HomeMy WebLinkAboutCP 01-08; BRISTOL COVE; Geotechnical Investigation for Northside Cove Drive; 2009-03-1610/16/2001 16:14 8582920771 LEIGHTON SAN DIEGO PAGE 01
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10/16/2001 16:14 6562920771 LEIGHTON SAN DIEGO PAGE 02
I&P3
^HD== Leighton and Associates
3. S> 6 1 - 2 O O 1 QEOTECHNICAL CONSULTANTS
October 16,2001
Project No. 040437-001
To: Bristol Cove, IXC
2880 Pio Pico Drive
Carlsbad, California, 92008
Subject GeotechnicaJ Investigation for the Proposed Development of Lot 13 of Map No. 5162,
Cove Drive, Carlsbad, California
In accordance with your request, we have performed a preliminary geotechnical investigation for the
proposed development for Lot 13 of Map No. 5162, located on the north side of Cove Drive in Carlsbad,
California. The purpose of our investigation was to evaluate the existing site geotechnicai conditions and
provide geotechnicai conclusions and recommendations relative to the proposed development of the site.
This report presents the results of our subsurface investigation and geotechnicai analysis, review of
available geotechnicai and geologic reports and maps applicable to the general vicinity of the property, and
provides a summary of our conclusions and recommendations.
Based on the results of our investigation and review of the geologic and geotechnicai reports pertinent to the
site, the proposed development is considered feasible from a geotechnicai standpoint provided the
recommendations summarized in this report are implemented during site grading and construction.
If you have any questions regarding our report, please contact this office. We appreciate this opportunity to
be of service.
Respectfully submitted,
LEIGmXJN AND ASSOCIATES, INC.
William D- Olson, RCE 45283
Senior Project Engineer
Distribution: (8) Addressee
Randall K WagneV, CEG 1612
Director of Geology
3934 Murphy Canyon Road, #8205, San Diego, CA 92123-4425
(858) 292-8030 • FAX (858) 292-0771 • www.lvlghton0eo.com
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040495-001
TATTLE OF CONTENTS
Section ?m
1.0 INTRODUCTION ._.„.
1.1 PURKKE AND score OF SERVICES ............................................................................ - .................................... 1
12 SnEDESOOWlONA^ PROPOSE I«^ .................................... „ .................... . ................................ 3
13 SURFACE fcWESTlGATOJNArrolJ^BaRATORYl^^ ..................................................................................... 3
2.0 GEOTfcCHNICAL CONDITIONS _______ ........................ ----------------------- .. ------------- ................ ------------------ 5
2.1 SITE GEOLOGY ................................................................................................................................................. 5
2.2.7 Artificial Older Fill Soils (Map Symbols Afo) ........................................................................................ 5
2.2.2 Santiago Formation (Map Symbol-Tsa) ....... , .................................................. ..... ............... ............... ... 5
2.3 GEOLOGIC STRUCTURE ................................................................................................................................... 5
2.4 GROUNPWATER .......................... . .................................................................................................................... 6
2.5 FAULTWG ............. „ .................................................................................................... . .................................... 6
2.6 SEISMIC ccwsmatATioNs ............................................................................... „ ............................................... 6
2.6.1 Uniform Building Code Seismic Parameters .......................................................... . .............................. 7
2.7 ENC3^EERINOCHARACTBWSTniC8OTCW-SlTESOE»S ..................... „ ................................................................... 7
2.7.1 Expansion Potential. ............................................................................................................................. 7
2.7.2 Soluble Svlfate Content ......................................................................................................................... 7
3.0 CONCLUSIONS ................ ____________________________________ ....... „ .......................... ----------------------------------------------- 9
4.0 RECOMMENDATIONS _____________ ............. „„„ ____ ............... ________________________ „ ______ . __________________ . ___ 10
4.1 EARTHWORK 10
4.1.1 Site Preparation }Q
4.1.2 Removal and decompaction of Potentially Compressible Soils 70
4.1.3 Excavations ]Q
4.1.4 Fill Placement and Compaction //
4.2 FOUNDATION DESIGN CONSIDERATIONS , "^ n
4.2,1 Post-Tensioned Foundation Design Considerations , 11
4-2.2 Slab Subgrade Soil Presaturation 73
4.2.3 Seismic .Design Parameters,.,,,.,,,..,,, 13
4.2.4 Foundation Setback From Slope Faces J4
4.3 CEMENT TYPE FOR CONSTRUCTJOK ....^..1^
4.4 LATBRAL EARTH PRESSURE AND RETAINING WALL DESIGN ....!..." 14
4.5 STJRPACEDRAI^CffiANDWTMAlNTENAWE Z!!.!"!I".!1..!I1""!"!"Z!""Z1 ."". 174.6 GRADED SLOPES ".'."..I.*,... ."''"^"". J7
4.7 PLAN RBVDEW AND CONSTRUCTION OBSERVATION '„.,', !"l.!r..".l...!!!.".!".......!!!!!."ZI!!", 17
5.0 LIMITATIONS __ 18
-i-
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TABLE
FIGURES
FIGURE 1 - SITE LOCATION MAP - PAGE 2
FIGURE 2 - GEOTBCHNICAL MAP - PAGE 4
FIGURE 3 - RETAINING WAIL BACKFILL AND DRAINAGE DETAIL - PACE 16
TABLES
TABUS i - POST-TENSIONED FOUNDATION DESIGN RECOMMENDATIONS - PAGE 12
TABLE 2 - LATERAL EARTH PRESSURES - PAGE is
APPENDICES
APPENDIX A - REFERENCES
APPENDIX B - BORING LOGS
APPENDIX c - LABORATORY TEST PROCEDURES AND TEST RESULTS
APPENDIX D - GENERAL EARTHWORK AND GRADING SPECIFICATIONS FOR ROUGH-GRADING
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1.0 INTRODUCTION
1.1 Purpose and Scope of Services
This report presents the results of our preliminaiy geotechnical investigation Bar Lot 13 of Map
No. 5162, located on the north side of Cove Drive in Carlsbad, California (Figure 1). The purpose of
our preliminary investigation •was to evaluate the pertinent geotechnical conditions at the she and to
provide design criteria for the proposed development The scope of services for our preliminary
geotechnical investigation included the following:
• Review of pertinent geotechnical/geologic documents regarding soil related conditions at the
site (Appendix A).
• Review of available aerial photographs and geologic and topographic maps (Appendix A),
• Excavation and logging of three small-diameter borings to evaluate the existing subsurface
geotechnical conditions. The borings were drilled to a maximum depth of approximately 31
feet, sampled, and logged by a geologist from our firm. The borings wore backfilled
subsequent to their excavation. Logs of the borings are presented in Appendix B. The
approximate locations of the borings are shown on Figure 2.
• Laboratory testing of representative samples of the onsite soils (Appendix C).
• Geotechnical analysis of the data accumulated
• Preparation of this geotechnical report presenting the results of our subsurface investigation
and laboratory testing and providing preliminary conclusions and recommendations
(including the General Earthwork and Grading Specifications for Rough Grading presented
as Appendix D) relative to the proposed development of the site.
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NORTH
BASE MAP: Thomas Bros.
Window*, San Diego County, 1996, Page 1106 1000 2000 4000
Scale In Feet
Lot 13 of Hap No. S1S2
Carlsbad. California
SITE
LOCATION
MAP
Project No.
040437-001
Date
October 2001 Figure No. 1
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1.2 §fre Description and Proposed Development
The subject site (Lot 13 of Map No. $162) which is an irregular shaped property, is located north of
Agua Hedionda Lagoon in the Carlsbad, California (Figure 1). The site is bounded by Cove Drive to
the east, Adams Street to the west, an open space lot to the south and on existing residential structure
to the north. Existing elevations of the site are not known but are anticipated to range from
approximately 15 to 40 feet mean sea level (msl). There are not any known existing improvements
on the site with the exception of the northeast-facing manufactured slope and minor landscaping on
the slope face.
Grading plans or an as-graded geotechnica] report were not available for the site, Cove Drive or the
adjacent residential lots. However, we anticipate that Cove Drive and Die associated lots were graded
in the 1970'$ or earlier. Based on our subsurface investigation, fill soils on the order of 5 to 10 feet
are present on the relatively level portion of the site,
Development plans have not been prepared at this time, however, we anticipate grading of the site
will include minor grading and construction of a retaining wall for a single-family residential
structure and associated improvements on the lot. In addition, we anticipate the residential structure
will be two-stories in height with a concrete slab-on-grade foundation and wood and stucco
construction.
1-3 Surface Investigation and Laboratory Testing
Our field investigation consisted of the excavation, logging and sampling of three small-diameter
exploratory borings excavated to a maximum depth of approximately 31 feet across the site. Logs of
the borings are presented in Appendix B. The approximate locations of the borings are shown on the
Geotechnical Map (Figure 2), Following the field investigation, the borings Were backfilled with
soils from the excavation of the borings.
Limited laboratory testing was performed on representative soil samples of the soils on the site. The
laboratory tests included moisture and density determinations, soluble sulfate content,
hydrocollapse, and expansion index tests. A discussion of the tests performed and a summary of the
results are presented in Appendix C.
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G E M D
Af O Artificial f 1U - oUter
T«H,ry Santiago fwwrtioh (dreltd where burled)
Appro*. l-tartts of fill «|ueH«l »*er« a*«t»in)
\
-A \ Afo
20 40
3) e in Feet
GEOTECHNICAL MAP
Lot 13 of Hap No. 5162
Carlsbad, California
Project No,
Scale
Engr./Geol.
Drafted By
Date
Leightan and Associates, Inc.
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2.0 GgQTECHNlCAL CONDITIONS
2.1 She Geology
Hie subject she is located within the coastal sub-province of the Peninsular Ranges Geomorphk
Province, near the western edge of the southern California batholith. The topography at the edge of
the batholith changes from the rugged landforms developed on the batholith to the more subdued
landforms that typify die softer sedimentary formations of the coastal plain such as those present on
the site. Specifically, the site is underlain by older artificial fill soils that are considered
undocumented and the Santiago Formation, The approximate areal limits of each of the geologic
units are indicated on the Geotechru'cal Map (Figure 2). Each of the geologic units present on the site
is described below (youngest to oldest).
2.2.1 Artificial Older Fill Soils (Map Symbols Afo)
Our subsurface investigation indicated that the relatively level portion of the lot consists of
approximately 5 to 10 feet of artificial older fill. No documentation of these fill soils was
available. Consequently, we consider the fill soils to be undocumented and recommend that
the fill soils be removed and recojmpacted prior to the placement of improvements on the
lot. The fill soils were found to consist of orange-brown, brown and gray-brown, damp to
moist, medium dense silty to clayey sands with minor amounts of stiff sandy clay. These
older fill soils were desiccated and are considered compressible and should be removed to
competent material and recompacted during the development of the she.
2.2.2 Santiago Formation (Map Svmbol-Tsal
The Tertiary-aged Santiago Formation was encountered beneath the fill soils while the
existing northeast-facing slope on the lot is anticipated to also consist of the Santiago
Formation. As encountered during our investigation, the Santiago Formation generally
consisted of mottled gray and light brown, moist to saturated, medium dense to dense, silty
to clayey fine to coarse sandstone.
2J Geologic Structure
Review of the geologic literature applicable to the site (Appendix A) and our professional
experience on nearby sites with similar soils indicates the on-site formational unit is generally flat
hying and thickly interbedded. Bedding is anticipated to dip generally 5 to 10 degrees or less toward
the west No faults have been mapped on the site nor were any encountered during our field study.
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040437-001
2.4 Gjtmndwater
Ground water was encountered in Borings B-l and B-3 at a depth of approximately 11.5 feet below
the existing ground surface (or an approximate elevation of 4 feet msl). No surface water or seepage
conditions were observed on the she. Seasonal fluctuations of surface water and ground water
should be expected. The ground water encountered on the site is not anticipated to be a constraint to
development provided the recommendations presented herein are implemented during the grading
and construction of the proposed development.
2.5 Emitting
Our discussion of faults relative to the site is prefaced with a discussion of California legislation and
state policies concerning the classification and land-use criteria associated with faults. By definition
of the California Mining and Geology Board, an active fault is a fault that has had surface
displacement within Holocene time (about the last 11,000 years). The State Geologist has defined a
potentially active fault as any fault considered to have been active during Quaternary time (last
1,600,000 years), but that has not been proven to be active or inactive. This definition is used in
delineating Fault-Rupture Hazard Zones as mandated by the Alquist-Priolo Earthquake Fault Zoning
Act of 1972 and as most recently revised in 1997. The intent of mis act is to assure thai unwise
urban development does not take place across the traces of active faults. Based on our review of die
Fault-Rupture Hazard Zones, the site is not located within any Fault-Rupture Hazard Zone as created
by the Alquist-Priolo Act (Hart, 1997).
North San Diego County, like the rest of southern California, is seismically active as a result of
being located near the active margin between the North American and Pacific tectonic plates. The
principal source of seismic activity is movement along the northwest-trending regional fault zones
such as the San Andreas, San Jacinto, Coronado Banks (offshore) and Elsinore Faults Zones, as well
as along less active faults such as the Rose Canyon Fault Zone.
Our review of geologic literature pertaining to the site and general vicinity indicates there are no
known major or active feufts on or in the immediate vicinity of die she (Haranan, 1975, Weber, 1982*
and Jennings, 1994). Evidence for faulting was not encountered during our field investigation. The
nearest known active fault is the Rose Canyon Fault Zone (RCFZ) which is considered a Type B
seismic source per the 1997 Uniform Building Code (UBC) and is located approximately 5.0 mites
(S.I kilometers) west of the site. The closest Type A seismic source is the Temecula segment of the
Elsinore Fault Zones located approximately 24.0 miles (38.7 kilometers) to the northeast.
2.6 Seismic Considerations
The principal seismic considerations for most structures in southern California are surface rupturing
of fault traces, damage caused by ground shaking or seismically induced ground settlement The
possibility of damage due to ground rupture is considered low since active faults are not known to
cross the site. The seismic hazard most likely to impact the site is ground-shaking resulting from an
earthquake on one of the major regional faults, The effects of seismic shaking can be reduced by
adhering to the most recent edition of the Uniform Building Code and design parameters of the
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Structural Engineers Association of California.
Liquefaction of cohesion less soils can be caused by strong vibratory motion due to earthquakes.
Research and historical data indicate thai loose granular soils underlain by a near-surfece
groundwater table are most susceptible to liquefaction, while me stability of most silry clays and
clay$ is not adversely affected by vibratory motion. Because of the dense nature of the formational
material below the groundwater table, it is our opinion the potential for liquefaction or seismicalry
induced dynamic settlement at the site due to the design earthquake is low. Due to the site's
elevation of approximately 15 feet above mean sea level and relatively sheltered location on the
north side of the Agua Hedionda lagoon, the hazard from seiches and tsunamis is also considered to
below.
2.6.1 Uniform Building Code Seismic Parameters
The site is located within Seismic Zone 4. The Rose Canyon Fault Zone (which is located
S.O miles or 8.1 kilometers from the site) is considered a Type B seismic source per the
1997 UBC, Table 16-O). The soil profile type at the site is anticipated to be Type SD while
the near source factors of N« * 1.0 and Nv - 1.2 are considered appropriate based on the
criteria of the 1997 UBC.
2.7
Based on the results of our geotechnical investigation, laboratory testing of representative on-site
soils, and our professional experience on nearby sites with similar soils, the engineering
characteristics of the on-site soils are discussed below.
2.7.1 Expansion Potential
The majority of the onsite soils are expected to have a low to medium expansion
potential per the Uniform Building Code (UBC) criteria. However, highly expansive
clayey soils may be present on the site. If highly expansive soils are encountered during
the future grading operations, the highly expansive soils should be placed outside the
proposed building limits.
2.7.2 Soluble Suifate Content
The National Association of Corrosion Engineers (NACE) defines corrosion as "a
deterioration of a substance or its properties because of a reaction with its environment".
From a geotechnical viewpoint, the "environment" is the prevailing foundation soils and
the "substances" are reinforced concrete foundations or various types of metallic buried
elements such as piles, pipes, etc., which are in contact with or within close vicinity of the
soils.
In general, soil environments that are detrimental to concrete have high concentrations of
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soluble sutfates. Table 19-A-4 of UBC, 1997 provides specific guidelines for the concrete
mix-design when the soluble sulfate content of the soil exceeds 0.1 percent by weight or
1000 parts per million (ppm). The results of our laboratory tests on representative soils
from the site indicate a soluble sulfate content of 0.18 percent which in aggregate suggests
that the concrete should be designed in accordance with the moderate category of Table 19-
A-4 of the 1997 UBC. The test results are provided in Appendix C.
u
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3.QCQNCLUS10NS
Based on the results of our update geotechnicai investigation at the subject site, it is our professional
opinion mat the proposed development of the site is feasible from a geotechnicai standpoint, provided the
following conclusions and recommendations are incorporated into the project plans, specifications, and
followed during site grading and construction.
The following is a summary of the geotechnicai factors that may effect development of the site.
• Based on our subsurface exploration and review of pertinent geologic reports, the site is underlain by
artificial older fill soils and the Santiago Formation.
• Our analysis indicates that the artificial older fill soils are compressible in their present state and will
require removal and recompaction within areas of the proposed development
• ft is anticipated that the on-site soils can be excavated with conventional construction equipment
• The existing on-site soils appear to be suitable for reuse as fill after it is removed to competent material
(provided they are placed in accordance with the recommendations presented herein).
• The expansion potential of the building pad soils is anticipated to be in the low to medium expansive
range (per UBC Criteria).
• Groundwater WHS encountered in Borings B-l and B-3 at a depth of approximately 11.5 feet (or
approximately 4 feet msl). Groundwater is not anticipated to be a constraint to development.
• Active faults are not known to exist on or in the immediate vicinity of the site.
• The main seismic hazard that may affect the site is from ground shaking from one of the active
regional faults.
• Due to the fine-grained and cohesive characteristics of the subsurface soils and lack of groundwatcr,
the potential for liquefaction at the site is considered to be low.
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4.1 Earthwork
We anticipate that earthwork at me site will consist of site preparation, removals of unsuitable soils,
fill placement, and trench excavation and backfill. We recommend the earthwork on-site be
performed in accordance with the following recommendations, the City of Carlsbad grading
requirements, and tile General Earthwork and Grading Specifications for Rough-Grading (GEGS)
included in Appendix D. In case of conflict, the following recommendations shall supersede those
included as part of Appendix D.
4.1.1 SitePreparation
Prior to grading, the site should be cleared of surface obstructions, any existing debris,
vegetation, and unsuitable material. Vegetation and debris should be removed and properly
disposed of offsitc. Holes results from the removal of buried obstructions that extend below
finished site grades should be replaced with suitable compacted fill material.
Areas to receive fill and/or other surface improvements (after the removals of unsuitable
material are made) should be scarified to a minimum depth of 12 inches, brought to above-
optimum moisture contents, and recompacted to at least 90 percent relative compaction
(based on American Standard! of Testing and Materials [ASTMJ Test Method D1557).
4.1.2 Removal and Recompaction of Potentially Compressible Soils
Due to the undocumented and potentially compressible characteristics of die on-site fill
soils, we recommend these soils be removed to competent material and recompacted
within 10 feet of the building footprint or envelope. The depth of the unsuitable material
[ removal is estimated to be on the order of approximately 4 to 10 feet in depth below die
k existing ground surface.
The unsuitable material removed from the excavation may be used as fill on the site. Prior
; to the placement of the soil as fill, it should be moisture conditioned (as needed) to obtain
a 2-percent above-optimum moisture content, and recompacted to a minimum 90 percent
relative compaction (based oft ASTM Test Method D1557). The actual depth and extent
of the required removals should be determined during grading operations by the
geotechnical consultant.
4.1.3 Excavations
; Excavations of the on-site soils may be accomplished with conventional earthwork
*' equipment It is not anticipated that oversized rock (greater than 8 inches in maximum
dimension) will be encountered. All excavations should be carried out in accordance with
I current OSHA requirements.
u
. 1
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L i
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4.1.4 Fill Placement and Compaction
The on-site soils are generally suitable for use as compacted fill provided they are free of
organic material, trash or debris, and rock fragments larger than 8 inches in maximum
dimension. The fill Should be brought to 2-percent above-optimum moisture content and
compacted in uniform lifts to at least 90 percent relative compaction based on the
laboratory maximum dry density (ASTM Test Method D1557). The optimum lift thickness
required to produce a uniformly compacted fill will depend on me type and size of
compaction equipment used. In general, fill should be placed in lifts not exceeding 8 inches
in compacted thickness. Placement and compaction of fill should be performed in general
accordance with the current City of Carlsbad grading ordinances, sound construction
practices, and the General Earthwork and Grading Specifications of Rough-grading
presented in Appendix D.
4.2 Foundation Desjgn Considerations
A two-story structure is tentatively planned for the she. Foundations and slabs of the proposed
structure are assumed to be founded on soils possessing a low to medium expansion potential
(less than 90 per UBC Test Method 18-2). This should be confirmed as necessary during site
grading based on the actual soils exposed at finish grade.
The proposed foundation system should be designed by the structural engineer in accordance
with the following geotechnical parameters in accordance with governing codes and ordinances.
4.2.1 Post-Tensioned Foundation Design Considerations
We recommend that the post-tensioned slab of the structure be designed it] accordance with
the following design parameters presented on Table 1 and the criteria of the 1997 edition of
the Uniform Building Code (ICBO, 1997). A post-tensioned foundation system designed
and constructed m accordance with the recommendations provided in this report is
expected to be structurally adequate for the support of the structure planned at Die subject
site provided our recommendations presented herein are followed. Adhering to the design
and maintenance recommendations presented in finis report will help ensure mat expansive
soil-related effects to the structure, if any, are limited to cosmetic distresses, with no
adverse impact to the overall structural integrity of the structure.
Please note that UBC Chapter 18 is based on certain climatological assumptions with
regard to soil-moisture conditions around and beneath the post-tensioned slabs. Soil-
moisture change below slabs is the major factor in expansive soil problems. The UBC
design method does not contain specific provisions to account for the effects of
irrigation, presaturation, or other non-climate-related influences oa the moisture content
of subgrade soils. We have, therefore, modified our geotechnical parameters to account
for reasonable irrigation practices, aftd a reasonable degree of maintenance and
responsibility. If proper landscaping and irrigation practices are not adhered to, then
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h
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some degree of distress is likely to occur. Based on our previous experience, Ms distress
typically does not impact the structural integrity of the structure.
We also recommend that soil-moisture around the immediate perimeter of the slab be
maintained at near optimum-moisture content (or above) during construction and up to
occupancy of the structure. The owner should be informed and educated regarding the
importance of maintaining a constant level of soil-moisture and should be made aware of
the potential negative consequences of both excessive watering, as well as allowing
expansive soils to lose moisture (i.e., the soil will undergo shrinkage as it dries up,
followed by swelling during the winter, rainy season or when irrigation is resumed,
resulting in distress to improvements and structures). Our recommendations for
landscaping and surface drainage are provided later in this report.
Table 1
Post-Tensioned Foundation Design Recommendations
Design Criteria
Edge Moisture Variation, e™
Differential Swell, yn
Acceptable Design Deflection
Center Lift:
Edge Lift:
Center Lift:
Edge Lift:
Minimum Perimeter Foundation Embedment
Modules of Subgrade Reaction
Expansion Index (UBC 18-I-B)
Medium
5.5 feet
4.0 feet
2.5 inches
1.0 inches
Structural Engineer/Architect
and governing codes
18 inches
120 pci
I.E
Long-term differential settlement is anticipated to occur at the site due to tine variability
of the engineered fill material. This settlement is not considered an impact to the
proposed structure as long as it is factored into the design of die foundation. In order to
simplify our recommendations for the foundation, we have factored in the anticipated
settlement into the geotechtucal perimeters for foundation design.
The post-tensioned foundation and slab should be designed in accordance with the design
presented above, acceptable deflection criteria determined by die structural engineer, and/or
architect and governing codes.
The slab should be underlain by a minimum of 2 inches of clean sand (sand equivalent
greater than 30) which is in turn underlain by a vapor barrier and an additional 2 inches of
clean sand. The vapor barrier should be sealed at all penetrations and laps. Moisture vapor
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transmission may be additionally reduced by use of concrete additives. Moisture barriers
can retard, but not eliminate moisture vapor movement from me underlying soils up
through the slabs. We recommend that the floor covering installer test the moisture vapor
flux rate prior to attempting applications of the flooring. '"Breathable" floor coverings
should be considered if the vapor flux rates are high. A slipsheet or equivalent should be
utilized above the concrete slab if crack-sensitive floor coverings (such as ceramic tiles,
etc.) are to be placed directly on the concrete slab.
Our experience indicates that use of reinforcement in slabs and foundations will generally
reduce the potential for drying and shrinkage cracking. However, some cracking should be
expected as the concrete cures. Minor cracking is considered normal; however, it is often
aggravated by a high water/cement ratio, high concrete temperature at the time of
placement, small nominal aggregate size, and rapid moisture loss due to hot, dry and/or
windy weather conditions during placement and curing. Cracking due to temperature and
moisture fluctuations can also be expected. The use of low slump concrete (not exceeding 4
to S inches at the time of placement) can reduce the potential for shrinkage cracking and die
action of tenskming the tendons can close small shrinkage cracks. In addition to the careful
control of water/cement ratios and slump of concrete, application of 50 percent of the
design post-tensioning load within three to four days of slab pour may be an effective
method of reducing the cracking potential.
4.2.2 S|ab Subprade Solj Presaturation
The slab subgrade soils underlying the post-tensioned foundation system should be
presoaked to a minimum 130 percent of the optimum moisture content to a depth of at least
j 18 inches prior to placement of the moisture barrier and slab concrete.
Presoaking or moisture conditioning may be achieved in a number of ways. But based on
our professional experience, we have found that minimizing the moisture loss on the pad
(by periodic wetting to keep the upper portion of the pad from drying out) and/or
T berming the building pad and flooding for a short period of time (days to a few weeks)
are some of the more efficient ways to meet the presoaking recommendations. If flooding
is performed, a couple of days to let the upper portion of the pad dry out and form a crust
, so equipment can be utilized should be anticipated. In addition, we recommend the
contractor place the final 2 feet of material at significantly over optimum moisture
' content to reduce the difficulties associated with presoaking.
* 4.2.3 Seismic peyjgn Parameters
The site lies within Seismic Zone 4, as defined in the UBC, 1997 edition. The nearest
known active fault is the Rose Canyon Fault Zone, which is located approximately
5.0 miles (8.1 kilometers), west of the she. The Mowing data should be considered for the
! seismic analysis of the proposed structure:
L
• Causative Fault Rose Canyon Fault Zone
' • Vfo^ifnum Magnitude: 6.9
.. i
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040437-001
• Seismk Source Type*. B
• Seismic Zone Factor. 0.40
• Soil Profile Type: So
• Near Source Factors: KB"" l.O/Nv5* 1.2
4.2.4
We recommend a minimum horizontal setback distance from the face of descending
slopes for all structural footings and settlement-sensitive structures. This distance is
measured from the outside edge of the footing, horizontally to the descending slope face
(or to the face of a retaining wall) and should be a minimum of H/2, where H is the slope
height (in feet). The setback should not be less than 10 feet and need not be greater than
IS feet. Please note that the soils within the structural setback area possess poor lateral
stability, and improvements (such as retaining walls, sidewalks, fences, pavements, etc.)
constructed within this setback area may be subject to lateral movement and/or
differential settlement
4.3 Cement Tvpe for Construction
The soluble sulfate content testing of a representative sample of the onshe soil indicates the soils
possess a moderate soluble sulfate content based on the 1997 UBC criteria. Concrete in contaxrt
with the oil-site soils should be designed in accordance with Table 19-A-4 of the 1997 UBC. This
should be confirmed by laboratory testing of the finish grade soils upon completion of the site
grading operations,
4.4 Lateral Earth Pressure and Retaining Wall Design
Embedded structural walls should be designed for lateral earth pressures exerted on them. The
magnitude of these pressures depends on the amount of deformation that the wall can yield under
load. If the wall can yield enough to mobilize the full shear strength of the soil, it can be designed
for "active" pressure. If the wall cannot yield under the applied load, the shear strength of the soil
cannot be mobilized and the earth pressure will be higher. Such walls should be designed for "at
rest" conditions. If a structure moves toward the soils, the resulting resistance developed by the soil
' is the "passive" resistance.
To design an unrestrained wall, such as cantilever wall, the active earth pressure may be used.
' For a restrained retaining wall, such as a basement wall, the at-rest pressure should be used.
Further, for sliding resistance, the friction coefficient of 0,35 may be used at the concrete and
soil interface. In combining the total lateral resistance, the passive pressure or the frictional
1 resistance should be reduced by 50 percent.
I Wall footings may be designed in accordance with structural considerations. The passive resistance
; i> value may be increased by one-third when considering loads of short duration including wind or
seismic loads. The horizontal distance between foundation elements providing passive resistance
! I should be a minimum of three times the depth of the elements to allow roll development of this
i. L
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040437-001
ri
passive pressure. The total depth of retained earth for design of cantilever walls should be the
vertical distance below die ground surface measured at the wall face for stem design or measured at
the heel of the footing for overturning and sliding. All retaining structures should be provided with
a drainage blanket or drains (as indicated in Figure 3) and appropriately waterproofed. Surcharge
loading effects from adjacent structures should be evaluated by the geotechnical and structural
engineers.
For design purposes, the recommended equivalent fluid pressure for each case for walls founded
above the static groundwater and backfilled with imported select granular soils having a sand
equivalent of 30 or greater is provided in Table 2. The equivalent fluid pressure values assume free-
draining conditions. If conditions other than those assumed above are anticipated, the equivalent
fluid pressure values should be provided on an individual-case basis by the geotechnical engineer.
All retaining wall structures should be provided with appropriate water proofing and drainage. The
outlet pipe should be sloped to drain to a suitable outlet, Typical drainage design is illustrated on
Figure 3.
All excavations should be made in accordance with me most current OSHA requirements. The
granular and native backfill soils should be compacted to at least 90 percent relative compaction
(based on ASTM Test Method D1557). The walls should be constructed and backfilled as soon as
possible after back cut excavation. Prolonged exposure of back cut slopes may result in some
localized slope instability.
Foundations for retaining walls in competent properly compacted fill should be embedded at least
18 inches below lowest adjacent grade. At this depth, an allowable bearing capacity of 2,000 psf
may be assumed.
Table 2
Lateral Earth Pressures
Conditions
Active
At-Rest
Passive
Equivalent Fluid Weight (pcf)
Import Soil with a SB of 30 or Greater
Level Backfill
35
55
350
2:1 Backfill
60
70
350
1 r
u
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RETAINING WALL BACKFILL AND
DRAINAGE DETAIL
For Mqxlmum 8-Foot High Walls with
On-Slte Soils Having Very Low to Low
Expansion Potentials (i.e. on Expansion
Index of 50 or Lass)
OPTION 1: PIPE SURROUNDED WITH
CLASS 2 PERMEABLE MATERIAL
WITH PROPER
SURFACE MADUCt
_L
(SEE GENERAL NOTES)
12* MINIMUM
CUSS ZPERMEMLEnUEH
(SEE
OPTION 2: GRAVEL WRAPPED
IN FILTER FABRIC
4 INCH tNAMETtt
PERFORATED PIPE
(SEC NOTE 3)
Class 2 Filter
FV CaflraM
5igy» Sin
1"
3/4"
3/R"
No. 4
No. 8
No. 30
Mo. 50
No. 200
SLOPE
OR LEVEL
HLICR fWRJC(SKNOTti)
X n> IK MCH sacCMVCL VMPPED IN nun
HMC
Material GradoHon
PotsSnn
100
90-100
40-100
25-40
18-33
5-15
0-7
0-J
GENERAL NOTES:
* Waterproofing should da provided when mature nvitence problem through the troll it undesirable
* Water proofing of trie tuoHs tx not under pun/tow 6f Ifa gtolecrtnical •ngirmfr
* AH dfoint should /torn o grcfunl of 1 ptftenl minfnum
* OulM portion of the tutitralfi thouU /taw a 4-fric/i (Kometef toKd pip* dlftHargtd inte a suttabte disposal art* designed
Ay ttm pnjttt tn^nttf- Trie BvaOeoin p&f t*cuu to eecasaibtt for mointentncf (reddrnf)
* Otkfr suftdron bectfm opthms Or* 3Ubj»Ct to the rvrifm By tht gtetechnlcet tngtrmf OrM modMcoUt* at tfrsfga
poromften
(jotes:
1) Sana should Hove e s«nd acfuAntoot of JO of greater and may to aaruffied by water jeUing
3) f Cu. ft. oaf II, el t/4~ to I 1/2-ifart s!z» oriofl wrapfea in fitter fabric
j; Pipe type should to AST* Dltt? juvyMMh OvteOm Sfynn» (ASS) SORJ5 or ASM 01789 fofrmyt Chloride pttath
fPlrC). Sctieduh *0. Armeo ASOOO PVC, or opf>ft»*e fquhnltrit. Pipe srtt»atl to iratollfa with perforations tfe-n- Pertontiora
should to 3/8 irich in diameter placed of the end* of o IX-negroe art in (wo rows at J-inch on center (staggered)
4) nter fabric should to U!rafi HONC or Opprend eetmolent
5) Weepholt shout* to J-!neh minimum diameter and provided el 10-foot momHurn Inlrnvla. If exposure is Bemtitfed,
wMphoMs shauM to located 12 ificfms atom finished grade. If txpeture is not permitted tucn as for a mU adjacent to a
*Hte*alk/cvrb, a pipe under me fldemolk to to aiienorgft) through the curb face or etiulmlanl shouU be pnnrided, for o
tiosem*nt~tyt* •«#. o proper MMrUvl oulM system should to provided
S) (titoininq noil plena should to mw'mrtrf end approved by tr+ geotechtiicel
7) Walls over tiffit feet in Htifht ore tvojfct to o $pec!ol re*if» by the oeoieehfncol engineer and modilieotiore to the
above
RETAINING WALL BACKFILL
AND SUBDRAIN DETAIL
(rev. June 2000)
PROJECT NO.
PROJECT NAME un 13 I LJU
Leighton and Associates, Inc. Figure No. 3
IB/16/2001 16:14 8582920771 LEIGHTON SAN DIEGO PAGE 21
040437-001
4.5 Surface Drainage and Lot Maintenance
Positive drainage of surface water away from the structure is very important. No water should be
allowed to pond adjacent to the building. Positive drainage may be accomplished by providing
drainage away from the building at a gradient of at least 2 percent for a distance of at least S feet,
and further maintained by a swale of drainage path at a gradient of at least 1-percent Where
necessary, drainage paths may be shortened by use of area drains and collector pipes. Have gutters
also help reduce water infiltration into the subgrade soils if the downspouts are properly connected
to appropriate outlets.
Planters with open bottoms adjacent to buildings should be avoided, if possible. Planters should not
be designed adjacent to buildings unless properly designed with a liner to prevent moisture
variations below the foundation.
4.6 Graded Slopes
It is recommended that all graded slopes within the development be planted with drought-tolerant
ground cover vegetation as soon as practical to protect against erosion by reducing runoff velocity.
Deep-rooted vegetation should also be established to protect against surficial slumping,
Oversteepening of existing slopes should be avoided during fine grading and construction unless
supported by appropriately designed retaining structures,
We recommend terrace drains on the slopes be designed by the civil engineer and be constructed in
.. ^ accordance with current County of San Diego specifications. Design of surface drainage provisions
''"*' is within the purview of me project civil engineer.
4.7 Plan Review and Construction Observation
Final project drawings should be checked by Leighton and Associates before grading to see that
the recommendations in this report are incorporated in project plans. Construction observation of
all onsite excavations and field density testing of all compacted fill should be performed by a
representative of this office. We recommend that a geologist map all excavations during grading
for the presence of potentially adverse geologic conditions. All footing excavations should be
reviewed by this office prior to placing steel or concrete.
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040437-001
5.0 LIMITATIONS
The conclusions and recommendations in this report are based in part upon data that were obtained from
a limited number of observations, site visits, excavations, samples, aad tests. Such information is by
necessity incomplete. The nature of many sites is such thai differing geotechnical or geological
conditions can occur within small distances and under varying climatic conditions. Changes in
subsurface conditions can and do occur over time. Therefore, the findings, conclusions, and
recommendations presented in this report can be relied upon only if Leighton and Associates has the
opportunity to observe the subsurface conditions during grading and construction of the project, in order
to confirm that our preliminary findings are representative for the site.
V
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