3.2_-_Murray_Street_Bridge_Final_RW-9_FR_Soldier_Pile.pdf

FINAL FOUNDATION REPORT

FOR EARTH RETAINING SYSTEMS

Retaining Wall No. 9

Murray Street Bridge Seismic Retrofit and Barrier Replacement

City of Santa Cruz, California Crawford File No. 2009-0272-3

Bridge No.: 36C0108

Prepared by:

Crawford & Associates, Inc.

4701 Freeport Boulevard Sacramento, CA 95822

July 2023

Prepared for:

TRC

10680 White Rock Road, Suite 100

Rancho Cordova, CA 95670

Corporate Office: 4701 Freeport Boulevard | Sacramento, CA 95822 | (916) 455-4225 Modesto: 1165 Scenic Drive, Suite A | Modesto, CA 95350 | (209) 312-7668 Pleasanton: 6200 Stoneridge Mall Road, Suite 330 | Pleasanton, CA 94588 | (925) 401-3515 Ukiah: 100 North Pine Street | Ukiah, CA 95482| (707) 240-4400

July 19, 2023 Crawford File No. 2009-0272-3

Cameron Pinkerton, PE

TRC

10680 White Rock Road, Suite 100 Rancho Cordova, CA 95670-6061

Subject: FINAL FOUNDATION REPORT FOR EARTH RETAINING SYSTEMS

Retaining Wall No. 9 Murray Street Bridge Seismic Retrofit and Barrier Replacement City of Santa Cruz, California

Dear Mr. Pinkerton, Crawford & Associates, Inc. (Crawford) prepared this Final Foundation Report for Earth Retaining Systems (ERS) associated with the Murray Street Bridge Seismic Retrofit and Barrier Replacement Project in the City of Santa Cruz, California. Crawford prepared this report in accordance with our agreements between TRC and Crawford (previously Taber Consultants).

This report contains the results of our subsurface exploration and conclusions and recommendations for the design of the proposed Retaining Wall No. 9 (RW-9). Foundation recommendations for the new bridge foundations, approach roadway, and Retaining Wall No.’s 11, 17 and 18 are reported under a separate cover. This report updates and supersedes our Draft Foundation Report for ERS, dated December 2021.

Thank you for the opportunity to be part of your design team. Should questions arise concerning foundation conditions at the site, please do not hesitate to call on us. An opportunity to review and comment on plans and specifications insofar as they rely on this report is an integral part and a condition of our recommendations.

Sincerely, Crawford & Associates, Inc., Reviewed by:

David P. Castro, PE W. Eric Nichols, PE, CEG Senior Engineer Principal

FINAL FOUNDATION REPORT FOR ERS Crawford File: 2009-0272-3 Retaining Wall No. 9 July 19, 2023 Murray Street Bridge Seismic Retrofit and Barrier Replacement City of Santa Cruz, California

TABLE OF CONTENTS

1 INTRODUCTION

1.1 PURPOSE

1.2 GEOTECHNICAL SERVICES

2 PROJECT DESCRIPTION

3 GEOTECHNICAL INVESTIGATION

3.1 2013 EXPLORATION

3.2 2015 EXPLORATION

3.3 DRILLING AND SAMPLING

3.4 LOGGING

3.5 SAMPLER PENETRATION RESISTANCE (N-VALUE)

3.6 BOREHOLE ABANDONMENT

3.7 BORING LOCATIONS AND ELEVATIONS

4 LABORATORY TESTING PROGRAM

5 GEOTECHNICAL CONDITIONS

5.1 GEOLOGY

5.2 SURFACE CONDITIONS

5.3 SUBSURFACE CONDITIONS

6 GROUNDWATER

7 AS-BUILT FOUNDATION DATA

8 SCOUR DATA

9 CORROSION EVALUATION

10 SEISMIC INFORMATION

10.1 SHEAR WAVE VELOCITY

10.2 GROUND MOTION HAZARD

10.3 OTHER SEISMIC HAZARDS

10.3.1 SURFACE FAULT RUPTURE

10.3.2 LIQUEFACTION POTENTIAL

10.3.3 SEISMICALLY INDUCED SETTLEMENT

10.3.4 SEISMIC SLOPE INSTABILITY AND LATERAL SPREADING

10.3.5 TSUNAMI

11 GEOTECHNICAL RECOMMENDATIONS

11.1 GEOTECHNICAL ENGINEERING PARAMETERS

11.2 SOLDIER PILE WALL (RW-9)

11.2.1 SOLDIER PILES

11.2.2 WALL LAGGING AND BACKFILL

11.2.3 LATERAL EARTH PRESSURES

11.2.4 WALL DRAINAGE

11.2.5 SURFACE DRAINAGE

12 NOTES FOR SPECIFICATIONS

12.1 GENERAL CONDITIONS

12.2 CIDH FOUNDATIONS

12.3 CONSTRUCTION MONITORING

12.4 EXISTING FACILITIES

13 NOTES FOR CONSTRUCTION

13.1 EXCAVATION AND SHORING

13.2 SOLDIER PILES

13.3 DEWATERING

14 RISK MANAGEMENT

15 LIMITATIONS

LIST OF TABLES

Table 1: Proposed RW-9 Summary Table 2: Subsurface Exploration Summary Table 3: Subsurface Materials Table 4: Summary of Groundwater Data Table 5: Correlated Shear Wave Velocity Table 6: Idealized Geotechnical Engineering Parameters Table 7: Equivalent Fluid Weights

APPENDIX I

Figure 1: Vicinity Map Figure 2: Locations of Field Tests Figure 3: Geologic Map Figure 4: Fault Map

APPENDIX II

Log of Test Borings (LOTB) – Retaining Wall 9

APPENDIX III

Laboratory Test Results

1 INTRODUCTION

1.1 PURPOSE

Crawford & Associates, Inc. (Crawford) prepared this Final Foundation Report for the Earth Retaining System (ERS) associated with the Murray Street Bridge Seismic Retrofit and Barrier Replacement project in City of Santa Cruz, California. Services for this project were completed in accordance with the agreement, dated July 12, 2021. The purposed of this report presents the results of our subsurface exploration and testing, engineering analysis, conclusions and recommendations for use in design and construction of the proposed Retaining Wall No. 9 (RW- 9).

This report updates and supersedes our Draft Foundation Report for ERS, dated December 9, 2021. Information and comments on that study are incorporated herein, as appropriate.

Geotechnical design recommendations for proposed new bridge foundations, approach roadway, and Retaining Wall No.'s 11, 17 and 18 are reported under separate covers.

1.2 GEOTECHNICAL SERVICES

To prepare this report, Crawford:

• Discussed the project with the design team from TRC;

• Reviewed in-house file data regarding this project/site that included:

• Preliminary Foundation Investigation – Seismic Retrofit, dated May 5, 1999 (Taber Consultants Job Number 1R2/396/66.5);

• Preliminary Design Report – Draft, dated November 19, 2010 (Taber Consultants Job Number 2009-0272-1);

• Reviewed final draft project plans (100% submittal) for Murray Street Bridge Seismic Retrofit and Barrier Replacement, date 4-21-23, prepared by TRC (sheets RW-1 to RW- 5);

• Reviewed published geologic, seismic, and tsunami maps and literature pertaining to the project site;

• Subcontracted StrongMotions, Inc. to complete a report of Site-Specific Seismic Response Spectrum of Free-Surface Horizontal Motion for Murray Avenue Bridge (Report Number: SMI-12-2010), dated August 30, 2010;

• Completed geotechnical test borings at the site on; March 27 (A-13-011, D-13-012A, A- 13-012), 2013; February 9 (A-15-017A), February 10 (R-15-017B, HA-15-018A); February 11 (15-018B, A-15-019) 2015;

• Completed laboratory tests on soil/rock samples obtained during subsurface explorations;

and

• Performed geotechnical engineering evaluation and analysis.

Limitations of this study are discussed in the final section of this report.

2 PROJECT DESCRIPTION

The project site is located at Murray Street where it crosses Woods Lagoon in the City of Santa Cruz, California. The site coordinates at proposed RW-9 are approximately 36.9679°N and 122.0043°W. The site vicinity is shown on Figure 1 in Appendix I.

According to TRC, the current project datum for vertical control is the National Geodetic Vertical Datum of 1929 (NGVD 29)). Unless otherwise noted, elevations presented in this report are with respect to NGVD 29.

The proposed soldier pile RW-9 is 121 ft 10 inches, located west of Woods Lagoon and along an existing cut-slope on the south side of Murray Street and is shown on the final project plans (100% submittal) “Murray Street Bridge Seismic Retrofit and Barrier Replacement Retaining Wall No. 9 Layout” drawing prepared by TRC (dated 4-21-23). The soldier piles are shown as 30-inch diameter drilled holes with a steel HP14x117 steel pile core and filled with concrete. The plans also show timber lagging. The finished grade in front of RW-9 is shown at 2.0 ft (minimum) above the bottom of wall (i.e., bottom of the wall lagging). A maximum 1.5H:1V slope is indicated above the planned retaining wall. Proposed RW-9 information is in the table below.

Table 1: Proposed RW-9 Summary

Lay Out Line (ft)

M-Line Design Height

(ft) Length1

(ft) Soldier Pile

Spacing

Bottom of Lagging Elevation

(ft) Station

(ft) Offset

(ft)

10+00.0 (Begin Wall) 9+70.92 19.70 Right

8 to 12 121.83 2 at 4.7 ft and 19 at 5.92 ft 28.67 to 29.71

11+21.83 (End Wall) 10+95.39 25.15 Right

1) Measured along the Retaining Wall Layout Line. Total length of RW-9 measured along LOL = 121.83 ft.

A private residence is located at the top of the existing cut-slope and adjacent to Murray Street.

An existing 30-inch diameter sewer force main crosses below the lagoon on the south side of the existing bridge. The sewer force main will be abandoned and replaced by a new sewer force main located on the north side of the bridge. Overhead utility lines are also present along Murray Street in close proximity to the existing bridge.

3 GEOTECHNICAL INVESTIGATION

The following provides a generalized description of the drilling, sampling and logging methods and procedures used for the 2013 and 2015 field exploration and testing phases of this project completed by Taber Consultants (now Crawford). The boring locations are shown on Figure 2 in Appendix I. The Log of Test Borings (LOTB) drawings for RW-9 in Appendix II provide more specific soil/rock descriptions, elevations, boring details and explanation of engineering geology descriptors and terms used to log the rock cores.

3.1 2013 EXPLORATION

Borings A-13-011, D-13-012A and A-13-012 were completed on March 27, 2013 by Taber Drilling to help characterize subsurface foundation conditions for the bridge retrofit. The logged test borings (A-13-011 and A-13-012) were advanced using solid auger and/or mud-rotary drilling methods through soil and rock. In A-13-011, the underlying bedrock was cored with an HQ (3.8-inch O.D.) diamond bit core barrel. The 2013 test borings were drilled with a track-mounted CME 55 drill rig.

3.2 2015 EXPLORATION

Between February 10 and 11, 2015, four test borings (A-15-017A, R-15-017B, HA-15-018A, A- 15-018B, A-15-019) were drilled at the site to obtain subsurface data for the proposed retaining wall and bridge retrofit. The borings were drilled by Taber Drilling and advanced using solid auger and/or mud-rotary drilling methods through soil and rock. The 2015 borings were drilled with a trailer mounted CME 45 drill rig.

3.3 DRILLING AND SAMPLING

A summary of the subsurface exploration completed for the retaining wall study is shown in Table 2.

Table 2: Subsurface Exploration Summary

Project Element

Boring Number

Completion Date

Drill Rig

Type Hammer

Type Hammer

Efficiency

Ground Surface

Elevation (ft)

Boring Depth

(ft)

RW-9

A-13-011 3-27-2013 CME 55

Track Automatic 87 30.7 25.1

D-13-012A 3-27-2013 CME 55

Track Automatic 87 31.6 5.0

A-13-012 3-27-2013 CME 55

Track Automatic 87 31.6 5.5

A-15-017A 2-9-2015 CME 45

Trailer Automatic 104 32.1 4.0

R-15-017B 2-10-2015 CME 45

Trailer Automatic 104 31.8 40.3

HA-15-018A 2-10-2015 Hand Auger N/A N/A 48.4 4.9

A-15-018B 2-11-2015 CME 45

Trailer Automatic 104 48.3 31.9

A-15-019 2-11-2015 CME 45

Trailer Automatic 104 31.1 41.0

DCP = dynamic cone penetrometer

Soil samples were recovered by means of 2.0-inch O.D. Standard Penetration Test (SPT) split-spoon sampler (ASTM D1586) with 1.4-inch I.D. stainless steel liners and a 3.0-inch O.D.

“California” split-spoon sampler (ASTM D3550) with 2.4-inch I.D. stainless steel liners. The samplers were advanced with standard 350 ft-lb striking force using a 140-lb automatic hammer and a drop height of 30 inches. The dynamic cone penetrometer boring (D-13-012A) was similarly advanced. Weathered bedrock in A-13-011 was cored with a HQ (3.8-inch O.D.) diamond bit core barrel. A hammer energy calibration test was not performed specifically for this project/site.

The hammer used for the drive samples during the 2013 subsurface exploration is recorded to have a hammer energy efficiency (ERi) of 87% based on an energy calibration completed on April 9, 2012. The hammer used for the drive samples during the 2015 subsurface exploration is recorded to have a hammer energy efficiency (ERi) of 104% based on an energy calibration completed on November 18, 2014.

Drive samples taken in the borings were typically collected at approximate 5 ft intervals and as otherwise directed by the field engineer/geologist. At each test interval, the sampler was driven 18 inches (or until sampler refusal criterion was met), and the blows necessary to advance the sampler each 6 inches of penetration were recorded. The sample refusal criterion is defined as 50 or more blows with less than 6 inches of sampler advancement. For reference, the Log of Test Borings for the retaining wall is included in Appendix II.

Bulk samples of soil cuttings were also collected at selected depths from the borings.

3.4 LOGGING

The borings were logged and earth materials field-classified by an engineer/geologist as to consistency, color, texture and gradation on the bases of penetration resistance, examination of samples and observation of drill cuttings. Where rock cores were obtained, the percent recovery and Rock Quality Designation (RQD) were recorded for the recovered portions of rock.

Selected portions of recovered soil/rock samples from the driven samplers were retained in sealed containers for laboratory testing and reference. Rock cores were retained in core boxes for reference. The bulk soil samples collected from auger cuttings were placed in plastic bags for laboratory testing and reference.

Groundwater observations were also made in the borings during and at completion of drilling operations.

3.5 SAMPLER PENETRATION RESISTANCE (N-VALUE)

The in-situ sampler penetration resistance (N-value) in blows per foot was recorded in the field to obtain an approximate measure of the dynamic resistance of the soil. The N-value was recorded as the number of hammer blows necessary to drive the sampler the final 12-inches of the 18-inch sample interval, unless refusal was met.

The SPT N-value adjusted to 60% hammer energy (N60) is routinely used to provide an index of the apparent density of cohesionless soils and sometimes (albeit less reliably) to estimate the consistency of cohesive soils. The energy-corrected N60 value normalized for effective overburden stress referred to as (N1)60 is typically used to correlate soil strength parameters and bearing characteristics.

For a non-standard sampler (i.e., non-SPT sampler), the in-situ N-value was corrected to an Equivalent SPT N-value using values published in Caltrans Geotechnical Manual (March 2021), then adjusted to provide an Equivalent N60 and/or Equivalent (N1)60 value that can be correlated to soil strength and bearing characteristics for use in geotechnical analysis.

The in-situ (uncorrected) N-values are shown on the Log of Test Borings (LOTB) drawings provided in Appendix II.

3.6 BOREHOLE ABANDONMENT

The test borings completed for this study were backfilled with cement grout at the completion of drilling. The shorter borings (A-13-012, D-13-012A, A-15-017A, HA-15-018A) were backfilled with soil cuttings. The borings were also surface sealed with asphalt-concrete cold patch where drilled within paved areas.

3.7 BORING LOCATIONS AND ELEVATIONS

The details and locations of test borings completed by Taber Consultants are shown on the Log of Test Borings (LOTB) drawings in Appendix II.

Taber Consultants’ personnel located the borings in the field relative to existing site features and subsequently to project stationing. The boring elevations for the 2013 and 2015 field studies were surveyed in the field by Taber personnel. The datum for the elevations shown for the borings completed by Earl and Wright, Inc. are with respect to mean lower low water (datum conversion to NGVD29 unknown). Glen Wade, P.G., and Amanda Kahn, G.I.T. were the field geologists for the respective 2013 and 2015 field studies.

4 LABORATORY TESTING PROGRAM

The following laboratory tests were performed on soil and/or rock samples obtained from the test explorations:

• Moisture Content - Dry Density (ASTM D2216 / D2937),

• Unconfined Compressive Strength (ASTM D2166),

• Point Load Index (ASTM D5731).

Laboratory test locations are referenced on the LOTB drawings in Appendix II and laboratory test reports are provided in Appendix III.

5 GEOTECHNICAL CONDITIONS

5.1 GEOLOGY

In the vicinity of the Murray Street Bridge, published1 geologic mapping shows three geologic units as follows:

• Basin deposits (Holocene);

• Lowest emergent coastal terrace deposits (Pleistocene); and

• Siltstone and Sandstone – Purisima Formation (Pliocene and upper Miocene).

1 Brabb, Earl E., Geologic Map of Santa Cruz County, California, United States Geological Survey (USGS), Miscellaneous Investigation Series, Map I-1905, Scale 1:62,500, 1989.

The higher elevations at the margins of the lagoon are mapped as “Lowest emergent coastal terrace deposits” of Pleistocene-age. Materials of this geologic unit are described as semiconsolidated, generally well-sorted sand with a few thin, relatively continuous layers of gravel. Locally it includes many small areas of fluvial and colluvial silt, sand and gravel.

Underlying this unit is siltstone and sandstone of the Pliocene and upper Miocene-age Purisima Formation. This geologic unit is described as very thick bedded yellowish-gray tuffaceous and diatomaceous siltstone containing thick interbeds of bluish-gray, semifriable, fine-grained andesitic sandstone.

The “Basin deposits” of Holocene-age are shown overlaying the Purisima Formation within the lagoon and along the lower reaches of the slopes bordering the lagoon. These deposits are described as unconsolidated, plastic, silty clay and clay rich in organic material. Locally this unit includes interbedded thin layers of silt and silty sand.

No active fault traces are shown on published mapping to cross the site. Site seismicity and information on local faults and seismic sources is presented in Section 10. A Geologic Map is shown as Figure 3 in Appendix I.

5.2 SURFACE CONDITIONS

The existing Murray Street Bridge is 35±feet wide by 540±feet long structure is aligned east-west and consists of nine 60-foot spans of reinforced concrete deck on precast, prestressed concrete I-girders. The substructure is comprised of two-column bents and open-style seat abutments. The structure was damaged in the 1989 Loma Prieta earthquake. Bent-2 sustained some transverse lateral displacement and as much as 2-inches of settlement. Subsequent repair work included strengthening displaced columns and excavating to visually observe pile conditions at Bents-2, -3 and -4. Bent-6 was extended on the south side of the bridge. It is not known if damage to the railroad bridge also occurred in 1989.

Upstream (north) of the bridge, Woods Lagoon is aligned northeast-southwest; downstream (south) of the bridge, Woods Lagoon is aligned north-south. The bridge is located just downstream (south) of the confluence of a southerly flowing tributary stream that enters into Woods Lagoon from its west bank. A Santa Cruz County Regional Transportation Commission (SCCRTC) railroad bridge that also crosses over Woods Lagoon is located 20-35±feet north of the Murray Street Bridge.

Woods Lagoon is the location of the Santa Cruz Small Craft Harbor. Construction of the bridge and the harbor included placement of artificial fill along the margins of the lagoon and dredging of a boat channel below the bridge. A previous structure known as the East Cliff Drive Bridge that crossed over Woods Lagoon was also demolished/removed when the existing bridge and harbor improvements were completed.

An existing 30-inch diameter sewer force main crosses below the lagoon on the south side of the bridge. Overhead utility lines are also present along Murray Street in close proximity to the existing bridge.

5.3 SUBSURFACE CONDITIONS

Earth materials encountered in the borings completed for this study are considered consistent with the published geologic mapping. Refer to the LOTBs in Appendix II for specific soil/rock descriptions, blow count data and elevations. Refer to 2018 Caltrans Standard Plans A10F, A10G and A10H that provide an explanation of terms and engineering geology descriptors used to log the soil and rock. A general description of the materials logged in our borings are shown in Table 3.

Table 3: Subsurface Materials

Unit Depth Range

(ft) Elev.

(ft) Generalized Soil/Rock Layer Descriptions

1 0 – 15 48 to 31 very stiff sandy clay with gravel, medium dense silty/clayey sand, very loose silty sand and sand with gravel.

2 14 - 41 31 to 6 variably weathered sedimentary rock (sandstone) poorly indurated, intensely weathered, soft, (excavated as very dense silty sand in most borings)

Notes: The locations/depths/elevations shown in the table are approximate. Based on boring data rock surface descends slightly from west to east.

The upper portions of the underlying rock unit were penetrated with light-weight drilling equipment using tungsten-carbide tipped drill bits. Some cemented zones were highly resistant to this drilling method. Where attempted, Standard Penetration Test (SPT) samplers typically achieved only partial penetration of the rock unit. In boring A-13-011 where the rock was cored with a diamond drill bit, the recovery varied from 0 to 100% and Rock Quality Designation varied from 0 to 96%.

Point Load Index test results on intensely to moderately weathered rock core samples from boring A-13-011 typically indicate weak rock with correlated uniaxial compressive strengths that range from 200 to 300 pounds/square inch (psi). It was noted that these samples were soft and broke apart with light pressure. Factors accounting for the variability in Point Load strength include rock composition, fracturing, grain size and weathering characteristics. The Point Load Index test results are included in Appendix III.

6 GROUNDWATER

The groundwater conditions encountered in the borings completed for RW-9 are presented in Table 4.

Table 4: Summary of Groundwater Data

Boring Number

Ground Surface

Elevation (ft)

Depth to Groundwater

(ft)

Groundwater Elevation

(ft) Date

Measured

A-13-011 30.7 7.1 23.7 3/27/2013

D-13-012A 31.6 Not encountered N/A N/A

Boring Number

Ground Surface

Elevation (ft)

Depth to Groundwater

(ft)

Groundwater Elevation

(ft) Date

Measured

A-13-012 31.7 4.0 27.6 3/27/2013

A-15-017A 32.0 Not encountered N/A N/A

R-15-017B 31.8 10.7 21.1 2/10/2015

HA-15-018A 48.4 Not encountered N/A N/A

A-15-018B 48.3 23.6 24.7 2/11/0215

A-15-019 31.1 15.7 15.4 2/11/2015

Groundwater levels can fluctuate due to changes in precipitation, irrigation and other factors.

The groundwater elevation used for design was 25 ft.

7 AS-BUILT FOUNDATION DATA

No as-built retaining wall foundation plans were available to review for this project at the time of this report was prepared.

8 SCOUR DATA

Scour is not indicated to be a consideration for retaining wall design.

9 CORROSION EVALUATION

For structural elements, Caltrans2 defines a corrosive environment as an area where the soil has either a chloride concentration of 500 ppm or greater, a sulfate concentration of 1,500 ppm or greater, or has a pH of 5.5 or less. With the exception of MSE wall design, Caltrans does not require soil and water to be tested for chlorides and sulfates if the minimum resistivity is greater than 1,100 ohm-cm.

The project is located in a marine environment and within a tidal channel (chloride concentration in seawater is approximately 19,000 mg/l) that is considered corrosive and the design of reinforced concrete and steel foundation elements should consider potential exposure to corrosive salt water and marine atmosphere. Section 12 of the 2021 Caltrans guidelines provides information regarding corrosion mitigation measures for structural elements and lists additional Caltrans guideline documents regarding corrosion mitigation.

10 SEISMIC INFORMATION

10.1 SHEAR WAVE VELOCITY

For this project element, a correlated shear wave velocity (VS30) in the upper 100 ft of the soil profile equal to 256 meters/second (about 840 feet/second) is considered appropriately

2 Caltrans Corrosion Guidelines Version 3.2, May 2021.

conservative for use in RW-9 design. This value corresponds to a “stiff" soil with180 m/s < VS30 < 360 m/s (600 ft/s < VS30 < 1200 ft/s).

The VS30 value was determined for this site based on the subsurface data obtained from the 2013 and 2015 test borings completed for/near RW-9 and correlations with SPT blow count N-values corrected for hammer efficiency using the equations outlined by Caltrans3. The correlated VS30 values estimated from the 2013 and 2015 test boring logs are shown in Table 5.

Table 5: Correlated Shear Wave Velocity

10.2 GROUND MOTION HAZARD

For RW-9, the Caltrans ARS Online (V3.0.2)4 web-based tool was used to calculate the probabilistic acceleration response spectra for the site based on criteria outlined in Appendix B of Caltrans Seismic Design Criteria Version 2.0 (SDCv2).

A probabilistic evaluation approach is used to determine the Safety Evaluation Earthquake (SEE) design spectrum taken as the spectrum based on the 2008 USGS Seismic Hazard Map for the 5% in 50 years probability of exceedance (or 975-year return period). The SEE design spectrum was developed using the Caltrans ARS Online Tool V3.0.2. We used site coordinates of 36.9679°N and 122.0043°W for RW-9.

Caltrans structure design practice requires an increase to spectra due to fault proximity (near-fault factor) and when the site is located over a deep sedimentary basin (basin factor).

The near-fault adjustment factor is applied for locations with a site to rupture plane distance (Rrup) of 25 km (15.6 miles) or less to the causative fault and is based on the deaggregated mean distance for spectral acceleration at a period of 1.0 second. The basin factor does not apply to this site, and the near-fault adjustment factor does apply to the site.

The mean magnitude value reported by Caltrans ARS Online V3.0.2 is not used in the ground motion calculation. It is included to support simplified liquefaction analysis and is obtained from a hazard deaggregation performed at the Peak Ground Acceleration (PGA).

3 California Department of Transportation, Methodology for Developing Design Response Spectrum for Use in

Seismic Design Recommendations, Appendix A, November 2012.

4 https://arsonline.dot.ca.gov/, accessed 10/14/2021.

Project Element

Boring Number

Top of Boring Elevation

(ft)

Bottom of Boring

Elevation (ft)

Total Boring Depth

(ft)

Correlated Shear Wave Velocity in

Upper 100 ft

VS30

(m/sec)

VS30

(ft/sec)

RW-9 A-13-011 30.7 5.6 25.1 256 840

RW-9 R-15-017B 31.8 -8.5 40.3 318 1,043

RW-9 A-15-018B 48.3 16.4 31.9 351 1,152

RW-9 A-15-019 31.1 -9.9 41.0 327 1,073

Based on the above information, we recommend RW-9 design using the SEE Design Acceleration Response Spectrum in accordance with the following Caltrans SDC parameters:

• Shear Wave Velocity, VS30: 256 meters/second (840 feet/second);

• Peak Ground Acceleration (PGA): 0.6 g;

• Magnitude (M) at PGA: 7.0; and

• Mean Site-to-Fault Distance at 1.0 Second: 19.2 km / 11.9 mi.

10.3 OTHER SEISMIC HAZARDS

10.3.1 SURFACE FAULT RUPTURE

The project site is neither located within an Alquist–Priolo Earthquake Fault Zone (EFZ), nor within 1,000 feet of an unzoned fault (not located in an Alquist–Priolo EFZ) that is Holocene (11,000 years) in age or younger and no faults with displacement in the last 15,000 years (Holocene-Latest Pleistocene age or younger) are mapped by the California Geological Survey (CGS) or the United States Geological Survey (USGS) within or through the project area.

Based on the foregoing, surface fault rupture within the project limits is not expected unless a previously unmapped fault that crosses beneath the site were to rupture. In our opinion the structure is not susceptible to surface fault rupture hazard and no additional fault study is required (Caltrans Memo to Designer 20-10). We show nearby faults on Figure 4 in Appendix I.

10.3.2 LIQUEFACTION POTENTIAL

Soil liquefaction can occur when saturated, relatively loose sand and specific soft, fine-grained saturated soils (typically within the upper 50 feet) are subject to ground shaking strong enough to create soil particle separation that results from increased pore pressure. This separation and subsequent pore pressure dissipation can lead to decreased soil shear strength and settlement.

Liquefaction is known to occur in soils ranging from low plasticity silts to gravels. However, soils most susceptible to liquefaction are clean sands to silty sands and non-plastic silts. Granular soils with SPT blow count (N1)60 ≥ 30, rock and most clay soil are not liquefiable.

Based on the soil, bedrock and groundwater conditions encountered in the test borings completed for RW-9, the potential for liquefaction does not exist for this project element and is not a geotechnical consideration for RW-9 foundation design.

10.3.3 SEISMICALLY INDUCED SETTLEMENT

During a seismic event, ground shaking can cause densification of loose to medium dense granular soil above the water table that can result in settlement of the ground surface. Based on the data generated for this project element, intact bedrock along the layout line of RW-9 is at expected to be at shallow depth. The magnitude of seismically-induced ground settlement (< 0.5-inch, should it occur) is expected to be insignificant and is not expected to adversely impact the retaining wall foundations established in the underlying bedrock.

10.3.4 SEISMIC SLOPE INSTABILITY AND LATERAL SPREADING

Proposed RW-9 is located along an existing cut-slope that exposes intact sedimentary rock. No indications of slope instability were observed at the existing cut-slope and the cut-slope appears to be performing well. Therefore, seismic slope stability at RW-9 is not a design consideration.

Similarly, competent/intact bedrock is present at shallow depth along the RW-9 layout line and lateral spreading is not a foundation design consideration.

10.3.5 TSUNAMI

The west coast of California is subject to tsunami hazard. A tsunami is a large sea wave or series of waves generated by the sudden displacement in the sea floor that can be produced by an earthquake, submarine landslide, volcanic activity, or other event that causes large-scale disturbance of the sea floor.

Woods Lagoon and the Santa Cruz Small Craft Harbor are located within a mapped tsunami inundation zone5. The likely source for Tsunami generation at the project site is an earthquake (distant source). In March 2011, the Santa Cruz Harbor sustained damage from tsunami wave surges triggered by the March 11, 2011 magnitude 9 Tōhoku Earthquake that occurred in the north-western Pacific Ocean located about 50 miles from the east coast of Japan. While that event caused damage to boats and docks within the Santa Cruz Small Craft Harbor, no damage was reported to have occurred to the existing Murray Street Bridge.

Based on the foregoing, there is a potential for tsunami and inundation hazard at the project site. However, the proposed RW-9 location is outside the mapped inundation hazard zone, therefore potential hazard from a tsunami at the proposed RW-9 location is considered low.

Should there be important design considerations with respect to tsunami hazard for this project/site, additional study would be required.

11 GEOTECHNICAL RECOMMENDATIONS

Subsurface foundation conditions at the project site are considered to be adequately stable and suitable for construction of the proposed soldier pile retaining wall (RW-9). Based on the location of the proposed wall line, we expect that intact bedrock (sedimentary rock) will be present at/below the planned bottom of the retaining wall lagging and that vertical CIDH pile elements will achieve support entirely within the underlying bedrock.

The risk of fault rupture hazard is not a design consideration. The proposed retaining wall is located on the bluff above the lagoon and is outside the limits of tsunami hazard. No other over-riding geologic hazards (e.g., liquefaction, seismically-induced settlement, downdrag, landslides, subsidence, etc.) were identified by either published geologic mapping or site exploration/reconnaissance performed for this project element.

The subsurface data suggest that the underlying (i.e., buried) rock surface is generally consistent throughout the proposed RW-9 location and the rock surface descends from west to east.

5 Tsunami Inundation Map For Emergency Planning, State of California – County of Santa Cruz, Santa Cruz

Quadrangle, Scale: 1:24,000, July 1, 2009

The proximity of structures/facilities located in the project vicinity, particularly the residence above the retaining wall, existing bridge, and underground/overhead utilities will require consideration during construction.

11.1 GEOTECHNICAL ENGINEERING PARAMETERS

For permanent wall design, we recommend the retained soils be modeled as a granular soil and the embedded soil (i.e., the underlying sedimentary bedrock) be conservatively modeled as a very dense granular soil. The recommended idealized geotechnical parameters for use in soldier pile wall design are shown in Table 6.

Table 6: Idealized Geotechnical Engineering Parameters

Soil Average

N60 (blows/foot)

Unit Weight g pcf

Friction Angle f degrees

Cohesion c psf p-y Modulus, k (lb/in3)

Soil 11 (Retained Section) 38 120 34 0 90

Soil 22 (Embedded Section) > 60 125 42 0 225

1) Upper Unit native fine-grained soil modeled as granular soil for retaining wall design.

2) Lower Unit sedimentary bedrock modeled as very dense sand for retaining wall design.

11.2 SOLDIER PILE WALL (RW-9)

With use of the idealized geotechnical engineering parameters provided in Table 6, the earth pressure distributions for permanent non-gravity cantilevered walls shown in Figure 3.11.5.6-1 of AASHTO LRFD BDS (8th Edition) may be used for retaining wall design, except the active pressure against the back of the soldier pile as shown in the cited figure can be neglected. This approach is considered appropriately conservative for retaining wall design since the soldier piles will be embedded into intact sedimentary rock. The top of wall deflection should not exceed one inch.

For RW-9, the sequence of construction of the soldier pile wall is expected to proceed with installation of the vertical (soldier) piles followed by installation of the lagging from the top down with excavation in front of the wall as lagging progresses.

11.2.1 SOLDIER PILES

For vertical 30-inch diameter CIDH (soldier) pile elements we recommend at least 2B spacing Section 19-3.02C(2), where B is the CIDH pile diameter. All piles are recommended to be embedded a minimum of 23 ft into intact sedimentary bedrock (modeled as very dense granular soil) measured from the bottom of the lagging or a minimum of two pile diameters below the required structural design pile tip elevation (to be determined by the designer), whichever is lower.

For design, pile “fixity” can be taken at 6 ft below the bottom of the lagging.

For axial capacity analysis of soldier piles embedded in intact bedrock (Unit 2), use a unit side resistance equal to 1.5 ksf to determine the nominal axial (compressive) resistance. Neglect the

Murray Street Bridge Seismic Retrofit and Barrier Replacement City of Santa Cruz, California contribution from the uppermost 5 ft below the bottom of the lagging. The designer may apply an appropriate resistance factor (LRFD) to the calculated nominal axial resistance, as applicable.

11.2.2 WALL LAGGING AND BACKFILL

Install timber lagging between steel H-pile flanges to retain the slope. Wall lagging is recommended to be installed in a “top down” manner in lifts. Each lift is recommended to consist of a maximum 5 ft cut to expose the wall line and allow for installation of lagging. Excavation behind the front of wall face should be the minimum necessary to install wall lagging tight against the drainage element. The drainage element needs to be tight against the back of cut without creating voids as discussed in Section 11.2.4.

If/where needed, it is recommended materials that meet Structure Backfill requirements per Section 19 of Caltrans 2022 Standard Specifications be used for retaining wall backfill. Place and compact the Structure Backfill in accordance with Sections 19-3.03E(1) and 19-3.03E(3) of Caltrans Standard Specifications. Compact the Structure Backfill behind the wall to 90% (per CTM 216) relative compaction.

Any material used as backfill should be approved by the soils engineer. On-site soils are not expected to meet Structure Backfill requirements. Unless further testing at time of construction confirms suitability for use as Structure Backfill, on-site soils should be excluded from use as retaining wall backfill.

11.2.3 LATERAL EARTH PRESSURES

The recommended nominal (i.e., ultimate) equivalent fluid weights (EFW) for design of the soldier pile wall are provided in Table 7. The designer should apply appropriate resistance factors (LRFD) to these values as applicable.

Table 7: Equivalent Fluid Weights

Wall Element Soil Type

Slope Condition

Active Static

EFW

(pcf)

Passive Static

EFW

(pcf)

Active Seismic

EFW

(pcf)

Retained Soil (behind wall lagging) Soil 1 1½ :1 (H:V) 100 NA 51

Embedded Soil (soldier piles) Soil 2 level NA 400 NA

NA = Not Applicable

For earth pressure against the back of the retaining wall use an active equivalent fluid weight (EFW) of 104 pcf to calculate lateral force on the wall. Use a triangular pressure distribution and apply the controlling static resultant earth pressure at a distance of H/3 from the base of the wall (bottom of lagging). This value is based on retained soils (Soil 1) under fully drained condition and sloped backfill at 1.5H:1V behind the retaining wall.

For seismic design, calculate the resultant of incremental lateral soil pressure due to seismic loading based on an equivalent fluid pressure of 53 pcf6. Use a triangular pressure distribution and apply the magnitude of the resultant at H/3 from the base of the wall (bottom of lagging).

Add the resultant of the seismic earth pressure to the resultant of the static active earth pressure.

For surcharge loads, apply an additional uniform lateral load behind the wall that is the greater of: 0.3-times the design surcharge pressure, or 0.3-times a minimum surcharge of 240 psf.

To evaluate resistance to lateral CIDH (soldier) pile foundation movement, use a passive pressure of 400 pounds per square foot, per foot of depth, for the embedded soil (i.e., sedimentary bedrock modeled as a very dense granular soil). Distribute the passive pressure using a triangular distribution below the base of the wall (i.e., bottom of lagging), limited to a maximum nominal passive pressure of 7 ksf. The passive pressure can be applied over three times the pile diameter. Neglect the active pressure against the back of the soldier piles since they are embedded into intact bedrock.

11.2.4 WALL DRAINAGE

Installation of a drainage element placed against between the back of the wall lagging and exposed cut is recommended to provide relief of seepage and control fines migration. Wall drainage element should be provided by means of a prefabricated drainage panel (e.g., geocomposite wall drain, Caltrans Standard Specification 96-1.02C) attached to the wall lagging with filter fabric backing. The filter fabric used should meet the requirements of Class C filter fabric (Caltrans Standard Specification 96-1.02B).

The drainage element should extend from the base of the wall (bottom of lagging) to within 18-inches of finished surface at the top of the wall. Drainage element installation is anticipated to be from top down and placed behind the wall as each section is excavated. Separate drainage elements will need to be installed for each lagged section. Retaining wall design should also include vertical spacing (e.g., minimum 0.25-inch gap) between lagging. Provide an underdrain along the bottom of the wall (bottom of lagging) to collect the wall drainage behind the retaining zone and gravity flow to solid drainpipe outlet(s) away from the wall.

11.2.5 SURFACE DRAINAGE

Surface water should be directed away from the construction area and also prevented from draining over the edge or at the ends of the wall without provisions to control erosion. The sidewalk in front of the wall should be sloped to drain away from the wall face and prevent surface water from ponding.

12 NOTES FOR SPECIFICATIONS

This section is provided to assist the designer develop the geotechnical related Standard Special Provisions (SSPs) for this project element. Before using the information provided in this section, the designer should read and review the report to comprehend the contents and intent of the geotechnical design.

6 Determined based on equations presented in Agusti, G.C. and Sitar, N., Seismic Earth Pressures on Retaining

Structures in Cohesive Soils, Technical Report CA13-2170, California Department of Transportation, 08/14/2013

12.1 GENERAL CONDITIONS

Retaining wall construction should conform to Caltrans 2022 Standard Specifications unless otherwise stated in the “Special Provisions”. We include the following Caltrans Standard Specification sections as a minimum (non-comprehensive) to be included into the project plans and specifications.

• Section 47 Earth Retaining Structures

• Section 19-3.03B(1) Structure Excavation: General

• Section 19-3.03B(2) Structure Excavation: Soldier Pile Walls

• Section 19-3.03E(1) Structure Backfill: General

• Section 19-3.03E(3) Structure Backfill: Soldier Pile Walls

• Section 19-3.03J Soldier Pile Walls

• Section 49-1.03 Difficult Pile Installation

• Section 49-3.02C(2) Construction: Drilled Holes

• Section 49-4.01 Steel Soldier Piling: General

• Section 49-3.02A(3)(b) CIDH Concrete Piling: Submittals: Pile Installation Plan

• Section 49-3.02C(1) Construction: General

• Section 49-4.03A Difficult Drilling Conditions

• Section 49-4.03B Steel Soldier Piling: Drilled Holes

• Section 49-4.03C Steel Soldier Piling: Steel Soldier Piles

• Section 90-1.02G(6) Mixing and Transporting Concrete: Quantity of Water/Slump

• Section 90-1.02H Mixing and Transporting Concrete: Corrosive Environments

For the project described herein, we also recommend that the foundation report, log of test borings and legend, and any subsequent addenda be included with project documents during the bidding process for reference purposes.

12.2 CIDH FOUNDATIONS

To help avoid cross communication between the CIDH pile excavations, stagger the construction/installation of the piles so that no two holes are open at the same time within 4 pile diameters. No drilled hole should remain open overnight. The sequence of soldier pile construction/installation should be specified in the contract documents.

Add to Section 49-1.03:

Expect difficult pile installation due to the conditions shown in the following table:

Pile location Conditions Bridge no. Support location

36C0108 Abutment 1 Intensely to moderately weathered, moderately to slightly fractured, soft to moderately soft, fine grained sandstone

Add to Section 49-3.02C(1):

If the piling center-to-center spacing is less than 4 pile diameters, do not drill holes or install casing for an adjacent pile until 24 hours have elapsed after concrete placement in the preceding pile and your prequalification test results for the concrete mix design show that the concrete will attain at least 1800 psi compressive strength at the time of drilling or installation.

Add to Section 49-4.03A:

Difficult drilling conditions through variable rock (intensely to moderately weathered, soft to moderately soft sandstone) is anticipated during construction.

12.3 CONSTRUCTION MONITORING

Heavy construction equipment and drilling operations for RW-9 will be in close proximity to the existing residence located above the existing cut-slope. While there are typically less impacts associated with vibration/settlement of drilled shaft installation/construction versus pile driving activities, advance notification of residence/business owners, pre-construction condition surveys, and vibration monitoring plans promote "good will" and can reduce the risk of claims.

Therefore, for residences/structures/facilities within a 1,000 ft radius of RW-9, we recommend construction term monitoring as follows:

1. Conduct a pre-construction survey to record the exterior and (if possible) interior of existing facilities (particularly the residence located above RW-9) which are identified as settlement sensitive. The condition of each facility should be documented with use of video, photographs and written descriptions. Where existing cracks are apparent, the use of crack-monitors (or “tell-tales”) should be employed as a means to determine whether cracking is exacerbated by RW-9 construction operations.

2. Daily observation of critical facilities for cracking, settlement or other damage that might be occurring as a result of construction activities.

3. Conduct a thorough post-construction review of existing facilities for cracking, settlement or other damage which might be present after construction. The condition of each facility should be documented with use of video, photographs and written descriptions.

12.4 EXISTING FACILITIES

Significant existing utilities and structures may be located within the planned improvement area.

Existing single-family home structure may be impacted and monitoring prior, during and post construction is discussed above. Overhead utility lines are also present along Murray Street in close proximity to the existing bridge. Overhead utilities were observed throughout the project area. Existing utilities will need to be protected or moved/replaced prior to construction.

13 NOTES FOR CONSTRUCTION

13.1 EXCAVATION AND SHORING

The soils and bedrock to the planned bottom of the retaining wall lagging are anticipated to be rippable by typical heavy-duty excavation equipment. The contractor is responsible for design, construction, and safety of all excavations and shoring in accordance with CalOSHA requirements.

13.2 SOLDIER PILES

Based on boring data, it expected that excavation of the sedimentary bedrock within CIDH (soldier) pile excavations can be achieved using heavy-duty foundation drilling equipment. Intact bedrock may contain resistant zones/layers that may require rock auger, air tools, or other methods to excavate. The contractor should anticipate variable drilling conditions in all CIDH pile excavations due to the variable hardness of the bedrock. Variable drilling conditions may include alternating between soft and hard drilling techniques.

Temporary casing is permitted where necessary to help control water or to prevent caving of hole excavation.

Excavation of the CIDH piles, placement of the steel soldier pile, and concrete pour should be completed in one continuous operation.

Before mobilizing to the site, the foundation contractor should prepare and submit a detailed work plan for the engineer’s review and approval. The work plan should state explicitly any assumptions that the contractor has made regarding earth materials and foundation construction conditions. The work plan should include details of proposed equipment, personnel, materials, methods and order of work. The actual tools and equipment used during CIDH (soldier) pile excavation/installation should be documented in the construction records.

13.3 DEWATERING

Groundwater was encountered in the borings completed for this study. The soils and upper portions of the underlying bedrock unit are expected to be capable of transmitting seepage to open excavations. The lower portions of the underlying bedrock unit are expected to have a relative lower permeability, but still capable of transmitting seepage to open excavations.

Due to the presence of groundwater encountered in the borings completed for RW-9, the installation of the soldier piles will likely require dewatering of the borehole before the concrete is placed. Alternatively, current Caltrans practice for soldier pile construction does allow the use of mineral, synthetic, and water slurry. Mineral and synthetic slurry should comply with Section 49-3.02B(6) and water slutty should comply with Section 49-4.02B.

14 RISK MANAGEMENT

Our experience, and that of our profession, clearly indicates the risks of costly design, construction, and maintenance problems can be significantly lowered by retaining the Geotechnical Engineer of Record to provide additional services during design and construction.

For this project, Crawford should be retained as the Geotechnical Engineer of Record to:

• Review and provide comments on the final plans and specifications, insofar as they rely upon this report, prior to construction bidding to verify consistency with the recommendations contained herein;

• Review soldier pile workplan prior to construction;

• Review soldier pile installation during construction in order to confirm anticipated bearing materials and provide additional or alternate recommendations if necessary.

• Review vibration and noise monitoring submittals and data; and

• Update this report if design changes occur, 2 years or more lapse between this report and construction, and/or site conditions have changed.

Should there be significant change in the project or should soil/rock conditions different from those described in this report be encountered during construction, this office should be contacted/notified for evaluation and supplemental recommendations as necessary or appropriate.

Crawford cannot be responsible for any other parties’ interpretation of our report and recommendations contained herein, as well as subsequent addendums, letters, and discussions. If others perform the construction observation, they should review this report and either accept the conclusions and recommendations herein as their own or provide alternative recommendations.

15 LIMITATIONS

The conclusions and recommendations of this study are professional opinion based upon the indicated project criteria and the limited data described herein. It is recognized there is potential for variation in subsurface conditions and modification of conclusions and recommendations might emerge from further, more detailed study. This report is intended only for the purpose, site location, and project description indicated and construction in accordance with Caltrans practice.

As changes in appropriate standards, site conditions and technical knowledge cannot be adequately predicted; review of recommendations by this office for use after a period of two years is a condition of this report.

A review by this office of any foundation…