Cornerstone Earth IncProject Name Proposed 303 Baldwin Avenue
Retail, Office, and Residential Building Prelim
Location 303 Baldwin Avenue San Mateo, California
Client Prometheus Real Estate Group
Client Address 1900 S. Norfolk Street, Suite 150 San Mateo,
CA
Project Number 307-23-1
Project Engineer Geotechnical Project Manager
Scott E. Fitinghoff, P.E., G.E. Principal Engineer Quality
Assurance Reviewer
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1.3 Exploration Program
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2
1.5 Environmental Services
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2
2.1 Geological Setting
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2
2.2 Regional Seismicity
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3 Table 1: Approximate Fault Distances
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3
SECTION 3: SITE CONDITIONS
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4
3.1 Surface Description
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4
3.2 Subsurface Conditions
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4 3.2.1 Plasticity/Expansion Potential
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4 3.2.2 In-Situ Moisture Contents
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5
3.3 Ground Water
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5
3.4 CORROSION SCREENING
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5 3.4.1 Preliminary Soil Corrosion Screening
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6
SECTION 4: GEOLOGIC HAZARDS
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6
4.1 Fault Rupture
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6
4.3 Liquefaction
Potential-------------------------------------------------------------------------------------
7 4.3.1 Background
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7 4.3.2 Analysis
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7 4.3.3 Summary
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8 4.3.4 Ground Rupture Potential
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8
4.4 Lateral Spreading
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4.6 Tsunami/seiche
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4.7 Flooding
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9
SECTION 5: CONCLUSIONS
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5.2 Design-Level Geotechnical Investigation
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SECTION 6: EARTHWORK
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6.2 Below-Grade Excavations
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14 6.2.1 Temporary Shoring
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SECTION 7: FOUNDATIONS
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7.1 Summary of Recommendations
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15
7.2 Seismic Design Criteria
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17 Table 4: CBC Site Categorization and Site Coefficients
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7.3 Shallow Foundations
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17 7.3.1 Mat Foundations
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7.3.2 Mat Foundation Construction Considerations
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Mat Foundations – With Ground Improvement
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SECTION 10: LIMITATIONS
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FIGURE 3: REGIONAL FAULT MAP FIGURE 4A TO 4C: LIQUEFACTION ANALYSIS
SUMMARY – CPT-01 TO CPT-03 APPENDIX A: FIELD INVESTIGATION APPENDIX
B: LABORATORY TEST PROGRAM
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Project Name Proposed 303 Baldwin Avenue Retail, Office, and
Residential Building
Location 303 Baldwin Avenue San Mateo, California
SECTION 1: INTRODUCTION This geotechnical feasibility evaluation
was prepared for the sole use of Prometheus Real Estate Group for
the proposed 303 Baldwin Avenue Retail, Office, and Residential
Building project in San Mateo, California. The location of the site
is shown on the Vicinity Map, Figure 1. The purpose of this study
was to evaluate the existing subsurface conditions and develop an
opinion regarding potential geotechnical concerns that could impact
the proposed development. The preliminary geotechnical
recommendations contained in this report are for your forward
planning, cost estimating, and preliminary project design. 1.1
PROJECT DESCRIPTION We understand that a three- to five-story
retail/office/residential building with a two- to three- level
below-grade garage is planned for the site. We understand the
planned development will be of concrete-framed construction for the
below-grade parking and at-grade podium and wood and steel frame
construction for the portion of the structure above grade.
Appurtenant parking, utilities, landscaping and other improvements
necessary for site development are also planned. Structural loads
are not known at this time; however, loads are anticipated to be
typical for these types of structures. Grading for site development
is anticipated to consist of excavation of 20 feet for two levels
below-grade and cuts on the order of 25 feet for mat foundations,
shoring, ramps drainage, and mat subgrade stabilization. Grading
for site development is anticipated to consist of excavation of 30
feet for three levels below-grade and cuts on the order of 35 feet
for mat foundations, shoring, ramps, drainage, and mat subgrade
stabilization. 1.2 SCOPE OF SERVICES Our scope of services was
presented in our proposal dated November 2, 2016 and consisted of
field and laboratory programs to evaluate physical and engineering
properties of the subsurface soils, engineering analysis to prepare
recommendations for site work and grading, building
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foundations, flatwork, retaining walls, and pavements, and
preparation of this report. Brief descriptions of our exploration
and laboratory programs are presented below. 1.3 EXPLORATION
PROGRAM Field exploration consisted of one boring drilled on
November 16, 2016 with truck-mounted, hollow-stem auger drilling
equipment and three Cone Penetration Tests (CPT’s) advanced on
November 14, 2016. The boring was drilled to a depth of
approximately 50 feet; the CPT’s were advance to depths of
approximately 52½ to 67 feet. Seismic shear wave velocity
measurements were collected from CPT’s-1 and 3. The boring (Boring
EB-1) was advanced adjacent to CPT-2 for direct collection and
evaluation of physical samples to correlated soil behavior. The
borings and CPT’s were backfilled with cement grout in accordance
with local requirements; exploration permits were obtained as
required by local jurisdictions. The approximate locations of our
exploratory boring and CPT’s are shown on the Site Plan, Figure 2.
Details regarding our field program are included in Appendix A. 1.4
LABORATORY TESTING PROGRAM In addition to visual classification of
samples, the laboratory program focused on obtaining data for
foundation design and seismic ground deformation estimates. Testing
included moisture contents, dry densities, washed sieve analyses,
Plasticity Index tests, a consolidation test, and a suite of
corrosion tests. Details regarding our laboratory program are
included in Appendix B. 1.5 ENVIRONMENTAL SERVICES We understand
that environmental services for the project are being provided by
others. If environmental concerns are present, the environmental
consultant should review our geotechnical recommendations for
compatibility with the environmental concerns. SECTION 2: REGIONAL
SETTING 2.1 GEOLOGICAL SETTING The San Francisco peninsula is a
relatively narrow band of rock at the north end of the Santa Cruz
Mountains separating the Pacific Ocean from San Francisco Bay. This
represents one mountain range in a series of northwesterly-aligned
mountains forming the Coast Ranges geomorphic province of
California that stretches from the Oregon border nearly to Point
Conception. In the San Francisco Bay area, most of the Coast Ranges
have developed on a basement of tectonically mixed Cretaceous- and
Jurassic-age (70- to 200-million years old) rocks of the Franciscan
Complex. Locally these basement rocks are capped by younger
sedimentary and volcanic rocks. Most of the Coast Ranges are
covered by still younger surficial deposits that reflect geologic
conditions of the last million years or so.
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Movement on the many splays within the San Andreas Fault system has
produced the dominant northwest-oriented structural and topographic
trend seen throughout the Coast Ranges today. This trend reflects
the boundary between two of the Earth’s major tectonic plates: the
North American plate to the east and the Pacific plate to the west.
The San Andreas Fault system and its major branch faults are about
40 miles wide in the Bay area and extends from the San Gregorio
Fault near the coastline to the Coast Ranges-Central Valley blind
thrust at the western edge of the Great Central Valley as shown on
the Regional Fault Map, Figure 3. The San Andreas Fault is the
dominant structure in the system, nearly spanning the length of
California, and capable of producing the highest magnitude
earthquakes. Many other subparallel or branch faults within the San
Andreas system are equally active and nearly as capable of
generating large earthquakes. Right-lateral movement dominates on
these faults but an increasingly large amount of thrust faulting
resulting from compression across the system is no being identified
also. 2.2 REGIONAL SEISMICITY The San Francisco Bay area region is
one of the most seismically active areas in the Country. While
seismologists cannot predict earthquake events, geologists from the
U.S. Geological Survey have recently updated earlier estimates from
their 2014 Uniform California Earthquake Rupture Forecast (Version
3) publication. The estimated probability of one or more magnitude
6.7 earthquakes (the size of the destructive 1994 Northridge
earthquake) expected to occur somewhere in the San Francisco Bay
Area has been revised (increased) to 72 percent for the period 2014
to 2043 (Aagaard et al., 2016). The faults in the region with the
highest estimated probability of generating damaging earthquakes
between 2014 and 2043 are the Hayward (33%), Rodgers Creek (33%),
Calaveras (26%), and San Andreas Faults (22%). In this 30-year
period, the probability of an earthquake of magnitude 6.7 or larger
occurring is 22 percent along the San Andreas Fault and 33 percent
for the Hayward or Rodgers Creek Faults. The faults considered
capable of generating significant earthquakes are generally
associated with the well-defined areas of crustal movement, which
trend northwesterly. The table below presents the State-considered
active faults within 25 kilometers of the site. Table 1:
Approximate Fault Distances
Fault Name
San Andreas (1906) 3.2 5.1 San Gregorio 10.1 16.2
Monte Vista-Shannon 10.9 17.6 Hayward (Total Length) 14.0
22.6
A regional fault map is presented as Figure 3, illustrating the
relative distances of the site to significant fault zones.
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SECTION 3: SITE CONDITIONS 3.1 SURFACE DESCRIPTION The project site
is located at 303 Baldwin Avenue in San Mateo, California. The site
is currently occupied by a one-story market building and
surrounding asphalt concrete parking lot and landscaping areas. The
site is bounded by residential and commercial development to the
north, North B Street to the east, Baldwin Avenue to the south, and
North Ellsworth Avenue to the west. Surface pavements at our
exploration location generally consisted of one inch of asphalt
concrete over 3 inches of aggregate base. Based on visual
observations, the existing pavements are in fair to good condition
with minor cracking observed. Based on our review of aerial
photographs, geologic maps (Pampeyan, 1994), and experience on
adjacent project, we understand that an underground culvert is
present along the eastern corner of the site. Based on our review
of available maps and photos, it appears the culvert runs along the
northeast side parallel to B street and along the southeast side
parallel to Baldwin Avenue. The culvert is a concrete box type with
an open bottom positioned about 12 to 14 feet below the ground
surface. 3.2 SUBSURFACE CONDITIONS Below the surface pavements, our
exploratory borings (EB-1 and paired CPT-2) generally encountered
very stiff lean clay with varying amounts of sand to a depth of
approximately 17 feet. Beneath the lean clay, our boring
encountered medium dense silty sand to a depth of 20 feet underlain
by dense poorly graded sand with silt and gravel to a depth of
approximately 25½ feet. Beneath the sand, our boring encountered
medium stiff to stiff lean clay with sand to a depth of 30 feet
underlain by medium dense clayey sand to a depth of 34½ feet.
Beneath the medium dense clayey sand our boring encountered stiff
to very stiff lean clay with varying amounts of sand to a depth of
42½ feet underlain by dense to very dense clayey sand to the
maximum boring depth of 50 feet. Below the surface pavements, our
CPTs encountered primarily clays with varying amounts of sands and
silts with interbedded sand lenses to a depth of approximately 40
feet. Below a depth of approximately 40 feet our CPTs encountered
primarily dense to very dense sands with clay and gravel and
interbedded silt and clay lenses to the maximum depth explored of
approximately 67 feet, or practical refusal. 3.2.1
Plasticity/Expansion Potential We performed two Plasticity Index
(PI) tests on representative samples. Test results were used to
evaluate expansion potential of surficial soils, and near the
bottom of the proposed excavation. The results of the surficial PI
tests indicated a PI of 13 at a depth of 2½ feet and a PI of 23 at
a depth of 26 feet, indicating low to moderate expansion potential
to wetting and
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drying cycles. We anticipate the soils between 20 and 25 feet, and
30 and 35 feet will have low to moderate expansion potential. 3.2.2
In-Situ Moisture Contents Laboratory testing indicated that the
in-situ moisture contents within the upper 25 feet are over the
estimated laboratory moisture. The in-situ moisture content is
estimated to be approximately 5 percent over in the upper 5 feet,
approximately 8 percent over between 5 to 10 feet, approximately 8
percent over between 10 and 15 feet, approximately 6 percent over
between 15 and 20 feet, approximately 2 to 7 percent over between
20 and 25 feet, approximately 10 to 15 percent over between 25 and
30, and approximately 20 percent over between 30 and 35 feet. 3.3
GROUND WATER Ground water was encountered in some of our
explorations (Boring EB-1 and CPT-2) at depths of approximately 16
to 19 feet below current grades. All measurements were taken at the
time of drilling and may not represent the stabilized levels that
can be higher than the initial levels encountered. Based on our
previous explorations in the area and ground water data reported on
GeoTracker, we estimate stabilized levels of groundwater are
estimated to be on the order of 14 to 15 feet below existing grade.
We recommend a high ground water level of 12 feet be used for
design to account for potential fluctuation in ground water levels.
Fluctuations in ground water levels occur due to many factors
including seasonal fluctuation, underground drainage patterns,
regional fluctuations, and other factors. 3.4 CORROSION SCREENING
We tested two samples collected at depths of 1½ to 5½ feet for
resistivity, pH, soluble sulfates, and chlorides. The laboratory
test results are summarized in Table 2. Table 2: Summary of
Corrosion Test Results
Sample/Test Location Number
Sulfate (% dry wt)
EB-1 1½ 7.5 4,169 6 0.0053 EB-1 5½ 7.3 1,443 10 0.0023
Notes: (1) Laboratory resistivity measured at 100% saturation Many
factors can affect the corrosion potential of soil including
moisture content, resistivity, permeability, and pH, as well as
chloride and sulfate concentration. Typically, soil resistivity,
which is a measurement of how easily electrical current flows
through a medium (soil and/or water), is the most influential
factor. In addition to soil resistivity, chloride and sulfate ion
concentrations, and pH also contribute in affecting corrosion
potential.
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3.4.1 Preliminary Soil Corrosion Screening Based on the laboratory
test results summarized in Table 2, the soils are considered
moderately to severely corrosive to buried metallic improvements
(Palmer, 1989). Other corrosion parameters (pH and chloride
content) do not indicate a significant contribution to corrosion
potential to buried metallic structures. In accordance with the
2013 CBC, Chapter 19, Section 1904A.2, alternative cementitious
materials for sulfate exposure shall be in accordance with the
following:
ACI 318-11 - Table 4.2.1, and Table 4.3.1 Based on the laboratory
test results, no cement type restriction is required, although, in
our opinion, it is generally a good idea to include some sulfate
resistance and to maintain a relatively low water-cement ratio. We
have summarized applicable design values and parameters from ACI
318, Table 4.3.1 below in Table 3 for your information. We
recommend the structural engineer and a corrosion engineer be
retained to confirm the information provided and for additional
recommendations, as required. Table 3: Sulfate Soil Corrosion
Design Values and Parameters (1)
Category
Cementitious Materials (2)
S, Sulfate < 0.10 S0 not applicable no type restriction Notes:
(1) above values and parameters are from on ACI 318-08, Table 4.2.1
and Table 4.3.1
(2) cementitious materials are in accordance with ASTM C150, ASTM
C595 and ASTM C1157 SECTION 4: GEOLOGIC HAZARDS 4.1 FAULT RUPTURE
As discussed above several significant faults are located within 25
kilometers of the site. The site is not located within a
State-designated Alquist Priolo Earthquake Fault Zone. As shown in
Figure 3, no known surface expression of fault traces is thought to
cross the site; therefore, fault rupture hazard is not a
significant geologic hazard at the site. 4.2 ESTIMATED GROUND
SHAKING Moderate to severe (design-level) earthquakes can cause
strong ground shaking, which is the case for most sites within the
Bay Area. A peak ground acceleration (PGA)M was estimated for
analysis using a value equal to FPGA x PGA, as allowed in the 2013
and soon to be adopted 2016 editions of the California Building
Code. For our liquefaction analysis we used a PGAM of 0.763g.
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4.3 LIQUEFACTION POTENTIAL The site is not currently mapped by the
State of California, but is within a zone mapped as having a
moderate to very high liquefaction potential by the Association of
Bay Area Governments (ABAG, 2011). Our field and laboratory
programs addressed this issue by sampling potentially liquefiable
layers to depths of at least 50 feet, performing visual
classification on sampled materials, evaluating CPT correlations,
and performing various tests to further classify the soil
properties. 4.3.1 Background During strong seismic shaking,
cyclically induced stresses can cause increased pore pressures
within the soil matrix that can result in liquefaction triggering,
soil softening due to shear stress loss, potentially significant
ground deformation due to settlement within sandy liquefiable
layers as pore pressures dissipate, and/or flow failures in sloping
ground or where open faces are present (lateral spreading) (NCEER
1998). Limited field and laboratory data is available regarding
ground deformation due to settlement; however, in clean sand layers
settlement on the order of 2 to 3 percent of the liquefied layer
thickness can occur. Soils most susceptible to liquefaction are
loose, non-cohesive soils that are saturated and are bedded with
poor drainage, such as sand and silt layers bedded with a cohesive
cap. 4.3.2 Analysis As discussed in the “Subsurface” section above,
several sand layers were encountered below the design ground water
depth of 12 feet. Following the procedures in the 2008 monograph,
Soil Liquefaction During Earthquakes (Idriss and Boulanger, 2008)
and in accordance with CDMG Special Publication 117A guidelines
(CDMG, 2008) for quantitative analysis, these layers were analyzed
for liquefaction triggering and potential post-liquefaction
settlement. These methods compare the ratio of the estimated cyclic
shaking (Cyclic Stress Ratio - CSR) to the soil’s estimated
resistance to cyclic shaking (Cyclic Resistance Ratio - CRR),
providing a factor of safety against liquefaction triggering.
Factors of safety less than or equal to 1.3 are considered to be
potentially liquefiable and capable of post-liquefaction
re-consolidation. The CSR for each layer quantifies the stresses
anticipated to be generated due to a design- level seismic event,
is based on the peak horizontal acceleration generated at the
ground surface discussed in the “Estimated Ground Shaking” section
above, and is corrected for overburden and stress reduction factors
as discussed in the procedure developed by Seed and Idriss (1971)
and updated in the 2008 Idriss and Boulanger monograph. The soil’s
CRR is estimated from the in-situ measurements from CPT’s and
laboratory testing on samples retrieved from our borings. SPT “N”
values obtained from hollow-stem auger borings were not used in our
analyses, as the “N” values obtained are unreliable in sands below
ground water. The tip pressures are corrected for effective
overburden stresses, taking into consideration both the ground
water level at the time of exploration and the design ground water
level, and stress reduction versus depth factors. The CPT method
utilizes the soil behavior type index (IC) to estimate the
plasticity of the layers.
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In estimating post-liquefaction settlement at the site, we have
implemented a depth weighting factor proposed by Cetin (2009).
Following evaluation of 49 high-quality, cyclically induced, ground
settlement case histories from seven different earthquakes, Cetin
proposed the use of a weighting factor based on the depth of
layers. The weighting procedure was used to tune the surface
observations at liquefaction sites to produce a better model fit
with measured data. Aside from the better model fit it produced,
the rationale behind the use of a depth weighting factor is based
on the following: 1) upward seepage, triggering void ratio
redistribution, and resulting in unfavorably higher void ratios for
the shallower sublayers of soil layers; 2) reduced induced shear
stresses and number of shear stress cycles transmitted to deeper
soil layers due to initial liquefaction of surficial layers; and 3)
possible arching effects due to nonliquefied soil layers. All these
may significantly reduce the contribution of volumetric settlement
of deeper soil layers to the overall ground surface settlement
(Cetin, 2009). The results of our CPT analyses (CPT-1, CPT-2, and
CPT-3) are presented on Figures 4A to 4C of this report. 4.3.3
Summary Our analyses indicate that several layers could potentially
experience liquefaction triggering that could result in soil
softening and post-liquefaction total settlement ranging from
approximately ½ to 1½ inches based on the Ishihara and Yoshimine
(1990) method. Total post-liquefaction settlement will be reduced
to approximately ½ to 1 inch for the upper 20 feet removed for a
two- level below-grade basement. Total post-liquefaction settlement
will be reduced to approximately ¼ to 1 inch for the upper 30 feet
removed for a three-level below-grade basement. As discussed in SP
117A, differential movement for level ground sites over deep soil
sites will be up to about two-thirds of the total settlement
between independent foundation elements. In our opinion,
differential settlements are anticipated to be on the order of 1
inch or less for at-grade, and -inch or less for two- to
three-levels below-grade over a horizontal distance of 30 feet.
4.3.4 Ground Rupture Potential The methods used to estimate
liquefaction settlements assume that there is a sufficient cap of
non-liquefiable material to prevent ground rupture or sand boils.
For ground rupture to occur, the pore water pressure within the
liquefiable soil layer will need to be great enough to break
through the overlying non-liquefiable layer, which could cause
significant ground deformation and settlement. The work of Youd and
Garris (1995) indicates that the 12-foot thick layer of
non-liquefiable cap for at-grade construction is sufficient to
prevent ground rupture. For below- grade excavations, the
non-liquefiable caps over the thin potentially liquefiable layers
below the basement foundations are sufficient to prevent ground
rupture; therefore the above total settlement estimates are
reasonable. 4.4 LATERAL SPREADING Lateral spreading is
horizontal/lateral ground movement of relatively flat-lying soil
deposits towards a free face such as an excavation, channel, or
open body of water; typically lateral
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spreading is associated with liquefaction of one or more subsurface
layers near the bottom of the exposed slope. As failure tends to
propagate as block failures, it is difficult to analyze and
estimate where the first tension crack will form. There are no open
faces within a distance considered susceptible to lateral
spreading; therefore, in our opinion, the potential for lateral
spreading to affect the site is low. 4.5 SEISMIC
SETTLEMENT/UNSATURATED SAND SHAKING Loose unsaturated sandy soils
can settle during strong seismic shaking. Because the soils
encountered at the site above the expected minimum basement cuts of
20 feet were predominantly stiff to very stiff clays and medium
dense to dense sands, in our opinion, the potential for significant
differential seismic settlement affecting the proposed improvements
is low. 4.6 TSUNAMI/SEICHE The terms tsunami or seiche are
described as ocean waves or similar waves usually created by
undersea fault movement or by a coastal or submerged landslide.
Tsunamis may be generated at great distance from shore (far field
events) or nearby (near field events). Waves are formed, as the
displaced water moves to regain equilibrium, and radiates across
the open water, similar to ripples from a rock being thrown into a
pond. When the waveform reaches the coastline, it quickly raises
the water level, with water velocities as high as 15 to 20 knots.
The water mass, as well as vessels, vehicles, or other objects in
its path create tremendous forces as they impact coastal
structures. Tsunamis have affected the coastline along the Pacific
Northwest during historic times. The Fort Point tide gauge in San
Francisco recorded approximately 21 tsunamis between 1854 and 1964.
The 1964 Alaska earthquake generated a recorded wave height of 7.4
feet and drowned eleven people in Crescent City, California. For
the case of a far-field event, the Bay area would have hours of
warning; for a near field event, there may be only a few minutes of
warning, if any. A tsunami or seiche originating in the Pacific
Ocean would lose much of its energy passing through San Francisco
Bay. Based on the study of tsunami inundation potential for the San
Francisco Bay Area (Ritter and Dupre, 1972), areas most likely to
be inundated are marshlands, tidal flats, and former bay margin
lands that are now artificially filled, but are still at or below
sea level, and are generally within 1½ miles of the shoreline. The
site is approximately 1 mile inland from the San Francisco Bay
shoreline, and is approximately 26 to 29 feet above mean sea level.
Therefore, the potential for inundation due to tsunami or seiche is
considered low. 4.7 FLOODING Based on our internet search of the
Federal Emergency Management Agency (FEMA) flood map public
database, the site is located within Zone X, an area determined to
be outside the 2%
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annual chance flood. We recommend the project civil engineer be
retained to confirm this information and verify the base flood
elevation, if appropriate. SECTION 5: CONCLUSIONS 5.1 SUMMARY From
a geotechnical viewpoint, the project is feasible provided the
concerns listed below are addressed in the project design and
construction. The preliminary recommendations that follow are
intended for conceptual planning and preliminary design. A
design-level geotechnical investigation should be performed once
site development plans are prepared indicating the depth and
location of proposed structures. The design-level investigation
findings will be used to confirm the preliminary recommendations
and develop detailed geotechnical recommendations for design and
construction. Descriptions of each geotechnical concern with brief
outlines of our preliminary recommendations follow the listed
concerns. Potential settlement of compressible clays below 25½ feet
Potential for liquefaction-induced settlements Ground improvement
to reduce settlement below foundations Shallow ground water,
excavation, and construction below the ground water table
Hydro-static uplift pressures for buildings with basements below
the ground water table Differential movement at on-grade to
on-structure transitions
5.1.1 Potential Settlement of Compressible Clays below 25½ Feet As
discussed in the “Subsurface” section, moderately compressible
clays were encountered at a depth of approximately 25½ to 30 feet
in our boring EB-1 and correlated with CPT-2. Depending on the
foundation loads and bottom of mat foundation, their presence may
require ground improvement to reduce total and differential
settlement. For planning purposes, due to the presence of the high
ground water table, potentially compressible clays, and anticipated
static and seismic settlement, support of the structure on a mat
foundation is recommended on a preliminary basis provided the
estimated settlement is tolerable from a structural standpoint.
Ground improvement beneath the mat foundation may be needed to
mitigate potentially compressible soils and potentially liquefiable
soils as discussed below. Alternatively, support of the structure
on a deep foundation system consisting of auger cast piles or
drilled piers or micro piles could be considered as an alternative
to a mat foundation system to control settlement and uplift.
Preliminary foundation recommendations are presented in the
“Foundations” section of the report. 5.1.2 Potential For
Liquefaction-Induced Settlements As discussed, our liquefaction
analysis indicates that there is a potential for liquefaction of
localized sand layers during a significant seismic event. Although
the potential for liquefied sands to vent to the ground surface
through cracks in the surficial soils is low, our analysis
indicates that liquefaction-induced settlement at the ground
surface on the order of ½ to 1½ inch
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could occur, resulting in differential settlement up to 1-inch.
Total post-liquefaction settlement will be reduced to approximately
½ to 1 inch for the upper 20 feet removed for a two-level
below-grade basement and total post-liquefaction settlement will be
reduced to approximately ¼ to 1 inch for the upper 30 feet removed
for a three-level below-grade basement, resulting in differential
settlement on the order of -inch over a horizontal distance of 30
feet. Foundations should be designed to tolerate the anticipated
total and differential settlements. Preliminary foundation
recommendations are presented in the “Foundations” section. 5.1.3
Ground Improvement to Reduce Settlement Below Foundations Based on
the subsurface soil conditions encountered at the site and if the
mat foundation cannot be designed tolerate the anticipated total
and differential settlement, ground improvement may be used to
reduce the estimated total and differential settlement due to
static loading and liquefaction. The intent of the ground
improvement design would be to increase the density of the loose to
medium dense sands and reduce the compressibility of the clay soil
by laterally displacing and/or densifying the existing in-place
soils and reinforcing the clays. The degree to which the density is
increased will depend on the improvement method and spacing. In
addition to increasing the density, the methods listed above, could
provide an additional increase in bearing capacity and soil
stiffness at the individual improvement locations, which could be
taken into consideration during evaluation of the post-construction
settlement. Further details and recommendations are provided in the
“Foundations” section below. 5.1.4 Shallow Ground Water,
Excavation, and Construction Below the Ground Water Table Shallow
ground water was measured at depths ranging from approximately 16
to 19 feet below the existing ground surface and high ground water
in the area is estimated to be on the order of 12 feet. Our
experience with similar sites in the vicinity indicates that
shallow ground water could significantly impact grading and
underground construction. Obviously, constructing a two- to
three-level below grade basement needs to be designed to withstand
hydrostatic pressure. In our experience, supporting the below-grade
structure on a mat foundation designed to resist uplift hydrostatic
pressures, static and seismic settlements appears to be feasible
for the subsurface conditions encountered at the site provided the
estimated settlement can be tolerated from a structural viewpoint.
Depending on the weight of the structure providing resistance to
uplift, drilled and post grouted anchors may need to be
incorporated into the foundation to resist uplift. Further
discussion of this issues is presented in the “Foundations” section
of this report. Dewatering and shoring of the basement excavation
will be required at the site during construction and should be
anticipated. Carefully planned and implemented temporary dewatering
should be anticipated for the construction of this project.
Typically, permanent dewatering of the below grade basement is not
desired due to potential construction complications such as
settlement of adjacent structures and long term maintenance and
costs of the site.
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As the planned basement excavation will extend below the current
ground water level, we anticipate the need for stabilization of the
excavation bottom where construction activities are planned.
Further details are provided in the “Anticipated Earthworks”
section of this report. Based on the site conditions encountered
during our investigation, the cuts may be supported by shoring with
tie-backs, braced excavations, or potentially other methods such as
soil mixed columns or cut-off walls with H-beams or auger assisted
installed sheet piles. Because of the groundwater table depth,
shoring combined with temporary dewatering may be needed to control
the water inflow for some shoring systems. And some shoring methods
such as the use of wooden lagging may be problematic for
installation because of the water seepage and potential flowing
sands and may not be feasible below the water table. Where
excavations will extend more than about 10 feet, restrained shoring
will most likely be required to limit detrimental lateral
deflections and settlement behind the shoring. In addition to soil
earth pressures, the shoring system will need to support adjacent
loads such as construction vehicles and incidental loading,
existing structure foundation loads, and street loading.
Underpinning of the adjacent structures may be need depending on
the proximity of the excavation to the property line. We recommend
that the contractor implement a monitoring program to monitor the
effects of the construction on nearby improvements, including the
monitoring of cracking and vertical movement of adjacent
structures, nearby streets, sidewalks, parking and other
improvements. In critical areas, we recommend that inclinometers or
other instrumentation be installed as part of the shoring system to
closely monitor lateral movement. A discussion of the general
shoring issues are provided in the “Earthwork” section of this
report. 5.1.5 Hydro-static Uplift Pressures and Water Proofing for
Buildings with Basements Below the Ground Water Table As previously
discussed, it is our opinion that ground water could be encountered
during construction at depths ranging from approximately 14 to 15
feet below current grades. However, for design purposes including
hydrostatic uplift and waterproofing, we recommend a design depth
to ground water to be 12 feet based on available data. Where
portions of the mat foundation and related basement structures
extend below the design ground water level, including bottoms of
mat foundations, they should be water proofed and designed to
resist potential hydrostatic uplift pressures. Further
recommendations are provided in the “Hydrostatic Uplift and
Waterproofing” section below. 5.1.6 Differential Movement At
On-grade to On-Structure Transitions Proposed improvements,
including the driveway into the below-grade basement levels, will
transition from on-grade support to on-structure support. On-grade
to on-structure transition areas may experience increased
differential movement due to a variety of causes, including
difficulty in achieving compaction of retaining wall backfill
closest to the wall. We recommend consideration be given to where
engineered fill is placed behind retaining walls extending to near
finished grade, and that subslabs be included beneath flatwork or
pavers that can
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cantilever at least 3 feet beyond the wall. If surface improvements
are included that are highly sensitive to differential movement,
additional measures may be necessary. 5.2 DESIGN-LEVEL GEOTECHNICAL
INVESTIGATION The preliminary recommendations contained in this
feasibility study were based on limited site development
information and limited exploration. As site conditions may vary
significantly between the small-diameter boring and CPT’s performed
during this investigation, and more detailed design information
will become available as development plans are finalized, we also
recommend that we be retained to 1) perform a design-level
geotechnical investigation, once detailed site development plans
are available; 2) to review the geotechnical aspects of the project
structural, civil, and landscape plans and specifications, allowing
sufficient time to provide the design team with any comments prior
to issuing the plans for construction; and 3) be retained to
provide geotechnical observation and testing during earthwork and
foundation construction. SECTION 6: EARTHWORK 6.1 ANTICIPATED
EARTHWORK MEASURES On a preliminary basis, we recommend that any
existing foundations, slabs and/or abandoned underground utilities
be removed entirely and the resulting excavations backfilled with
engineered fill. As a two- to three-level below grade parking is
currently planned for the site, we expect all undocumented fills
will be removed during basement excavation and geotechnically they
will not be an issue with this project. On-site soils below the
paved surface layer appear to be suitable for use as fill at the
site. As discussed in the “Subsurface” section in this report, the
in-situ moisture contents are up to about 8 percent over the
estimated laboratory optimum in the upper 25 feet of the soil
profile. Additionally, the in-situ moisture contents increase to
about 10 to 15 percent over the estimated laboratory optimum
between 25 and 30 feet and up to about 20 percent over the
estimated laboratory optimum between 30 and 25 feet. The contractor
should anticipate drying the soils prior to reusing them as fill,
and this includes the material from the basement excavation. In
addition, repetitive construction loading may de-stabilize the
soils which is why subgrade stabilization at the bottom of the
basement excavation is recommended. Imported fill material for use
as general fill should be predominantly granular with a Plasticity
Index of 15 or less. All fill as well as scarified soils in those
areas to receive fill or slabs-on- grade should be compacted to at
least 90 percent relative compaction as determined by ASTM Test
Designation D-1557, latest edition; and be at least 2 percent above
optimum. Areas of fill placed behind basement or retaining walls
where surface improvements are planned and/or where improvements
will transition from on-grade support to overlying the basements
should be compacted to 95 percent. The upper 6 inches of subgrade
in pavement areas and all aggregate base materials should be
compacted to at least 95 percent relative compaction (ASTM D-1557,
latest edition). Utility trench backfill should be compacted to at
least 95 percent relative compaction (ASTM D-1557, latest edition)
by mechanical means only.
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As the planned basement excavation will extend below the current
ground water level, we recommend that the contractor plan for
stabilization of the excavation bottom to provide a working
platform upon which to construct the foundation. This may include
excavating an additional 12 to 18 inches below subgrade, placing a
layer of stabilization fabric (Mirafi 500X or approved equivalent)
at the bottom, and backfilling with clean, crushed rock. The
crushed rock should be consolidated in place with vibratory
equipment. Rubber tired and heavy track equipment should not be
allowed to operate on the exposed subgrade; the crushed rock should
be stockpiled and pushed out over the stabilization fabric. Because
of the water table, we anticipated that chemically treating the
bottom with lime treatment may not be feasible due to the concern
of additional water inflow during the time frame needed for the
mixing, curing and compaction. We anticipate a significant amount
of soil off haul will be required for this project. Off haul soils
are anticipated to consist of primarily clays with interbedded sand
layers that are above optimum moisture content. Analytical testing
of soils will be required prior to off haul of soil. This should be
anticipated in the budgeting for this project. Some poorly graded
sands were encountered in our exploratory boring EB-1 around 20 to
25½ feet below the surface. Depending on the final depth of
basement excavation, the sands may slough during excavation,
trenching, and shoring. Deeper utility excavation trenches and
basement excavations will need to be shored and/or dewatered.
Surface water runoff should not be allowed to pond adjacent to
building foundations, slabs-on- grade, or pavements. Hardscape
surfaces adjacent to structures should slope at least 2 percent
towards suitable discharge facilities; landscape areas adjacent to
structures should slope at least 3 percent away from buildings. 6.2
BELOW-GRADE EXCAVATIONS Below-grade excavations on the order of 8
to 10 feet deep, if considered, may be constructed with temporary
slopes in accordance with OSHA requirements if space allows.
Alternatively, temporary shoring may support the planned cuts up to
35 feet. The choice of shoring method should be left to the
contractor’s judgment based on experience, economic considerations
and adjacent improvements such as utilities, pavements, and
foundation loads. Temporary shoring should support adjacent
improvements without distress and should be the contractor’s
responsibility. A pre-condition survey including photographs and
installation of monitoring points for existing site improvements
should be included in the contractor’s preliminary cost estimate.
6.2.1 Temporary Shoring Based on the site conditions encountered
during our investigation, the cuts may be supported by shoring with
tie-backs, braced excavations, or potentially other methods such as
soil mixed columns or cut-off walls with H-beams or auger assisted
installed sheet piles. Because of the groundwater table depth,
shoring combined with temporary dewatering may be needed to control
the water inflow for some shoring systems. And some shoring methods
such as the use
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of wooden lagging may be problematic for installation because of
the water seepage and potential flowing sands and may not be
feasible below the water table. Where excavations will extend more
than about 10 feet, restrained shoring will most likely be required
to limit detrimental lateral deflections and settlement behind the
shoring. It is noted that the use of tie- backs will need to be
coordinated with avoiding the existing culvert in the city right of
way and the feasibility of installation of tie-backs outside of the
property needs to be verified. In addition to soil earth pressures,
the shoring system will need to support adjacent loads such as
construction vehicles and incidental loading, existing structure
foundation loads, and street loading. Underpinning of the adjacent
buildings may be needed as part of the shoring plan for the project
depending on the proximity of the basement excavation to the
property line. We performed our borings with hollow-stem auger
drilling equipment and as such were not able to evaluate the
potential for caving soils, which can create difficult conditions
during soldier beam, tie-back, or soil nail installation; caving
soils can also be problematic during excavation and lagging
placement. The contractor is responsible for evaluating excavation
difficulties prior to construction. Where relatively clean sands
were encountered during our exploration, pilot holes performed by
the contractor may be desired to further evaluate these conditions
prior to the finalization of the shoring budget. As previously
mentioned, we recommend that a monitoring program be developed and
implemented to evaluate the effects of the shoring on adjacent
improvements. All sensitive improvements should be located and
monitored for horizontal and vertical deflections and distress
cracking based on a pre-construction survey. The above
considerations are for the use of the design team during conceptual
planning and preliminary design. Additional subsurface exploration
and engineering analysis should be performed during the
design-level geotechnical investigation to develop shoring design
parameters, as needed. A California-licensed civil or structural
engineer must design and be in responsible charge of the temporary
shoring design. SECTION 7: FOUNDATIONS 7.1 SUMMARY OF
RECOMMENDATIONS In our opinion and on a preliminary basis, due to
proximity of ground water to the basement bottom, the proposed
structures may be supported on a mat foundation or a mat foundation
overlying ground improvement provided the estimated settlement is
tolerable from a structural viewpoint. In our opinion, spread
footings are not feasible because they are estimated to settle more
than the structure can tolerate and the basement will be subject to
hydrostatic uplift pressures. Hold-Down anchors may be necessary to
resist hydrostatic uplift pressures. Deep foundations are also
feasible but are judged to be more costly so we have not included
recommendations in this report for them. This assumption should be
verified by the construction estimators assisting the owner in
providing preliminary costs for the project. Preliminary
recommendations for mat foundations are discussed in the following
sections.
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7.2 SEISMIC DESIGN CRITERIA The project structural design should be
based on the 2016 California Building Code (CBC), which provides
criteria for the seismic design of buildings in Chapter 16. The
“Seismic Coefficients” used to design buildings are established
based on a series of tables and figures addressing different site
factors, including the soil profile in the upper 100 feet below
grade and mapped spectral acceleration parameters based on distance
to the controlling seismic source/fault system. Shear wave velocity
measurements performed at CPT-1 and CPT-3 to depths of
approximately 52½ and 64 feet, respectively, resulted in an average
shear wave velocity of 1024 and 899 feet per second (or 312 and 274
meters per second), respectively. Therefore, we have classified the
site as Soil Classification D. The mapped spectral acceleration
parameters SS and S1 were calculated using the USGS computer
program Design Maps, located at
http://geohazards.usgs.gov/designmaps/us/application.php, based on
the site coordinates presented below and the site classification,
which appear to be valid for the 2016 CBC. The table below lists
the various factors used to determine the seismic coefficients and
other parameters. Table 4: CBC Site Categorization and Site
Coefficients Classification/Coefficient Design Value Site Class D
Site Latitude 37.56785° Site Longitude -122.32524° 0.2-second
Period Mapped Spectral Acceleration1, SS 1.941g 1-second Period
Mapped Spectral Acceleration1, S1 0.910g Short-Period Site
Coefficient – Fa 1.0 Long-Period Site Coefficient – Fv 1.5
0.2-second Period, Maximum Considered Earthquake Spectral Response
Acceleration Adjusted for Site Effects - SMS
1.941g
1.364g
0.2-second Period, Design Earthquake Spectral Response Acceleration
– SDS 1.294g 1-second Period, Design Earthquake Spectral Response
Acceleration – SD1 0.910g
1For Site Class B, 5 percent damped. 7.3 SHALLOW FOUNDATIONS Based
on our preliminary investigation, soils between a depth of
approximately 20 and 32 feet vary significantly across the site
from compressible medium stiff clays to liquefiable sands. On a
preliminary basis, and until a design level investigation can be
performed, we have provided two mat foundation options we believe
may be feasible for planning and design purposes.
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7.3.1 Mat Foundations On a preliminary basis, the proposed
residential and commercial structure can potentially be supported
on a conventionally reinforced concrete mat foundation. Mats should
be designed in accordance with the 2016 California Building Code.
As discussed, building loads are not yet known; however, we assumed
average preliminary mat contact pressures ranging from 650 to 850
pounds per square foot for a 3 to 5 story building with two levels
of below grade parking, and 800 to 1,000 pounds per square foot for
a 3 to 5 story building with three levels of below-grade parking.
To reduce potential differential movement, on a preliminary basis,
mats can likely be designed for a maximum average aerial bearing
pressure on the order of 1,000 to 1,200 pounds per square foot
(psf) for dead plus live loads; at column or wall loading, the
maximum localized allowable bearing pressure should be limited to
about 2,000 psf. When evaluating wind and seismic conditions,
allowable bearing pressures may be increased by one-third. On a
preliminary basis, we estimate that the total static mat foundation
post-construction settlement will be on the order of ¾-inch for a
below grade mat at a depth of 25 feet below current site grades and
on the order of -inch for a below grade mat at a depth of 35 feet.
In addition, estimated seismic differential settlement will be on
the order of -inch or less between across a horizontal distance of
30 feet for both a two- and three-level below grade basement.
Therefore, the mat would need to be designed to tolerate total
differential settlement up to 1¼ inches between center and edge of
mat for a two-level below grade basement and approximately 1-inch
of total differential settlement between center and edge of mat for
a three- level below grade basement. Detailed settlement analysis
should be performed during the design-level geotechnical
investigation to confirm these settlement estimates. Alternatively,
recommendations for deep foundations can be provided in the
design-level investigation if the estimated settlement is not
tolerable from a structural viewpoint for mat foundations. We have
not provided recommendations for PT mats because we anticipate that
there will not be enough room for post tensioning in the basement
excavation. 7.3.2 Mat Foundation Construction Considerations The
mat foundation will be constructed near or below the current ground
water level and even if temporary dewatering is included, the soil
above the water table will be at near saturated conditions.
Subgrade stabilization may be required as discussed in the
“Earthwork” section above. 7.3.3 Mat Foundations – With Ground
Improvement Based on the subsurface soil conditions encountered at
the site and if the mat foundation cannot be designed tolerate the
anticipated differential settlement, ground improvement may be used
to reduce the estimated differential settlement due to static and
seismic loading to less than 1 inch. The design allowable bearing
pressures will be dependent on the final ground
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improvement details including the stiffness and spacing of ground
improvement elements; however, substantial improvement in bearing
capacity would be expected. As discussed above, a mat foundation
may be used in combination with ground improvement to reduce the
total and differential settlements (seismic and static). Ground
improvement, such as impact piers, stone columns, drilled
displacement columns, or other similar methods, could be used to
improve the subsurface soils such that the combined static and
seismic differential settlements are reduced to less than 1 inches,
enabling the structure to be supported on a mat foundation, in our
opinion. 7.4 HYDROSTATIC UPLIFT AND WATERPROOFING As previously
discussed, it is our opinion that ground water could be encountered
during construction at depths ranging from approximately 14 to 15
feet below current grades. However, for design purposes including
hydrostatic uplift and waterproofing, we recommend a design depth
to ground water to be 12 feet based on available data. Where
portions of the mat foundation and related basement structures
extend below the design ground water level, including bottoms of
mat foundations, they should be designed to resist potential
hydrostatic uplift pressures by constructing a thicker concrete mat
foundation, installing ground anchors, or other methods. Basement
walls and the bottom of the mat foundation extending below design
ground water should be waterproofed and designed to resist
hydrostatic pressure for the full wall height below the design
ground water depth. Where portions of the basement walls extend
above the design ground water level, a drainage system may be
added. It may be necessary to construct a “Rat Slab” on the
stabilized bottom of the basement excavation as part of the water
proofing system. In addition, the portions of the structures
extending below design ground water should be waterproofed to limit
moisture infiltration, including mat foundation/thickened slab
areas, all construction joints, and any retaining walls. We
recommend that a waterproofing specialist design the waterproofing
system. SECTION 10: LIMITATIONS This report, an instrument of
professional service, has been prepared for the sole use of
Prometheus Real Estate Group specifically to support the design of
the 303 Baldwin Avenue Retail, Office, and Residential Building
Prelim project in San Mateo, California. The opinions, conclusions,
and preliminary recommendations presented in this report have been
formulated in accordance with accepted geotechnical engineering
practices that exist in Northern California at the time this report
was prepared. No warranty, expressed or implied, is made or should
be inferred. Preliminary recommendations in this report are based
upon the soil and ground water conditions encountered during our
limited subsurface exploration. Preparation of a design-level
investigation is anticipated to provide additional information and
refine the preliminary recommendations presented herein. If
variations or unsuitable conditions are encountered
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during the construction phase, Cornerstone must be contacted to
provide supplemental recommendations, as needed. Prometheus Real
Estate Group may have provided Cornerstone with plans, reports and
other documents prepared by others. Prometheus Real Estate Group
understands that Cornerstone reviewed and relied on the information
presented in these documents and cannot be responsible for their
accuracy. Cornerstone prepared this report with the understanding
that it is the responsibility of the owner or his representatives
to see that the recommendations contained in this report are
presented to other members of the design team and incorporated into
the project plans and specifications, and that appropriate actions
are taken to implement the geotechnical recommendations during
construction. Conclusions and recommendations presented in this
report are valid as of the present time for the development as
currently planned. Changes in the condition of the property or
adjacent properties may occur with the passage of time, whether by
natural processes or the acts of other persons. In addition,
changes in applicable or appropriate standards may occur through
legislation or the broadening of knowledge. Therefore, the
conclusions and recommendations presented in this report may be
invalidated, wholly or in part, by changes beyond Cornerstone’s
control. This report should be reviewed by Cornerstone after a
period of three (3) years has elapsed from the date of this report.
In addition, if the current project design is changed, then
Cornerstone must review the proposed changes and provide
supplemental recommendations, as needed. An electronic transmission
of this report may also have been issued. While Cornerstone has
taken precautions to produce a complete and secure electronic
transmission, please check the electronic transmission against the
hard copy version for conformity. Recommendations provided in this
report are based on the assumption that Cornerstone will be
retained to provide observation and testing services during
construction to confirm that conditions are similar to that assumed
for design, and to form an opinion as to whether the work has been
performed in accordance with the project plans and specifications.
If we are not retained for these services, Cornerstone cannot
assume any responsibility for any potential claims that may arise
during or after construction as a result of misuse or
misinterpretation of Cornerstone’s report by others. Furthermore,
Cornerstone will cease to be the Geotechnical- Engineer-of-Record
if we are not retained for these services. SECTION 11: REFERENCES
Aargard, B.T., Blair, J.L., Boatwright, J., Garcia, S.H., Harris,
R.A., Michael, A.J., Schwartz, D.P., and DiLeo, J.S., 2016,
Earthquake outlook for the San Francisco Bay Region 2014-2043 (ver.
1.1, August 2016): U.S. Geological Survey Fact Sheet 2016-3020, 6
p., http://dx.doe.org/10.3133/fs20163020.
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Association of Bay Area Governments (ABAG), 2011, Interactive
Liquefaction Hazard Map: http://quake.abag.ca/gov/liquefaction/
Boulanger, R.W. and Idriss, I.M., 2004, Evaluating the Potential
for Liquefaction or Cyclic Failure of Silts and Clays, Department
of Civil & Environmental Engineering, College of Engineering,
University of California at Davis. Brabb, E.E. and J.A. Olson,
1986, Map Showing Faults and Earthquake Epicenters in San Mateo
County, California: U.S. Geological Survey Map I-1257-F. California
Building Code, 2016, Structural Engineering Design Provisions, Vol.
2. California Department of Conservation Division of Mines and
Geology, 1998, Maps of Known Active Fault Near-Source Zones in
California and Adjacent Portions of Nevada, International
Conference of Building Officials, February, 1998. California
Division of Mines and Geology, 1974, Special Studies Zones, San
Mateo Quadrangle. California Division of Mines and Geology (2008),
“Guidelines for Evaluating and Mitigating Seismic Hazards in
California, Special Publication 117A, September. Cetin, K.O.,
Bilge, H.T., Wu, J., Kammerer, A.M., and Seed, R.B., Probablilistic
Model for the Assessment of Cyclically Induced Reconsolidation
(Volumetric) Settlements, ASCE Journal of Geotechnical and
Geoenvironmental Engineering, Vo. 135, No. 3, March 1, 2009.
Federal Emergency Management Administration (FEMA), 2015, FIRM City
of San Mateo, California, Community Panel #0603280154F. Idriss,
I.M., and Boulanger, R.W., 2008, Soil Liquefaction During
Earthquakes, Earthquake Engineering Research Institute, Oakland,
CA, 237 p. Ishihara, K., 1985, Stability of Natural Deposits During
Earthquakes: Proceedings Eleventh International Conference on Soil
Mechanics and Foundation Engineering, San Francisco. Ishihara, K.
and Yoshimine, M., 1992, Evaluation of Settlements in Sand Deposits
Following Liquefaction During Earthquakes, Soils and Foundations,
32 (1): 173-188. Lew, M. et al, 2010, Seismic Earth Pressures on
Deep Building Basements, Proceedings, SEAOC Convention, Indian
Wells, CA. Pampeyan, E.H., 1994, Geologic map of the Montara
Mountain and San Mateo 7.5-minute quadrangles, San Mateo County,
California: U.S. Geological Survey Miscellaneous Investigation
Series Map I-2390, scale 1:24,000. Portland Cement Association,
1984, Thickness Design for Concrete Highway and Street Pavements:
report.
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Schwartz, D.P. 1994, New Knowledge of Northern California
Earthquake Potential: in Proceedings of Seminar on New Developments
in Earthquake Ground Motion Estimation and Implications for
Engineering Design Practice, Applied Technology Council 35-1. Seed,
H.B. and I.M. Idriss, 1971, A Simplified Procedure for Evaluation
soil Liquefaction Potential: JSMFC, ASCE, Vol. 97, No. SM 9, pp.
1249 – 1274. Seed, H.B. and I.M. Idriss, 1982, Ground Motions and
Soil Liquefaction During Earthquakes: Earthquake Engineering
Research Institute. Seed, Raymond B., Cetin, K.O., Moss, R.E.S.,
Kammerer, Ann Marie, Wu, J., Pestana, J.M., Riemer, M.F., Sancio,
R.B., Bray, Jonathan D., Kayen, Robert E., and Faris, A., 2003,
Recent Advances in Soil Liquefaction Engineering: A Unified and
Consistent Framework., University of California, Earthquake
Engineering Research Center Report 2003-06. Southern California
Earthquake Center (SCEC), 1999, Recommended Procedures for
Implementation of DMG Special Publication 117, Guidelines for
Analyzing and Mitigating Liquefaction Hazards in California, March.
State of California Department of Transportation, 2015, Highway
Design Manual, December 31, 2015. Tokimatsu, K., and Seed, H.
Bolton, 1987, Evaluation of Settlements in Sands due to Earthquake
Shaking, ASCE Journal of Geotechnical Engineering, Vol. 113, August
1987, pp. 861-878. United States Geological Survey, 2014, U.S.
Seismic Design Maps, revision date June 23, available at
http://earthquake.usgs.gov/hazards/designmaps/usdesign.php. Working
Group on California Earthquake Probabilities, 2015, The Third
Uniform Earthquake Rupture Forecast, Version 3 (UCERF), U.S.
Geologic Survey Open File Report OF-2006-1037, scale 1:200000.
Youd, T.L. and C.T. Garris, 1995, Liquefaction-Induced
Ground-Surface Disruption: Journal of Geotechnical Engineering,
Vol. 121, No. 11, pp. 805 - 809. Youd, T.L. and Idriss, I.M., et
al, 1997, Proceedings of the NCEER Workshop on Evaluation of
Liquefaction Resistance of Soils: National Center for Earthquake
Engineering Research, Technical Report NCEER - 97-0022, January 5,
6, 1996. Youd et al., 2001, “Liquefaction Resistance of Soils:
Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on
Evaluation of Liquefaction Resistance of Soils,” ASCE Journal of
Geotechnical and Geoenvironmental Engineering, Vo. 127, No. 10,
October, 2001.
Project Number
Figure Number
San Mateo, CA November 2016
SITE
Approximate location of cone penetration test (CPT)
Legend
N
San Mateo, CA November 2016
Approximate
T ra
a u
lt M
a p
Base by California Geological Survey - 2010 Fault Activity Map of
California (Jennings and Bryant, 2010)
N
Project Title 12 FEET
Project No. 0.10 (Inches)
Project Manager 50 FEET
Earthquake Magnitude (Mw) 7.9
PGA (Amax) 0.763 (g)
LDI2 1.35 L/H 38.6
Ground Water Depth at Time of Drilling (feet) 16 LDI1 Corrected for
Distance 0.44 (4 < L/H < 40)
Design Water Depth (feet) 12
Ave. Unit Weight Above GW (pcf) 125 0.2 to 0.9 feet
Ave. Unit Weight Below GW (pcf) 120 1Not Valid for L/H Values <
4 and > 40. 2LDI Values Only Summed to 2H Below Grade.
EXPECTED RANGE OF DISPLACEMENT
SEISMIC PARAMETERS
San Andreas
De pt
h (fe
De pt
h (fe
De pt
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Project Title 12 FEET
Project No. 0.00 (Inches)
Project Manager 50 FEET
Earthquake Magnitude (Mw) 7.9
PGA (Amax) 0.763 (g)
LDI2 0.00 L/H 80.0
Ground Water Depth at Time of Drilling (feet) 16 LDI1 Corrected for
Distance 0.00 (4 < L/H < 40)
Design Water Depth (feet) 12
Ave. Unit Weight Above GW (pcf) 125 0.0 to 0.0 feet
Ave. Unit Weight Below GW (pcf) 120 1Not Valid for L/H Values <
4 and > 40. 2LDI Values Only Summed to 2H Below Grade.
4B
0
5
10
15
20
25
30
35
40
45
50
De pt
h (fe
De pt
h (fe
Project Title 12 FEET
Project No. 0.00 (Inches)
Project Manager 50 FEET
Earthquake Magnitude (Mw) 7.9
PGA (Amax) 0.763 (g)
LDI2 0.08 L/H 25.0
Ground Water Depth at Time of Drilling (feet) 16 LDI1 Corrected for
Distance 0.03 (4 < L/H < 40)
Design Water Depth (feet) 12
Ave. Unit Weight Above GW (pcf) 125 0.0 to 0.1 feet
Ave. Unit Weight Below GW (pcf) 120 1Not Valid for L/H Values <
4 and > 40. 2LDI Values Only Summed to 2H Below Grade.
EXPECTED RANGE OF DISPLACEMENT
SEISMIC PARAMETERS
San Andreas
De pt
h (fe
De pt
h (fe
PROPOSED 303 BALDWIN AVENUE RETAIL, OFFICE, AND RESIDENTIAL
BUILDING PRELIM 307-23-1
Page A-1
APPENDIX A: FIELD INVESTIGATION The field investigation consisted
of a surface reconnaissance and a subsurface exploration program
using truck-mounted, hollow-stem auger drilling equipment and
20-ton truck-mounted Cone Penetration Test equipment. One
8-inch-diameter exploratory borings were drilled on November 16,
2016 to a depth of approximately 50 feet. Three CPT soundings were
also performed in accordance with ASTM D 5778-95 (revised, 2002) on
November 14, 2016, to depths ranging from approximately 52½ to 67
feet. The approximate locations of exploratory borings and CPTs are
shown on the Site Plan, Figure 2. The soils encountered were
continuously logged in the field by our representative and
described in accordance with the Unified Soil Classification System
(ASTM D2488). Boring logs, as well as a key to the classification
of the soil and bedrock, are included as part of this appendix.
Boring and CPT locations were approximated using existing site
boundaries, and other site features as references. Boring and CPT
elevations were not determined. The locations of the borings and
CPTs should be considered accurate only to the degree implied by
the method used. Representative soil samples were obtained from the
borings at selected depths. All samples were returned to our
laboratory for evaluation and appropriate testing. The standard
penetration resistance blow counts were obtained by dropping a
140-pound hammer through a 30-inch free fall. The 2-inch O.D.
split-spoon sampler was driven 18 inches and the number of blows
was recorded for each 6 inches of penetration (ASTM D1586).
2.5-inch I.D. samples were obtained using a Modified California
Sampler driven into the soil with the 140-pound hammer previously
described. Relatively undisturbed samples were also obtained with
2.875-inch I.D. Shelby Tube sampler which were hydraulically
pushed. Unless otherwise indicated, the blows per foot recorded on
the boring log represent the accumulated number of blows required
to drive the last 12 inches. The various samplers are denoted at
the appropriate depth on the boring logs. The CPT involved
advancing an instrumented cone-tipped probe into the ground while
simultaneously recording the resistance at the cone tip (qc) and
along the friction sleeve (fs) at approximately 5-centimeter
intervals. Based on the tip resistance and tip to sleeve ratio
(Rf), the CPT classified the soil behavior type and estimated
engineering properties of the soil, such as equivalent Standard
Penetration Test (SPT) blow count, internal friction angle within
sand layers, and undrained shear strength in silts and clays. A
pressure transducer behind the tip of the CPT cone measured pore
water pressure (u2). Graphical logs of the CPT data is included as
part of this appendix. Field tests included an evaluation of the
unconfined compressive strength of the soil samples using a pocket
penetrometer device. The results of these tests are presented on
the individual boring logs at the appropriate sample depths.
Attached boring and CPT logs and related information depict
subsurface conditions at the locations indicated and on the date
designated on the logs. Subsurface conditions at other locations
may differ from conditions occurring at these boring and CPT
locations. The passage of time may result in altered subsurface
conditions due to environmental changes. In addition,
PROPOSED 303 BALDWIN AVENUE RETAIL, OFFICE, AND RESIDENTIAL
BUILDING PRELIM 307-23-1
Page A-2
any stratification lines on the logs represent the approximate
boundary between soil types and the transition may be
gradual.
Poorly-Graded Gravelly Sand
ON NO 4. SIEVE
*
NUMBER OF BLOWS OF 140 LB HAMMER FALLING 30 INCHES TO DRIVE A 2
INCH O.D.
(1-3/8 INCH I.D.) SPLIT-BARREL SAMPLER THE LAST 12 INCHES OF AN
18-INCH DRIVE
(ASTM-1586 STANDARD PENETRATION TEST).
>12% FINES
* UNDRAINED SHEAR STRENGTH IN KIPS/SQ. FT. AS DETERMINED BY
LABORATORY
TESTING OR APPROXIMATED BY THE STANDARD PENETRATION TEST,
POCKET
PENETROMETER, TORVANE, OR VISUAL OBSERVATION.
Well Graded Gravelly Sand
ON NO 4. SIEVE
SOIL GROUP NAMES & LEGEND
Figure Number A-1
PI PLOTS >"A" LINE
PI PLOTS <"A" LINE
0
10
20
30
40
50
60
70
80
ORGANIC
INORGANIC
ORGANIC
GRAVELS
-
-
-
-
-
-
-
-
-
R-VALUE
CA
CD
CN
CU
DS
PP
(3.0)
RV
SA
MC-1B
MC
MC-3B
MC
MC-5B
SPT
SPT-7
SPT-8
115
109
119
1328
33
27
35
22
36
47
36
1 inch asphalt concrete over 3 inches aggregate base Sandy Lean
Clay (CL) very stiff, moist, dark brown, fine to coarse sand, some
fine subangular to subrounded gravel, low plasticity Liquid Limit =
31, Plastic Limit = 18
Lean Clay with Sand (CL) very stiff, moist, brown, fine to medium
sand, moderate plasticity
Sandy Lean Clay (CL) very stiff, moist, brown, fine to medium sand,
low plasticity
Silty Sand (SM) medium dense, moist to wet, brown, fine to coarse
sand, some fine subangular to subrounded gravel
Poorly Graded Sand with Silt and Gravel (SP-SM) dense, wet, gray
brown, fine to coarse sand, fine to coarse subangular to subrounded
gravel
18
21
16
10
DRILLING CONTRACTOR Exploration Geoservices, Inc.
GROUND WATER LEVELS:
DATE STARTED 11/16/16 DATE COMPLETED 11/16/16 BORING DEPTH 50
ft.GROUND ELEVATION
LATITUDE LONGITUDE
UNCONFINED COMPRESSIONS Y
PROJECT NUMBER 307-23-1
BORING NUMBER EB-1 PAGE 1 OF 2
This log is a part of a report by Cornerstone Earth Group, and
should not be used as a stand-alone document. This description
applies only to the location of the exploration at the time of
drilling. Subsurface conditions may differ at other locations and
may change at this location with time. The description presented is
a simplification of actual conditions encountered. Transitions
between soil types may be gradual.
UNDRAINED SHEAR STRENGTH, ksf
50 6"
Lean Clay with Sand (CL) medium stiff to stiff, moist, brown, fine
sand, moderate plasticity Liquid Limit = 40, Plastic Limit =
17
increasing sand content Clayey Sand (SC) medium dense, moist,
brown, fine to coarse sand, some fine subrounded gravel
becomes wet
Sandy Lean Clay (CL) very stiff, moist, brown, fine to medium sand,
some fine subangular to subrounded gravel, low plasticity
Lean Clay with Sand (CL) stiff, moist, brown, fine to medium sand,
moderate plasticity
Clayey Sand (SC) dense to very dense, moist, brown, fine to coarse
sand, some fine subangular to subrounded gravel
Bottom of Boring at 50.0 feet.
28
consol
23
32
18
36
18
10
26
PROJECT NUMBER 307-23-1
BORING NUMBER EB-1 PAGE 2 OF 2
This log is a part of a report by Cornerstone Earth Group, and
should not be used as a stand-alone document. This description
applies only to the location of the exploration at the time of
drilling. Subsurface conditions may differ at other locations and
may change at this location with time. The description presented is
a simplification of actual conditions encountered. Transitions
between soil types may be gradual.
UNDRAINED SHEAR STRENGTH, ksf
UNCONSOLIDATED-UNDRAINED TRIAXIAL
Cornerstone Earth Group Project 303 Baldwin Avenue Building
Operator KK-RB Filename SDF(375).cpt Job Number P6803 Cone Number
DDG1333 GPS Hole Number CPT-01 Date and Time 11/14/2016 6:44:41 AM
Maximum Depth 52.66 ft EST GW Depth During Test 16.00 ft
Net Area Ratio .8
Cone Size 10cm squared Soil Behavior Referance*Soil behavior type
and SPT based on data from UBC-1983
0
10
20
30
40
50
60
70
FRICTION TSF 0 10
5 - clayey silt to silty clay
6 - sandy silt to clayey silt
7 - silty sand to sandy silt
8 - sand to silty sand
9 - sand
CPT DATA
D EP
TH (ft )
SO IL
IO R
TY PE
Cornerstone Earth Group Project 303 Baldwin Avenue Building
Operator KK-RB Filename SDF(376).cpt Job Number P6803 Cone Number
DDG1333 GPS Hole Number CPT-02 Date and Time 11/14/2016 7:57:20 AM
Maximum Depth 66.93 ft EST GW Depth During Test 16.00 ft
Net Area Ratio .8
Cone Size 10cm squared Soil Behavior Referance*Soil behavior type
and SPT based on data from UBC-1983
0
10
20
30
40
50
60
70
FRICTION TSF 0 10
5 - clayey silt to silty clay
6 - sandy silt to clayey silt
7 - silty sand to sandy silt
8 - sand to silty sand
9 - sand
CPT DATA
D EP
TH (ft )
SO IL
IO R
TY PE
Cornerstone Earth Group Project 303 Baldwin Avenue Building
Operator KK-RB Filename SDF(377).cpt Job Number P6803 Cone Number
DDG1333 GPS Hole Number CPT-03 Date and Time 11/14/2016 9:31:28 AM
Maximum Depth 64.30 ft EST GW Depth During Test 16.00 ft
Net Area Ratio .8
Cone Size 10cm squared Soil Behavior Referance*Soil behavior type
and SPT based on data from UBC-1983
0
10
20
30
40
50
60
70
FRICTION TSF 0 10
5 - clayey silt to silty clay
6 - sandy silt to clayey silt
7 - silty sand to sandy silt
8 - sand to silty sand
9 - sand
CPT DATA
D EP
TH (ft )
SO IL
PROPOSED 303 BALDWIN AVENUE RETAIL, OFFICE, AND RESIDENTIAL
BUILDING PRELIM 307-23-1
Page B-1
APPENDIX B: LABORATORY TEST PROGRAM The laboratory testing program
was performed to evaluate the physical and mechanical properties of
the soils retrieved from the site to aid in verifying soil
classification. Moisture Content: The natural water content was
determined (ASTM D2216) on 12 samples of the materials recovered
from the borings. These water contents are recorded on the boring
logs at the appropriate sample depths. Dry Densities: In place dry
density determinations (ASTM D2937) were performed on 7 samples to
measure the unit weight of the subsurface soils. Results of these
tests are shown on the boring logs at the appropriate sample
depths. Washed Sieve Analyses: The percent soil fraction passing
the No. 200 sieve (ASTM D1140) was determined on two samples of the
subsurface soils to aid in the classification of these soils.
Results of these tests are shown on the boring logs at the
appropriate sample depths. Plasticity Index: Two Plasticity Index
determinations (ASTM D4318) were performed on samples of the
subsurface soils to measure the range of water contents over which
this material exhibits plasticity. The Plasticity Index was used to
classify the soil in accordance with the Unified Soil
Classification System and to evaluate the soil expansion potential.
Results of these tests are shown on the boring logs at the
appropriate sample depths. Consolidation: One consolidation test
(ASTM D2435) was performed on a relatively undisturbed sample of
the subsurface clayey soils to assist in evaluating the
compressibility property of this soil. Results of the consolidation
test are presented graphically in this appendix. Soluble Sulfate:
Two soluble sulfate determination (California Test Method No.
417-Modified) was performed on a sample of the subsurface soil to
water soluble sulfate content. Results of this test are attached is
this appendix.
303 Baldwin Avenue Retail, Office, and Residential Buildings
San Mateo, CA
S y m
Sandy Lean Clay (CL)13EB-1 31 18182.0 —
Lean Clay with Sand (CL)23EB-1 40 172826.0 —
Job No.: Boring: Run By: MD Client: Sample: Reduced: PJ Project:
Depth, ft.: Checked: PJ/DC Soil Type: Date: 12/5/2016
Assumed Gs 2.75 Initial Final 30.8 22.5 89.3 106.1 0.922 0.618 92.0
100.0
Void Ratio: % Saturation:
0.0
5.0
10.0
15.0
20.0
25.0
St ra
CTL # Date: PJ Client: Project:
Remarks: Chloride pH Sulfide Moisture
As Rec. Min Sat. mg/kg mg/kg % Qualitative At Test Dry Wt. Dry Wt.
Dry Wt. EH (mv) At Test by Lead %
Boring Sample, No. Depth, ft. ASTM G57 Cal 643 ASTM G57 ASTM D4327
ASTM D4327 ASTM D4327 ASTM G51 ASTM G200 Temp °C Acetate Paper ASTM
D2216
EB-1 1A 1.5 - - 4,169 6 53 0.0053 7.5 - - - 16.4 Dark Olive Brown
Sandy CLAY w/ Gravel
EB-1 2A 5.5 - - 1,443 10 23 0.0023 7.3 - - - 26.6 Dark Olive Gray
Sandy CLAY
Corrosivity Tests Summary
Tested By:
Section 3: Site conditions
Section 4: Geologic Hazards
4.6 Tsunami/seiche
4.7 Flooding
5.1.1 Potential Settlement of Compressible Clays below 25½
Feet
5.1.2 Potential For Liquefaction-Induced Settlements
5.1.3 Ground Improvement to Reduce Settlement Below
Foundations
5.1.4 Shallow Ground Water, Excavation, and Construction Below the
Ground Water Table
5.1.5 Hydro-static Uplift Pressures and Water Proofing for
Buildings with Basements Below the Ground Water Table
5.1.6 Differential Movement At On-grade to On-Structure
Transitions
5.2 Design-Level Geotechnical Investigation
7.3 Shallow Foundations
7.3.1 Mat Foundations
7.3.3 Mat Foundations – With Ground Improvement
7.4 Hydrostatic Uplift and Waterproofing
Section 10: Limitations
Section 11: References
307-23-1 Figure 4A.pdf
PM Summary - FIGURE
307-23-1 Figure 4B.pdf
PM Summary - FIGURE
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