CA BLM 2020(1) San Vicente Bridges_Geotech Rpt FINAL 12.3.19.pdf

San Vicente Bridges

CA BLM 2020(1)

Cotoni-Coast Dairies National Monument Santa Cruz County, California

Final Geotechnical Report

Report No. CA-LX-2020-19-01

Prepared by:

Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch December 2019 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

Devin T. Dixon, P.E., Geotechnical Engineer

Report reviewed by: ____________________________________________________________ James M. Arthurs, P.E., Ph.D., Geotechnical Engineer

Approved for distribution by: ____________________________________________________

Marilyn D. Dodson, P.E., Lead Geotechnical Engineer

Date: December 3, 2019

Electronic Distribution N:\CA\2020(1)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Design Bridge i

San Vicente Bridges, CA BLM 2020(1) December 2019 Page i

Table of Contents

SECTION ONE --INTRODUCTION

SECTION TWO --GEOLOGY AND SEISMICITY

2.1. REGIONAL GEOLOGY

2.2. SITE GEOLOGY

2.3. SEISMICITY

2.4. SEISMIC DESIGN PARAMETERS

2.5. GEOLOGIC HAZARDS

SECTION THREE --FIELD EXPLORATION

3.1. SUBSURFACE EXPLORATION PROGRAM

3.2. LABORATORY TESTING PROGRAM OF SOIL

3.3. ROCK CHARACTERISTICS

Generalized Strength Criterion for Intact Rock

Rock Characterization

SECTION FOUR --ANALYSIS AND RECOMMENDATIONS

4.1. SITE CONDITIONS

4.2. SHALLOW FOUNDATIONS

4.2.1 Bearing Resistance

4.2.2 Sliding and Lateral Stability

4.2.3 Settlement Estimation

4.2.4 Scour Potential

Depth of Influence and Frost Protection

Erosion and Corrosion Protection

4.2.7 Construction Considerations

SECTION FIVE --LIMITATIONS

SECTION SIX --REFERENCES

ii

San Vicente Bridges, CA BLM 2020(1) December 2019 Page ii

TABLES

Table 2.1: Summary of Nearby Seismic Source Faults

Table 2.2: Summary of Seismic Parameters for Site Class B

Table 3.1: Summary of Subsurface Explorations

Table 3.2: Summary of Laboratory Testing Results

Table 3.3: Summary of Uniaxial Compression Test Results of Siltstone from Monterey Bay Sanctuary Scenic Trail Project, CA FLAP SCR T5 (1) & (2)

Table 3.4: Strength Parameters Calculated for Rock Based on Hoek Brown Criterion

Table 4.1: Summary of Spread Footing Design Information

Table 4.2: Resistance Factors for Abutment Design

Table 4.3: Allowable Bearing Pressures for Comparable Geology

Table 4.4: Bearing Values for Shallow Foundations

PLATES

Plate 1: Project Vicinity Map Plate 2: Geology Map Plate 3: Geotechnical Exploration Locations

APPENDICES

Appendix A: Field Exploration Program Appendix B: Laboratory Test Results Appendix C: Photographs Appendix D: Special Contract Requirements

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 1

SECTION ONE--INTRODUCTION

This report presents the results of the geotechnical investigation conducted for the San Vicente Bridges, CA BLM 2020(1), replacement and removal project and provides bridge foundation recommendations. The bridges cross over San Vicente Creek on San Vicente Road approximately

1.0 and 1.9 miles northeast of Davenport, California within the Cotoni-Coast Dairies National Monument, as shown on Plate 1, Project Vicinity Map.

The two bridges have deteriorated to such an extent that Bridge 1 has been arbitrarily load posted to 3 tons due to deck damage and Bridge 2 has been closed to all traffic. Access to the bridges is currently controlled by a locked gate and only accessible to local residents and the Bureau of Land Management (BLM). The road provides access to private property owners and BLM administrative use. San Vicente Road may potentially be opened to the public as a trail in the future thus the replacement structure should be designed to accommodate pedestrians, cyclists, and equestrians.

Public vehicular use is not anticipated but the replacement structure will provide for BLM vehicular access and BLM heavy construction equipment access.

The original bridges were constructed in the late 1960’s with a timber decking performed in the early 1980’s. Bridge 1 is oriented roughly west to east with San Vicente Creek roughly flowing from north to south. The bridge spans about 29 feet consisting of a 12-foot wide timber deck which rests on longitudinal timber nailing boards. The nailing boards then rest on four W12x65 steel girders. There are no bridge railings, however there are curbs made of 8’x8’ timbers on 8’x8’ blocks. Both abutments at Bridge 1 are concrete stem wall abutments. Bridge 2 is oriented roughly south to north with San Vicente Creek roughly flowing from northeast to southwest. The bridge consists of a timber deck with no longitudinal timber running planks for the wheel lines. The deck is 12-feet wide and the length of the structure is approximately 40-feet. The superstructure consists of a railcar flanked by one upstream and one downstream W16x40 beam. There are no bridge railings, however there are curbs made of 4’x4’ timber resting directly on the deck. Both abutments consist of concrete stem walls, approximately 8-feet high, sitting on spread footings. The condition of both abutments is such that they are not reusable. Between Bridges 1 and 2, San Vicente Road is less a road and more of a trail. The road between the structures is unlikely to be more than 10-feet wide, with locations that are closer to 8-feet in width. The road is blocked at numerous locations by fallen trees and cut slope slides that have blocked a portion of the roadway. The overgrowth of vegetation has also made this path difficult to traverse. The roadway approaches at the bridges consist of a gravel/dirt unpaved surface and traffic estimates indicate a current and projected ADT of less than 100.

Replacement of Bridge 1 (San Vicente Creek Bridge) is a critical need for the BLM management in the area. Proposed construction work at Bridge 1 includes the following:

1. Removal of the existing 29-foot long structure with timber decking on four steel girders.

2. Replacement with a 51-foot long structure meeting the Q5 design flood hydraulic capacity and

14-foot minimum clear width from inside to inside of railing. The channel will be widened and restored to accommodate natural flow characteristics of the channel.

3. The new structure will be designed in accordance with AASHTO LRFD Bridge

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 2

Specifications. The structure will be designed for HL-93 live load.

4. Aggregate surface approaches at both ends of the bridge.

5. Bridge railing will meet requested equestrian and bicycle height requirements while providing minimum TL-1 load rating for occasional vehicle use. Object Markers, Type 3 will be installed at each corner.

6. Structure type proposed will meet FHWA Every Day Counts (EDC) initiative with respect to prefabricated bridge elements and systems.

Given the location of Bridge 2 and the estimated cost, its replacement is not a priority. Therefore, it will be removed (including abutments and grouted riprap) and be graded to match existing terrain and revegetated. Considerations related to the abutments for Bridge 1 are presented in Section 4 of this report.

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SECTION TWO--GEOLOGY AND SEISMICITY

2.1. REGIONAL GEOLOGY

The San Vicente bridges are located within the California Coast Ranges section of the Pacific Border physiographic province. The California Coast Ranges is comprised of numerous north-south trending mountain ranges and spans 400 miles from Santa Barbara County in the south, to Del Norte County in the north. Of these ranges, the San Vicente bridges lie within the Santa Cruz Mountains, which comprise the part of the California Coast Ranges that extends for about 75 miles from the Golden Gate on the northwest, to the Pajaro River on the southeast.

This area, southwest of the San Andreas Fault and northeast of the San Gregorio fault, is comprised of the most complete Tertiary stratigraphic section in the California Coast Ranges. The strata predominantly consist of marine clastic sedimentary rock sequences, displaying multiple marine cycles of transgression and regression. Largely, the sequences are very thick and massive, ranging from 3,000 feet to over 20,000 feet. In general, the coarser-grained sedimentary rocks (gravel beds and sandstone) were deposited in a near-shore, shallow-marine environment, while the finer-grained sedimentary rocks (siltstone, mudstone, and shale) were deposited in a relatively deeper marine environment, most likely in the neritic zone.

Topography of the area is moderately rugged, with elevations ranging from sea level to 2,600 feet along the crest of Ben Lomond Mountain. The area is characterized by deep weathering and dense vegetation, making fresh bedrock exposures rare and largely restricted to canyons, cliffs, and road cuts. Slumps and slides are a common occurrence in the area, especially where the slopes are underlain by mudstone (Clark, 1981).

2.2. SITE GEOLOGY

The bridge site lies to the west of the Ben Lomond Divide along San Vicente Creek. The creek flows southward directly into the Pacific Ocean and has cut a narrow steep-sided canyon in the slope of the mountain.

The bridge site is underlain by the Santa Cruz Mudstone, which was deposited as the area submerged below the zone of effective wave and current action. The Santa Cruz Mudstone is a medium to thick-bedded, faintly laminated, siliceous organic mudstone (Brabb, 1997). It locally grades to a sandy siltstone and may contain spheroidal dolomite concretions. Where exposures are fresh the mudstone displays an olive gray to dark yellowish brown color. It then weathers to a yellowish gray to nearly white color with blocky fractures (Clark, 1981).

A geologic map of the project area is presented in Plate 2.

2.3. SEISMICITY

Numerous seismic source faults are mapped within 40 miles of the project area. The critical active faults to the project site are based on the Caltrans ARS Online tool and may be found in Table 1

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(CalTrans, 2017; U.S. Geological Survey, 2019). These fault systems largely lie to the east and west of the project site, varying in length from approximately 29 to 81 miles. They may all generally be described as right lateral fault systems striking northwest and dipping to the east. San Andreas and San Gregorio are the most active fault systems, exhibiting slip rates ranging from

0.276 to 0.669 inches per year, while the remaining faults are much less active with slip rates as low as 0.00394 inches per year.

Table 2.1: Summary of Nearby Seismic Source Faults

FAULT OR FAULT

ZONE

MAXIMUM

MOMENT

MAGNITUDE

DISTANCE FROM

CENTER OF

PROJECT

FAULT

PARALLEL SLIP

RATE

FAULT

LENGTH AGE

(Mmax) (miles) (inch/year) (miles) (years) San Gregorio (San Gregorio Section) 7.4 3.52 0.276 81.4 <15,000

Zayante-Vergeles Lower

2011 CFM 7.0 6.1 0.00394 54.06 <15,000

Zayante-Vergeles Upper

2011 CFM 7.0 7.09 0.00394 54.06 <15,000

Monterey Bay- Tularcitos (Monterey

Bay section)

7.2 7.93 0.0236 29.1 <15,000

San Andreas (Santa Cruz Mts) 2011 CFM 8.0 14.66 0.669 46.6 <15,000

2.4. SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the San Vicente Bridges site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 7th Edition, 2014, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 100-year return period).

The 1000-year return period uniform hazard spectrum for the San Vicente Bridges Project site, located at 37.024078°N latitude & 122.186181°W, was obtained in accordance with the AASHTO ground motion maps.

Based on the material encountered and tested during excavation of test pits, in addition to the presence of shallow bedrock (5 to 10 feet in depth), the site is classified as Class B according to the site class definitions specified in Table 3.10.3.1-1 of AASHTO. The recommended spectral acceleration coefficient values for probabilistic design are summarized in Table 2.

Table 2.2: Summary of Seismic Parameters for Site Class B

Horizontal Peak Ground Acceleration, (PGA) 0.605 g Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 1.408 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.621 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.0 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.0 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.0

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 5

Factored Horizontal Peak Ground Acceleration, (As) 0.605 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 1.408 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.621 g

Seismic Zone Zone 4

Based on the long acceleration coefficient SD1 value of 0.621, the bridge site is assigned to seismic hazard Zone 4 in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 in AASHTO.

2.5. GEOLOGIC HAZARDS

Geologic hazards exist from both the natural environment of the project site and from existing and proposed construction of the bridge. The main geologic hazards that may exist within the vicinity of the project limits include slope instability, seismic events, and flooding. In addition, hard bedrock is mapped near the surface in the project area and may be encountered in shallow excavations for bridge foundations.

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SECTION THREE--FIELD EXPLORATION

3.1. SUBSURFACE EXPLORATION PROGRAM

A CFLHD geotechnical engineer observed a subsurface investigation of Bridge 1 on August 6th, 2019. The test pit excavations allowed for characterization of the subsurface materials at the proposed bridge abutment locations. Nearby surface outcrops of bedrock supported the decision to utilize a backhoe to excavate test pits to the shallow bedrock. Test pits excavated at each abutment on Bridge 1 investigated the subsurface at the proposed bridge structure location and determined the depth to bedrock. Disturbed soil grab samples were obtained from the excavated material at various depths. Test pit locations are shown on Plate 3, Geotechnical Exploration Locations.

Subsurface test pit conditions were logged and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing.

Logs of the explorations and laboratory test results are presented in Appendix A and B, respectively. A summary of the field exploration is provided below in Table 3.1.

Table 3.1: Summary of Subsurface Explorations

EXPLORATION

DESIGNATION

ABUTMENT

LOCATION

APPROXIMATE

ELEVATION (ft)

DEPTH TO

BEDROCK (ft)

DEPTH TO

GROUNDWATER (ft)

TP19-01

Bridge 1 Eastern

Abutment (Abutment 2)

108 5.0 No groundwater encountered.

TP19-02

Bridge 1 Western

Abutment (Abutment 1)

107 9.8 7.0

Note: Ground elevations were estimated from Google Earth.

3.2. LABORATORY TESTING PROGRAM OF SOIL

Soil grab samples from the test pits were tested in the laboratory to support the field classifications and to provide an estimate of the engineering characteristics and mechanical properties of the soil.

Laboratory tests included moisture content (AASHTO T255), sieve analysis (AASHTO T11 and T27), classification (AASHTO M145 and ASTM D 2487), Atterberg limits (AASHTO T89 and T90), and soil pH (AASHTO T 289). When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. A summary of the soil test results is shown in Table 3.2.

Visual classification and soil test results indicate a general soil profile of poorly graded sand with gravel to well graded gravel with varying amounts of sand and silt, classifying as SP and GW-GM by USCS and A-1-a (0) and A-1-b (0) by AASHTO.

The results of the laboratory testing program and the grain-size distribution curve are presented in Appendix B.

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Table 3.2: Summary of Laboratory Testing Results

Test Pit ID Location Field Number

Depth Interval

(ft)

Grain Size Distribution

L iq ui d

L im it

Pl as tic ity

In de x

M oi st ur e pH

Soil Classification

G ra ve l, Sa nd

Fi ne s, U

SC

S

A A

SH

T

O

TP19-01

Bridge 1 East

Abutment (Abutment 2)

Grab-01 0-5 56 36.3 7.7 NV NP 19.7 - GW-GM A-1-a (0)

TP19-02

Bridge 1 West

Abutment (Abutment 1)

Grab-01 0-2 48 39.7 12.3 NV NP 29.5 - GW-GM A-1-a (0)

Grab-02 2 26 71.4 2.6 NV NP 6.9 8.6 SP A-1-b (0)

N/A Streambed Streambed Sample 0-1.5 2 96.9 1.1 NV NP 23.9 - SP A-1-b (0)

(-) Indicates that not enough sample was provided for testing.

3.3. ROCK CHARACTERISTICS

A natural rock mass is rarely a continuous, isotropic, homogeneous material. It can be highly variable and may be very difficult to characterize in a generalized manner. A rock mass is generally composed of intact or weathered blocks of rock separated by discontinuities such as joints or bedding planes. The discontinuity and weathering characteristics help establish the design recommendations for a footing on rock. These characteristics include, but are not limited to, the rock lithology, strength properties, deformation properties, frequency of discontinuities (RQD), spacing of discontinuities, orientation of discontinuities, aperture, filling, condition, degree of weathering, and the ground water condition. This information was observed at limited bottom of test pit locations. No drilling or geophysical tests were conducted. No laboratory testing of rock was conducted.

As discussed in Section 3.1, test excavations (test pits) were the chosen exploration method to investigate the subsurface at the Bridge 1 abutments. Limited data becomes available with the use of test pits as in-situ geotechnical testing/sampling, such as SPT or Shelby tubes, cannot be performed. However, test pits can be used to determine the geology and, sometimes, the water table of certain sites. It is also possible to differentiate between bedrock and large boulders in the shallow subsurface through the use of test pit excavations. The test pits excavated at the Bridge 1 abutments revealed some cobbles and boulders, from approximately 0.5 to 3 feet in diameter, in the upper stratums followed by a medium hard to hard sedimentary rock layer encountered at 5 and 9.8 feet at the east and west abutments, respectively. This rock layer was classified in the field as siltstone and could likely be identified as part of the Santa Cruz Mudstone formation that is well-known along the northern Santa Cruz coast (see Section 2.2). The following sections discuss the strength properties based on the field investigation and prior experience on another project in the area.

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3.3.1. Generalized Strength Criterion for Intact Rock

Samples were not taken of the rock encountered at the Bridge 1 test pits, therefore laboratory testing such as the uniaxial (unconfined) compression test could not be performed. In the field, the rock could be fractured with about 1-2 blows from a rock hammer and it was estimated that the rock corresponded with the description of “medium strong rock,” R3, as described in the FHWA Rock Description Guidelines. The range of relative strength for R3 rock as listed in this reference is between 3,500-7,250 psi. However, the rock observed in the field at the Bridge 1 site appeared to have slight to moderate weathering.

For comparison with tests conducted in the same geologic formation only three miles from this project site, another FHWA project, the Monterey Bay Scenic Sanctuary Trail, CA FLAP SCR T5

(1) and (2), was examined. This project scope proposed the construction of a segment of the Monterey Bay Sanctuary Scenic Trail in Santa Cruz County, CA. The project investigated the construction of Segment 5 of this trail, which passes through the area just to the south of the Cabrillo Highway along the railroad right of way near Davenport, just a few miles from the San Vicente bridges sites. The geotechnical investigation for this project took place between August 23 and 25, 2016 and included drilling a total of fourteen exploratory borings and installing seven monitoring wells. Laboratory tests were completed on select soil and rock samples recovered for the field exploration program in general accordance with the American Association of State Highway and Transportation Officials (AASHTO) and ASTM testing methods. Uniaxial compression testing of three of the rock samples was conducted on the siltstone, Santa Cruz mudstone, type rock in accordance with ASTM D 2938. The test results can be seen below in Table 3.3.

Table 3.3: Summary of Uniaxial Compression Test Results of Siltstone from Monterey Bay Sanctuary Scenic Trail Project, CA FLAP SCR T5 (1) & (2)

Boring Number Depth Interval (ft) Compressive Strength (psi)

BH16-06

16 1,420 20 1,480 32 2,440

These results are significantly lower values compared to the range of relative strength for R3 rock described in the FHWA Rock Description Guidelines (2017) as mentioned previously. This can likely be attributed to the weathering of the rock over time. It can be reasonably assumed that these values are more representative of the rock encountered at the San Vicente Bridge 1 test pits due to their close proximity to the project site and similarity in material composition. Based on site observations and this correlation to rock tests in the same siltstone formation, the rock strength for the San Vicente Creek Bridge site is conservatively assumed to be approximately 1,420 psi for design. This value is equal to the lowest compressive strength, 1,420 psi, given for the Monterey Bay Sanctuary Scenic Trail project. This is based on the assumption that a similar degree of weathering has occurred on the rock at San Vicente compared to the Monterey Bay project due to the erodible nature the San Vicente Creek and Pacific Ocean tend to have on shallow subsurface

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 9 features. This value is within the range of R2, “weak rock,” as described in Appendix A under the Central Federal Lands Rock Core Description Field Reference.

3.3.2. Rock Characterization

The Geologic Strength Index (GSI) (Hoek and Marinos, 2000) system was used to characterize the rock based on surface geologic mapping and previous boring information from the Monterey Bay project. The GSI is a slight modification of the Rock Mass Rating system (RMR) (AASHTO, 2014) which is more appropriate for the bridge foundation evaluation at this site. The GSI is the algebraic sum of ratings assigned for the following rock mass properties:

• Intact rock strength

• Rock Quality Designation (RQD)

• Discontinuities spacing

• Discontinuities condition

• Presence of groundwater

For this project, the GSI was estimated based on rock outcrops nearby and the rock encountered at the bottom of the test pits by visual inspection. The rock composition was identified as siltstone with moderate weathering and groundwater was encountered at the test pit located near the west abutment at approximately 7 feet below the ground surface. The estimated GSI values given these assumptions corresponds to a range of 30 to 40 (Hoek and Marinos, 2000). A GSI value of 35 was then assumed for the rock. Other pertinent rock properties include the Hoek-Brown constant, mi, rock unit weight, γ, and the damage factor, D. These values were assumed to be 7 for siltstone, 150 pcf, and 0 assuming the rock will remain relatively undisturbed, respectively. For intact rock, the following equations were then used to determine the appropriate curve fitting coefficients:

mb = mi ∗ exp �GSI−100

For GSI > 25: α = 0.5 + 1 ∗ (exp �−GSI

� − exp �− 20 s = exp �

SI − 100

9 − 3 ∗ D �

A summary of the assumed rock properties and the Hoek Brown criteria calculations is presented in Table 3.4 below. These values are directly related to the bearing resistance discussed in Section 4.2.1.

Table 3.4: Strength Parameters Calculated for Rock Based on Hoek Brown Criterion

GSI 𝛔𝛔𝐜𝐜𝐜𝐜 (psi) 𝐬𝐬 𝐦𝐦𝐛𝐛 𝐦𝐦𝐜𝐜 𝛂𝛂 35 1,420 7.302*10-4 0.6869 7 0.516

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SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

This section provides a characterization of the site conditions, analysis, and recommendations for the geotechnical aspects of project design and construction. Recommendations provided include foundations, abutment design, and construction considerations for San Vicente Creek Bridge (Bridge 1).

4.1. SITE CONDITIONS

West Abutment (Abutment 1)

The existing west abutment is a cast-in-place concrete abutment as described in Section 1.

According to the 2017 inspection report, the abutment appears to have a soil foundation with a concrete footing.

The subsurface conditions were investigated at the west abutment by excavating test pit TP19-02.

The soils encountered broadly consisted of granular fill material, gravel and sand. Both cobbles and boulders, up to approximately 1.5 feet in size, were observed during the excavation. This overburden material was underlain by slightly to moderately weathered siltstone rock at a depth of approximately 10 feet. Groundwater, which is greatly affected by seasonal, flow, and weather effects, was recorded at 7 feet.

Laboratory tests were performed on the granular material and indicated no plasticity with a fines content of 12.3 and 2.6 percent based on the two samples taken. Rock samples were not taken from the field. The relative strength of the rock was estimated to be classified as R3, medium strong rock, to R2, weak rock, based on the ISRM method (1978 & 1981).

The area is highly vegetated to the point of substantial overgrowth. Cobbles and boulders, similar in size to those observed in the test pits, are scattered throughout the channel and line the banks.

East Abutment (Abutment 2)

The existing east abutment is also a cast-in-place concrete abutment. From the inspection report, it is assumed that the east abutment has a bedrock foundation that the concrete stem wall is anchored.

The subsurface conditions were investigated at the east abutment by excavating test pit TP19-01.

The soils encountered consisted of gravel with silt and sand. Cobbles and boulders were also present with boulders ranging in size from 0.5 to 3 feet in size. The top of moderately weathered siltstone was at approximately 5 feet. Groundwater was not encountered.

Laboratory tests were performed on the gravel material and indicated no plasticity with a fines content of 7.7 percent based on the one sample taken. Similar to the west abutment, the relative strength of the rock was estimated to be classified as R3, medium strong rock, to R2, weak rock.

A large tree exists near the southeast corner of the structure. Similar site conditions observed at the west abutment, such as overgrown vegetation and the presence of rocks, are present at the east

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 11 abutment.

4.2. SHALLOW FOUNDATIONS

Due to the competent nature of the rock and near-surface exposure, the bridge is anticipated to be supported on spread footings bearing on shallow bedrock. The most significant construction challenges for this type of work typically consist of excavation problems and the proper removal of weathered or disturbed material to provide effective contact between the footing and bedrock (Coduto, 2001). The selection of a shallow foundation supported on rock, as with foundations bearing on soils, includes checking bearing, sliding, and overturning resistance, and settlement potential. A summary of the spread footing design information is presented below in Table 4.1.

Table 4.1: Summary of Spread Footing Design Information

ABUTMENT

LOCATION

ESTIMATED

BEDROCK

ELEVATION (ft)

EMBEDMENT INTO

COMPETENT BEDROCK

(inches) West Abutment (Abutment 1) 989 9.0 (min.)

East Abutment (Abutment 2) 994 9.0 (min)

Note: Survey information based on assumed coordinates (not actual ground elevation).

Note: In the case of over-excavation due to variable subsurface, space between the footing and ground shall be filled with concrete.

Please refer to “Alternate Footing Embedment Detail” on Sheet S4 of the plans for further information.

4.2.1. Bearing Resistance

The abutment footing locations are anticipated to be located on rock. Based on the subsurface materials encountered at the project site and the methods used for determining soil strength parameters, the bearing resistance factors recommended for the long-term stability condition were selected from the LRFD AASHTO Manual (2017), Table 10.5.5.2.2-1 and are presented in Table

4.2 below.

Table 4.2: Resistance Factors for Abutment Design

LIMIT STATE

RESISTANCE FACTORS, φ

BEARING

RESISTANCE

SHEAR

RESISTANCE TO

SLIDING

PASSIVE PRESSURE

RESISTANCE TO

SLIDING

Strength 0.45 0.80 0.5

Service and Extreme Event 1.00 1.00 1.00

At the bridge abutments, bedrock is expected to be encountered beneath the overburden materials at relatively shallow depths (≤ 10 feet). As discussed in Section 3.3.1, a reasonable assumption of the rock strength, considering weathering effects, is approximately 1,420 psi. A resistance factor of

0.45 should be used when bearing on bedrock as the limiting bearing resistance at the strength limit

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 12 state. The equation used to calculate the bearing resistance based on the Hoek Failure Criterion Method is shown below.

𝑞𝑞𝑢𝑢𝑢𝑢𝑢𝑢 = 𝜎𝜎𝑐𝑐𝑐𝑐 �√𝑠𝑠 + ��𝑚𝑚𝑏𝑏 √𝑠𝑠 + 𝑠𝑠�� where, qult = Nominal Bearing Resistance at the abutments mb and s = Criterion parameters defined in Table 3.4 σci = Uniaxial Compressive Strength of the intact rock

From this equation, the factored bearing resistance was calculated to be approximately 106 psi (~15.27 ksf). This value does not exceed the allowable stress of the concrete used in the structural foundation which is equal to 4,500 psi as indicated under the “Concrete” heading on Sheet S2 of the plans.

Table 4.3 below presents allowable bearing pressures for rock types similar to that encountered at the San Vicente site according to Section 8.4.8.2 of the NHI Soils and Foundations Reference Manual, Volume II. It can be noted that these values are larger than the factored bearing resistance previously calculated.

Table 4.3: Allowable Bearing Pressures for Comparable Geology (modified after FHWA NHI-06-089 Soils and Foundations, Volume II)

ROCK TYPE & TYPE OF BEARING MATERIAL ALLOWABLE BEARING PRESSURE,

psi (ksf)

Friable sandstone and claystone, tertiary, Oakland, CA 111.11-277.78 (16-40)

Friable sandstone (Pico formation), quaternary 138.89-277.78 (20-40) Weathered or broken bedrock of any kind except argillaceous rock (shale); RQD less than 25 (soft rock) 222.22-333.33 (32-48)

Sedimentary rock; hard cemented shales, siltstone, sandstone, limestone without cavities (medium hard-sound rock) 416.67-694.44 (60-100)

Presumptive values of the service limit state bearing resistance for various types of rock are presented in AASHTO Table C10.6.2.6.1-1. Based on this table, a service limit state bearing resistance of 40 ksf was selected for the siltstone assuming 1 inch of settlement. The resistance factor for the extreme limit state was taken as 1.0, based on AASHTO Section 10.5.5.3.3, and the extreme limit state bearing resistance was calculated to be 34 ksf. A summary of these values, including the strength limit state, is presented in Table 4.4.

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 13

Table 4.4: Bearing Values for Shallow Foundations

EXTREME EVENT LIMIT STATE STRENGTH LIMIT STATE SERVICE LIMIT STATE

34 ksf 15 ksf 40 ksf

4.2.2. Sliding and Lateral Stability

As indicated in Table 4.1, the foundations are to be embedded a minimum of 9 inches in the rock.

Because of this, sliding should not control the foundation design. However, for mass concrete on clean, sound rock a coefficient of friction, tan δ, of 0.70 with a friction angle, δ, of 35° should be used. Lateral loads imposed on the abutments due to lateral earth pressure and horizontal seismic forces may be resisted by the friction developed between the cast-in-place concrete footings and the supporting rock/soils with a resistance factor of 0.80. The passive pressure for free draining backfill can also be calculated using the properties mentioned. For the passive pressure a resistance factor of 0.50 for LRFD is recommended.

4.2.3. Settlement Estimation

Elastic settlements are estimated to be less than ½ inch for footings bearing on fair to very good rock, according to the Geomechanic Classification system and Section 10.6.2.4.4 of AASHTO (2017).

4.2.4. Scour Potential

Contraction scour and local abutment scour depths were not estimated by the hydraulics engineer due to the presence of shallow bedrock. The bedrock was assumed to be non-erodible, however clastic sedimentary rocks, such as siltstone and mudstone, have the potential to erode with time.

Resistance to scour is mainly a function of the composition of the rock. Sedimentary rock, such as that at the San Vicente Creek Bridge site, has been known to have a small portion of less stable minerals present in its composition, making it low to medium susceptible to erodibility.

4.2.5. Depth of Influence and Frost Protection

The depth of stress influence is a function of the footing width and is a factor in assessing both settlement and bearing capacity. The bearing capacity assumed in Section 4.2.1 should be used in sizing the footings and determining the stress induced in the ground.

Spread footings should be designed to bear below the frost depth for local conditions. For coastal California, the average frost depth is approximately 5 inches (National Oceanic and Atmospheric Administration, NOAA).

4.2.6. Erosion and Corrosion Protection

Riprap protection is not necessary in the area around the abutments per guidance from the hydraulics engineer. For additional information please refer to the Final Hydraulic Report dated

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 14

October 22, 2019.

Based on guidelines outlined in subsection 704.8(b) of FP-14, a pH below 5 or above 10, a resistivity below 3,000 ohm-cm and, a sulfate and chloride concentrations above 200 and 100 parts per million respectively, are representative of an aggressive soil environment. One sample at TP19- 02 was tested for pH per AASHTO T 289 and was recorded with a pH of 8.6. Therefore, the on-site soils are anticipated to have a low corrosion potential.

4.2.7. Construction Considerations

It is required that all foundation excavations be inspected by a CFLHD geotechnical engineer prior to the installation of any steel or concrete according to the Section 208.08(a) Special Contract Requirement (SCR) presented in Appendix D. Temporary shoring through overburden soils is the responsibility of the Contractor.

It is anticipated the foundations can be excavated with conventional excavation equipment.

Boulders will be present in the overburden material along with granular soils. Though not encountered during the site investigation the Contractor should be prepared to excavate boulders 5 feet or greater in diameter with appropriate heavy-duty equipment to break up and remove rock, such as hydraulic breakers. Blasting is not anticipated to be necessary. Weathered or disturbed material must be removed from the excavation to ensure good contact between the footing and the bedrock, per Section 208.08(a) of the FP-14. Water is likely to be encountered during the foundation excavation and dewatering may be required. Actual bedrock elevation may be variable based on the subsurface geology and observed field conditions. The “Alternate Footing Embedment Detail” as shown on Sheet S4 of the plans may be used if the bedrock surface is irregularly shaped or if the depth to bedrock is between the anticipated depths discussed in this report or 3 feet beneath them. If the depth to bedrock is below 3 feet of the anticipated depths, then the CO must be notified prior to construction.

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 15

SECTION FIVE--LIMITATIONS

The recommendations in this report are based on the data obtained from limited exploratory excavations, field review, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 16

SECTION SIX--REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2014, LFRD BRIDGE DESIGN SPECIFICATIONS, 7th Edition.

Brabb, Earl, E., 1997, Digital Database Prepared by S. Graham, C. Wentworth, D. Kinfong, R Graymer, and J. Blissenbach, “Geologic Map of Santa Cruz County, California,” Department of the Interior U.S. Geologic Survey (USGS).

Coduto, Donald P., 2001, “Foundation Design: Principles and Practices,” Second Edition. Prentice- Hall.

Clark, Joseph C., 1981, “Stratigraphy, Paleontology, and Geology of the Central Santa Cruz Mountains, California Coast Ranges,” Geologic Survey Professional Paper 1168, Department of the Interior U.S. Geologic Survey (USGS).

Federal Highway Administration (FHWA), 2007, Geotechnical Technical Guidance Manual, dated May.

Federal Highway Administration (FHWA), 2008, Federal Lands Highway Project Development and Design Manual (PDDM), dated March.

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14.

Federal Highway Administration (FHWA), 2017, Soil Description and Identification Guidelines, dated December.

Federal Highway Administration (FHWA), 2017, Rock Characterization Guidelines, dated December.

Federal Highway Administration (FHWA), 2017, “Monterey Bay Sanctuary Scenic Trail, CA FLAP SCR T5 (1) & (2), Draft Geotechnical Report,” dated July.

Federal Highway Administration (FHWA), 2019, “San Vicente Bridges Final Hydraulic Report,” dated October.

Gaudinski, Ranger. “The Rocks of Santa Cruz’s North Coast: The Mighty Mudstone.” Online, <http://www.mobileranger.com/santacruz/the-rocks-of-santa-cruzs-north-coast-the-mighty-mudstone/>.

Hoek E. & Marinos, P., 2000. “GSI: A Geological Friendly Tool for Rock Mass Strength Estimation.”

Moss, Andrew & Swanier, Ryan, 2017, “San Vicente Creek Bridge 1 Bridge Inspection Report,” U.S. Department of Interior, Bureau of Land Management.

“National Seismic Hazard Map,” United States Geological Survey (USGS), 2008, website:

<http://earthquake.usgs.gov/research/hazmap>.

Online, USGS, <http://geohazards.usgs.gov/cfusion/qfault/query_main_AB.cfm>.

Online, USGS, <http://geohazards.usgs.gov/cfusion/hazfaults_2014_search/query_main.cfm>.

http://earthquake.usgs.gov/research/hazmap http://geohazards.usgs.gov/cfusion/qfault/query_main_AB.cfm http://geohazards.usgs.gov/cfusion/hazfaults_2014_search/query_main.cfm

San Vicente Bridges, CA BLM 2020(1) December 2019 Page 17

Online, “California Frost Line,” < https://www.hammerpedia.com/california-frost-line/>. Source:

National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce.

State of California, 2017, Caltrans ARS Online (v2.3.09), http://dap3.dot.ca.gov/ARS_Online/index.php

PLATES

APPENDIX A

FIELD EXPLORATION PROGRAM

APPENDIX A

FIELD EXPLORATION PROGRAM

A.1 INTRODUCTION

The Central Federal Lands Highway Division (CFLHD) Geotechnical Section completed a field exploration program for San Vicente Bridges, CA BLM 2020(1), on August 6th, 2019. The scope of work for the field exploration program included excavating two test pits. Each component of the field exploration program was coordinated and observed by a Geotechnical Engineer from CFLHD. Field exploration location plans, as well as individual test pit logs are attached. These logs represent a compilation of field and laboratory data and description of the soil samples by CFLHD Geotechnical personnel. The methods used to conduct the field exploration program are described below. Photos of excavation equipment and field exploration activities are included in Appendix C. Representative soil samples collected during the field exploration program were transported to the CFLHD Materials Laboratory in Lakewood, Colorado for testing. A summary of the laboratory testing program is provided in Appendix B.

A.2 EXPLORATIONS

Test Pit Excavations

The Bureau of Land Management (BLM) provided the excavation services for the test pits. Each test pit was excavated using a John Deere mini-excavator.

Test pits were completed on August 6th, 2019. A total of two test pits (TP19-01 & TP19-

02) were completed to depths ranging from approximately five to ten feet below ground surface to further define subsurface conditions. The location and elevation of individual test pits were estimated using mobile phone GPS location services and Google Earth.

Test pit locations are listed on individual test pit logs and are show below in Table A-1.

Following excavation activities, field personnel reclaimed the pits by refilling the pits with excavated soil.

TABLE A-1

FIELD EXPLORATION LOCATIONS

Location Easting Northing Exploration Type

Exploration Designation

Approx. Depth (ft)

East Abutment 572386.03 4097852.65 Test Pit TP19-01 5 West

Abutment 572384.61 4097852.30 Test Pit TP19-02 9.8

A.3 SOIL AND ROCK SAMPLING

Test Pit Explorations

Disturbed soil and rock grab samples were obtained from the excavated materials or the excavation walls at various intervals. Representative portions of grab samples were placed in plastic baggies and transported to the CFLHD Materials Laboratory for testing.

Following excavation, the test pit was logged by Geotechnical personnel.

A.4 SOIL AND ROCK CLASSIFICATION SYSTEM

During the completion of borings and test pit excavations, CFLHD Geotechnical personnel collected soil/rock samples and prepared field logs of the borings. Soil identification and descriptions, as shown on the field logs, are based on ASTM D2488, a systematic process for identifying and describing individual soil samples by visual and manual means. When sufficient laboratory testing was completed, select samples from borings were classified using the Unified Soil Classification System (USCS) and American Association of State Highway and Transportation Officials (AASHTO) soil classification system. Both the visual soil identification system and the referenced soil classification systems are summarized in the attached Soil Classification Field Reference. Rock samples were classified based on the stratigraphic structure, rock strength, degree of weathering, and other properties. The rock classification system is summarized in the attached Rock Classification Field Reference.

G rfc

Te xt

!S O

IL

C

A

LS

S

IF

IC

A

TI

O

N C

H A

R T

L

E G

E N

D

3/

/2

3:

:5

P

M

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

PT

Unified Soil Classification System

SOIL CLASSIFICATION CHART & LEGEND

SILTY GRAVELS WITH OR WITHOUT SAND

HIGHLY ORGANIC SOILS

POORLY-GRADED SANDS WITH OR WITHOUT

GRAVEL

CLEAN SANDS

WITH LESS THAN

15% FINES

GRAVELS WITH

15% OR MORE

FINES

GRAVELS

MORE THAN HALF

COARSE

FRACTION IS

LARGER THAN NO.

4 SIEVE

MAJOR DIVISIONS TYPICAL NAMES

WELL-GRADED GRAVELS WITH OR WITHOUT

SAND

POORLY-GRADED GRAVELS WITH OR

WITHOUT SAND

ORGANIC SILTS OR CLAYS OF HIGH

PLASTICITY WITH OR WITHOUT SAND OR

GRAVEL

PEAT AND OTHER HIGHLY ORGANIC SOILS

CLAYEY SANDS WITH OR WITHOUT GRAVEL

C O

A R

S E

-G R

A

IN

E D

S O

IL

S

M O

R E

T H

A N

H A

LF

IS

C O

A R

S E

R T

H A

N N

O . 2

S

IE

V

E

NOTE: Coarse-grained soils with between 5% and 12% passing the No.200 sieve and fine-grained soils with limits plotting in the gray zone on the plasticity chart have dual classifications.

SILTS AND CLAYS

LIQUID LIMIT 50% OR LESS

Unconfined Compression No Recovery

SILTS AND CLAYS

LIQUID LIMIT GREATER THAN 50%

UC

NR

FI

N

E -G

R A

IN

E

D S

O

IL

S M

O R

E T

H A

N H

A

LF

IS

F

IN

E

R T

H A

N N

O . 2

S

IE

V

E

SANDS WITH 15%

OR MORE FINES

SANDS

MORE THAN HALF

COARSE

FRACTION IS

FINER THAN NO. 4

SIEVE SIZE

INORGANIC CLAYS OF HIGH PLASTICITY

WITH OR WITHOUT SAND OR GRAVEL

Liquid Limit Plastic Index Moisture Content Dry Density Non Plastic No Valve

Water Level at Time of Drilling

Stabilized Water Level

CLEAN GRAVELS

WITH LESS THAN

15% FINES

Abbreviations

WELL-GRADED SANDS WITH OR WITHOUT

GRAVEL

CLAYEY GRAVELS WITH OR WITHOUT SAND

INORGANIC SILTS OF LOW TO MEDIUM

PLASTICITY WITH OR WITHOUT SAND OR

GRAVEL

INORGANIC CLAYS OF LOW TO MEDIUM

PLASTICITY WITH OR WITHOUT SAND OR

GRAVEL

LL

PI

W

DD

NP

NV

SILTY SANDS WITH OR WITHOUT GRAVEL

ORGANIC SILTS OR CLAYS OF LOW TO

MEDIUM PLASTICITY WITH OR WITHOUT

SAND OR GRAVEL

INORGANIC SILTS OF HIGH PLASTICITY WITH

OR WITHOUT SAND OR GRAVEL

Use primary colors or hyphenated compound primary colors. Use "mottled" or "streaked" if necessary.

4. Color Dry Dry to touch, dusty Moist Damp but no visible water Wet Visible free water

5. Moisture Content

Flat Width/Thickness >3 Elongated Length/Width >3

Flat & Elongated Meets both of the above

Applies to sand, gravel, cobbles and boulders.

Length, width and thickness refer to the greatest, intermediate and least dimensions, respectively.

6c. Particle Shape

N Density N Consistency

Unconfined Compressive Strength qu

(tsf)

Undrained Compressive Strength su

(tsf)

Behavior

0-4 Very Loose 0-1 Very Soft <0.25 <0.125 Extrudes between fingers when squeezed

5-10 Loose 2-4 Soft 0.25-0.50 0.125-0.25 Remolded by light finger pressure

11-30 Medium Dense 5-8 Firm 0.50-1.00 0.25-0.50

Imprinted easily with fingers, remolded by strong finger pressure

31-50 Dense 9-15 Stiff 1.00-2.00 0.50-1.00 Imprinted with considerable finger pressure, indented by finger nail

>50 Very Dense 16-30 Very Stiff 2.00-4.00 1.00-2.00 Barely imprinted by fingers or indented by finger nail

>30 Hard >4.00 >2.00 Not imprinted by fingers or difficult to indent with finger nail

SAND & GRAVEL SILT AND CLAY

3. Consistency and Density:

Angular Sharp edges and relatively plane sides.

Subangular Same as angular with rounded edges.

Subrounded Nearly plane sides but well-rounded corners and edges.

Rounded Smooth curved sides and no edges.

Well-

Rounded Very Smooth surfaces, spherical or ovular, no edges.

6b. Particle Angularity Applies to coarse sand, gravel, cobbles and boulders.

Sieve < #200 Flour or smaller

Fine > #200 to #40 Flour to sugar Medium #40 to #10 Sugar to rock salt Course #10 to #4 Rock salt to pea-sized

Fine #4 to 3/4 in. Pea-sized to thumb Coarse 3/4 in. to 3 in. Tumb to fist Cobble 3 in. to 12 in. Fist to basketball

Boulders > 12 in. Larger than Basketball

Material

Gravel

6a. Particle Size

Silt or Clay

Sand

Particle Size Approximate Scale

Example, fine-grained soil: Lean CLAY with Sand (CL)- stiff, brown, moist, medium plasticity, laminated Example, coarse-grained soil: Silty SAND with Gravel (SM)-medium dense, gray, wet, fine to coarse grained, angular to subangular gravel, weakly cemented Example, fine-grained soil (Long Form):

Clayey GRAVEL with SAND (CL-ML)- loose to soft, dark brownish green to pale brownish gray, wet; fine to medium grained, angular, flat sand; coarse grained, rounded elongated gravel, some chert, trace coarse gravel, and cobbles, medium plasticity, dessicated, weak cementation, low dry strength, rapid dilatancy, moderate HCL reaction, hydrocarbon odor, iron oxide staining, alluvium fill, (Quaternary Alluvium), Additional Description.

1. Group Name (Pg. 3) 9.

2. Group Symbol (Pg. 3) 10.

3. Consistency / Relative Density (Pg. 1) 11.

4. Color (Pg. 1) 12.

5. Moisture (Pg. 1) 13.

6. Particle Size / Shape / Angularity (Pg. 1) 14.

7. Plasticity (Pg. 2) 15.

8. Structure (Pg. 2)

Odor (Pg. 2)

Staining (Pg. 2)

Cementation (Pg. 2) Organics Dry Strength (Pg. 2) Dilatancy (Pg. 2) HCL Rxn (Pg. 2)

Weak Crumbles with little finger pressure Moderate Crumbles with considerable finger pressure Strong Will not crumble with finger pressure

Intact coarse-grained soil

9. Cementation

Nonplastic Thread cannot be rolled at any water content.

Low The thread can barely be rolled and the lump cannot be formed when drier than the plastic limit.

Medium The thread is easy to roll and not much time is required to reach the plastic limit. The thread cannot be rerolled after reaching the plastic limit. The lump crumbles when drier than the plastic limit.

High It takes considerable time kneading and rolling to reach the plastic limit. The thread can be rerolled several times after reaching the plastic limit. The lump can be formed without crumbling when drier than the plastic limit.

Toughness test: Shape the specimen into an elongated pat and rolled on a smooth surface or between the palms into a thread ~1/8”. If the sample is too wet to roll, it should be allowed to dry. Fold the thread and reroll repeatedly until the thread crumbles at a diameter of ~1/8”. This will be near the plastic limit. Note the pressure required to roll the thread near the plastic limit and the strength of the thread. After the thread crumbles, the pieces should be lumped together and kneaded until the lump crumbles. Note the toughness during kneading.

On the basis of observations made during the toughness test, describe plasticity.

7. Plasticity

None No visible reaction Weak Some reaction, bubbles forming slowly Strong Violent reaction, with bubbles forming immediately

13. HCL Reaction 14. Odor 15. Staining None None

Chemical Hyrodcarbon Hyrodcarbon Iron Oxide

Organic

None No visible change Slow Water appears slowly during shaking and does not disappear or disappears slowly during squeezing.

Rapid Water appears quickly during shaking and disappears quickly during squeezing.

12. Dilatancy

1. Mold soil, adding water if necessary, into ~1/2” diameter ball with soft but not sticky consistency.

2. Smooth in palm of one hand with knife blade. Shake horizontally, striking the side of the hand vigorously against the other hand several times. Note the reaction of water appearing on the surface.

4. After Dilatancy has been determined perform the Toughness test (see explanation…