Palo Verde TR 32279 BCPR2022-0008 - Geotechnical Report
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December 17, 2021
J.N. 21-468
WILLIAMS HOMES
24911 Avenue Stanford
Santa Clarita, California 91355
Attention: Ms. Autumn Gasparella
Subject: Geotechnical Design and Construction Recommendations, Palo Verde Project,
Tracts 33336 (Lots 1 – 23) and 32279 (Lots 2, 9 – 12, 16 – 22, 29 and 30), Cherrywood
Place and Rosewood Court, North of Avenue 58 and East of Paseo Del Lago, City of
La Quinta, Riverside County, California
References: See Attached List
Dear Ms. Gasparella:
At your request, Petra Geosciences, Inc. (Petra) is providing herewith our geotechnical design and
construction recommendations for the subject 37 residential units within Tracts 33336 (Lots 1 through 23)
and 32279 (Lots 2, 9 – 12, 16 – 22, 29 and 30 ) of the Palo Verde project located in the city of La Quinta,
California. The purpose of this report is to present geotechnical recommendations for site precise/ remedial
grading, and for the design and construction of the foundations for the proposed residential structures and
other site improvements located within these tracts that are based on the current site conditions and the
2019 CBC requirements.
This report is based on our review of the preliminary and as-graded geotechnical reports prepared by various
consultants, our supplemental sampling and laboratory testing, the requirements of the 2019 California
Building Code (2019 CBC), and our engineering judgment and professional opinion. It is our understanding
that as-graded reports have been approved by the City of La Quinta.
Petra has reviewed the referenced reports and we are in general concurrence with their findings, conclusions
and recommendations contained therein, except where to be noted in our subsequent reports and letters.
Petra hereby accepts the referenced reports as the basis for our geotechnical services on the site subject to
modifications as to be dictated by our findings during our independent investigation, as well as those
dictated by the project plans, specifications and applicable codes.
BCPR2022-0008
PALO VERDE / TRACT
CONSTRUCTION PLANS
(SFDX2) - 2019 CODES
10/05/2022
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SITE GENERAL OVERVIEW
The study area considered under the purview of this report, which is comprised of two tracts, Tracts 33336
and 32279, is located north of Avenue 58 and west of Paseo Del Lago in the city of La Quinta, Riverside
County, California. Tract 33336, which is comprised of 23 lots, is bisected by Cherrywood Place in the
north-south direction. Tract 32279, which is comprised of 30 lots, is bisected by Rosewood Court, also in
the north-south direction. The two streets merge in their southerly terminus and connect to Avenue 58. The
project site is relatively flat, and the elevation difference is on the order of 4 to 7 feet, increasing in the
northerly direction.
Previous Site Investigation and Grading
Earth Systems Southwest (ESS) performed the initial geotechnical engineering investigation within
Tract 32279 in 2004 (ESS, 2004). While their boring locations and logs and laboratory test results were not
available, it appears they drilled 5 exploratory to depths varying from 14 to 51.5 feet below ground surface.
In January 2005, ESS published the results of their geotechnical investigation performed within Tract 33336
(ESS, 2005). In the latter investigation, ESS drilled 6 exploratory borings to the same depth ranges and, in
both reports, concluded the subject sites are suitable for the proposed development.
In February 2006, Stoney-Miller Consultants, Inc. (SMC) provided a Geotechnical Consultant of Record
and a review of rough grade letter for Tract 33336 (SMC, 2006a, b). And, in June and August of 2006, they
provided a foundation design parameters letter for Tracts 32279 and 33336 and a pavement
recommendation letter for Tract 33336 (SMC, 2006c, d).
SMC provided observation and testing during rough grading of the subject tracts which reportedly took
place between March and April 2006 for Tract 32279 and April and May 2006 for Tract 33336 (SMC,
2006e, f). During the earthwork process, total of 4 to 6 feet of fill was placed within these tracts that include
the remedial over-excavation thicknesses.
Current Site Condition
A representative of Petra Visited the sites on December 3, 2021 to observe the existing conditions. The
tracts appear to be in relatively the same condition as when the site development was halted in the 2008
timeframe. There are 16 homes built on Tract 32279 with 14 lots unbuilt, 3 of the lots (Lots 2, 29 and 30)
have foundations constructed on them. No homes or foundation have been constructed on the 23 lots of
Tract 33336. The unbuilt lots were stabilized with a spray-on polymer and show little to no erosion. Walls
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constructed at the site showed no observable distress. The tracts are relatively free of trash and debris. Some
erosion control sandbags have shown signs of deterioration.
Rosewood Court in Tract 32279 has paved streets and utilities. Cherrywood Place in Tract 33336 is not
paved, and utilities were not observed.
Supplemental Sampling and Laboratory Testing
Several bulk soil samples were collected by a representative of Petra from the building pad surfaces to
depths on the order of 24± inches below pad grades for confirmation laboratory testing. The laboratory
program consisting of testing pad grade soils for expansion index and general corrosion potential (sulfate,
chloride, pH, resistivity). The laboratory tests results are presented in Plate 1 and reflected in the following
sections.
Precise Grading Plan Review
Proposed Construction and Precise Grading
Preparation for a new plan for site precise grading plan is currently in progress. However, we understand
that minimal cut and fill operations are planned for the site and, therefore, site finish elevation will be within
6+ inches of the current elevations. Further, the construction will likely consist of building two-story,
single-family homes with attached garages, as well as various site improvement activities including
placement of main and lateral utility lines, block walls, etc. The single-family residences will be of wood-
frame construction with first floor slabs constructed on-grade. Concrete driveways and walkways will
provide access to the adjacent street and to the front doors. The building pads will be graded to collect any
surface water and deliver it to the curb and gutter of the adjacent street
CONCLUSIONS AND RECOMMENDATIONS
Feasibility
Based on the current design, as-graded conditions and our current understanding of the project, the proposed
residential construction within the site is feasible from a geotechnical standpoint provided they are
performed in accordance with the City of La Quinta requirements and our following recommendations.
However, it should be noted that the precise grading plan, once available, should be reviewed by this firm
for verification of the following conclusions and recommendations.
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Geotechnical Recommendations
Building Pad Preparation and Precise Grading
The building pad surficial soils have become dry, slightly desiccated and eroded since the completion of
rough grading; therefore, building pad re-conditioning will be required to reprocess all disturbed, desiccated
and eroded surficial soils to create suitable pads for the construction of the proposed improvements. That
is, following clearing/grubbing operations, the majority of the subject building pad surface soils should be
scarified to a depth of at least 6 to 8 inches, thoroughly moisture-conditioned to near two percent above
optimum and then recompacted in-place to a minimum relative compaction of 90 percent based on ASTM
D 1557. Some deeper scarification/removal and recompaction should also be anticipated based on the field
condition at the time of this remedial grading.
Based on our observation, all pads, with the exception of Lots 2, 29 and 30 within Tract 32279, should be
re-conditioned as described above. However, based on the grading industry standard, the surficial
remediation process is essentially required after maximum of 6 months of the last grading. It is our
understanding that the existing foundation system in Lots 2, 29 and 30 within Tract 32279 will be
demolished and removed in their entirety. The depth of overexcavation/removal and replacement should be
extended to include all disturbed soils and that will be established during the remedial grading operation.
It is recommended that the deeper erosional gullies (i.e., those deeper than 8 inches), if any, that would not
be eliminated during pad and slope restoration should be widened as necessary to provide equipment access,
cleared of all loose soil and/or desiccated soil material and debris, an d backfilled with properly moisture
conditioned onsite soils and compacted to a minimum relative compaction of 90 percent. Following the
remedial grading, the site may be brought to proposed finish elevation and proper surface drainage may be
established. A representative of the project geotechnical consultant should be present during clearing,
remedial grading and fill placement operations.
It should be noted that the sequence of precise grading, as recommended above, is left to the grading
contractor’s discretion, however, our experience indicates that for conditions where site surficial soils exist
at a moisture content well below optimum, the grading operation may be performed more efficiently if the
soils are thoroughly moisture conditioned utilizing a temporary sprinkler system prior to scarification and
recompaction.
Exposed bottom surfaces areas to receive compacted fill should be observed and approved by the project
geotechnical consultant prior to fill placement. No fill should be placed without p rior approval from the
geotechnical consultant. The project geotechnical consultant should also be present onsite during re-
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processing and/or grading operations to document proper placement and adequate compaction of fill, as
well as to observe compliance with the other recommendations presented herein.
Importing of Fill
Depending on the requirements of the forthcoming precise grading plan, it is anticipated that some pads
may need to receive maximum of 6 inches of fill to achieve to finish pad elevation. If the required fill will
be imported from outside of the area, potential import sources should be evaluated prior to importing to the
site such that non-expansive soils that are free of deleterious materials will be used.
Foundation Design Considerations
Faulting
Based on our review of the referenced geologic maps and literature, no active faults are known to project
through the property. Furthermore, the site does not lie within the boundaries of an “Earthquake Fault Zone”
as defined by the State of California in the Alquist-Priolo Earthquake Fault Zoning Act (CGS, 2018). The
Alquist-Priolo Earthquake Fault Zoning Act (AP Act) defines an active fault as one that “has had surface
displacement within Holocene time (about the last 11,000 years).” The main objective of the AP Act is to
prevent the construction of dwellings on top of active faults that could displace the ground surface resulting
in loss of life and property.
However, it should be noted that according to the USGS Unified Hazard Tool website and/or 2010 CGS
Fault Activity Map of California, the San Andreas Fault Zone, located approximately 8.3 miles northeast
of the site, would probably generate the most severe site ground motions and, therefore, is the majority
contributor to the deterministic minimum component of the ground motion models. [The subject site is
located at a distance of less than 9.5 miles (15 km) from the surface projection of this fault system, which
is capable of producing a magnitude 7 or larger events with a slip rate along the fault greater than 0.04 inch
per year. As such, the site should be considered as a Near-Fault Site in accordance with ASCE 7-16,
Section 11.4.1.]
Seismic Design Parameters
Earthquake loads on earthen structures and buildings are a function of ground acceleratio n which may be
determined from the site-specific ground motion analysis. Alternatively, a design response spectrum can be
developed for certain sites based on the code guidelines. To provide the design team with the parameters
necessary to construct the design acceleration response spectrum for this project, we used two computer
applications. Specifically, the first computer application, which was jointly developed by Structural
Engineering Association of California (SEAOC) and California’s Office of Statewide Health Planning and
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Development (OSHPD), the SEA/OSHPD Seismic Design Maps Tool website, https://seismicmaps.org, is
used to calculate the ground motion parameters. The second computer application, the United Stated
Geological Survey (USGS) Unified Hazard Tool website, https://earthquake.usgs.gov/hazards/interactive/,
is used to estimate the earthquake magnitude and the distance to surface projection of the fault.
To run the above computer applications, site latitude and longitude, seismic risk category and knowledge
of site class are required. The site class definition depends on the direct measurement and the ASCE 7-16
recommended procedure for calculating average small-strain shear wave velocity, Vs30, within the upper
30 meters (approximately 100 feet) of site soils.
A seismic risk category of II was assigned to the proposed buildings in accordance with 2019 CBC, Table
1604.5. No shear wave velocity measurement was performed at the site, however, the subsurface materials
at the site appears to exhibit the characteristics of stiff soils condition for Site Class D designation.
Therefore, an average shear wave velocity of 750 feet per second for the upper 100 feet was assigned to the
site based on engineering judgment and geophysical experience. As such, in accordance with ASCE 7-16,
Table 20.3-1, Site Class D (D- Default as per SEA/OSHPD software) has been assigned to the subject site.
The following table, Table 1, provides parameters required to construct the seismic response coefficient,
Cs, curve based on ASCE 7-16, Article 12.8 guidelines. A printout of the computer output is attached in
Appendix A.
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TABLE 1
Seismic Design Parameters
Ground Motion Parameters Specific Reference Parameter
Value Unit
Site Latitude (North) - 33.6292 °
Site Longitude (West) - -116.2583 °
Site Class Definition Section 1613.2.2 (1), Chapter 20 (2) D-Default (4) -
Assumed Seismic Risk Category Table 1604.5 (1) II -
Mw - Earthquake Magnitude USGS Unified Hazard Tool (3) 7.3 (3) -
R – Distance to Surface Projection of Fault USGS Unified Hazard Tool (3) 13.4 (3) km
Ss - Mapped Spectral Response Acceleration
Short Period (0.2 second) Figure 1613.2.1(1) (1) 1.50 (4) g
S1 - Mapped Spectral Response Acceleration
Long Period (1.0 second) Figure 1613.2.1(2) (1) 0.60 (4) g
Fa – Short Period (0.2 second) Site Coefficient Table 1613.2.3(1) (1) 1.2 (4) -
Fv – Long Period (1.0 second) Site Coefficient Table 1613.2.3(2) (1) Null (4) -
SMS – MCER Spectral Response Acceleration Parameter
Adjusted for Site Class Effect (0.2 second) Equation 16-36 (1) 1.800 (4) g
SM1 - MCER Spectral Response Acceleration Parameter
Adjusted for Site Class Effect (1.0 second) Equation 16-37 (1) Null (4) g
SDS - Design Spectral Response Acceleration at 0.2-s Equation 16-38 (1) 1.200 (4) g
SD1 - Design Spectral Response Acceleration at 1-s Equation 16-39 (1) Null (4) g
To = 0.2 SD1/ SDS Section 11.4.6 (2) Null s
Ts = SD1/ SDS Section 11.4.6 (2) Null s
TL - Long Period Transition Period Figure 22-14 (2) 8 (4) s
PGA - Peak Ground Acceleration at MCEG (*) Figure 22-9 (2) 0.572 g
FPGA - Site Coefficient Adjusted for Site Class Effect (2) Table 11.8-1 (2) 1.2 (4) -
PGAM –Peak Ground Acceleration (2)
Adjusted for Site Class Effect Equation 11.8-1 (2) 0.687 (4) g
Design PGA ≈ (⅔ PGAM) - Slope Stability (†) Similar to Eqs. 16-38 & 16-39 (2) 0.458 g
Design PGA ≈ (0.4 SDS) – Short Retaining Walls (‡) Equation 11.4-5 (2) 0.480 g
CRS - Short Period Risk Coefficient Figure 22-18A (2) 0.915 (4) -
CR1 - Long Period Risk Coefficient Figure 22-19A (2) 0.895 (4) -
SDC - Seismic Design Category (§) Section 1613.2.5 (1) Null (4) -
References:
(1) California Building Code (CBC), 2019, California Code of Regulations, Title 24, Part 2, Volume I and II.
(2) American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI), 2016, Minimum Design Loads and Associated
Criteria for Buildings and Other Structures, Standards 7-16.
(3) USGS Unified Hazard Tool - https://earthquake.usgs.gov/hazards/interactive/
(4) SEI/OSHPD Seismic Design Map Application – https://seismicmaps.org
Related References:
Federal Emergency Management Agency (FEMA), 2015, NEHERP (National Earthquake Hazards Reduction Program)
Recommended Seismic Provision for New Building and Other Structures (FEMA P-1050).
Notes:
* PGA Calculated at the MCE return period of 2475 years (2 percent chance of exceedance in 50 years).
† PGA Calculated at the Design Level of ⅔ of MCE; approximately equivalent to a return period of 475 years (10 percent chance o f
exceedance in 50 years).
‡ PGA Calculated for short, stubby retaining walls with an infinitesimal (zero) fundamental period.
§ The designation provided herein may be superseded by the structural engineer in accordance with Section 1613.2.5.1, if applic able.
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Discussion
General
Owing to the characteristics of the subsurface soils, as defined by Site Class D-Default designation, and
proximity of the site to the sources of major ground shaking, the site is expected to experience strong ground
shaking during its anticipated life span. Under these circumstances, where the code-specified design
response spectrum may not adequately characterize site response, the 2019 CBC typically requires a site-
specific seismic response analysis to be performed. This requirement is signified/identifi ed by the “null”
values that are output using SEA/OSHPD software in determination of short period, but mostly, in
determination of long period seismic parameters, see Table 1.
For conditions where a “null” value is reported for the site, a variety of design approaches are permitted by
2019 CBC and ASCE 7-16 in lieu of a site-specific seismic hazard analysis. For any specific site, these
alternative design approaches, which include Equivalent Lateral Force (ELF) procedure, Modal Response
Spectrum Analysis (MRSA) procedure, Linear Response History Analysis (LRHA) procedure and
Simplified Design procedure, among other methods, are expected to provide results that may or may not be
more economical than those that are obtained if a site-specific seismic hazards analysis is performed. These
design approaches and their limitations should be evaluated by the project structural engineer.
Seismic Design Category
Please note that the Seismic Design Category, SDC, is also designated as “null” in Table 1. For condition
where the mapped spectral response acceleration parameter at 1 – second period, S1, is less than 0.75, the
2019 CBC, Section 1613.2.5.1 allows that seismic design category to be determined from Table 1613.2.5(1)
alone provided that all 4 requirements concerning fundamental period of structure, story drift, seismic
response coefficient, and relative rigidity of the diaphragms are met. Our interpretation of ASCE 7-16 is
that for conditions where one or more of these 4 conditions are not met, seismic design category should be
assigned based on: 1) 2019 CBC, Table 1613.2.5(1), 2) structure’s risk category and 3) the value of SDS, at
the discretion of the project structural engineer.
Equivalent Lateral Force Method
Should the Equivalent Lateral Force (ELF) method be used for seismic design of structural elements, the
value of Constant Velocity Domain Transition Period, Ts, is estimated to be 1.7 seconds and the value of
Long Period Transition Period, TL, is provided in Table 1 for construction of Seismic Response Coefficient
– Period (Cs -T) curve that is used in the ELF procedure.
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As stated herein, the subject site is considered to be within a Site Class D-Default. A site-specific ground
motion hazard analysis is not required for structures on Site Class D-Default with S1 > 0.2 provided that
the Seismic Response Coefficient, Cs, is determined in accordance with ASCE 7-16, Article 12.8 and
structural design is performed in accordance with Equivalent Lateral Force (ELF) procedure.
Allowable Soil Bearing Capacities
Pad Footings
An allowable soil bearing capacity of 1,500 pounds per square foot may be utilized for design of isolated
24-inch-square footings founded at a minimum depth of 12 inches below the lowest adjacent final grade
for pad footings that are not a part of the slab system and are used for support of such features as roof
overhang, second-story decks, patio covers, etc. This value may be increased by 20 percent for each
additional foot of depth and by 10 percent for each additional foot of width, to a maximum value of 2,500
pounds per square foot. The recommended allowable bearing value includes both dead and live loads, and
may be increased by one-third for short duration wind and seismic forces.
Continuous Footings
An allowable soil bearing capacity of 1,500 pounds per square foot may be utilized for design of continuous
footings founded at a minimum depth of 12 inches below the lowest adjacent final grade . This value may
be increased by 20 percent for each additional foot of depth and by 10 percent for each additional foot of
width, to a maximum value of 2,500 pounds per square foot . The recommended allowable bearing value
includes both dead and live loads, and may be increased by one-third for short duration wind and seismic
forces.
Footing Settlements
Based on the allowable bearing values provided above and settlement potential of existing fill, total
settlement of the footings under the anticipated loads is estimated to be less than ½ inches. Differential
settlement is expected to be less than ½ inch over a horizontal span of 40 feet. The majority of the
anticipated settlement is likely to take place as footing loads are applied or shortly thereafter. A differential
settlement of 2 inches over a horizontal distance of 40 feet should be considered in design under the ‘seismic
‘dry sand’ settlement.
Lateral Resistance
A passive earth pressure of 250 pounds per square foot per foot of depth, to a maximum value of
2,500 pounds per square foot, may be used to determine lateral bearing resistance for footings. In addition,
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a coefficient of friction of 0.30 times the dead load forces may be used between concrete and the supporting
soils to determine lateral sliding resistance. The above values may be increased by one-third when designing
for transient wind or seismic forces. It should be noted that the above values are based on the condition
where footings are cast in direct contact with compacted fill or competent native soils. In cases where the
footing sides are formed, all backfill placed against the footings upon removal of forms should be
compacted to at least 90 percent of the applicable maximum dry density.
Guidelines for Footings and Slabs on-Grade Design and Construction
Expansive Soil Conditions
The results of laboratory testing performed by Petra on representative samples of near-surface soils within
the subject tracts indicate that the soils underlying the subject units predominantly exhibit a Very Low
expansion potential (Expansion Index less than or equal to 20). Thus, the site soils may generally be
classified as "non-expansive" as defined in Section 1803.5.3 of the 2019 CBC. As such, the design of slabs
on-grade is considered to be exempt from the procedures outlined in Sections 1803.5.3 and 1808.6.2 of the
2019 CBC and may be performed using any method deemed rational and appropriate by the project
structural engineer. Test results performed by Petra and other consultants on the samples obtained from the
tracts are summarized on Plate I.
The following minimum recommendations are presented herein for conditions where the project design
team may require geotechnical engineering guidelines for design and construction of footings and slabs on-
grade the project site.
The design and construction guidelines that follow are based on the above soil conditions and may
be considered for reducing the effects of variability in fabric, composition and, therefore, the
detrimental behavior of the site soils such as excessive short- and long-term total and differential
settlements. These guidelines have been developed on the basis of the previous experience of this
firm on projects with similar soil conditions. Although construction performed in accordance with
these guidelines has been found to reduce post-construction movement and/or distress, they
generally do not positively eliminate all potential effects of variability in soils characteris tics and
future settlement.
It should also be noted that the suggestions for dimension and reinforcement provided herein are
performance-based and intended only as preliminary guidelines to achieve adequate performance
under the anticipated soil conditions. However, they should not be construed as replacement for
structural engineering analyses, experience and judgment. The project structural engineer,
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architect and/or civil engineer should make appropriate adjustments to slab and footing
dimensions, and reinforcement type, size and spacing to account for internal concrete forces (e.g.,
thermal, shrinkage and expansion) as well as external forces (e.g., applied loads) as deemed
necessary. Consideration should also be given to minimum design criteria as dictated by local
building code requirements.
Post-Tensioned Slabs on-Grade System (Very Low Expansion)
In consideration of the expansion index of less than or equal to 20, as predominantly exhibited by onsite
soils, any rational and appropriate procedure may be chosen by the project structural engineer for the design
of post-tensioned slabs on-grade. Should the design engineer choose to follow the most current procedure
published by the Post-Tensioning Institute (PTI DC10.5-12), the following minimum design criteria are
provided Table 2, below.
TABLE 2
Post-Tensioned Slab on-Grade Design Parameters for PTI Procedure
Soil Information
Approximate Depth of Constant Suction, feet 9
Approximate Soil Suction, pF 3.9
Inferred Thornthwaite Index: -20
Average Edge Moisture Variation Distance, em in feet:
Center Lift
Edge Lift
9.0
4.7
Anticipated Swell, ym in inches:
Center Lift
Edge Lift
0.25
0.45
Modulus of Subgrade Reaction
The modulus of subgrade reaction for design of load bearing elements depends on the size of the element
and soil-structure interaction. However, as a fist level of approximation, this value may be assumed to be
125 pounds per cubic inch.
Minimum Design Recommendations
The soil values provided above may be utilized by the project structural engineer to design post-tensioned
slabs on-ground in accordance with Section 1808.6.2 of the 2019 CBC and the PTI publication. Thicker
floor slabs and larger footing sizes may be required for structural reasons and should govern the design if
more restrictive than the minimum recommendations provided below:
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1. Exterior continuous footings for both one-story and two-story structures should be founded at a
minimum depth of 12 inches below the lowest adjacent finished ground surface. Interior footings
may be founded at a minimum depth of 10 inches below the tops of the adjacent finish floor slabs.
2. In accordance with Table 1809.7 of 2019 CBC for light-frame construction, all continuous footings
should have minimum widths of 12 inches for one- and two-story construction. We recommend all
continuous footings should be reinforced with a minimum of two No. 4 bars, one top and one
bottom. Alternatively, post-tensioned tendons may be utilized in the perimeter continuous footings
in lieu of the reinforcement bars.
3. A minimum 12-inch-wide grade beam founded at the same depth as adjacent footings should be
provided across the garage entrances or similar openings (such as large doors or bay windows).
The grade beam should be reinforced in a similar manner as provided above.
4. Exterior isolated pad footings intended for support of roof overhangs such as second-story decks,
patio covers and similar construction should be a minimum of 24 inches square and founded at a
minimum depth of 18 inches below the lowest adjacent final grade. The pad footings should be
reinforced with No. 4 bars spaced a maximum of 18 inches on centers, both ways, placed near the
bottoms of the footings. Exterior isolated pad footings may need to be connected to adjacent pad
and/or continuous footings via tie beams at the discretion of the project structural engineer.
5. The thickness of the floor slabs should be determined by the project structural engineer with
consideration given to the expansion potential of the onsite soils; however; we recommend that a
minimum slab thickness of 4 inches be considered.
6. As an alternative to designing 4-inch-thick post-tensioned slabs with perimeter footings as
described in Item 1 and 2 above, the structural engineer may design the foundation system using a
thickened slab design. The minimum thickness of this uniformly thick slab should be 8 inches. The
engineer in charge of post-tensioned slab design may also opt to use any combination of slab
thickness and footing embedment depth as deemed appropriate based on their engineering
experience and judgment.
7. Living area concrete floor slabs and areas to receive moisture sensitive floor covering should be
underlain with a moisture vapor retarder consisting of a minimum 10-mil-thick polyethylene or
polyolefin membrane that meets the minimum requirements of ASTM E96 and ASTM E1745 for
vapor retarders (such as Husky Yellow Guard®, Stego® Wrap, or equivalent). All laps within the
membrane should be sealed, and at least 2 inches of clean sand should be placed over the membrane
to promote uniform curing of the concrete. To reduce the potential for punctures, the membrane
should be placed on a pad surface that has been graded smooth without any sharp protrusions. If a
smooth surface cannot be achieved by grading, consideration should be given to lowering the pad
finished grade an additional inch and then placing a 1-inch-thick leveling course of sand across the
pad surface prior to the placement of the membrane.
At the present time, some slab designers, geotechnical professionals and concrete experts
view the sand layer below the slab (blotting sand) as a place for entrapment of excess
moisture that could adversely impact moisture-sensitive floor coverings. As a preventive
measure, the potential for moisture intrusion into the concrete slab could be reduced if the
concrete is placed directly on the vapor retarder. However, if this sand layer is omitted,
appropriate curing methods must be implemented to ensure that the concrete slab cures
uniformly. A qualified materials engineer with experience in slab design and construction
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should provide recommendations for alternative methods of curing and supervise the
construction process to ensure uniform slab curing. Additional steps would also need to be
taken to prevent puncturing of the vapor retarder during concrete placement.
8. Garage floor slabs should be designed in a similar manner as living area floor slabs. Consideration
should be given to placement of a moisture vapor retarder below the garage slab, similar to that
provided in Item 7 above, should the garage slab be overlain with moisture sensitive floor covering.
9. Presaturation of the subgrade below floor slabs will not be required; however, prior to placing
concrete, the subgrade below all dwelling and garage floor slab areas should be thoroughly
moistened to achieve a moisture content that is at least equal to or slightly greater than optimum
moisture content. This moisture content should penetrate to a minimum depth of 12 inches below
the bottoms of the slabs.
10. The minimum footing dimensions and reinforcement recommended herein may be modified
(increased or decreased subject to the constraints of Chapter 18 of the 2019 CBC) by the structural
engineer responsible for foundation design based on his/her calculations, engineering experience
and judgment.
Foundation Excavation Observations
Foundation excavations should be observed by a representative of this firm to document that they have been
excavated into competent engineered fill soils prior to the placement of forms, reinforcement or concrete.
Following remedial/precise grading, the presence of rock, up to 12 inches diameter, if any, in the compacted
fill may require the use of forms when pouring concrete. The excavations should be trimmed neat, level
and square. All loose, sloughed or moisture-softened soils and/or any construction debris should be
removed prior to placing of concrete. Excavated soils derived from footing and/or utility trenches should
not be placed in building slab-on-grade areas or exterior concrete flatwork areas unless the soils are
compacted to at least 90 percent of maximum dry density.
Soil Corrosivity Screening
As a screening level study, limited chemical and electrical tests were performed on samples considered
representative of the onsite soils to identify potential corrosive characteristics of these soils. The common
indicators associated with soil corrosivity include water-soluble sulfate and chloride levels, pH (a measure
of acidity), and minimum electrical resistivity.
It should be noted that Petra does not practice corrosion engineering; therefore, the test results,
opinion and engineering judgment provided herein should be considered as general guidelines
only. Additional analyses would be warranted, especially, for cases where buried metallic building
materials (such as copper and cast or ductile iron pipes) in contact with site soils are planned for
the project. In many cases, the project geotechnical engineer may not be informed of these choices.
Therefore, for conditions where such elements are considered, we recommend that other, relevant
WILLIAMS HOMES December 17, 2021
Palo Verde Project / La Quinta J.N. 21-468
Page 14
project design professionals (e.g., the architect, landscape architect, civil and/or structural
engineer) also consider recommending a qualified corrosion engineer to conduct additional
sampling and testing of near-surface soils during the final stages of site grading to provide a
complete assessment of soil corrosivity. Recommendations to mitigate the detrimental effects of
corrosive soils on buried metallic and other building materials that may be exposed to corrosive
soils should be provided by the corrosion engineer as deemed appropriate.
In general, a soil’s water-soluble sulfate levels and pH relate to the potential for concrete degradation;
water-soluble chlorides in soils impact ferrous metals embedded or encased in concrete, e.g., reinforcing
steel; and electrical resistivity is a measure of a soil’s corrosion potential to a variety of buried metals used
in the building industry, such as copper tubing and cast or ductile iron pipes. Table 3, below, presents a
values of individual test results by Petra and other consultants for the project with an interpretation of
current code indicators and guidelines that are commonly used in this industry. The table includes the code-
related classifications of the soils as they relate to the various tests, as well as a general recommendation
for possible mitigation measures in view of the potential adverse impact on various components of the
proposed structures in direct contact with site soils. The guidelines provided herein should be evaluated and
confirmed, or modified, in their entirety by the project structural engineer, corrosion engineer and/or the
contractor responsible for concrete placement for structural concrete used in exterior and interior footings,
interior slabs on-ground, garage slabs, wall foundations and concrete exposed to weather such as driveways,
patios, porches, walkways, ramps, steps, curbs, etc.
TABLE 3
Soil Corrosivity Screening Results
Test Test Results Classification General Recommendations
Soluble Sulfates
(Cal 417)
0.0189 to 0.0414
percent S01 Min. fc’= 2,500 psi No water/cement ratio restrictions
pH
(Cal 643) 7.7 - 8.5
Slightly to
Moderately
Alkaline
Type I-P (MS) Modified or Type II Modified cement
Soluble Chloride
(Cal 422) 65 to 3,900 ppm C12
C24
Residence: Increase concrete cover thickness; fc’(2)
should not be less than 2,500 psi. Consult a corrosion
engineer. Spas/Decking: water/cement ratio 0.40, fc’ =
5,000 psi
Resistivity
(Cal 643)
405 to 2,800
ohm-cm
Corrosive to
Extremely
Corrosive3
Protective wrapping/coating of buried pipes; corrosion
resistant materials; or cathodic protection. Consult a
corrosion engineer.
Notes:
1. ACI 318-14, Section 19.3
2. ACI 318-14, Section 19.3
3. Pierre R. Roberge, “Handbook of Corrosion Engineering”
4. Exposure classification C2 applies specifically to swimming pools/spas and appurtenant concrete elements
WILLIAMS HOMES December 17, 2021
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Page 15
As depicted in Table 3 and Plate 1, one sample, obtained from Lot 7, Tract 33336, exhibited an
excessive level of soluble chloride content. Currently, this is considered a ‘hot spot’ for soluble
chloride and may require special recommendations by a corrosion engineer for the entire project.
However, considering other test results, this high value may be limited to surficial soils on and
within the vicinity of Lot 7. It is highly recommended that at the end of precise grading additional
surficial samples be obtained from this area for further verification. If, upon further testing, this
condition persists, protective measures such as encasing post-tensioned cables in plastic sheathing
may become necessary. Further, a corrosion engineer should also examine the results and provide
appropriate mitigative recommendations.
Post-Grading Considerations
Precise Grading and Drainage
Surface and subsurface drainage systems consisting of sloping concrete flatwork and graded swales will be
constructed on the subject lots to collect and direct all surface water to the curb and gutter of the adjacent
streets. In addition, the ground surface around the proposed buildings should be sloped to provide a positive
drainage gradient away from the structures. The purpose of the drainage systems is to prevent ponding of
surface water within the level areas of the site and against building foundations and associated site
improvements. The drainage systems should be properly maintained throughout the life of the proposed
development.
Section 1804.4 of the 2019 CBC requires that "The ground immediately adjacent to the foundation shall be
sloped away from the building at a slope of not less than one unit vertical in 20 units horizontal (5-percent
slope) for a minimum distance of 10 feet (3048 mm) measured perpendicular to the face of the wall."
Further, “Swales used for this purpose shall be sloped a minimum of 2 percent where located within 10 feet
(3048 mm) of the building foundation”.
These provisions fall under the purview of the Design Civil Engineer. However, exceptions to allow
modifications to these criteria are provided within the same section of the code as "Where climatic or soil
conditions warrant, the slope of the ground away from the building foundations is permitted to be reduced
to not less than one unit in 48 units horizontal (2-percent slope).” This exemption provision appears to fall
under the purview of the Geotechnical Engineer-of-Record.
It is our understanding that the state-of-the-practice for projects in various cities and unincorporated areas
of Ventura Counties, as well as throughout Southern California, has been to construct earthen slopes at 2
WILLIAMS HOMES December 17, 2021
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Page 16
percent minimum gradient away from the foundations and at 1 percent minimum for earthen swale
gradients. Structures constructed and properly maintained under those criteria have performed
satisfactorily. Therefore, considering the semi-arid climate, site soil conditions and an appropriate irrigation
regime, Petra considers that the implementation of 2 percent slopes away from the structures and 1 percent
swales to be acceptable for the subject lots.
It should be emphasized that the homeowners are cautioned that the slopes away from the structures and
swales to be properly maintained, not to be obstructed, and that future improvements not to alter established
gradients unless replaced with suitable alternative drainage systems. Further, where the flow line of the
swale exists within five feet of the structure, adjacent footings shall be deepened appropriately to maintain
minimum embedment requirements, measured from the flow line of the swale.
Utility Trench Backfill
All utility trench backfill should be compacted to a minimum relative compaction of 90 percent . Trench
backfill materials should be placed in approximately 8- to 12-inch thick maximum lifts, moisture
conditioned as necessary to achieve near optimum moisture conditions, and mechanically compacted in
place with a hydra-hammer, pneumatic tamper or similar equipment to achieve a minimum relative
compaction of 90 percent. A representative of this firm should probe and test the backfills to document the
adequate compaction has been achieved.
For shallow trenches where pipe or utilities might be damaged by mechanical compaction equipment,
imported sand having a Sand Equivalent (SE) value of 30 or greater may be used for backfill. Sand backfill
materials should be watered to achieve optimum (or above) moisture conditions, and then tamped with
hand-operated pneumatic tampers to ensure proper consolidation of the backfill. No specific relative
compaction will be required; however, observation, probing and, if deemed necessary, testing should be
performed by a representative of this firm to verify that the backfill is adequately compacted and will not
be subject to excessive settlement.
Where an exterior or interior utility trench is proposed in a direction that is parallel to a building footing,
the bottom of the trench should not extend below a 1:1 plane projected downward from the bottom edge of
the adjacent footing. Where this condition occurs, the adjacent footing should be deepened or the trench
backfilled and compacted prior to construction of the footing.
Masonry Block Screen Walls
Footings for masonry block walls may be designed in accordance with the bearing and lateral resistance
values provided previously for building footings. However, as a minimum, the wall footings should be
WILLIAMS HOMES December 17, 2021
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embedded at a minimum depth of 18 inches below the lowest adjacent final grade. The footings should also
be reinforced with a minimum of four No. 4 bars, two near top and two near bottom. In order to minimize
the potential for unsightly cracking related to the possible effects of differential settlement and/or
expansion, positive separations (construction joints) should also be provided in the block walls at each
corner and at horizontal intervals of approximately 20 to 25 feet. The separations should be provided in the
blocks and not extend through the footings. The footings should be poured monolithically with continuous
reinforcement bars to serve as effective “grade beams” below the walls.
Exterior Concrete Flatwork
General
Near-surface compacted fill soils within the site indicate expansion index ranging from 3 to 20 which is a
borderline value between Very Low and Low expansion potential. For this reason, we recommend that
additional testing of subgrade soils be performed at the completion of precise grading in order to provide
specific recommendations for all exterior concrete flatwork. However, due to typical project scheduling
constraints, it may not be feasible to collect additional samples of subgrade soils for testing to verify their
expansion potential immediately prior to pouring concrete. For this reason, we recommend that all exterior
concrete flatwork such as sidewalks, patio slabs, large decorative slabs, concrete subslabs that will be
covered with decorative avers, private and/or public vehicular driveways and/or access roads within and
adjacent to the site be designed by the project architect and/or structural engineer with consideration given
to mitigating the potential cracking and uplift that can develop in soils exhibiting expansion index values
that fall in the upper band of Very Low category.
The guidelines that follow should be considered as minimums and are subject to review and revision by the
project architect, structural engineer and/or landscape consultant as deemed appropriate . If sufficient time
will be allowed in the project schedule for verification sampling and testing prior to the concrete pour, the
test results generated may dictate that a somewhat less conservative design could be used.
Subgrade Preparation
Compaction
To reduce the potential for distress to concrete flatwork, the subgrade soils below concrete flatwork areas
to a minimum depth of 12 inches (or deeper, as either prescribed elsewhere in this report or determined in
the field) should be moisture conditioned to at least equal to, or slightly greater than, the optimum moisture
content and then compacted to a minimum relative compaction of 90 percent. Where concrete public roads,
concrete segments of roads and/or concrete access driveways are proposed, the upper 6 inches of subgrade
soil should be compacted to a minimum 95 percent relative compaction.
WILLIAMS HOMES December 17, 2021
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Pre-Moistening
As a further measure to reduce the potential for concrete flatwork cracking, subgrade soils should be
thoroughly moistened prior to placing concrete. The moisture content of the soils should be at least 1.2
times the optimum moisture content and penetrate to a minimum depth of 12 inches into the subgrade.
Flooding or ponding of the subgrade is not considered feasible to achieve the above moisture conditions
since this method would likely require construction of numerous earth berms to contain the water.
Therefore, moisture conditioning should be achieved with sprinklers or a light spray applied to the subgrade
over a period of few to several days just prior to pouring concrete. Pre-watering of the soils is intended to
promote uniform curing of the concrete, reduce the development of shrinkage cracks and reduce the
potential for differential expansion pressure on freshly poured flatwork. A representative of the project
geotechnical consultant should observe and verify the density and moisture content of the soils, and the
depth of moisture penetration prior to pouring concrete.
Thickness and Joint Spacing
To reduce the potential of unsightly cracking, concrete walkways, patio-type slabs, large decorative slabs
and concrete subslabs to be covered with decorative pavers should be at least 4 inches thick and provided
with construction joints or expansion joints every 6 feet or less. Private driveways that will be designed for
the use of passenger cars for access to private garages should also be at least 4 inches thick and provided
with construction joints or expansion joints every 10 feet or less. Concrete pavement that will be designed
based on an unlimited number of applications of an 18-kip single-axle load in public access areas, segments
of road that will be paved with concrete (such as bus stops and cross-walks) or access roads and driveways,
which serve multiple residential units or garages, that will be subject to heavy truck loadings should have
a minimum thickness of 5 inches and be provided with control joints spaced at maximum 10-foot intervals.
A modulus of subgrade reaction of 125 pounds per cubic foot may be used for design of the public and
access roads.
Reinforcement
All concrete flatwork having their largest plan-view panel dimensions exceeding 10 feet should be
reinforced with a minimum of No. 3 bars spaced 18 inches for 4-inch-thick slabs and No. 4 bars spaced 24
inches for 5-inch-thick slabs on centers, both ways. Alternatively, the slab reinforcement may consist of
welded wire mesh of the sheet type (not rolled) with 6x6/W1.4xW1.4 designations for 4-inch-thick slabs
and 6x6/W2.9xW2.9 designations for 5-inch-thick slabs in accordance with the Wire Reinforcement
Institute (WRI). The reinforcement should be properly positioned near the middle of the slabs. All foot and
equipment traffic on the reinforcement should be avoided or reduced to a minimum.
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The reinforcement recommendations provided herein are intended as a guideline to achieve
adequate performance for anticipated soil conditions. As such, this guideline may not satisfy
certain acceptable approaches, e.g. the area of reinforcement to be equal to or greater that 0.2
percent of the area of concrete. The project architect, civil and/or structural engineer should make
appropriate adjustments in reinforcement type, size and spacing to account for concrete internal
(e.g., shrinkage and thermal) and external (e.g., applied loads) forces as deemed necessary.
Edge Beams (Optional)
Where the outer edges of concrete flatwork are to be bordered by landscaping, it is recommended that
consideration be given to the use of edge beams (thickened edges) to prevent excessive infiltration and
accumulation of water under the slabs. Edge beams, if used, should be 6 to 8 inches wide, extend 8 inches
below the tops of the finish slab surfaces. Edge beams are not mandatory; however, their inclusion in
flatwork construction adjacent to landscaped areas is intended to reduce the potential for vertical and
horizontal movement and subsequent cracking of the flatwork related to uplift forces that can develop in
expansive soils.
Drainage
Drainage from patios and other flatwork areas should be directed to local area drains and/or graded earth
swales designed to carry runoff water to the adjacent streets or other approved drainage structures . The
concrete flatwork should be sloped at a minimum gradient of one percent, or as prescribed by project civil
engineer or local codes, away from building foundations, retaining walls, masonry garden walls and slope
areas.
Tree Wells
Tree wells are not recommended in concrete flatwork areas since they introduce excessive water into the
subgrade soils and allow root invasion, both of which can cause heaving and cracking of the flatwork.
Pavement Design
A sample of site soils was taken by SMC for R-Value testing. The sample exhibited and R-Value of 70.
Based on this result, and a Traffic Index (TI) of 5.0, SMC recommended (SMC, 2006d) a structural
pavement section consisting of 3 inches of hot mix asphalt concrete (AC) over 4.5 inches of crushed
aggregate base (AB).
WILLIAMS HOMES December 17, 2021
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Page 20
We agree with their assessment and recommend the same pavement section for the unpaved interior street
within the project.
Subgrade Preparation
Subgrade soils immediately below the aggregate base should be compacted to a minimum of 90 percent
relative compaction based on ASTM D 1557 to a depth of 12 inches or more. Final subgrade compaction
should be performed prior to placing the base material and after utility-trench backfills have been
compacted and tested. Subgrade shall be firm and unyielding, as exhibited by proof-rolling, prior to
placement of base. Should conditions be encountered where significantly higher moisture contents are
present within the subgrade soil, additional overexcavation may be required to achieve a firm, unyielding
excavation bottom. A jurisdictional relative compaction standard in excess of the aforementioned minimum
may exist and should be complied with, as applicable.
Crushed Aggregate Base and Asphaltic Concrete
Crushed Aggregate Base (CAB) should conform to Section 200-2.2 of the Standard Specifications for
Public Works Construction (Greenbook). It should be noted that the base thickness is predicated on the use
of Crushed Aggregate Base material. For conditions where either Crushed Miscellaneous Base or Processed
Miscellaneous Base Materials are used, a 10 percent increase in base section thickness should be
incorporated in the design and construction of the structural pavement section. The base materials should
be brought to a uniform moisture content near optimum and then compacted to at least 95 percent of ASTM
D 1557.
Optional Considerations
Typical project sequencing often involves construction of a pavement section consisting of the design
aggregate base course that is initially capped with an asphalt layer that is thinner than the final design
asphalt thickness. This practice provides for a temporary pavement surface during the construction period
when streets are commonly subjected to heavy construction traffic. In light of the inherent weakness of this
temporary structural pavement section and the damage that can occur to the pavement as a result of repeated
heavy construction loads, it is our professional opinion that the streets may be paved with the full
recommended structural section provided herein prior to the construction phase of the project. If needed
after construction, the pavement surface should be brushed off (micro cold planed), all divots and
depressions patched with a tack coat and hot mix asphalt and sealed with a Type II slurry seal.
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Plan Reviews, Future Improvements and/or Grading
Petra should review any subsequent plans when they become available and issue an addendum letter to this
report. If additional site improvements are considered in the future, our firm should be notified so that we
may provide design recommendations to mitigate movement, settlement and/or tilting of the structures.
Potential problems can develop when drainage on the pads and slopes is altered in any way such as
placement of fill and construction of new walkways, patios, landscape walls, swimming pools, spas and/or
planters. Therefore, it is recommended that we be engaged to review the final design drawings,
specifications and grading plan prior to any new construction. If we are not provided the opportunity to
review these documents with respect to the geotechnical aspects of new construction and grading, it should
not be assumed that the recommendations provided herein are wholly or impart applicable to the proposed
improvements.
Post Grading Observations and Testing
Petra should be notified at the appropriate times in order that we may provide the following observation
and testing services during the various phases of construction and precise grading.
1. Building Construction
• Observe all footing trenches when first excavated to verify adequate depth and competent
bearing conditions.
• Re-observe all footing trenches, if necessary, if trenches are found to be excavated to
inadequate depth and/or found to contain significant slough, saturated or compressible
materials.
2. Concrete Flatwork Construction
• Observe and test subgrade soils below all concrete flatwork areas to verify adequate
compaction, moisture content and moisture penetration prior to pouring concrete.
3. Utility Trench Backfill
• Observe and test placement of all utility trench backfill mains and laterals to verify adequate
compaction.
4. Precise Grading
• Observe and test placement of any fill to be placed on the subject lots to verify adequate
compaction.
WILLIAMS HOMES December 17, 2021
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Page 22
5. Masonry Screen Walls
• Observe all screen wall footing trenches when first excavated to verify adequate depth and
competent bearing conditions.
REPORT LIMITATIONS
This report is based on the proposed residential tract and the geotechnical observations made during our
literature review of the prior consultant’s as-graded (Leighton, 2019e and 2019f) and settlement monitoring
(Leighton, 2019d) reports and our limited soil laboratory testing within the site. No representatives of Petra
were present during the previous grading activities that have been completed to-date at this site. This report
has been prepared consistent with that level of care being provided by other professionals providing similar
services at the same locale and in the same time period. The contents of this report are professional opinions
and as such, are not to be considered a guaranty or warranty.
Based on our findings, the conclusions and recommendations presented herein and within the referenced
reports by our firm were prepared in conformance with generally accepted professional engineering
practices.
We appreciate this opportunity to be of service. If you have questions, please contact this office.
Respectfully submitted,
PETRA GEOSCIENCES, INC.
12/17/21
Siamak Jafroudi, PhD
Senior Principal Engineer
GE 2024
AP/SJ/lv
Attachments: References
Plate I – Chemical, Electrical and Expansion Index Test Data
Appendix A – Seismic Design Analyses
W:\2020-2025\2021\400\21-468\Reports\21-468 150 Foundation Design and Constrution Recommendations.docx
WILLIAMS HOMES December 17, 2021
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Page 23
REFERENCES
Earth Systems Southwest, 2004, Geotechnical Engineering Report, Proposed Residential Development,
Tract Number 32279, Avenue 58, West of Madison Street La Quinta, California, dated April 21.
______, 2005, Geotechnical Engineering Report, Proposed Residential Development, 8.11-Acre Portion of
APN 762-240-01 80-600, Avenue 58, La Quinta, California, dated January 25.
Schiff Associates, 2006, Laboratory Tests on Soil Samples, STD.PAC., La Quinta, dated April 18.
Stoney-Miller Consultants, Inc., 2006a, Geotechnical Consultant of Record, Tract 33336, La Quinta,
California, dated February 6.
______, 2006b, Geotechnical Review of Rough Grading Plans, Tract 33336, La Quinta, California, dated
February 28.
______, 2006c, Review of Foundation Design Parameters, Tracts 32279 and 33336, La Quinta, California,
dated June 13.
______, 2006d, Pavement Recommendations, Tract 33336, Mirage II of La Quinta, California, dated
August 8.
______, 2006e, Earthwork Observation and Testing Report Rough Grading, Lots 1 through 30, Tract 32279
La Quinta, California, dated August 24.
______, 2006f, Earthwork Observation and Testing Report Rough Grading, Lots 1 through 23, Tract 33336
La Quinta, California, dated August 24.
WILLIAMS HOMES December 17, 2021
Palo Verde Project / La Quinta J.N. 21-468
PLATE I
CHEMICAL, ELECTRICAL AND EXPANSION INDEX TEST DATA
LABORATORY DATA SUMMARY
Consultant Sample ID Sample Location Expansion
Index
Sulfate
Content
(%)
Chloride
Content
(PPM)
pH
Minimum
Resistivity
(ohm-cm)
Petra Tract 32279
Lot 11 2 - - - -
Lot 16 0 - - - -
Lot 22 0 0.0414 900 8.5 1,100
Lot 30 0 - - - -
Petra Tract 33336
Lot 2 0 - - - -
Lot 7 1 0.0189 3,900 8.5 810
Lot 17 0 - - - -
Lot 23 0 - - - -
SMC Tract 33336 Boring B-5, 1 - 4’ - 445 ppm 726 7.8 405
Schiff
#1 - - 243 ppm 90 7.7 2,200
#2 - - 158 ppm 65 7.9 2,800
#3 - - 246 ppm 105 7.8 1,900
APPENDIX A
SEISMIC DESIGN ANALYSES
12/16/21, 9:37 PM U.S. Seismic Design Maps
https://seismicmaps.org 1/2
21-468
Latitude, Longitude: 33.629167, -116.2583
Date 12/16/2021, 9:25:43 PM
Design Code Reference Document ASCE7-16
Risk Category II
Site Class D - Default (See Section 11.4.3)
Type Value Description
SS 1.5 MCER ground motion. (for 0.2 second period)
S1 0.6 MCER ground motion. (for 1.0s period)
SMS 1.8 Site-modified spectral acceleration value
SM1 null -See Section 11.4.8 Site-modified spectral acceleration value
SDS 1.2 Numeric seismic design value at 0.2 second SA
SD1 null -See Section 11.4.8 Numeric seismic design value at 1.0 second SA
Type Value Description
SDC null -See Section 11.4.8 Seismic design category
Fa 1.2 Site amplification factor at 0.2 second
Fv null -See Section 11.4.8 Site amplification factor at 1.0 second
PGA 0.572 MCEG peak ground acceleration
FPGA 1.2 Site amplification factor at PGA
PGAM 0.687 Site modified peak ground acceleration
TL 8 Long-period transition period in seconds
SsRT 1.629 Probabilistic risk-targeted ground motion. (0.2 second)
SsUH 1.78 Factored uniform-hazard (2% probability of exceedance in 50 years) spectral acceleration
SsD 1.5 Factored deterministic acceleration value. (0.2 second)
S1RT 0.621 Probabilistic risk-targeted ground motion. (1.0 second)
S1UH 0.693 Factored uniform-hazard (2% probability of exceedance in 50 years) spectral acceleration.
S1D 0.6 Factored deterministic acceleration value. (1.0 second)
PGAd 0.572 Factored deterministic acceleration value. (Peak Ground Acceleration)
CRS 0.915 Mapped value of the risk coefficient at short periods
CR1 0.895 Mapped value of the risk coefficient at a period of 1 s
12/16/21, 9:37 PM U.S. Seismic Design Maps
https://seismicmaps.org 2/2
DISCLAIMER
While the information presented on this website is believed to be correct, SEAOC /OSHPD and its sponsors and contributors assume no responsibility or
liability for its accuracy. The material presented in this web application should not be used or relied upon for any specific application without competent examination
and verification of its accuracy, suitability and applicability by engineers or other licensed professionals. SEAOC / OSHPD do not intend that the use of this
information replace the sound judgment of such competent professionals, having experience and knowledge in the field of practice, nor to substitute for the
standard of care required of such professionals in interpreting and applying the results of the seismic data provided by this website. Users of the information from
this website assume all liability arising from such use. Use of the output of this website does not imply approval by the governing building code bodies responsible
for building code approval and interpretation for the building site described by latitude/longitude location in the search results of this website.
12/16/21, 9:21 PM Unified Hazard Tool
https://earthquake.usgs.gov/hazards/interactive/1/5
Uni ed Hazard Tool
Input
U.S. Geological Survey - Earthquake Hazards Program
Please do not use this tool to obtain ground motion parameter values for the design code
reference documents covered by the U.S. Seismic Design Maps web tools (e.g., the
International Building Code and the ASCE 7 or 41 Standard). The values returned by the two
applications are not identical.
Edition
Dynamic: Conterminous U.S. 2014 (u…
Latitude
Decimal degrees
33.6292
Longitude
Decimal degrees, negative values for western longitudes
-116.2583
Site Class
259 m/s (Site class D)
Spectral Period
Peak Ground Acceleration
Time Horizon
Return period in years
2475
12/16/21, 9:21 PM Unified Hazard Tool
https://earthquake.usgs.gov/hazards/interactive/2/5
Hazard Curve
View Raw Data
Hazard Curves
Time Horizon 2475 years
Peak Ground Acceleration
0.10 Second Spectral Acceleration
0.20 Second Spectral Acceleration
0.30 Second Spectral Acceleration
0.50 Second Spectral Acceleration
0.75 Second Spectral Acceleration
1.00 Second Spectral Acceleration
2.00 Second Spectral Acceleration
3.00 Second Spectral Acceleration
4.00 Second Spectral Acceleration
5.00 Second Spectral Acceleration
1e-2 1e-1 1e+0
Ground Motion (g)
1e-14
1e-13
1e-12
1e-11
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
Annual Frequency of Exceedence
Uniform Hazard Response Spectrum
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Spectral Period (s)
0.0
0.5
1.0
1.5
2.0
2.5
Ground Motion (g)
Spectral Period (s): PGA
Ground Motion (g): 0.7627
Component Curves for Peak Ground Acceleration
Time Horizon 2475 years
System
Grid
Interface
Fault
1e-2 1e-1 1e+0
Ground Motion (g)
1e-11
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
Annual Frequency of Exceedence
12/16/21, 9:21 PM Unified Hazard Tool
https://earthquake.usgs.gov/hazards/interactive/3/5
Deaggregation
Component
Total
ε = (-∞ .. -2.5)
ε = [-2.5 .. -2)
ε = [-2 .. -1.5)
ε = [-1.5 .. -1)
ε = [-1 .. -0.5)
ε = [-0.5 .. 0)
ε = [0 .. 0.5)
ε = [0.5 .. 1)
ε = [1 .. 1.5)
ε = [1.5 .. 2)
ε = [2 .. 2.5)
ε = [2.5 .. +∞)
5
15
25
35
Closest Distance, rRup (km)
45
55
65
75
85
95
9
8 .5
8
7 .5
M a g n it u d e (M w )
7
6 .5
6
5 .5
5
4 .5
5
% Contribution to Hazard
10
15
20
5
15
25
35
45
55
Closest Distance, rRup (km)
65
75
85
95
9
8 .5
8
7 .5
7
6 .5
M a g n it u d e (M w )
6
5 .5
5
4 .5
12/16/21, 9:21 PM Unified Hazard Tool
https://earthquake.usgs.gov/hazards/interactive/4/5
Summary statistics for, Deaggregation: Total
Deaggregation targets
Return period:2475 yrs
Exceedance rate:0.0004040404 yr⁻¹
PGA ground motion:0.76265298 g
Recovered targets
Return period:3085.8314 yrs
Exceedance rate:0.00032406177 yr⁻¹
Totals
Binned:100 %
Residual:0 %
Trace:0.1 %
Mean (over all sources)
m:7.18
r:13.97 km
ε₀:1.7 σ
Mode (largest m-r bin)
m:7.34
r:13.35 km
ε₀:1.7 σ
Contribution:13.29 %
Mode (largest m-r-ε₀ bin)
m:7.34
r:13.42 km
ε₀:1.67 σ
Contribution:12.37 %
Discretization
r:min = 0.0, max = 1000.0, Δ = 20.0 km
m:min = 4.4, max = 9.4, Δ = 0.2
ε:min = -3.0, max = 3.0, Δ = 0.5 σ
Epsilon keys
ε0:[-∞ .. -2.5)
ε1:[-2.5 .. -2.0)
ε2:[-2.0 .. -1.5)
ε3:[-1.5 .. -1.0)
ε4:[-1.0 .. -0.5)
ε5:[-0.5 .. 0.0)
ε6:[0.0 .. 0.5)
ε7:[0.5 .. 1.0)
ε8:[1.0 .. 1.5)
ε9:[1.5 .. 2.0)
ε10:[2.0 .. 2.5)
ε11:[2.5 .. +∞]
12/16/21, 9:21 PM Unified Hazard Tool
https://earthquake.usgs.gov/hazards/interactive/5/5
Deaggregation Contributors
Source Set Source Type r m ε0 lon lat az %
UC33brAvg_FM31 System 35.79
San Andreas (Coachella) rev [2]13.42 7.66 1.56 116.157°W 33.715°N 44.38 29.09
San Jacinto (Anza) rev [5]28.28 8.01 2.01 116.513°W 33.490°N 236.76 2.60
San Jacinto (Clark) rev [1]26.51 7.82 2.06 116.428°W 33.438°N 216.57 2.30
UC33brAvg_FM32 System 35.59
San Andreas (Coachella) rev [2]13.42 7.65 1.56 116.157°W 33.715°N 44.38 28.85
San Jacinto (Anza) rev [5]28.28 8.00 2.02 116.513°W 33.490°N 236.76 2.63
San Jacinto (Clark) rev [1]26.51 7.83 2.06 116.428°W 33.438°N 216.57 2.24
UC33brAvg_FM31 (opt)Grid 14.32
PointSourceFinite: -116.258, 33.679 7.04 5.87 1.56 116.258°W 33.679°N 0.00 2.63
PointSourceFinite: -116.258, 33.679 7.04 5.87 1.56 116.258°W 33.679°N 0.00 2.61
PointSourceFinite: -116.258, 33.688 7.86 5.78 1.72 116.258°W 33.688°N 0.00 1.42
PointSourceFinite: -116.258, 33.688 7.86 5.78 1.72 116.258°W 33.688°N 0.00 1.41
UC33brAvg_FM32 (opt)Grid 14.30
PointSourceFinite: -116.258, 33.679 7.04 5.87 1.56 116.258°W 33.679°N 0.00 2.63
PointSourceFinite: -116.258, 33.679 7.04 5.87 1.56 116.258°W 33.679°N 0.00 2.61
PointSourceFinite: -116.258, 33.688 7.86 5.78 1.72 116.258°W 33.688°N 0.00 1.42
PointSourceFinite: -116.258, 33.688 7.86 5.78 1.72 116.258°W 33.688°N 0.00 1.41