, Chula Vista, California, dated June 2, 2006 (Work Order 400948B4).
Pacific Soils Engineering, Inc., 2003, Feasibility-Level Geotechnical Report, Parcels A, B
and C, Otay Ranch, Chula Vista, California, dated August 22, 2003 (Work Order
400948).
Petersen, M. D., Bryant, W. A., Cramer, C. H., Cao, T., Reichle, M. S., Frankel, A. D.,
Lienkaemper, J. J., McCrory, P. A., and Schwartz, D. P., 1996, Probabilistic seismic hazard
assessment for the State of California: California Division of Mines and Geology, open file
report 96-08,59 p.
Sadigh, K., Chang, Y., Egan, J.A., Makdisi, F., and Youngs, R.R., 1997, Attenuation
relationships for shallow crustal earthquakes based on California strong motion data: Seis.
Res. Let. v. 68, n.l.
Tan, S. S., 1995, Landslide hazards in the southern part of the San Diego metropolitan area,
San Diego, California, landslide identification map no. 33: California Department of
Conservation, Division of Mines and Geology, open file report 95-03.
October 22, 2010 Page A-3
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
REFERENCES continued
United States Department of Agriculture, 1953, Vertical black and white aerial photographs (photo
nos. AXN-9M-162, -163, -164, -165, and AXM 10M-4, -5) scale 1 :20,000; dated April 14,
1953.
Uniform Building Code, 1997, International Conference of Building Officials (lCBO), Volume
2, structural engineering design provisions: p. 2-30-2-35.
Walsh, S.L., and Derriere, T.A., 1991, Age and stratigraphy of the Sweetwater and Otay Formations,
San Diego County, California, in Abbott, P.L. and May, J.A. (eds.), Eocene geologic history of
the San Diego region: Society of Economic Paleontologists and Mineralogists, Pacific Section, v.
68, p. 131-148.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX B
SUBSURFACE INVESTIGATION
October 22, 2010 Page B-1
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ADVANCED GEOTECHNICAL SOLUTIONS, INC.
DESCRIPTION OF PSE’s SUBSURFACE INVESTIGATION
Pacific Soils Engineering, Inc. performed a subsurface field investigation for Otay Ranch Parcel "B"
in June of2004. Sixteen (16) borings were excavated utilizing a bucket auger drill rig (Plates A-1
through A-16). In addition, seventy-three (73) backhoe test pits were excavated (Table I). Boring and
test pit excavations ranged from three (3) to eighty (80) feet below existing grades. All excavations
were logged and sampled by representatives of PSE.
Representative bulk and "undisturbed" samples were obtained from the exploratory
excavations and delivered to PSE's laboratory for testing and analysis.
Undisturbed samples were obtained from the bucket auger borings by driving a sampling spoon into
the material. A split barrel type spoon, having an inside diameter of 2.50 inches, with a tapered
cutting tip at the lower end, was used. The barrel is lined with thin brass rings, each 1 inch in height.
The spoons penetrated into the soil approximately 12 inches. The lower portion of the sample (6
inches) was retained for testing. All samples in the natural field condition were sealed in airtight
containers and transported to PSE's laboratory.
Blow counts were noted for each "undisturbed" sample and are presented in the logs of the
borings (Plates A-1 through A-16).
The approximate locations of all exploratory excavations are shown on the enclosed plans
(Sheets 3 through 5 and 7).
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX C
LABORATORY TESTING
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
DESCRIPTION OF PSE's LABORATORY ANALYSES
Moisture Density Determinations
Moisture and density determinations were made by direct measurements on undisturbed samples to
provide in-situ information of the various materials. The results of these tests are shown on the logs
of borings [Plates A-1 through A-16 (Appendix B)] and Table II.
Compaction Characteristics
Maximum densities and optimum moistures were determined for selected samples in
accordance with ASTM: D-1557. Results are presented in Table II.
Hydrometer Analyses
Hydrometer grain size analyses were performed on the minus No. 10 sieve portion of selected
samples. These tests were used as an aid in soil classification. The results of these tests are shown on
Table II.
Expansion Index Testing
Expansion index testing was performed on selected samples in accordance with the expansion index
UBC Standard No. 18-2. Results are presented in Table II.
Direct Shear Tests
Direct shear tests were performed on samples, which were remolded to 90 percent of the laboratory
maximum density, and on undisturbed specimens. Samples were tested after inundation and
confinement for 24 hours. Tests were made under various normal loads at a constant rate of strain of
0.01 or 0.05 inches per minute. Shear test data are presented in Table II and on Plates B-1 through B-
14.
Chemica/Resistivity
Chemical/resistivity testing was conducted by others and is presented on Plates C-1 through C-13.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX D
SLOPE STABILITY ANALYSIS
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX E
GENERAL EARTHWORK SPECIFICATIONS AND
GRADING DETAILS
General Earthwork Specifications Page 1
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
GENERAL EARTHWORK SPECIFICATIONS
I. General
A. General procedures and requirements for earthwork and grading are presented herein. The
earthwork and grading recommendations provided in the geotechnical report are considered part
of these specifications, and where the general specifications provided herein conflict with those
provided in the geotechnical report, the recommendations in the geotechnical report shall govern.
Recommendations provided herein and in the geotechnical report may need to be modified
depending on the conditions encountered during grading.
B. The contractor is responsible for the satisfactory completion of all earthwork in accordance
with the project plans, specifications, applicable building codes, and local governing agency
requirements. Where these requirements conflict, the stricter requirements shall govern.
C. It is the contractor’s responsibility to read and understand the guidelines presented herein and
in the geotechnical report as well as the project plans and specifications. Information presented
in the geotechnical report is subject to verification during grading. The information presented on
the exploration logs depict conditions at the particular time of excavation and at the location of
the excavation. Subsurface conditions present at other locations may differ, and the passage of
time may result in different subsurface conditions being encountered at the locations of the
exploratory excavations. The contractor shall perform an independent investigation and evaluate
the nature of the surface and subsurface conditions to be encountered and the procedures and
equipment to be used in performing his work.
D. The contractor shall have the responsibility to provide adequate equipment and procedures to
accomplish the earthwork in accordance with applicable requirements. When the quality of work
is less than that required, the Geotechnical Consultant may reject the work and may recommend
that the operations be suspended until the conditions are corrected.
E. Prior to the start of grading, a qualified Geotechnical Consultant should be employed to
observe grading procedures and provide testing of the fills for conformance with the project
specifications, approved grading plan, and guidelines presented herein. All clearing and
grubbing, remedial removals, clean-outs, removal bottoms, keyways, and subdrain installations
should be observed and documented by the Geotechnical Consultant prior to placing fill. It is the
contractor’s responsibility to apprise the Geotechnical Consultant of their schedules and notify
the Geotechnical Consultant when those areas are ready for observation.
F. The contractor is responsible for providing a safe environment for the Geotechnical
Consultant to observe grading and conduct tests.
II. Site Preparation
A. Clearing and Grubbing: Excessive vegetation and other deleterious material shall be
sufficiently removed as required by the Geotechnical Consultant, and such materials shall be
General Earthwork Specifications Page 2
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
properly disposed of offsite in a method acceptable to the owner and governing agencies. Where
applicable, the contractor may obtain permission from the Geotechnical Consultant, owner, and
governing agencies to dispose of vegetation and other deleterious materials in designated areas
onsite.
B. Unsuitable Soils Removals: Earth materials that are deemed unsuitable for the support of fill
shall be removed as necessary to the satisfaction of the Geotechnical Consultant.
C. Any underground structures such as cesspools’, cisterns, mining shafts, tunnels, septic tanks,
wells, pipelines, other utilities, or other structures located within the limits of grading shall be
removed and/or abandoned in accordance with the requirements of the governing agency and to
the satisfaction of the Geotechnical Consultant. Environmental evaluation of existing conditions
is not the responsibility of the Geotechnical Consultant.
D. Preparation of Areas to Receive Fill: After removals are completed, the exposed surfaces shall
be processed or scarified to a depth of approximately 8 inches, watered or dried, as needed, to
achieve a generally uniform moisture content that is at or near optimum moisture content. The
scarified materials shall then be compacted to the project requirements and tested as specified.
E. All areas receiving fill shall be observed and approved by the Geotechnical Consultant prior to
the placement of fill. A licensed surveyor shall provide survey control for determining elevations
of processed areas and keyways.
III. Placement of Fill
A. Suitability of fill materials: Any materials, derived onsite or imported, may be utilized as fill
provided that the materials have been determined to be suitable by the Geotechnical Consultant.
Such materials shall be essentially free of organic matter and other deleterious materials, and be
of a gradation, expansion potential, and/or strength that is acceptable to the Geotechnical
Consultant. Fill materials shall be tested in a laboratory approved by the Geotechnical
Consultant, and import materials shall be tested and approved prior to being imported.
B. Generally, different fill materials shall be thoroughly mixed to provide a relatively uniform
blend of materials and prevent abrupt changes in material type. Fill materials derived from
benching should be dispersed throughout the fill area instead of placing the materials within only
an equipment-width from the cut/fill contact.
C. Oversize Materials: Rocks greater than 12 inches in largest dimension shall be disposed of
offsite or be placed in accordance with the recommendations by the Geotechnical Consultant in
the areas that are designated as suitable for oversize rock placement. Rocks that are smaller than
8 inches in largest dimension may be utilized in the fill provided that they are not nested and are
their quantity and distribution are acceptable to the Geotechnical Consultant and do not inhibit
the ability to properly compact fill materials.
General Earthwork Specifications Page 3
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
D. The fill materials shall be placed in thin, horizontal layers such that, when compacted, shall
not exceed 6 inches. Each layer shall be spread evenly and shall be thoroughly mixed to obtain a
near uniform moisture content and uniform blend of materials.
E. Moisture Content: Fill materials shall be placed at or above the optimum moisture content or
as recommended by the geotechnical report. Where the moisture content of the engineered fill is
less than recommended, water shall be added, and the fill materials shall be blended so that a
near uniform moisture content is achieved. If the moisture content is above the limits specified
by the Geotechnical Consultant, the fill materials shall be aerated by discing, blading, or other
methods until the moisture content is acceptable.
F. Each layer of fill shall be compacted to the project standards in accordance to the project
specifications and recommendations of the Geotechnical Consultant. Unless otherwise specified
by the Geotechnical Consultant, the fill shall be compacted to a minimum of 90 percent of the
maximum dry density as determined by ASTM Test Method: D1557-09.
G. Benching: Where placing fill on a slope exceeding a ratio of 5 to 1 (horizontal to vertical), the
ground should be keyed or benched. The keyways and benches shall extend through all
unsuitable materials into suitable materials such as firm materials or sound bedrock or as
recommended by the Geotechnical Consultant. The minimum keyway width shall be 15 feet and
extend into suitable materials, or as recommended by the geotechnical report and approved by
the Geotechnical Consultant. The minimum keyway width for fill over cut slopes is also 15 feet,
or as recommended by the geotechnical report and approved by the Geotechnical Consultant. As
a general rule, unless otherwise recommended by the Geotechnical Consultant, the minimum
width of the keyway shall be equal to ½ the height of the fill slope.
H. Slope Face: The specified minimum relative compaction shall be maintained out to the finish
face of fill and stabilization fill slopes. Generally, this may be achieved by overbuilding the slope
and cutting back to the compacted core. The actual amount of overbuilding may vary as field
conditions dictate. Alternately, this may be achieved by backrolling the slope face with suitable
equipment or other methods that produce the designated result. Loose soil should not be allowed
to build up on the slope face. If present, loose soils shall be trimmed to expose the compacted
slope face.
I. Slope Ratio: Unless otherwise approved by the Geotechnical Consultant and governing
agencies, permanent fill slopes shall be designed and constructed no steeper than 2 to 1
(horizontal to vertical).
J. Natural Ground and Cut Areas: Design grades that are in natural ground or in cuts should be
evaluated by the Geotechnical Consultant to determine whether scarification and processing of
the ground and/or overexcavation is needed.
K. Fill materials shall not be placed, spread, or compacted during unfavorable weather
conditions. When grading is interrupted by rain, filing operations shall not resume until the
Geotechnical Consultant approves the moisture and density of the previously placed compacted
fill.
General Earthwork Specifications Page 4
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
IV. Cut Slopes
A. The Geotechnical Consultant shall observe all cut slopes, including fill over cut slopes, and
shall be notified by the contractor when cut slopes are started.
B. If adverse or potentially adverse conditions are encountered during grading, the Geotechnical
Consultant shall investigate, evaluate, and make recommendations to mitigate the adverse
conditions.
C. Unless otherwise stated in the geotechnical report, cut slopes shall not be excavated higher or
steeper than the requirements of the local governing agencies. Short-term stability of the cut
slopes and other excavations is the contractor's responsibility.
V. Drainage
A. Backdrains and Subdrains: Backdrains and subdrains shall be provided in fill as
recommended by the Geotechnical Consultant and shall be constructed in accordance with the
governing agency and/or recommendations of the Geotechnical Consultant. The location of
subdrains, especially outlets, shall be surveyed and recorded by the Civil Engineer.
B. Top-of-slope Drainage: Positive drainage shall be established away from the top of slope. Site
drainage shall not be permitted to flow over the tops of slopes.
C. Drainage terraces shall be constructed in compliance with the governing agency requirements
and/or in accordance with the recommendations of the Civil Engineer.
D. Non-erodible interceptor swales shall be placed at the top of cut slopes that face the same
direction as the prevailing drainage.
VI. Erosion Control
A. All finish cut and fill slopes shall be protected from erosion and/or planted in accordance with
the project specifications and/or landscape architect's recommendations. Such measures to
protect the slope face shall be undertaken as soon as practical after completion of grading.
B. During construction, the contractor shall maintain proper drainage and prevent the ponding of
water. The contractor shall take remedial measures to prevent the erosion of graded areas until
permanent drainage and erosion control measures have been installed.
VII. Trench Excavation and Backfill
A. Safety: The contractor shall follow all OSHA requirements for safety of trench excavations.
Knowing and following these requirements is the contractor's responsibility. All trench
excavations or open cuts in excess of 5 feet in depth shall be shored or laid back. Trench
excavations and open cuts exposing adverse geologic conditions may require further evaluation
General Earthwork Specifications Page 5
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
by the Geotechnical Consultant. If a contractor fails to provide safe access for compaction
testing, backfill not tested due to safety concerns may be subject to removal.
B. Bedding: Bedding materials shall be non-expansive and have a Sand Equivalent greater than
30. Where permitted by the Geotechnical Consultant, the bedding materials can be densified by
jetting.
C. Backfill: Jetting of backfill materials to achieve compaction is generally not acceptable.
Where permitted by the Geotechnical Consultant, the bedding materials can be densified by
jetting provided the backfill materials are granular, free-draining and have a Sand Equivalent
greater than 30.
VIII. Geotechnical Observation and Testing During Grading
A. Compaction Testing: Fill will be tested and evaluated by the Geotechnical Consultant for
evaluation of general compliance with the recommended compaction and moisture conditions.
The tests shall be taken in the compacted soils beneath the surface if the surficial materials are
disturbed. The contractor shall assist the Geotechnical Consultant by excavating suitable test pits
for testing of compacted fill.
B. Where tests indicate that the density of a layer of fill is less than required, or the moisture
content is not within specifications, the Geotechnical Consultant shall notify the contractor of the
unsatisfactory conditions of the fill. The portions of the fill that are not within specifications shall
be reworked until the required density and/or moisture content has been attained. No additional
fill shall be placed until the last lift of fill is tested and found to meet the project specifications
and approved by the Geotechnical Consultant.
C. If, in the opinion of the Geotechnical Consultant, unsatisfactory conditions, such as adverse
weather, excessive rock or deleterious materials being placed in the fill, insufficient equipment,
excessive rate of fill placement, results in a quality of work that is unacceptable, the consultant
shall notify the contractor, and the contractor shall rectify the conditions, and if necessary, stop
work until conditions are satisfactory.
D. Frequency of Compaction Testing: The location and frequency of tests shall be at the
Geotechnical Consultant's discretion. Generally, compaction tests shall be taken at intervals
approximately two feet in fill height.
E. Compaction Test Locations: The Geotechnical Consultant shall document the approximate
elevation and horizontal coordinates of the compaction test locations. The contractor shall
coordinate with the surveyor to assure that sufficient grade stakes are established so that the
Geotechnical Consultant can determine the test locations. Alternately, the test locations can be
surveyed and the results provided to the Geotechnical Consultant.
F. Areas of fill that have not been observed or tested by the Geotechnical Consultant may have to
be removed and recompacted at the contractor's expense. The depth and extent of removals will
be determined by the Geotechnical Consultant.
General Earthwork Specifications Page 6
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
G. Observation and testing by the Geotechnical Consultant shall be conducted during grading in
order for the Geotechnical Consultant to state that, in his opinion, grading has been completed in
accordance with the approved geotechnical report and project specifications.
H. Reporting of Test Results: After completion of grading operations, the Geotechnical
Consultant shall submit reports documenting their observations during construction and test
results. These reports may be subject to review by the local governing agencies.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
APPENDIX F
HOMEOWNERS MAINTENANCE GUIDELINES
October 22, 2010 Page C-1
P/W 1009-05 Report No. 1009-05-B-2
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
HOMEOWNERS MAINTENANCE GUIDELINES
Homeowners are accustomed to maintaining their homes. They expect to paint their houses
periodically, replace wiring, clean out clogged plumbing, and repair roofs. Maintenance of the home
site, particularly on hillsides, should be considered on the same basis, or even on a more serious basis
because neglect can result in serious consequences. In most cases, lot and site maintenance can be
taken care of along with landscaping, and can be carried out more economically than repair after
neglect.
Most slope and hillside lot problems are associated with water. Uncontrolled water from a broken pipe,
cesspool, or wet weather causes most damage. Wet weather is the largest cause of slope problems,
particularly in California where rain is intermittent, but may be torrential. Therefore, drainage and
erosion control are the most important aspects of home site stability; these provisions must not be
altered without competent professional advice. Further, maintenance must be carried out to assure
their continued operation.
As geotechnical engineers concerned with the problems of building sites in hillside developments, we
offer the following list of recommended home protection measures as a guide to homeowners.
Expansive Soils
Some of the earth materials on site have been identified as being expansive in nature. As such, these
materials are susceptible to volume changes with variations in their moisture content. These soils will
swell upon the introduction of water and shrink upon drying. The forces associated with these volume
changes can have significant negative impacts (in the form of differential movement) on foundations,
walkways, patios, and other lot improvements. In recognition of this, the project developer has
constructed homes on these lots on post-tensioned or mat slabs with pier and grade beam foundation
systems, intended to help reduce the potential adverse effects of these expansive materials on the
residential structures within the project. Such foundation systems are not intended to offset the forces
(and associated movement) related to expansive soil, but are intended to help soften their effects on
the structures constructed thereon.
Homeowners purchasing property and living in an area containing expansive soils must assume a certain
degree of responsibility for homeowner improvements as well as for maintaining conditions around
their home. Provisions should be incorporated into the design and construction of homeowner
improvements to account for the expansive nature of the onsite soils material. Lot maintenance and
landscaping should also be conducted in consideration of the expansive soil characteristics. Of primary
importance is minimizing the moisture variation below all lot improvements. Such design, construction
and homeowner maintenance provisions should include:
Employing contractors for homeowner improvements who design and build in
recognition of local building code and site specific soils conditions.
Establishing and maintaining positive drainage away from all foundations, walkways,
driveways, patios, and other hardscape improvements.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
Avoiding the construction of planters adjacent to structural improvements. Alternatively,
planter sides/bottoms can be sealed with an impermeable membrane and drained away
from the improvements via subdrains into approved disposal areas.
Sealing and maintaining construction/control joints within concrete slabs and walkways
to reduce the potential for moisture infiltration into the subgrade soils.
Utilizing landscaping schemes with vegetation that requires minimal watering.
Alternatively, watering should be done in a uniform manner as equally as possible on all
sides of the foundation, keeping the soil "moist" but not allowing the soil to become
saturated.
Maintaining positive drainage away from structures and providing roof gutters on all
structures with downspouts installed to carry roof runoff directly into area drains or
discharged well away from the structures.
Avoiding the placement of trees closer to the proposed structures than a distance of one-
half the mature height of the tree.
Observation of the soil conditions around the perimeter of the structure during extremely
hot/dry or unusually wet weather conditions so that modifications can be made in
irrigation programs to maintain relatively constant moisture conditions.
Sulfates
On site soils were tested for the presence of soluble sulfates. Based on the results of that testing, the
soluble sulfate exposure level was determined to be “negligible” to “moderate” when classified in
accordance with the ACI 318-05 Table 4.3.1 (per 2007 CBC). As such, a concrete mix design should be
based on a “moderate” sulfate exposure (4,000 psi concrete with a water to cement ratio of 0.50).
Homeowners should be cautioned against the import and use of certain fertilizers, soil amendments,
and/or other soils from offsite sources in the absence of specific information relating to their chemical
composition. Some fertilizers have been known to leach sulfate compounds into soils otherwise
containing "negligible" sulfate concentrations and increase the sulfate concentrations in near-surface
soils to "moderate" or "severe" levels. In some cases, concrete improvements constructed in soils
containing high levels of soluble sulfates may be affected by deterioration and loss of strength.
Water - Natural and Man Induced
Water in concert with the reaction of various natural and man-made elements, can cause detrimental
effects to your structure and surrounding property. Rain water and flowing water erodes and saturates
the ground and changes the engineering characteristics of the underlying earth materials upon
saturation. Excessive irrigation in concert with a rainy period is commonly associated with shallow slope
failures and deep seated landslides, saturation of near structure soils, local ponding of water, and
transportation of water soluble substances that are deleterious to building materials including concrete,
steel, wood, and stucco.
Water interacting with the near surface and subsurface soils can initiate several other potentially
detrimental phenomena other then slope stability issues. These may include expansion/contraction
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
cycles, liquefaction potential increase, hydro-collapse of soils, ground surface settlement, earth material
consolidation, and introduction of deleterious substances.
The homeowners should be made aware of the potential problems which may develop when drainage is
altered through construction of retaining walls, swimming pools, paved walkways and patios. Ponded
water, drainage over the slope face, leaking irrigation systems, over-watering or other conditions which
could lead to ground saturation must be avoided.
Before the rainy season arrives, check and clear roof drains, gutters and down spouts of
all accumulated debris. Roof gutters are an important element in your arsenal against rain
damage. If you do not have roof gutters and down spouts, you may elect to install them.
Roofs, with their, wide, flat area can shed tremendous quantities of water. Without
gutters or other adequate drainage, water falling from the eaves collects against
foundation and basement walls.
Make sure to clear surface and terrace drainage ditches, and check them frequently during
the rainy season. This task is a community responsibility.
Test all drainage ditches for functioning outlet drains. This should be tested with a hose
and done before the rainy season. All blockages should be removed.
Check all drains at top of slopes to be sure they are clear and that water will not overflow
the slope itself, causing erosion.
Keep subsurface drain openings (weep-holes) clear of debris and other material which
could block them in a storm.
Check for loose fill above and below your property if you live on a slope or terrace.
Monitor hoses and sprinklers. During the rainy season, little, if any, irrigation is required.
Oversaturation of the ground is unnecessary, increases watering costs, and can cause
subsurface drainage.
Watch for water backup of drains inside the house and toilets during the rainy season, as
this may indicate drain or sewer blockage.
Never block terrace drains and brow ditches on slopes or at the tops of cut or fill slopes.
These are designed to carry away runoff to a place where it can be safely distributed.
Maintain the ground surface upslope of lined ditches to ensure that surface water is
collected in the ditch and is not permitted to be trapped behind or under the lining.
Do not permit water to collect or pond on your home site. Water gathering here will tend
to either seep into the ground (loosening or expanding fill or natural ground), or will
overflow into the slope and begin erosion. Once erosion is started, it is difficult to control
and severe damage may result rather quickly.
Never connect roof drains, gutters, or down spouts to subsurface drains. Rather, arrange
them so that water either flows off your property in a specially designed pipe or flows out
into a paved driveway or street. The water then may be dissipated over a wide surface or,
preferably, may be carried away in a paved gutter or storm drain. Subdrains are
constructed to take care of ordinary subsurface water and cannot handle the overload
from roofs during a heavy rain.
ADVANCED GEOTECHNICAL SOLUTIONS, INC.
Never permit water to spill over slopes, even where this may seem to be a good way to
prevent ponding. This tends to cause erosion and, in the case of fill slopes, can eat away
carefully designed and constructed sites.
Do not cast loose soil or debris over slopes. Loose soil soaks up water more readily than
compacted fill. It is not compacted to the same strength as the slope itself and will tend to
slide when laden with water; this may even affect the soil beneath the loose soil. The
sliding may clog terrace drains below or may cause additional damage in weakening the
slope. If you live below a slope, try to be sure that loose fill is not dumped above your
property.
Never discharge water into subsurface blanket drains close to slopes. Trench drains are
sometimes used to get rid of excess water when other means of disposing of water are not
readily available. Overloading these drains saturates the ground and, if located close to
slopes, may cause slope failure in their vicinity.
Do not discharge surface water into septic tanks or leaching fields. Not only are septic
tanks constructed for a different purpose, but they will tend, because of their construction,
to naturally accumulate additional water from the ground during a heavy rain.
Overloading them artificially during the rainy season is bad for the same reason as
subsurface subdrains, and is doubly dangerous since their overflow can pose a serious
health hazard. In many areas, the use of septic tanks should be discontinued as soon as
sewers are made available.
Practice responsible irrigation practices and do not over-irrigate slopes. Naturally, ground
cover of ice plant and other vegetation will require some moisture during the hot summer
months, but during the wet season, irrigation can cause ice plant and other heavy ground
cover to pull loose. This not only destroys the cover, but also starts serious erosion. In
some areas, ice plant and other heavy cover can cause surface sloughing when saturated
due to the increase in weight and weakening of the near-surface soil. Planted slopes
should be planned where possible to acquire sufficient moisture when it rains.
Do not let water gather against foundations, retaining walls, and basement walls. These
walls are built to withstand the ordinary moisture in the ground and are, where necessary,
accompanied by subdrains to carry off the excess. If water is permitted to pond against
them, it may seep through the wall, causing dampness and leakage inside the basement.
Further, it may cause the foundation to swell up, or the water pressure could cause
structural damage to walls.
Do not try to compact soil behind walls or in trenches by flooding with water. Not only is
flooding the least efficient way of compacting fine-grained soil, but it could damage the
wall foundation or saturate the subsoil.
Never leave a hose and sprinkler running on or near a slope, particularly during the rainy
season. This will enhance ground saturation which may cause damage.
Never block ditches which have been graded around your house or the lot pad. These
shallow ditches have been put there for the purpose of quickly removing water toward the
driveway, street or other positive outlet. By all means, do not let water become ponded
above slopes by blocked ditches.
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Seeding and planting of the slopes should be planned to achieve, as rapidly as possible, a
well-established and deep-rooted vegetal cover requiring minimal watering.
It should be the responsibility of the landscape architect to provide such plants initially
and of the residents to maintain such planting. Alteration of such a planting scheme is at
the resident's risk.
The resident is responsible for proper irrigation and for maintenance and repair of
properly installed irrigation systems. Leaks should be fixed immediately. Residents must
undertake a program to eliminate burrowing animals. This must be an ongoing program
in order to promote slope stability. The burrowing animal control program should be
conducted by a licensed exterminator and/or landscape professional with expertise in hill
side maintenance.
Geotechnical Review
Due to the presence of expansive soils on site and the fact that soil types may vary with depth, it is
recommended that plans for the construction of rear yard improvements (swimming pools, spas,
barbecue pits, patios, etc.), be reviewed by a geotechnical engineer who is familiar with local conditions
and the current standard of practice in the vicinity of your home.
In conclusion, your neighbor’s slope, above or below your property, is as important to you as the slope
that is within your property lines. For this reason, it is desirable to develop a cooperative attitude
regarding hillside maintenance, and we recommend developing a “good neighbor” policy. Should
conditions develop off your property, which are undesirable from indications given above, necessary
action should be taken by you to insure that prompt remedial measures are taken. Landscaping of your
property is important to enhance slope and foundation stability and to prevent erosion of the near
surface soils. In addition, landscape improvements should provide for efficient drainage to a controlled
discharge location downhill of residential improvements and soil slopes.
Additionally, recommendations contained in the Geotechnical Engineering Study report apply to all
future residential site improvements, and we advise that you include consultation with a qualified
professional in planning, design, and construction of any improvements. Such improvements include
patios, swimming pools, decks, etc., as well as building structures and all changes in the site
configuration requiring earth cut or fill construction.