Codorniz TR 32070 11-0083 (SFD) (Plans 1-3) 2010 Code Update - Geotechnical Engineering UpdateEarth Systems
Southwest
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SEP 2 6 Z011
BY:),
CITY OF LA QUINTA
BUILDING & SAFETY DEPT.
APPROVED
FOR CONSTRUCTION
2,1 2-00 BY-10
2-0 IrC) 46P6
Consulting Engineers and Geologists
RTT HOMES
P.O. Box 810
LA QUINTA, CALIFORNIA 92253
GEOTECHNICAL
ENGINEERING REPORT UPDATE WITH
SUPPLEMENTAL RECOMMENDATIONS
FOR TRACT 32070
SEC OF AVENUE 52 AND JEFFERSON STREET
LA QUINTA, CALIFORNIA
August 31, 2011
02011 Earth Systems Southwest
Unauthorized use or copying of this document is strictly prohibited
without the express written consent of Earth Systems Southwest.
File No.: 09386-04
Doc. No.: 11-08-776
Earth Systems
'`/ Southwest 79-811B Country Club Drive
Bermuda Dunes, CA 92203
(760)345-1588
(800)924-7015
FAX (760) 345-7315
August 31, 2011 File No.: 09386-04
Doc. No.: 11-08-776
RJT Homes
P.O. Box 810
La Quinta, California 92253
Attention: Mr. Chad Meyer
Subject: Geotechnical Engineering Report Update
with Supplemental Recommendations
Project: Tract 32070, Cordorniz
SEC Avenue 52 and Jefferson Street
La Quinta, California
References: 1. Earth Systems Southwest, Geotechnical Engineering Report, Proposed
Residential Development, SEC Avenue 52 and Jefferson Street, La Quinta,
California, File No.: 09386-01, Document No.: 03-10-847, dated October 30,
2003.
2. Earth Systems Southwest, Recommended Pavement Sections for Interior
Roadway, Tract 32070, Codorniz, Southeast Corner of Jefferson Street and
Avenue 52, La Quinta, California, File No.: 09386-02, Document No.: 06-04-
818R2, revision dated May 3, 2006.
As requested, Earth Systems Southwest [ESSW] performed a site visit on August 30, 2011 to
observe current site conditions, performed additional geologic review, and is providing
supplemental recommendations for the remaining lots of the proposed residential housing tract.
In light of recent research performed in the Coachella Valley by the United States Geological
Survey [USGS], the project site lies within an area of documented regional land subsidence. As
such, the recommendations in the original geotechnical engineering report are superseded by this
update.
Provided the recommendations presented below are incorporated into the final project plans and
specifications, it is our opinion that, from a geotechnical standpoint, the recommendations
provided in the project soils report remain applicable to the proposed project except as amended
and superseded below (subject to the Limitations presented within the project soils report and
below). The Limitations should be reviewed as they are integral to the understanding and use of
this report. The reader is directed to the referenced reports for a full discussion of geotechnical
findings and recommendations.
Site Conditions
During our site visit, the following general conditions were noted:
➢ All building pads have been rough -graded and appear in good condition with minimal
evidence of erosion.
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➢ Minor to moderate amounts of vegetation has grown on some lots.
➢ All streets have been partial -paved with asphalt -concrete [AC]. The streets show some
signs of distress in the form of shrinkage, fatigue cracking, oil loss, and occasional poor
construction practices (wood stake buried in AC). Solution voids were noted adjacent to
storm drain inlets where uncontrolled water undermined the curbing, gutter, and storm
drain box.
➢ Several utility covers/boxes located in the sections of road appear to have settled.
➢ Some cracking on perimeter walls was noted.
➢ Signs of distress related to subsidence were not noted.
Subsidence Discussion
ESSW recently became aware of a geotechnical issue (aerial subsidence due to groundwater
withdrawal) that could potentially affect future development of Tract 32070. The United States
Geological Survey [USGS] in conjunction with the Coachella Valley Water District [CVWD]
has been performing periodic monitoring of the Coachella Valley with respect to the potential for
aerial subsidence since 1996. The initial report concerned monitoring conducted between 1996
and 1998 was published in 2001 (Reference 2). That report indicated that subsidence exceeded
the measuring error of +/- 40 millimeters at only half of the 14 measuring locations, indicating
that "small amounts of land subsidence occurred at these monuments between 1996 and 1998"
(Reference 2, p. 1). The amount of subsidence ranged up to about 1/4-foot in three areas of Palm
Desert, Indian Wells, and La Quinta.
In 2002, the USGS issued a follow-up to the 2001 report with data through 2000 (Reference 3).
Measurements of subsidence and/or uplift were relatively inconclusive in the lower portion of
the Coachella Valley, with changes measuring less than 0.15 feet at most locations.
Interferometric data for the Palm Desert, Indian Wells and La Quinta areas were similar to the
data from 1996 to 1998.
In 2007, the USGS issued another follow-up report with data covering the period 1996 to 2005
(Reference 4). This report indicated that up to about 1 foot of subsidence had occurred in the
southern Coachella Valley between 2000 and 2005. InSAR measurements found subsidence
rates of 0.01 to 0.02 feet per month in portions of Palm Desert, Indian Wells and La Quinta.
Subsidence rates increased 2 to 4 times in these areas as compared to the 1996 to 2000 time
period. At the Codorniz site, the areal. settlement recorded in the referenced time period was
about 3 inches.
In October, 2010, the USGS presented a paper providing an update to the Coachella Valley
monitoring program (Reference 5). The 2010 report did not include the same GPS-based
measurements as the prior reports, but did provide InSAR data of the Cordorniz tract area. The
InSAR data is more qualitative than quantitative, so the absolute amount of subsidence cannot be
derived from the information, but it suggests that the areas of subsidence in the Palm Desert,
Indian Wells and La Quinta areas are getting larger and more pronounced. The InSAR data for
the area of the Cordorniz development is inconclusive due to land development activities that
occurred during the measurement time. However, the InSAR data does suggest that the
Cordorniz tract is in an area of continuing subsidence.
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Earth Systems became aware of the 2010 USGS report in June 2011, as part of a presentation we
put together regarding water resources in the Coachella Valley. Even though this is a newly
identified geotechnical issue for La Quinta, we believe it would be prudent to be proactive in
response to this information.
Geologic Overview
The project site is located along the southwestern margin of the Coachella Valley. Broad
expanses of alluvium and ancient Lake Cahuilla lake beds underlie the site. Bedrock hills exist
about 0.7 miles southwest of the site. Geologically, the site has been mapped as a mix of
Holocene and Pleistocene sand dune and lake bed deposits in excess of several hundred feet
deep.
Mapping presented by the California Division of Mines and Geology (1994) indicates a buried
southeast trending fault along the southwest margin of the Coachella Valley located near the
intersection of Avenue 52 and Jefferson Avenue. The fault is mapped as buried and queried and
is assumed to represent the range -front fault along the southwest margin of the subsiding valley.
The project site is not within a currently mapped Alquist-Priolo Earthquake fault zone or County
of Riverside fault zone.
IAs part of this update report, ESSW performed an aerial photograph lineament analysis of the
site area to evaluate if suspect features relating to subsidence and fissuring may have impacted
the site area in the past. Several aerial photographs dating back to 1939 were reviewed.
Lineament analysis suggests several weak vegetative and soil tonal lineaments in the immediate
r vicinity of the project site. These lineaments, as shown on 1939 and 1955 photographs, trend
I northwest to southeast and are reflective of the prevailing wind direction and sand dunes present
' before site development.
There are several known northwest to southeast trending fissures about 1 to 2 miles south of the
' Tract 32070 where damage has occurred due to differential settlement. Residences, streets and
golf course lakes have been adversely affected by these fissures, especially in the vicinity of
eastern La Quinta. Based upon our experience with the local fissuring in the Coachella Valley
area and the associated property damage, the apparent damaging settlement was on the order of 1
to two inches of offset; however, offset on the order of 5 inches (angular distortion 1:100) has
been observed along the more well defined lineaments to the south of the site.
The fissures noted near La Quinta (southwest and south of Tract 32070) are likely the result of
tensional stresses associated with areal subsidence due to groundwater withdrawal. Regional
studies by the United States Geologic Survey (2007 and 2010) suggest substantial subsidence in
the La Quinta area, with a general northwest to southeast subsidence basin trend. Margins of
subsidence zones experience surface tensional stress and have a higher propensity for fissuring.
The presence of the damaging fissures in the east La Quinta area are not considered active faults,
but rather the effects of differential settlement and aquifer compaction due to groundwater
withdrawal. The pronounced settlement may also be the result of differential settlement of
unequal depth of sediment over and adjacent to buried bedrock ridges now disguised by the
broad Holocene alluvial and lacustrine geomorphic surfaces.
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Summary of Lines of Evidence for Lineament Sources
• The project site is not located within a currently delineated Alquist-Priolo Earthquake
fault zone or Riverside County delineated fault zone. This indicates that active
faulting has not been defined by local or state agencies in the immediate proximity of
the site.
• A buried and queried fault, as shown on the CGS Fault Activity Map -Map Sheet 6
(1994) is located near the intersection of Avenue 52 and Jefferson Avenue. This fault
is not thought to be active or potentially active and is assumed to represent a buried
fault along the southwest margin of the subsiding valley.
• There is no geomorphic evidence to substantiate active faulting on the site, as the site
is essentially flat and level.
• Weak lineaments related to previous dune fields and the northwest to southeast
prevailing wind are observable on older photographs. These lineaments pertain to the
dune structure.
• Multiple aerial photograph lineaments are observable on historical photographs (circa
1939 to 1990) that have a northwest to southeast trend and occur south and southeast
of the project and occur one to two miles south of the project site.
• Known ground surface fissures exist south of Tract 32070 in the vicinity of eastern La
Quinta. Damage to streets, golf course lakes, and residences has occurred due to
differential settlement associated with fissuring.
• The fissuring is reasonably assumed to be related to areal subsidence associated with
groundwater withdrawal.
• It is possible that the subsidence and associated differential settlement as noted in the
La Quinta area may be the result of deep sediment compaction along buried faults or
deep bedrock steps, where there is a marked difference in thickness of the sediment
column over bedrock. Per Biehler (1964), the project area is along a steep Bouger
anomaly contour gradient along the southwest margin of the valley which suggests a
rapidly increasing depth of sediments progressing to the northeast.
In summary, the lines of evidence suggest that subsidence, not active fault rupture is the probable
genesis of the lineaments and associated fissuring south of Tract 32070. Therefore, it is our
professional opinion that there are no active faults within the project limits. Due to regional
groundwater withdrawal, areal subsidence is occurring and the site is in an area where fissuring
might occur as subsidence basins propagate northeastward. Note that the prediction of where
future fissures might occur is nearly impossible, as they are highly dependent on the volume of
groundwater pumping and pumping patterns which can induce differential tensional stresses
which may change with time.
The project is about a mile to two miles north of the area where multiple fissures are known.
This indicates that the region is or could experience additional tensional stresses that might result
in surface fissuring.
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The USGS studies referenced suggest that the subsidence areas are expanding. Thus the
occurrence of fissuring beyond currently recognized areas could occur based on the continued
over -drafting of the aquifers.
We understand that various lots in the Cordomiz development may be in the design and planning
stage. There is current uncertainty regarding the magnitude of subsidence that may occur within
the immediate area. As the degree of continued groundwater pumping, pumping patterns, and
their combined effect on the overlying soils is unknown, we believe it is prudent for future
homes to utilize a stiffened foundation to reduce the potential for distress due to differential
settlement until the risk from areal subsidence is more fully understood.
Supplemental Recommendations
The recommendations presented in the project soils report remain applicable to the proposed
project except as amended and superseded below. The recommendations of the project soils
report are amended and superseded in an effort to address the items noted during our site visit,
and the broadening of knowledge regarding the potential effects of subsidence in the project
area.
Site Grading
Since many years have passed since finish grading, we recommend some remedial grading prior
to precise grading and building construction. The building pad areas and unpaved roadways (and
any areas of fill placement) should be initially prepared by removing from the pad surface and
roadway any organic growth, cobbles/boulders, or other material deposited since grading. The
pad and roadway should then be overexcavated/scarified and moisture conditioned to near
optimum moisture content to a depth of at least one foot below the current grade and compacted
to a minimum of 90% relative compaction based upon ASTM D 1557. Roadways, curbing and
gutters should be compacted to at least 95% compaction in the upper 12 inches. Where required
to raise grade, non -expansive granular fills should be placed in maximum 8-inch lifts (loose) and
compacted to at least 90 percent relative compaction (ASTM D 1557) near optimum moisture
content (Roadways, curbs, and gutters should be compacted to at least 95% compaction in the
upper 12 inches). If the pad is to be lowered in elevation, the depth of moisture conditioning
may need to be increased.
Additionally, depending upon the depth of cut (if any), additional over -excavation and
compaction may be required, depending on the conditions observed at the time of grading.
ESSW should be retained to review final grading plans to evaluate if this is required. Any over -
steepened fill slopes should be graded to no steeper than 2h:ly. Fill should be benched into the
existing slope as fill is placed. Benches should be one foot in maximum height and depth into
the existing slope. A key -way should be constructed at the toe of the slopes with a width equal
to the height of the slope and a depth equal to one-half the slope height. Fill should be
compacted as above.
Site soils are susceptible to erosion. It is recommended that slopes be protected from erosion
through the use of plantings or facing. Recommendations should be provided from a licensed
landscape architect.
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Areas to receive pools and spas will likely penetrate below the depths of recompaction.
Therefore, pools and spas should be overexcavated such that they are supported by a minimum
of 2 feet of compacted soil below the bottom of pool shells and associated foundations
compacted to at least 90% relative compaction (ASTM D 1557). Compaction should be verified
by testing.
Loose stockpiles should be removed. Erosional features should be excavated to suitable
compacted soil and backfilled with compacted fill (90% relative compaction).
Within footing excavations, the bottom of the excavation should be stripped of rocks larger than
3 inches in maximum dimension, moisture conditioned to near optimum moisture content, and
recompacted to at least 90% relative compaction (ASTM D 1557). Voids resulting from the
removal of rocks should be filled with compacted fill placed as recommended.
Flatwork and Drivewaygrade Preparation
In the deck/flatwork areas, any organic growth, cobbles/boulders, loose or other material
deposited since grading should be removed. The subgrade should then be scarified a minimum
of 8 inches, moisture conditioned to near optimum moisture content, and compacted to at least
90% relative compaction (ASTM D 1557). Compaction should be verified by testing. Fill
placement, if required, should be performed as recommended in Site Grading above.
Slabs and Foundations
The mechanism of areal subsidence is deep seated and not readily addressed through soil
improvement techniques (i.e. increased grading or compaction). Although the current and future
magnitudes are unknown and related to many variables such as amount of groundwater
overdraft, homogeneous soil composition, buried subsurface natural structures, etc., it is our
experience that one method to reduce the severity of potential damage from differential
settlement is to utilize a stiffened foundation and slab. Stiffened foundations typically consist of
a post -tensioned slab with integral footings and/or grade beam footings with a waffle slab and
stiffened shear walls. Based upon these newly anticipated geologic conditions, it is our opinion
that Codorniz should consider the use of these types of measures to reduce the potential for
future home distress, given the current knowledge that subsidence is occurring.
A review of the USGS data indicates that the Codorniz project site is in the central margin of the
La Quinta (Area 3) zone. While the reported areal subsidence is inferred to be approximately 70
to 80 mm, the site is at the margin of the subsidence area, which can result in an area of greater
tensional stress and possible surface manifestation of earth fissures. It is our opinion, and that of
the City of La Quinta that, while predicting the location of surface ground ruptures as a result of
fissuring is difficult to impossible, the potential hazards from fissuring and continued subsidence
should be reduced. While, to date, no evidence of fissuring has been noted on the project site,
the potential of damage from fissuring exists and has been documented in this portion of the
Coachella Valley.
With respect to homes currently in design, it is our opinion that the recommendations provided in
the project soils reports (References 1 and 2 above), as supplemented and superseded below,
remain applicable provided the following recommendations are incorporated into the design and
construction.
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We recommend future foundation design consist of stiffened shallow footings incorporated with
a post tensioned slab and/or waffle slab with grade beams which are designed to accommodate a
differential settlement of at least 2 inches over a horizontal distance of 40 feet (angular distortion
on the order of 1:240). Differential movement resulting from aerial subsidence is not expected to
result in a complete unsupported loss of subgrade support, but, rather a tilting of the structure.
The differential settlement presented above is based upon current conditions and observations.
Groundwater overdraft is occurring in the Coachella Valley on a regional level and must be
addressed ultimately on a regional level through decreased pumping and increased recharge. It is
important to stress that increased pumping and continued groundwater overdraft may lead to
increased subsidence related settlement which may exceed the estimated design settlement
values presented above.
Footing Design
In our professional opinion, structure foundations should be founded on a structural mat that is a
flat -plate or waffled slab and uses either conventionally reinforced or post -tensioned tendons,
designed to accommodate the estimated differential settlement of 2 inches in a 40-foot span
(1:240 distortion ratio). Foundations should be bearing on a zone of properly prepared and
compacted soils placed as recommended above under "Site Grading."
Foundation design of.widths, depths, and reinforcing steel are the responsibility of the Structural
Engineer, considering the structural loading and the geotechnical parameters given in this report.
A minimum footing depth (below lowest adjacent grade within 3 feet) of 12 inches for single
story, and 18 inches for two to three stories should be maintained. Bearing values provided in
the referenced project soils report remain valid. A representative of ESSW should observe
foundation excavations before placement of reinforcing steel or concrete. Loose soil or
construction debris should be removed from footing excavations before placement of concrete.
Modulus of Subgrade Reaction
Structural mat rigidity can be estimated by using a modulus of subgrade reaction (ksl) of 200 pci
for the underlying subgrade.
Expected Settlement
Estimated total static settlement, based on footings founded on firm soils as recommended,
should be less than 1-inch. Differential settlement between exterior and interior bearing
members should be less than 3/4-inch. This settlement is exclusive of any subsidence related
settlement that may occur.
Sub rg ade
Concrete slabs -on -grade and flatwork should be supported by compacted soil placed in
accordance with the Site Grading section of this report. Slabs -on -grade should be designed to
accommodate the estimated settlements presented within.
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Vapor Retarder
In areas of moisture sensitive floor coverings, an appropriate vapor retarder should be installed to
reduce moisture transmission from the subgrade soil to the slab. For these areas, an impermeable
membrane (10-mil thickness) should underlie the floor slabs. The membrane should be covered
with 2 inches of sand to help protect it during construction and to aid in concrete curing. The
sand should be lightly moistened just before placing the concrete, but in no instance should
standing water be permitted. Low -slump concrete should be used to help reduce the potential for
concrete shrinkage. The effectiveness of the membrane is dependent upon its quality, the
method of overlapping, its protection during construction, and the successful sealing of the
membrane around utility lines.
The following minimum slab recommendations are intended to address geotechnical concerns
such as potential variations of the subgrade and are not to be construed as superseding any
structural design. The design engineer and/or project architect should ensure compliance
with SB800 with regards to moisture and moisture vapor.
Slab Thickness and Reinforcement
Slab thickness and reinforcement of slabs -on -grade are contingent on the recommendations of
the structural engineer or architect and the expansion index of the supporting soil and the
settlement recommendations given above. Based upon our findings, a modulus of subgrade
reaction of approximately 200 pounds per cubic inch can be used in concrete slab design for the
expected very low expansion subgrade.
Concrete slabs and flatwork should be a minimum of 4 inches thick (actual, not nominal). We
suggest that the concrete flatwork be reinforced with a minimum of No. 3 rebars at 18-inch
centers, both horizontal directions, placed at slab mid -height on suitable supports to resist
potential cracking. Welded wire mesh is not recommended. The thickness and reinforcing given
are not intended to supersede any structural requirements provided by the structural engineer
especially in regard to the potential for corrosion of reinforcing steel in a wet, pool environment.
The recommendations above are strictly to reduce the potential for cracking due to structural
loading and should be evaluated by the project structural engineer for applicability to the
proposed project.
The project architect or geotechnical engineer should observe all reinforcing steel in slabs during
placement of concrete to evaluate proper location within the slab. We recommend reinforcing
steel be self supported on spacers to ensure proper mid slab location during concrete placement.
We do not recommend lifting the reinforcing bar matrix into place as concrete is placed as voids
within the concrete can be created, mid -height elevation is rarely achieved, and the rebar
elevation is rarely uniform.
Control Joints
Control joints should be provided in all concrete slabs -on -grade at a maximum spacing of
36 times the slab thickness (12 feet maximum on -center, each way) as recommended by
American Concrete Institute [ACI] guidelines.
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Control joints should be provided around isolated footings to isolate them from the slab and
reduce the potential for cracking. All joints should form approximately square patterns to reduce
the potential for randomly oriented, contraction cracks. Contraction joints in the slabs should be
tooled at the time of the pour or saw cut (1/4 of slab depth) within 8 hours of concrete placement.
Construction (cold) joints should consist of thickened butt joints with %2-inch dowels at 18 inches
on center or a thickened keyed joint to resist vertical deflection at the joint. All construction
joints in exterior flatwork should be sealed to reduce the potential of moisture or foreign material
intrusion. These procedures will reduce the potential for randomly oriented cracks, but may not
prevent them from occurring.
Curing and Quality Control: The contractor should take precautions to reduce the potential for
curling of slabs in this and desert region using proper batching, placement, and curing methods.
Curing is highly affected by temperature, wind, and humidity. The recommendations of the ACI
should be applied to adequately cure the concrete. .Quality control procedures may be used,
including trial batch mix designs, batch plant inspection, and on -site special inspection and
testing.
Paving
The paved streets should be capped with the final layer of AC. All street sections should be in
accordance with the recommendations in the referenced project geotechnical engineering report.
Construction traffic should not be permitted on unfinished streets. Loading from heavy
construction traffic may cause damage to the AC and aggregate base on both un-capped and
finished. street surfaces. Street sections were designed for residential traffic only. Heavy
construction traffic should traverse along unpaved access ways. Capping should include the
placement of a Petromat and tack coat/binder installed between the AC base lift and capping
layer in accordance with Caltrans specifications (Caltrans Standard Specifications, 2006) to
reduce the potential for reflection cracking and to increase the adhesion between lifts. The
pavement should be thoroughly cleaned and allowed to dry, and have any embedded debris
removed prior to the application of the tack coat/binder. Due to current distress to the pavement
and aggregate base layer from traffic, weathering, and inability of runoff water to be conveyed
properly to drainage structures (curbs and gutters) due to the low street level below the gutters,
the pavement may have a reduced life span than originally designed, and may require additional
maintenance throughout its life. Voids resulting from runoff water which have undermined
drainage structures should be backfilled with compacted soil or slurry, and have the aggregate
base, pavement, and structures restored. The project civil engineer should confirm the asphalt
concrete pavement and aggregate base thickness were constructed in accordance with ESSW
recommendations (Reference No. 2 above).
Seismic Design Criteria
This site is subject to strong ground shaking due to potential fault movements along regional
faults including the San Andreas and San Jacinto faults. Engineered design and earthquake -
resistant construction increase safety and allow development of seismic areas. The minimum
seismic design should comply with the 2010 edition of the California Building Code and/or
Residential Code using the seismic coefficients given below.
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2010 CBC Seismic Parameters
Approximate Site Location: 33.6694°N, 116.2678°W
Site Class:
D
Table 1613.5.2
Spectral Earthquake Ground Motion
Short Period Spectral Response Ss:
1.50 g
Figure 1613.5
1 second Spectral Response, S I :
0.60 g
Figure 1613.5
Site Coefficient, Fa:
1.00
Table 1613.5.3(1)
Site Coefficient, F,:
1.50
Table 1613.5.3(2)
Design Earthquake Ground Motion
Short Period Spectral Response, SDs
1.00 g
1 second Spectral Response, SDI
0.60 g
The intent of the CBC lateral force requirements is to provide a structural design that will resist
collapse to provide reasonable life safety from a major earthquake, but may experience some
structural and nonstructural damage. A fundamental tenet of seismic design is that inelastic
yielding is allowed to adapt to the seismic demand on the structure. In other words, damage is
allowed. The CBC lateral force requirements should be considered a minimum design. The
owner and the designer may evaluate the level of risk and performance that is acceptable.
Performance based criteria could be set in the design. The design engineer should exercise
special care so that all components of the design are fully met with attention to providing a
continuous load path. An adequate quality assurance and control program is urged during
project construction to verify that the design plans and good construction practices are followed.
This is especially important for sites lying close to the major seismic sources.
Perimeter Walls
It is recommended the owner perform their own evaluation of the cracking observed and, if
necessary, develop a plan for mitigation. It is our experience that block walls of the type
constructed and observed, typically develop minor cracking of the type observed and can vary
greatly depending on the construction practice performed.
Utility Covers/Boxes
ESSW does not have documentation regarding compaction testing performed for the utility line
covers and boxes where settlement of backfill soils was observed. Compaction testing was
performed on an "as -requested" basis during construction. Where settlement of the backfill soils
has occurred, it is recommended that backfill soils be overexcavated to competent soil (defined a
having a relative compaction of 90% per ASTM D 1557) and re-backfilled with soil moisture
conditioned to near optimum moisture content, and compacted to a minimum of 90% relative
compaction per ASTM D 1557. The upper 12 inches of subgrade below street sections should be
compacted to 95% relative compaction.
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Site Drainage
The constructed pads and slopes are comprised of sandy soils which are susceptible to erosion.
Soils exposed to weathering will result in decomposition of surficial earth materials, thus
reducing shear strength properties. As these soils deteriorate, they can be expected to become
susceptible to surficial instability such as slumps, erosion, creep, and debris flow. It is
recommended that slopes be planted with native, drought resistant plants which cover the
majority of the slope area and which do not require slope watering to survive. Recommendations
for slope planting should be provided by a qualified landscape architect. Slope planting will
reduce the potential for soil erosion and debris flow. Additionally, pad, structure, plant
irrigation, or yard runoff should not be allowed to drain over the fill slope. Positive drainage or
other appropriate erosion control techniques should be provided by the civil designer to avoid
erosion of slopes and pads. Adequate provisions should be made to control and limit the flow of
runoff water across the site and over slopes. It is important that positive surface drainage be
provided to prevent ponding and/or saturation of the soils/rock in the vicinity of the slope.
It is highly recommended that landscape irrigation or other sources of water be collected
and conducted to an approved drainage device. Landscaping grades should be lowered
and sloped such that water drains to appropriate collection and disposal areas. All runoff
water should be controlled, collected, and drained into proper drain outlets. Control
methods may include curbing, ribbon gutters, 'V' ditches, or other suitable containment
and redirection devices.
In no instance should water be allowed to flow or pond against structures, slabs or
foundations. Adequate provisions should be employed to control and limit moisture
changes in the subgrade beneath foundations or structures to reduce the potential for soil
saturation. Landscape borders should not act as traps for water within landscape areas.
Potential sources of water such as piping, drains, broken sprinklers, etc, should be
frequently examined for leakage or plugging. Any such leakage or plugging should be
immediately repaired.
Use the minimum amount of landscape water required to sustain the life of grass turf and
plants. Grass turf and plants differ in water requirements. It should be recognized that, by
nature, silty or clayey topsoil used to sustain grass turf may impede the absorption
(infiltration) of water into and away from the grass turf which may cause increased runoff
if over -watered. Within the subject area, consideration should be given to using drought
resistant grass turf or artificial turf which requires little or no water. If less water is
required to sustain plants and grass turf, a reduction in the necessary amount and
magnitude of drainage and collection devices could possibly implemented.
The drainage pattern should be established at the time of final grading and maintained
throughout the life of the project. Additionally, drainage structures should be maintained
(including the de -clogging of piping) throughout their design life. Structural performance
is dependent on many drainage -related factors such as landscaping, irrigation, lateral
drainage patterns and other improvements.
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Drainage Structures
Maintenance of drainage systems can be the most critical element in determining the success of a
design. Due to general accessibility limitations which typically exist with drainage systems, they
must be protected from sediment -laden water both during and after construction to prevent
clogging of any filter medium, and the near structure soils. The potential for clogging can be
reduced by pre -treating structure inflow through the installation of maintainable forebays,
biofilters, or sedimentation chambers. In addition, sediment leaves, and debris must be removed
from inlets and traps, and basin bottoms on a regular basis.
Closing and Limitations
Except as modified in this report, it is our opinion that the referenced documents are applicable
to the proposed development. We make no representation as to the accuracy of the dimensions,
measurements, calculations, or any portion of the design.
Our evaluation of subsurface conditions at the site has considered subgrade soil and groundwater
conditions present at the time of our study. The influence(s) of post -construction changes to
these conditions such as introduction or removal of water into or from the subsurface will likely
influence future performance of the proposed project. The magnitude of the introduction or
removal, and the effect on the surface and subsurface soils is currently unknown. It should be
recognized that definition and evaluation of subsurface conditions are difficult. Judgments
leading to conclusions and recommendations are generally made with incomplete knowledge of
the subsurface conditions due to the limitation of data from field studies. The availability and
broadening of knowledge and professional standards applicable to engineering services are
continually evolving. As such, our services are intended to provide the Client with a source of
professional advice, opinions and recommendations based on the information available as
applicable to the project location and scope. Recommendations contained in this report are based
on our field observations and subsurface explorations, select published documents (referenced),
and our present knowledge of the proposed construction. If the scope of the proposed
construction changes from that described in this report, the conclusions and recommendations
contained in this report are not considered valid unless the changes are reviewed, and the
conclusions of this report are modified or approved in writing by ESSW.
ESSW intends that this update information be incorporated with the referenced geotechnical
engineering report. Conclusions, recommendations, and limitations provided in the referenced
reports, except where amended herein, remain valid and apply to this update report.
Recommendations contained in this report are based on our field observations from our
previously referenced studies, our current field observation, and our present knowledge of the
proposed construction. The scope of our geotechnical services did not include observation of
areas not accessible to a walking visual assessment nor any environmental site assessment for the
presence or absence of hazardous/toxic materials. It is possible that soil conditions could vary
between or beyond the points explored.
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If during construction, soil conditions are encountered which differ from those described herein,
we should be notified immediately in order that a review may be made and any supplemental
recommendations provided. In such an event, the contractor should promptly notify the owner
so that ESSW's geotechnical engineer can be contacted to confirm those conditions. We
recommend the contractor describe the nature and extent of the differing conditions in writing
and that the construction contract include provisions. for dealing with differing conditions.
Contingency funds should be reserved for potential problems during earthwork and foundation
construction.
If the scope of the proposed construction changes from that described in this report, the
conclusions and recommendations contained in this report are not considered valid unless the
changes are reviewed, and the conclusions of this report are modified or approved in writing by
ESSW.
This report is issued with the understanding that the owner or the owner's representative has the
responsibility to bring the information and recommendations contained herein to the attention of
the architect and engineers for the project so that; they are reviewed for applicability and
conformance to the current design and incorporated into the plans for the project. The owner or
the owner's representative also has the responsibility to take the necessary steps to see that the
general contractor and all subcontractors follow such recommendations. It is further understood
that the owner or the owner's representative is responsible for submittal of this report to the
appropriate governing agencies.
ESSW has striven to provide our services in accordance with generally accepted geotechnical
engineering practices in this locality at this time. No warranty or guarantee, express or implied,
is made. This report was prepared for the exclusive use of the Client and the Client's authorized
agents.
Grading and compaction operations should be performed in conjunction with observation and
testing. The recommendations provided in this report are based on the assumption that ESSW
will be retained to provide observation during the construction phase to evaluate our
recommendations in relation to the apparent site conditions at that time. If we are not accorded
this observation, ESSW assumes no responsibility for the suitability of our recommendations. In
addition, if there are any changes in the field to the plans and specifications, the Client must
obtain written approval from ESSW's engineer that such changes do not affect our
recommendations. Failure to do so will vitiate ESSW's recommendations. These services will
be performed on a time and expense basis in accordance with our agreed upon fee schedule once
we are authorized and contracted to proceed. Maintaining ESSW as the geotechnical consultant
from beginning to end of the project will provide continuity of services. The geotechnical
engineering firm providing tests and observations shall assume the responsibility of
Geotechnical Engineer of Record.
This report may be used only by the Client and the registered design professional in responsible
charge and only for the purposes stated for this specific engagement within a reasonable time
from its issuance, but in no event later than one (1) year from the date of the report. Land use,
site conditions (both on site and off site) or other factors may change over time, and additional
work may be required with the passage of time.
EARTH SYSTEMS SOUTHWEST
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Doc. No.: 11-08-776
Any party other than the client who wishes to use this report shall notify ESSW of such intended
use. Based on the intended use of the report, ESSW may require that additional work be
performed and that an updated report be issued. Non-compliance with any of these requirements
by the client or anyone else will release ESSW from any liability resulting from the use of this
report by any unauthorized party.
If you should you have any questions concerning our report, please do not hesitate to contact us
and we will be pleased to assist you.
Respectfully submitted,
EARTH SYSTEMS SOUTHWEST
Kevin L. Paul
Senior Engineer
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Mark S. Spyke
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EARTH SYSTEMS SOUTHWEST
August 31, 2011
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CITED REFERENCES
1. Caltrans Standard Specifications, 2006.
2. Sneed, M., Ikehara, M.E., Galloway, D.L., and Amelung, F., 2001. Detection and
Measurement of Land Subsidence Using Global Positioning System and Interferometric
Synthetic Aperture Radar, Coachella Valley, California, 1996-98, USGS Water
Resources Investigation Report 01-4193.
3. Sneed, M., Stork, S.V., and Ikehara, M.E., 2002. Detection and Measurement of Land
Subsidence Using Global Positioning System Surveying and Interferometric Synthetic
Aperture Radar, Coachella Valley, California, 1998-2000, USGS Water Resources
Investigations, Report 02-4239.
4. Sneed, M. and Brandt, J.T., 2007. Detection and Measurement of Land Subsidence Using
Global Positioning System Surveying and Interferometric Synthetic Aperture Radar,
Coachella Valley, California, 1996-2005, United States Geological Survey Scientific
Investigations Report 2007-5251.
5. Sneed, M., 2010. Measurement of Land Subsidence using Interferometry, Coachella
Valley, California, in Land Subsidence, Associated Hazards and the Role of Natural
Resources Development (Proceedings of EISOLS 2010, Queretaro, Mexico, 17 to 22
October, 2010), p. 260 to 263. IAHS Publ. 339, 2010.
ADDITIONAL REFERENCES
Biehler, Shawn, 1964, Geophysical Study of the Salton Trough of Southern California, Thesis by
Shawn Biehler, California Institute of Technology, Pasadena, California, 1964.
California Division of Mines and Geology, 1994, Fault Activity Map of California and Adjacent
Areas, California Geologic Data Map Series, Map No. 6.
California Division of Mines and Geology, 1986, Geologic Map of California, Santa Ana Sheet.
Shlemon, Roy J., and Davis, Paul, 1992, Ground Fissures in the Temecula Area, Riverside
County, California, Engineering Geology Practice in Southern California, Association of
Engineering Geologist Special Publication NO.4, pp 275-288.
Stewart, Craig A., et al, 1998, Earth Fissuring, Ground -Water Flow, and Ground -Water Quality
in the Chino Basin, California, Land Subsidence Case Studies and Current Research:
Proceedings of the Dr. Joseph F. Poland Symposium on Land Subsidence, Association of
Engineering Geologist Special Publication No. 8, pp 195-205.
Aerial Photoaranhs:
Whittier College -Fairchild Collection
Flight: C-6060
Frames: WR 341-342
Date: 10/4/1939
Scale: 1"=1500'
EARTH SYSTEMS SOUTHWEST
August 31, 2011
Riverside County Flood Control District
Frames: 737
Date: 05/26/1980
Scale: P=2150'
Riverside County Flood Control District
Frames: 685
Date: 12/15/1983
Scale: 1 "=1900'
Riverside County Flood Control District
Frames: 13-80
Date: 01/05/1990
Scale: 1 "=1700'
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EARTH SYSTEMS SOUTHWEST