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Appendix D: Geotechnical Report


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Copyright 2011 By ENGEO Incorporated.This Document May Not Be Reproduced InWhole Or In Part By Any Means Whatsoever,Nor May It Be Quoted Or Excerpted WithoutThe Express Written Consent Of ENGEOIncorporated

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GEOTECHNICAL EXPLORATIONJUNIPERO SERRA BOULEVARDTRAFFIC CALMING IMPROVEMENTS

SANTA CLARA COUNTY,CALIFORNIA

Submitted to:

Ms. Olga RodriguezAECOM Transportation

2905 Stender Way, Suite 82Santa Clara, California 95054

Prepared by:ENGEO Incorporated

May 27, 2011Project No. 9232.000.000


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GEOTECHNICAL

ENVIRONMENTAL

WATER RESOURCES

CONSTRUCTION SERVICES

6399 San Ignacio Avenue, Suite 150 San Jose, CA 95119 (408) 574-4900 Fax (888) 279-2698www.engeo.com

Project No.

9232.000.000

May 27, 2011

Ms. Olga Rodriguez

AECOM Transportation

2905 Stender Way, Suite 82Santa Clara, CA 95054

Subject: Junipero Serra Boulevard Traffic Calming ImprovementsSanta Clara County, California

GEOTECHNICAL EXPLORATION

Dear Ms. Rodriguez:

With your authorization, we performed a geotechnical exploration for the proposed trafficcalming improvements along Junipero Serra Boulevard in Santa Clara County, California.

The accompanying report contains our findings, conclusions, and recommendations for the

proposed project. It is our opinion that the proposed project is feasible from a geotechnicalstandpoint provided the recommendations in this report are implemented.

We are pleased to be of service to you on this project and will continue to consult with you andyour design team as the project progresses.

Sincerely,

ENGEO Incorporated

Andrew H. Firmin, PE Julia A. Moriarty, GE

ahf/jam/rc


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AECOM Transportation 9232.000.000Junipero Serra Boulevard Traffic Calming Improvements May 27, 2011

TABLE OF CONTENTS

Letter of TransmittalPage

1.0 INTRODUCTION .........................................................................................1

1.1 PURPOSE AND SCOPE.......................................................................................1

1.2 SITE LOCATION AND DESCRIPTION ...........................................................2

1.3 PROPOSED IMPROVEMENTS .........................................................................3

2.0 GEOLOGY AND SEISMICITY ..................................................................3

2.1 SITE GEOLOGY...................................................................................................3

2.2 REGIONAL FAULTING AND SITE SEISMICITY.........................................4

3.0 FIELD EXPLORATION...............................................................................43.1 FIELD LOGGING.................................................................................................5

3.2 LABORATORY TESTING..................................................................................6

3.3 SURFACE CONDITIONS....................................................................................6

3.4 SUBSURFACE CONDITIONS............................................................................7

3.5 GROUNDWATER.................................................................................................8

4.0 DISCUSSION AND CONCLUSIONS.........................................................8

4.1 SEISMIC HAZARDS............................................................................................8

4.2 EXISTING PAVEMENT CONDITIONS ASSESSMENT................................9

4.3 DRAINAGE CHANNEL BANK STABILITY ...................................................9

4.4 COMPRESSIBLE SOILS.....................................................................................94.5 EXISTING FILL..................................................................................................10

4.6 EXPANSIVE SOILS............................................................................................10

4.7 GROUNDWATER...............................................................................................10

4.8 FLOODING..........................................................................................................10

4.9 2010 CALIFORNIA BUILDING CODE SEISMIC PARAMETERS............11

4.10 CORROSIVITY CONSIDERATIONS .............................................................11

4.11 CONCLUSIONS ..................................................................................................13

5.0 RECOMMENDATIONS.............................................................................13

5.1 MONITORING AND TESTING........................................................................13

5.2 EXISTING UTILITY CROSSINGS..................................................................145.3 EXCAVATABILITY...........................................................................................14

5.4 TEMPORARY EXCAVATIONS.......................................................................14

5.5 TEMPORARY DEWATERING........................................................................14

5.6 SOFT SUBGRADE CONDITIONS...................................................................15

5.7 ENGINEERED FILL SELECTION AND PLACEMENT..............................15

5.8 RETAINING WALLS.........................................................................................155.8.1 Retaining Wall Design Recommendations................................................16


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5.8.1.1 Spread Footing Foundation............................................................165.8.1.2 Drilled Pier Foundation..................................................................17

5.8.2 Caltrans Standard Retaining Walls ............................................................18

5.8.3 Construction Considerations......................................................................185.9 R-VALUE RESULTS FOR PAVEMENT SECTION DESIGN .....................19

5.10 SURFACE DRAINAGE AND INFILTRATION OPPORTUNITIES ...........19

6.0 LIMITATIONS AND UNIFORMITY OF CONDITIONS......................19

SELECTED REFERENCES

FIGURESFigure 1 Vicinity Map

Figure 2 Site PlanFigure 3 Regional Geologic Map

Figure 4 Regional Faulting and Seismicity

APPENDIX A Boring Logs

APPENDIX B Laboratory Test Data


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1.0 INTRODUCTION1.1 PURPOSE AND SCOPEThe purpose of this geotechnical study was to provide geotechnical conclusions and

recommendations for design and construction of the proposed traffic calming improvements

along Junipero Serra Boulevard. We performed the following services.

Review of available literature and geologic maps for the study area;

Subsurface exploration consisting of three soil borings and one shallow core;

Laboratory testing of materials sampled during the field exploration;

Geotechnical data analyses;

Report preparation summarizing our conclusions and recommendations for the proposedimprovements.

As described in our proposal revise dated January 31, 2011, based on our geological andgeotechnical data gathered, our geotechnical report will address the following:

Physical properties of the typical soils encountered in the borings and results of laboratorytesting.

Discussion of geotechnical constraints such as compressible soils, expansive soils, existingfills, and shallow groundwater levels, as necessary.

Retaining wall recommendations, including foundation design criteria.

We were informed that the County of Santa Clara will address the following items based on the

information we gather, assess, and report herein:

Geological hazard assessment of the site and project area, such as faulting, landsliding,liquefaction, lateral spreading, tsunamis, and seiches.

Grading recommendations and engineered fill placement specifications.

Existing pavement conditions assessment.

Pavement design sections.


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We prepared this report exclusively for the AECOM Transportation and their design teamconsultants. ENGEO should review any changes made in the character, design or layout of the

project to modify the conclusions and recommendations contained in this report, as necessary.

1.2 SITE LOCATION AND DESCRIPTIONAs shown on the Landscape Concept Plan prepared by Sebastian & Associates and dated

December 3, 2009 (Job No. JSB 08020), the planned street improvements are alongJunipero Serra Boulevard between approximately 1,200 feet northwest of Santa Maria Avenue

and approximately 1,500 feet southeast of Santa Maria Avenue (roughly 2,700 total feet)

(Figures 1 and 2).

Photo 1 Junipero Serra Boulevard near Santa Maria Avenue looking southeast

An existing drainage channel runs adjacent to the southwestern side of Junipero Serra Boulevard

at the northwestern end of the project alignment. The channel extends roughly 5 feet below street

level, and contains approximately 1:1 (horizontal:vertical) to 2:1 slope bank walls, and roughlyruns in a southeast-northwest direction. The stretch of channel bank immediately adjacent toJunipero Serra Boulevard was lined with dry-stack concrete bags and that stretch of channel

contained a concrete bottom.

Topographic conditions off the edges of roadway along the remaining study area generallymatched existing pavement or were observed to have 2:1 or flatter upslopes or downslopes up to

2 to 3 feet in height off the edge of pavement.

According to the plans and regional topographic maps, the existing road gently slopes upward

from each end of the project alignment to a high point near the intersecting Santa Maria Avenue.Based on review of a topographic map of the area (USGS, 1997), the project alignment varies

from approximate Elevations of 180 to 210 feet.


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1.3 PROPOSED IMPROVEMENTSThe Landscape Concept Plan shows proposed traffic-calming improvements includingconstruction of the following.

A 10-foot-wide raised median along the project alignment.

A widened street with teardrop entry medians at the northwestern and southeastern ends ofthe project alignment.

A widened street with a diamond entry median at the Santa Maria Avenue intersection.

To accomplish the above improvements, the existing road will be widened up to approximately

23 feet off each side at the three above widening locations, and may be slightly widenedelsewhere at tapers and to improve the shoulders.

Based on correspondence with AECOM Transportation (AECOM), we understand construction

of site retaining walls will be necessary at some of the road widening sections. In particular, awall system is likely at the widened entry median at the northwestern end of the project

alignment adjacent to the existing drainage channel. To avoid impacting the existing drainage

channel bottom, we anticipate at least a portion of the wall system will comprise a 3-sided box

culvert or arch culvert with open bottom, with one side supporting the existing street soils andthe other side supporting the existing slope on the opposite side of the channel. For the retaining

wall system that extends beyond the ends of the culvert, we anticipate the wall will be situated in

between the existing channel bottom and the existing edge of pavement, therefore, a levelbackfill is anticipated with a 1:1 or slightly flatter foreground below the wall. Other segments of

site walls are anticipated in areas of planned cut or planned fill associated with the road widening

activities.

2.0 GEOLOGY AND SEISMICITY2.1 SITE GEOLOGYRegional mapping by Brabb (2000) depicts the majority of the subject site as underlain by theWhiskey Hill formation (Tw) consisting mostly of coarse-grained sandstone. The northwestern

end of the site is mapped as Holocene-age alluvial fan and fluvial deposits (Qhaf) in proximity tothe existing drainage channel, and the southeastern end of the site is mapped as the Santa Clara

formation (QTsc) (Figure 3).


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2.2 REGIONAL FAULTING AND SITE SEISMICITYThe site is not located within a State of California Earthquake Fault Hazard Zone (1982) for

active faulting; however, because of the presence of nearby active faults1

, the region isconsidered seismically active.

Numerous small earthquakes occur every year in the region, and large (>M7) earthquakes have

been recorded and can be expected to occur in the future. Figure 4 shows the approximatelocations of these faults and significant historic earthquakes recorded within the region.

The most common nearby active faults within 25 miles of the site (EQFault, Version 3.00) areprovided in the following table.

TABLE 1

Regional Faults

Fault Name

Approximate

Distance

(miles)

Approximate

Direction

from Site

Monte Vista - Shannon 1.7 Southwest

San Andreas 3.8 Southwest

San Gregorio 14.5 Southwest

Hayward 15.3 Northeast

Calaveras 19.4 Northeast

Ground motions are typically expressed as a fraction of the acceleration due to gravity (g). The

California Geological Survey (CGS) supports a web database that includes probabilistic peak

horizontal ground accelerations for the State of California. The probabilistic data are based onthe USGS/CGS Probabilistic Seismic Hazards Assessment (PSHA, 2002) model. The local faults

are estimated to cause a peak ground acceleration of approximately 0.59g at the site for a seismic

event that has an exceedance probability of 10 percent in 50 years.

3.0 FIELD EXPLORATIONThe sections below summarize our field exploration activities and laboratory testing; as well as

ground surface, subsurface, and groundwater conditions.

1An active fault is defined by the California Geological Survey as one that has had surface displacement within

Holocene time (about the last 11,000 years) (Hart, 1997).


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3.1 FIELD LOGGINGThe field exploration for this study consisted of drilling three exploratory borings and

performing one shallow core within the project alignment on May 12, 2011. Three solid flightauger boreholes (Borings 1-B1 through 1-B3) were drilled to a maximum depth of approximately14 feet below the existing ground surface. In addition, one shallow core (Core 1-P1) was

advanced along Junipero Serra Boulevard, using the solid flight auger drill bit to record the

existing thicknesses of the asphaltic concrete and underlying aggregate base using a field tape.

To minimize impact to existing traffic along Junipero Serra Boulevard and due to several

existing utility conflicts on the southwestern shoulder, our borings were advanced on thenortheastern paved shoulder. In addition, existing utility conflicts at the northwestern end of the

project near the existing drainage channel prohibited advancement of an exploratory boring on

either side of the street in this area.

Figure 2 presents the approximate locations of the exploratory borings and core obtained by

taping or pacing from existing features. As a result, the mapped locations should be considered

only as accurate as the methods used to determine them.

Photo 2 Field exploration

The borings and core were logged in the field by an ENGEO engineer and soil samples were

collected using either a 2-inch inside diameter (I.D.) California-type split-spoon sampler fitted

with 6-inch-long brass liners or a 2-inch outside diameter (O.D.) Standard Penetration Testsplit-spoon sampler. Bulk samples were also collected of near-surface drilling spoils. The

penetration of the samplers into the site materials were recorded as the number of blows needed

to drive the sampler 18 inches in 6-inch increments using an automatic-trip, 140-pound safety


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hammer falling a distance of 30 inches. The boring logs record blow count results as the actualnumber of blows required for the last 1 foot of penetration; no conversion factors have been

applied. We used the field logs to develop the report boring logs, which are presented in

Appendix A.

The logs depict subsurface conditions within the borings and core at the time the exploration was

conducted. Stratification lines on the logs represent the approximate boundaries between soil

types, and the transition may be gradual. It should be recognized that subsurface conditions atother locations may differ from conditions occurring at these boring locations. In addition, the

passage of time may also result in altered subsurface conditions.

3.2 LABORATORY TESTINGSelect soil samples recovered during drilling activities were tested to determine the following

soil characteristics:

TABLE 2Laboratory Testing

Soil Characteristic Testing Method Location of Results

Natural Unit Weight and Moisture Content ASTM D-2216 Appendix A

Grain Size Distribution ASTM D 422 Appendix B

Unconfined Compression ASTM D-2166 Appendix B

Resistance Value (R-Value) Caltrans 301 Appendix B

Sulfate Testing Caltrans 417 Appendix B

The laboratory test results are shown on the bore logs (Appendix A) with individual test resultspresented in Appendix B.

3.3 SURFACE CONDITIONSAt the time of our field work and as briefly described under Section 1.2, the project alignment

consisted of the paved Junipero Serra Boulevard extending approximately 1,200 feet northwest

of Santa Maria Avenue and approximately 1,500 feet southeast of Santa Maria Avenue (roughly2,700 total feet). The existing street consists of a two-lane road roughly 12-feet-wide in each

direction with a median up to approximately 4 feet in width. A paved bike lane and/or shoulderranging from approximately 5 to 10 feet in width exist on each side of the existing street. Heavyvehicular and bicycle traffic was present along the project alignment while we were on site.

Based on our observations, the existing road gently sloped upward from each end of the project

alignment to a high point near the intersecting Santa Maria Avenue. The existing pavementgenerally appeared to be in good condition, with some occasional meandering cracks generally

aligned in the direction of travel in the vehicular and bicycle lanes. Minor patch pave areas were


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also observed, anticipated in the alignment of existing utilities. The roadway surface alsoappeared to be crowned to sheet flow water off the road.

Also as described in Section 1.2, a drainage channel exists adjacent to the southwestern side ofJunipero Serra Boulevard at the northwestern end of the project alignment (Photo 3). Accordingto a brief review of historic aerial photographs (Google Earth), the drainage channel historically

traveled to the northeast, under Junipero Serra Boulevard. The channel was redirected to run

parallel to Junipero Serra Boulevard when the roadway was constructed several decades ago andcurrently drains in a southeast to northwest direction. The channel banks are roughly 5 feet high

and has slope gradients that vary from roughly 1:1 to 2:1 (horizontal:vertical). The stretch of

channel bank immediately adjacent to Junipero Serra Boulevard is lined with dry-stack concretebags and that portion of the channel contains a concrete bottom.

Photo 3 Drainage channel at northwestern end of project looking southeast

In addition, topographic conditions off the edges of roadway along the remaining study area

generally matched existing pavement or were observed to have 2:1 or flatter upslopes ordownslopes up to 2 to 3 feet in height off the edge of pavement.

3.4 SUBSURFACE CONDITIONSOur borings and core were advance in pavement areas and generally encountered approximately4 to 7 inches of asphaltic concrete (AC) over 8 to 10 inches of aggregate base (AB). The table

below presents our asphaltic concrete and aggregate base field measurements at Borings 1-B1through 1-B3 and Core 1-P1.


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TABLE 3Existing Asphaltic Concrete and Aggregate Base Thickness

Boring/Core

Location

Asphaltic Concrete

Thickness (inches)

Aggregate Base

Thickness (inches)1-B1 6 10

1-B2 4 8

1-B3 7 8

1-P1 6 10

Beneath the aggregate base, the borings generally encountered 6 to 7 feet of stiff or loose to

medium dense or hard sandy lean clay and clayey sand with varying amounts of fine gravel.

Beneath the sandy lean clay and clayey sand, sandstone and claystone bedrock wereencountered.

Although plasticity index testing was not performed on the sandy clay/clayey sands, our fieldvisual classification and laboratory testing (moisture/density and R-value) suggests these soils

have a moderately low to moderate expansion potential.

Variations in soil stratigraphy should be expected along the project alignment.

3.5 GROUNDWATERWe encountered groundwater in Borings 1-B1 and 1-B2 at depths of approximately 14 and

7 feet below existing grade, respectively. We did not encounter groundwater in Boring 1-B3

within the depth explored of 14 feet.

Fluctuations in groundwater levels occur seasonally and over a period of years because of

variations in precipitation, temperature, irrigation, and other factors. Based on the variation in

depth and occurrence of groundwater, we anticipate perched zones of groundwater may beencountered within the project alignment at shallow depths.

4.0 DISCUSSION AND CONCLUSIONS4.1 SEISMIC HAZARDSThe project alignment was not evaluated with respect to known geologic and seismic hazards

common to the Bay Area, such as faulting, landsliding, liquefaction, lateral spreading, tsunamis,

and seiches. We understand the County of Santa Clara will perform this evaluation for theproject alignment, as applicable.


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4.2 EXISTING PAVEMENT CONDITIONS ASSESSMENTAs previously discussed, the existing pavement generally appeared to be in good condition, with

some occasional meandering cracks generally aligned in the direction of travel in the vehicularand bicycle lanes, and minor areas of patch paving. We understand the County of Santa Clarawill perform a detailed existing pavement conditions assessment to provide recommendations for

pavement rehabilitation, overlay, and/or slurry seal.

4.3 DRAINAGE CHANNEL BANK STABILITYAs noted above, an existing drainage channel runs adjacent to the southwestern side ofJunipero Serra Boulevard at the northwestern end of the project alignment in an area planned for

road widening. The channel bank is roughly 5 feet high (below street level) and roughly runs in a

southeast to northwest direction with channel bank slope gradients of roughly 1:1 to 2:1.

The northeastern channel bank (supporting the street) is generally lined with dry-stack concrete

bags. The southwestern channel bank (slope to the southwest of the street) is lined with dry-stack

concrete bags or blocks for a portion of the bank, and an unlined creek bank with sparsevegetation on the remaining portion of the bank.

Although slope stability analyses of the channel banks were not performed, these embankmentsmay be susceptible to natural slope regression processes, as well as minor slope deformations

during strong ground shaking. However, we assume that most of the existing utilities are situated

at least 3 feet from the top of existing creek bank and at depths generally equal to or below thechannel bottom; therefore, the potential for damage to the existing utilities due to channel bank

instability should be low. Future street improvements in proximity to the channel or otherdownslopes will be supported by a planned retaining wall system(s) designed to accommodate

the downsloping condition.

If proposed improvements require excavating a portion of the unlined slope above and to the

southwest of the project alignment, or if the slope bank is planned to be graded to a conditionsteeper than the current gradient, the slope should be assessed for local stability. As applicable,

additional retaining walls, slope reconstruction and incorporation of geogrid soil reinforcement

could be considered to achieve suitable factors of safety. We can provide additional consultationregarding this issue as planning progresses and if requested.

4.4

COMPRESSIBLE SOILS

Compressible soils were not encountered in our borings within the existing Junipero Serra

Boulevard street sections. However, proposed improvements include street widening into

unpaved areas where near-surface compressible soils may be encountered. These soils may besusceptible to settlement/compression from increased loads imposed by fills and traffic. During

construction, we recommend potholing several locations along the planned widening areas and

select street pavement areas to identify if potentially compressible soils are present within the


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project alignment. If present, it is recommended that these soils be removed and backfilled asengineered fill. Additional recommendations are presented in a subsequent section.

4.5 EXISTING FILLWith exception to the existing pavement and aggregate base along the project alignment, existing

fills were not evident at the boring locations. However, existing fill materials should be

anticipated within utility trench backfill and likely small fills are present along select roadwayedges.

Existing fills in planned roadway widening areas could undergo vertical movement that is noteasily characterized and could ultimately be inadequate to effectively support the proposed

improvements. In general, undocumented fills encountered during construction under the

existing pavement should be observed, tested, and proof-rolled for stability. If yielding or

unsuitable conditions are encountered, localized subexcavation and replacement with engineeredfill or aggregate base may be necessary.

4.6 EXPANSIVE SOILSAs previously discussed, some of the clayey soils encountered in the borings below the asphaltic

concrete and aggregate base could be moderately expansive. Expansive soils shrink and swell asa result of moisture changes which can cause heaving and cracking of pavements. Successful

construction on expansive soils requires special attention during grading. It is imperative to keep

exposed soils moist until the street section is placed. Conventional grading operations andincorporating moisture and compaction fill placement specifications tailored to the expansive

characteristics of the soil are common, generally cost-effective measures to address theexpansive potential of soils.

4.7 GROUNDWATERAs discussed above, groundwater was encountered in two of the three borings at a depth asshallow as 7 feet below existing grade. Based on the variation in depth and occurrence of

groundwater, we anticipate perched zones of groundwater may be encountered within the project

alignment. As a result, and depending on excavation depths for proposed improvements, perchedgroundwater may be encountered during construction activities.

4.8

FLOODING

We did not provide an assessment of flooding or review existing FIRM maps. We understand the

Project Civil Engineer will assess if the site is located within the 100-year flood elevation.


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4.9 2010 CALIFORNIA BUILDING CODE SEISMIC PARAMETERSAs applicable, based on the subsurface conditions encountered and local seismic sources and

provided the site is prepared according to the recommendations contained herein, the following2010 California Building Code (CBC) seismic design parameters should be used for design.

TABLE 4

2010 CBC Seismic Information

ParameterDesign

Value

Site Class C

0.2 second Spectral Response Acceleration, SS 2.16

1.0 second Spectral Response Acceleration, S1 0.87

Site Coefficient, FA 1.00

Site Coefficient, FV 1.30

Maximum considered earthquake spectral response accelerations for short periods, SMS 2.16

Maximum considered earthquake spectral response accelerations for 1-second periods, SM1 1.13

Design spectral response acceleration at short periods, SDS 1.44

Design spectral response acceleration at 1-second periods, SD1 0.75

Long period transition-period, TL 12

Latitude = 37.41341; Longitude = -122.16853

4.10 CORROSIVITY CONSIDERATIONSAn evaluation of potential sulfate attack to concrete elements was conducted on two samples of

site soils ranging in depth from approximately 1 to 2 feet below existing grade. The twosamples were tested for water-soluble sulfate (SO4) in accordance with Caltrans Test Method

417 and yielded water-soluble sulfate concentrations of 84 mg/kg and 2,262 mg/kg (0.0084 and

0.2262 percentage by weight), from Borings 1-B1 and 1-B3, respectively.

The CBC references the 2008 American Concrete Institute Manual, ACI 318 (Chapter 4,

Sections 4.2 and 4.3) for concrete requirements. ACI Tables 4.2.1 and 4.3.1 provide the

following sulfate exposure categories and classes and concrete requirements in contact with soilbased upon the exposure risk.


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TABLE 5Sulfate Exposure Categories and Classes

Water- Soluble

Sulfate in Soil

Dissolved Sulfate in

Water

Sulfate

Exposure CategoryS

ExposureClass% by Weight mg/kg (ppm)

Not Applicable S0 SO4 < 0.10 SO4 < 150

Moderate S1 0.10 SO4< 0.20150 SO4 1,500

seawater

Severe S2 0.20 SO4 2.00 1,500 SO4 10,000

Very Severe S3 SO4 > 2.00 SO4 > 10,000

TABLE 6Requirements for Concrete by Exposure Class

Cement TypeExposure

Class

Max

w/cm

Min fc

(psi) ASTM

C150

ASTM

C595

ASTM

C1157

Calcium

ChlorideAdmixture

S0 N/A 2500No Type

restrictionNo Type restriction

No Type

restrictionNo restriction

S1 0.5 4000 II IP(MS), IS(


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If requested, we can provide additional sulfate testing to further define the locations of highsulfate exposure within the project alignment.

4.11 CONCLUSIONSIt is our opinion, based on this exploration and laboratory test results, that the proposed traffic

calming improvements are feasible from a geotechnical standpoint for the project site. Design

considerations and recommendations presented in this report and developed by the County ofSanta Clara should be incorporated into project design and implemented during construction.

Based on this geotechnical exploration, the main concerns from a geotechnical standpoint for theplanned project include:

Long-term stability of the existing drainage channel bank and retaining wall

design/construction adjacent to drainage corridors and containing downsloping foreground.

Street widening into unpaved areas that may encounter soft/compressible soils near-surfacesoils or shallow existing undocumented fills

Moderately expansive subgrade soils

5.0 RECOMMENDATIONSBased upon the above geotechnical concerns, our design and general construction

recommendations are presented in the following sections. The recommendations included in this

report, along with other sound engineering practices, should be incorporated in the design andconstruction of the project. In addition, relevant information and recommendations from the

following references were incorporated.

Division of Occupations Safety and Health (DOSH) (Cal/OSHA)

Occupational Safety and Health Administration (OSHA) Technical Manual (OTM) (1999)

The Engineer, County, and Contractor should review these references along with this report. Ifthere appears to be a conflict, this should be brought to the attention of the applicable agency and

Geotechnical Engineer.

5.1 MONITORING AND TESTINGIt is important that all construction activities be performed under the observation of the

Geotechnical Engineers field representative, in accordance with the recommendations containedherein and developed by the County of Santa Clara.


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5.2 EXISTING UTILITY CROSSINGSThe locations and depths of the existing utilities located adjacent to or over the proposed

improvements should be evaluated such that they are not damaged during construction.Protection of existing utility crossings in trenches should also be considered. Critical utilities

should be protected through cradling while less critical utilities could span trenches unprotected.

5.3 EXCAVATABILITYBased on our field exploration, it appears that mid-sized equipment, such as D6 dozers and

excavators, would be able to excavate site soils to planned depths. At this time (with exception to

possible deep foundations for the retaining walls), we do not anticipate that proposedimprovements will extend to bedrock.

5.4 TEMPORARY EXCAVATIONSBased on the soil data, it is our opinion that excavations should be either shored and/or shielded

if vertical cuts are constructed, or sloped and/or bench cut if existing constraints are not present.The Contractor should be familiar with applicable local, state, and federal regulations, and shall

conform to Cal/OSHA requirements for shoring and/or sloped excavations and utility

construction within the excavations. The design of appropriate shoring systems is theresponsibility of the Contractor and should be in conformance with the OTM and Cal/OSHA.

For vertical trench excavation or sloped excavations and/or benching, design and constructionshould be performed in accordance with the OTM. The contractors Competent Person is

responsible to confirm or adjust soil classifications based upon actual field conditions

encountered during construction. The Contractor should consider vehicular traffic, constructionequipment, soil stockpiles, etc. in determining their excavation plan. Excavated soils, ifstockpiled, should be situated away from the edge of excavations to reduce potential adverse

effects on excavation stability.

Temporary slopes and excavations will be subject to deformation, sloughing, and erosion and

should be backfilled as soon as practical.

5.5 TEMPORARY DEWATERINGAs previously discussed, perched groundwater may be encountered depending on the location

and depth of excavations. General recommendations are provided below.

The water level at the excavation locations should be maintained below the bottom of theexcavations. The selection of equipment and method should be determined by the contractor.

The dewatering system implemented should be selected so as to have minimal impact on the

groundwater level surrounding the proposed excavations. The dewatering system should bedesigned to prevent pumping soil fines with the discharge water into a location approved by the

County.


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5.6 SOFT SUBGRADE CONDITIONSIt is possible that soft subgrade conditions may be encountered at the bottom of the proposedexcavations or at street subgrade. Depending on the depth of excavations and the time of yearthat construction takes place, it may become necessary to perform subgrade stabilization to

mitigate such conditions. Excavations that bottom in soft, yielding soils may require additional

mitigation such as chemical treatment, placement of geotextile stabilization material (such asMirafi 600X) directly on subgrade, or overexcavation of up to 18 inches below planned subgrade

(if existing utilities allow) to encounter a firm soil base before backfilling with drier engineered

soil fill or compacted aggregate base within street areas. If yielding material is encountered at thebase of soft soil overexcavations, placing geotextile stabilization material at the base and up the

side walls of the overexcavation is recommended prior to backfill entirely with compacted

aggregate base up to street subgrade. Incorporating geogrid reinforcement, such as Tensar

triaxial geogrid, on subgrade and at mid-height of the aggregate base section, could also beconsidered.

ENGEO is available to provide additional consultation and recommendations based on actualfield conditions during site construction.

5.7 ENGINEERED FILL SELECTION AND PLACEMENTWe understand the County of Santa Clara will provide grading recommendations and engineered

fill placement specifications and materials for the project. The proposed improvements should beconstructed in accordance with these separate recommendations and requirements provided by

the County.

5.8 RETAINING WALLSAs previously discussed, we anticipate construction of site retaining walls be necessary at some

of the planned road widening sections. In particular, a wall system is likely at the widened entrymedian at the northwestern end of the project alignment adjacent to the existing drainage

channel. To avoid impacting the existing drainage channel bottom, we anticipate at least a

portion of the wall system will comprise a 3-sided box culvert or arch culvert with open bottomthat receives backfill behind and above the culvert to achieve planned pavement elevation.

For the retaining wall system that extends beyond the ends of the culvert, we anticipate the wallwill be situated in between the existing channel bottom and the existing edge of pavement,therefore, a level backfill is anticipated with a 1:1 or slightly flatter foreground below the wall.

Other segments of site walls are anticipated in areas of planned cut or planned fill associated

with the road widening activities.


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The following sections present general retaining wall design recommendations and CaltransStandard Plan recommendations. The foundation details and structural calculations for retaining

walls should be submitted for review by the Geotechnical Engineer.

5.8.1 Retaining Wall Design RecommendationsSince the site retaining walls will be supporting pavements, we recommend they be designed

using restrained, at-rest, drained earth pressures. If minor rotation at the top of wall is tolerable,then an active, drained earth pressure is feasible. As a result, retaining walls up to 10 feet in

height may be designed using the following drained equivalent fluid pressures as follows.

TABLE 7Equivalent Fluid Pressures

Backfill Slope Condition

(horizontal:vertical)

Active Earth Pressure

(pounds per cubic foot, pcf)

At-Rest Earth Pressure

(pounds per cubic foot, pcf)Level 50 70

3:1 60 80

2:1 70 90

1:1 80 100

An at-rest earth pressure of 70 pcf should be used in design of the culvert since the culvert walls

should not be allowed to rotate.

If portions of the walls or culvert are to be undrained, then an additional 30 pcf should be added

to the values above. For walls with a level foreground condition (for a distance of at least10 feet), the equivalent fluid pressures should extend to a depth of 1 foot below lowest adjacent

soil subgrade, or to the bottom of the wall foundation (footing or pier cap), whichever is deeper.If a downsloping condition is present, the earth pressure should extend to that depth necessary to

achieve a horizontal distance of at least 10 feet to the nearest free face. For site retaining walls

situated adjacent to and parallel with the existing drainage channel, the earth pressure shouldextend to a depth of at least 2 feet below the drainage channel bottom.

Appropriate safety factors (F.S.) against overturning (suggested minimum F.S. of 2.0) andsliding (minimum F.S. of 1.5) should be incorporated into the design calculations as well as

incorporation of surcharge loads from pavement and traffic, where applicable.

5.8.1.1 Spread Footing Foundation

The following recommendations apply to site walls supported on a spread footing foundation.

Retaining wall footings should be designed using an allowable bearing pressure of 2,000 pounds

per square foot (psf), assuming the footing is supported on firm soil. A minimum footing width

of 12 inches is recommended as well as a minimum embedment depth of 18 inches below lowest


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adjacent soil subgrade, or deepened as necessary to achieve at least 10 horizontal feet to thenearest free slope face. For the wall footing situated parallel to the existing northwestern

drainage channel, we recommend the footings be extended to at least 24 inches below the bottom

of the drainage channel to account for a limited amount of future scour or downcutting.

The friction factor for sliding resistance may be assumed as 0.30. Passive pressures acting on

foundations may be assumed as 250 pounds per cubic foot (pcf) provided that the area in front of

the retaining wall is level for a distance of at least 10 feet or three times the depth of foundationand keyway, whichever is greater. Passive resistance should start below the depth required for

lateral equivalent fluid pressure noted above. The passive pressure may be increased by1/3 for

transient loads such as wind or seismic.

Actual footing dimensions and reinforcement should be determined by the wall designer, based

on structural design considerations.

5.8.1.2 Drilled Pier Foundation

For walls supported on a drilled pier foundation system, the following pier design criteria shouldbe incorporated:

Pier diameter: Minimum 12 inches.

Pier depth: Minimum 6 feet.

Maximum allowable skin friction: 400 pounds per square foot (psf). This value may be

increased by1

/3 when considering seismic or windloads. Exclude the upper 2 feet of the pier shaft

from pier load capacity computations.

Minimum pier spacing: 3 pier diameters, center-to-center.

For a downsloping foreground condition, the lateral equivalent fluid pressures noted above

should continue down the pier shaft to that depth necessary to achieve at least 10 horizontal feet

to the nearest slope face, or to at least 2 feet below the bottom of the existing drainage channel,where applicable. The lateral earth pressures should span over the tributary width between piers.

An equivalent fluid weight of 250 pounds per cubic foot acting on two times the pier diametermay be used to evaluate passive resistance, starting below the depth required for lateralequivalent fluid pressure noted above. The passive pressure may be increased by 1/3 for transient

loads such as wind or seismic.

Actual pier depths, spacing, and reinforcement should be determined by the wall designer, based

on structural design considerations.


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5.8.2 Caltrans Standard Retaining WallsAs an alternative, retaining walls may be designed and constructed according to the Caltrans

Standard Plan (2006). Based on our review, Retaining Wall Type 1, Type 1A, Type 2, or Type 5may be used. For the proposed culvert and select walls in cut, 2:1 or slightly steeper slopes maybe present behind the retaining wall and traffic loading may be applied to the other side of the

wall. As such, Loading Cases I, II, and IV should be considered in design based on the Caltrans

Standard Plan and Bridge Standard Detail Sheets.

According to the standard plans, the walls may be supported on a spread footing or pile/pier

foundation. The walls should be designed to meet the minimum requirements on the standardplans based on the design recommendations provided in prior sections of this report for spread

footing and drilled pier foundations.

5.8.3 Construction ConsiderationsRetaining walls should be provided with drainage facilities to prevent the build-up of hydrostatic

pressures behind the walls. Wall drainage may be provided using a 4-inch-diameter perforatedpipe (SDR 35 or equivalent) embedded in either free-draining gravel surrounded by synthetic

filter fabric (minimum 6-ounce) or Class 2 permeable material. The width of the drain blanket

should be at least 12 inches, and the drain blanket should extend to about 1 foot below the soilsubgrades. The upper 1 foot of wall backfill to soil subgrade should consist of compacted site

soils. Drainage should be collected in solid pipes and directed to an outlet approved by the Civil

Engineer, such as a catch basin, storm drain manhole or into an existing drainage swale orchannel above normal high water elevation. Wall drainage recommendations provided in the

Caltrans Standard Plan may also be utilized provided that weep hole drains are spaced at 6 feeton-center, maximum. Pervious backfill material should meet the requirements provided above.

All backfill should be placed as engineered fill using light equipment to reduce possible

overstressing of the walls.

For drilled pier foundations, pier drilling and concrete placement should be coordinated so that

pier holes are left open a limited amount of time. Pier holes should not be allowed to desiccate

visibly before placing concrete. Pier holes should be cleaned of loose materials and tamped, andany water at the base of the pier hole should be pumped prior to concrete placement, or displaced

during concrete placement using the tremie method. Due to the depth of the piers and

groundwater considerations, we suggest pier concrete in general be tremie-placed to avoiddislodging soil from the sidewalls of the pier shaft.

We recommend that all footing excavations, pier hole drilling, and related construction be

performed under observation of the Geotechnical Engineer to confirm exposed soil conditionsand that the walls are constructed in accordance with the recommendations presented in this

report.


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5.9 R-VALUE RESULTS FOR PAVEMENT SECTION DESIGNResistance value (R-value) testing was performed on near-surface soil cuttings collected from

Borings 1-B1 through 1-B3. The results are presented in the table below.

TABLE 8

Boring LocationResistance Value

(R-value)

1-B1 17

1-B2 13

1-B3 17

We understand the County of Santa Clara will provide pavement section designs based on our

R-value testing. If desired, ENGEO can provide preliminary sections for use in land planning ifthe Traffic Index is provided.

5.10 SURFACE DRAINAGE AND INFILTRATION OPPORTUNITIESThe work area and finished grade must be positively graded at all times to provide for removal ofsurface water run-off and to prevent ponding of water or seepage during or after construction.

Ponding of stormwater must not be permitted during prolonged periods of inclement weather. All

surface water should be collected and discharged into the storm drain system or through anengineered water quality system. Landscape mounds must not interfere with this requirement.

Due to the generally high fines content tested in the site materials, the site soils encountered arenot expected to have adequate permeability values to handle storm water infiltration in grassy

swales or permeable pavers unless subdrainage is installed. Therefore, best management

practices should assume that little stormwater infiltration will occur at the site.

6.0 LIMITATIONS AND UNIFORMITY OF CONDITIONSThis report is issued with the understanding that it is the responsibility of the owner to transmit

the information and recommendations of this report to developers, owners, buyers, architects,

engineers, and designers for the project so that the necessary steps can be taken by thecontractors and subcontractors to carry out such recommendations in the field. The conclusions

and recommendations contained in this report are solely professional opinions.

The professional staff of ENGEO strives to perform its services in a proper and professionalmanner with reasonable care and competence but is not infallible. There are risks of earth

movement and property damages inherent in land development. We are unable to eliminate all

risks or provide insurance; therefore, we are unable to guarantee or warrant the results of ourservices.


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This report is based upon field and other conditions discovered at the time of preparation ofENGEOs report. This document must not be subject to unauthorized reuse; that is, reusing

without written authorization of ENGEO. Such authorization is essential because it requires

ENGEO to evaluate the documents applicability given new circumstances, not the least ofwhich is passage of time. Actual field or other conditions will necessitate clarifications,adjustments, modifications or other changes to ENGEOs documents. Therefore, ENGEO must

be engaged to prepare the necessary clarifications, adjustments, modifications or other changes

before construction activities commence or further activity proceeds. If ENGEOs scope ofservices does not include on-study area construction observation, or if other persons or entities

are retained to provide such services, ENGEO cannot be held responsible for any or all claims

arising from or resulting from the performance of such services by other persons or entities, andfrom any or all claims arising from or resulting from clarifications, adjustments, modifications,

discrepancies or other changes necessary to reflect changed field or other conditions.


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SELECTED REFERENCES

American Concrete Institute, 2005, Building Code Requirements for Structural Concrete

(ACI 318-05) and Commentary (ACI 318R-05).

Blake, T. F., 2000, EQFAULT, A Computer Program for the Deterministic Prediction of PeakHorizontal Acceleration from Digitized California Faults.

Brabb, E.E., Graymore, R.W., and Jones, D.L., 2000, Geology of Palo Alto, 30 x 60 minute

quadrangle, California: a digital database: U.S. Geological Survey, Open-File ReportOF-98-348, Scale 1:100000.

California Building Code, 2010.

California Department of Transportation, 1992, Highway Design Manual.

Occupational Safety & Health Administration (OSHA), 1999, Technical Manual.

SEAOC, 1996, Recommended Lateral Force Requirements and Tentative Commentary.

State of California, Division of Mines and Geology (CDMG), 1974, Potential Seismic Hazards in

Santa Clara County, California, Special Report 107, Plates 1 through 4.


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LIST OF FIGURES

Figure 1 Vicinity MapFigure 2 Site Plan

Figure 3 Regional Geologic Map

Figure 4 Regional Faulting and Seismicity

F

G

U

R

E


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RAFT

RAFT


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1-B1

1-B2

1-P1

1-B

1-B3

1-P1

RAFT


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Qhl

alf

Qhsc

Qhb

Qhfp

Qpoaf

Qpaf

QTsc

Tlad

Tm

Tpm

Tw

RAFT

RAFT


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PAL

SANG

REGORIO

ORTIGALITA

GREENVILLE

MONTEREYBAY

-TULARCITO

S

RELIZ

SAN

GREGORIO

SAN

ANDR

EAS

HAYWARD

POINTR

EYES

SANANDREAS

CONCORD

GREENVALLEY

VACA

SANJOAQUIN

ORTIGALITA

TOLAY

REEK

CORDELIA

MIDWAY

SILV

ERCREEK

SANJOSE

MONTEVIST

ASHANNON

BERROCAL

ZAYANTE

VERGELES

RELIZ

SARGENT

CALAVERAS

CARNEGIECORALHO

LLOW

SAN

SANBEN

ITO

CALAVARES

QUIENSABE

WESTNAPA

ANTIOCH

GREAT

VALLE

Y

FAULT

San

Santa Cruz Santa Clara

San MateoAlameda

Stanis

Contra Costa

San Joaquin

Marin

Solano

San

Francisco

SANANDRE

AS

SANANDREAS

DRAFT


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APPENDIX A

Key to Boring LogsBoring Logs

A

PP

E

N

D

X

A


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No recovery

6.5 inches asphaltic concrete over 10 inches aggregate base

SANDY LEAN CLAY TO CLAYEY SAND (CL-SC), brown,medium stiff to medium dense, moist,


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4 inches asphaltic concrete over 8 inches aggregate base

SANDY LEAN CLAY (CL), dark brown, medium stiff, moist,

007 appendix d geotechnical report

Documents