CA.FTFS.46N50.Salt Gulch Bridge.FINAL 4-20-2020_signatures.pdf

SALT GULCH BRIDGE

CA FS 1038(1)

SISKIYOU COUNTY, CA

FINAL Geotechnical Report Report No. CA-FX-46N50-20-01

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch April 2020 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

Distribution

Electronic:

N:\CA\FS Salt Gulch\Geotech\6_DraftDocs Project Management Project Development, Lead Designer Bridge

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TABLE OF CONTENTS

SECTION ONE - INTRODUCTION

1.1 BACKGROUND AND LOCATION

1.2 SCOPE AND PURPOSE

SECTION TWO - GEOLOGY AND SEISMICITY

2.1 REGIONAL GEOLOGY

2.2 SITE GEOLOGY

2.3 REGIONAL SEISMICITY

2.4 SEISMIC DESIGN PARAMETERS

2.5 GEOLOGIC HAZARDS

SECTION THREE - SUBSURFACE INVESTIGATION

3.1 SUBSURFACE EXPLORATION PROGRAM

3.2 LABORATORY TESTING PROGRAM

3.3 SUMMARY OF SITE CONDITIONS

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 FOUNDATIONS - SALT GULCH BRIDGE REPAIR

4.1.1 Bridge Foundation Selection

4.1.2 Site Characterization

4.1.3 Drilled Shaft Axial Resistance

4.1.4 Group Effects on Axial Resistance

4.1.5 Lateral Loads on Deep Foundations

4.1.6 Settlement

4.1.7 Scour Potential and Erosion

4.2 GRS BRIDGE ABUTMENT & CULVERT WING WALL DESIGN

4.2.1 Bearing Resistance for Abutment and Wing Wall Design

4.2.2 Settlement and Consolidation

4.2.3 Foundation Preparation

4.2.4 Lateral Loads

4.3 PERMANENT EARTHWORKS

4.3.1 Embankment Construction

4.3.2 Shrink/Swell Recommendations

4.3.3 GRS Backfill for Abutment

4.4 PRESUMED LANDSLIDE AREA

4.5 CONSTRUCTION CONSIDERATIONS

4.6 SPECIFICATIONS

4.7 DISCLAIMER/LIMITATIONS CLAUSE

SECTION FIVE - REFERENCES

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TABLES

TABLE 2.1 Summary of Nearby Seismic Source Faults TABLE 2.2 Summary of Seismic Parameters Corrected for Site Class C TABLE 3.1 Summary of the Field Exploration Program TABLE 4.1 Proposed Bridge Foundation Location TABLE 4.2 LPILE Parameters TABLE 4.3 Lateral Earth Pressures for Bridge Abutments TABLE 4.4 Resistance Factors for Abutment and Wall Foundation Design

PLATES

PLATE 1 Project Vicinity Map PLATE 2 Geology Map PLATE 3 Geotechnical Exploration Locations PLATE 4 Factored Axial Resistance for Drilled Shafts PLATE 5 Bearing Resistance for Concrete Wing Walls

APPENDICES

APPENDIX A Field Exploration Program APPENDIX B Laboratory Test Results APPENDIX C Photographs APPENDIX D Inclinometer Data APPENDIX E CH2M Hill Geotechnical Report for Salt Gulch Bridge and Slide APPENDIX F Excavation Characteristics of Rock & Rippability Charts

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Salt Gulch Bridge, CA FS 1038(1) April 2020

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering assessment and provides recommendations for the Salt Gulch Bridge repair work to be performed on Horse Creek Road (Forest Road 46N50) in Siskiyou County, California. The project proposes to repair the existing Salt Gulch Bridge, which is located within the Klamath National Forest. A site map is presented on Plate 1 of this report.

A brief history of the existing bridge was summarized by the CFL Bridge Team based on bridge inspection reports and other information in the chronology provided below:

1958 → Original bridge designed by Bureau of Public Roads. Per the bridge drawings, design vehicle loading was H20-S16-44 (same as HS20, 36-ton gross weight). Materials included 3000 psi concrete and Grade 40 rebar.

1972 → Bridge inspection report mentions cracking and movement issues with south abutment.

1974 → Bridge inspection report shows bridge load posted to 10 tons.

1975 → Geotechnical Slide Investigation Report prepared for Forest Service by CH2M Hill.

1976 → Forest Service rehabilitation construction project completed. The top portion of the original south abutment, which consisted of a concrete cap supported on columns on spread footings (not founded on rock), was cutoff and replaced with a widened end beam at the end of the bridge supported on a new concrete cap with H-pile foundations. A short timber approach span was added to traverse the opening left between the old abutment and a tieback soldier pile wall with timber lagging was added. Cracks near the bottom of the southernmost pier column were epoxied. Per the bridge drawings, design vehicle loading was HS20-44. Materials included 3000 psi concrete, Grade 40 rebar, and HP 10x42 piles with A36 steel.

1980’s → Timber approach spans replaced several times per bridge inspection reports.

-2010’s 2006 → Cracks in south abutment cap first noted in bridge inspection report.

2016 → Cracks have gotten progressively larger, bridge inspection report requests load rating.

2018 → Bridge load posted to 5 tons per bridge inspection report.

Due to the observed damage, the structural capacity of the bridge has been significantly reduced, limiting the vehicles that can use it. This project proposes to repair the bridge to meet loading requirements for logging industry and Forest Service vehicles, including fire control and emergency vehicles. The repair is proposed for the south abutment only. The work will include

Page | 2 construction of a new abutment with concrete wing walls and a reinforced soil embankment. No significant changes in grade are proposed for the new bridge structure. The topography adjacent to the bridge site is mountainous and highly vegetated. Approximately 30 culverts in the same watershed are also planned for replacement. Ancillary construction including armoring, signage, and revegetation will also be performed.

1.2 SCOPE AND PURPOSE

The scope of work included a geotechnical investigation, analysis, and recommendations for bridge foundations and landslide evaluation within the project limits for use in design and construction. Additionally, a previously identified landslide near the bridge abutment was investigated to determine what influence, if any, the landslide is having on the bridge movement.

This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, installation and data collection from inclinometers and ground water observation points, interpretation and correlation of field measurements, and geotechnical engineering analysis.

Specifically, this investigation was conducted to determine the soil profiles at the bridge location and develop recommendations concerning bridge foundations, retaining structures, geologic hazards, and construction considerations.

No pavements investigation was performed for this scope of work. We understand that the proposed pavement section will either be calculated based on assumed, conservative soil parameters or specified to “match existing.” We understand that a hydraulics report is being prepared under separate cover by the CFLHD Hydraulics Engineer. The hydraulics report includes discussion of flood impacts, such as bridge scour and water surface elevations.

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SECTION TWO - Geology and Seismicity

2.1 REGIONAL GEOLOGY

The Salt Gulch Bridge is located within the western Paleozoic and Triassic belt in the Klamath Mountains of Northern California and Southern Oregon (Donato et al., 1980). This broad north-south-trending belt is divided into three sub belts of rocks, referred to as the Rattlesnake Creek, Hayfork, and North Fork terranes. The Rattlesnake Creek and North Fork terranes are largely ophiolitic, indicating they formed as oceanic crust. While the Hayfork terrane is largely andesitic, suggesting origination as an island arc. The assemblage of the belt, as seen today, may be explained by the individual terranes being separate during formation, then later telescoped together along thrust faults (Irwin, 1972). Throughout most of the area occupied by this belt, the rocks can generally be characterized as metavolcanic and metasedimentary rocks that have been regionally metamorphosed to green schist and amphibolite facies.

Within the western Paleozoic and Triassic belt lies another sub terrane known as the Condrey Mountain Terrane. The Condrey Mountain Terrane is a roughly circular area of approximately 250 square miles that has been exposed through a structural window; with low-angle folded thrust faults on the eastern and southern margins, and a high-angle fault to the west. Lithologically, the terrane is characterized by the Condrey Mountain Schist, a highly foliated low-grade metamorphic schist (green schist facies). Higher grade metamorphic rocks (amphibolite facies) and ultramafic rocks are in thrust-fault contact with the Condrey Mountain Schist to the south, east, and west (Hotz, 1979).

2.2 SITE GEOLOGY

The site is mapped as underlain by the aforementioned Condrey Mountain Schist. The schist is of two main types, a quartz-muscovite schist (commonly graphitic), and an actinolite-chlorite schist.

The quartz-muscovite schist is the most abundant variety of the Condrey Mountain Schist and is of sedimentary origin. It is gray to brownish-gray in color, fine-grained, and portrays well-developed schistosity along which it easily splits, revealing shiny cleavage surfaces coated with muscovite. The less common actinolite-chlorite schist is believed to have formed from the metamorphism of basaltic tuffs and flows. It is greenish-gray to grayish-green, fine-grained, and finely laminated (Hotz, 1979). A geology map of the project area is presented in Plate 2.

2.3 REGIONAL SEISMICITY

No seismic source faults are mapped within 40 miles of the project area, but numerous critical active faults exist outside this range. The critical active faults to the project site may be found in

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TABLE 2.1 (CalTrans, 2019; U.S. Geological Survey, 2019). These faults lie largely to the west and east of the project site, and vary in length from 26.7 to 468.5 miles. To the east lies the Cedar Mountain Fault System (Ikes Mountain section), which is a normal fault system striking northwest and dipping strongly (50°-70°) to the east, with slip rates less than .00787 inches per year. The western fault systems (Big Lagoon Fault Zone – Bald Mountain, Cascadia Subduction Zone, Trinidad fault, and McKinleyville fault) are all thrust faults generally striking northwest with a northeast to easterly dip. The fault systems to the west, although farther from the project site, appear to be more active and exhibit higher slip rates, ranging from .0236 to 1.38 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) Cascadia Subduction

Zone 8.3 74.2 1.38 468.5 <15,000

Big Lagoon - Bald Mountain 7.5 56.4 0.0394 77.05 <750,000

Trinidad 7.5 62.9 0.0591 - <15,000 Cedar Mountain (Ikes

Mountain section) 7 47.8 <0.00787 26.7 <15,000

McKinleyville 7.2 68.8 0.0236 - <15,000

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the Salt Gulch Bridge 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 Salt Gulch Bridge Project site, located at 41.872683°N latitude and 123.085325°W longitude, was obtained in accordance with the AASHTO ground motion maps.

Based on the material encountered and tested during drilling, the average Standard Penetration Test (SPT) blow count (blows per foot) (ASTM D1586) for the top 100 feet of the soil profile was estimated to be greater than 50 blows per foot. Therefore, the site is classified as Class C 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 2.2.

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TABLE 2.2 - Summary of Seismic Parameters Corrected for Site Class C Horizontal Peak Ground Acceleration, (PGA) 0.195 g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 0.455 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.204 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.2 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.2 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.596

Factored Horizontal Peak Ground Acceleration, (As) 0.234 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.546 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.325 g

Seismic Zone Zone 3

Based on the long acceleration coefficient SD1 value of 0.325, the bridge site is assigned to seismic hazard “Zone 3” 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, debris flows, flooding, and seismic events. In particular, debris flows and slope instability may occur during construction. This risk is particularly high during the late winter and early spring months, due to snowmelt and high precipitation periods. Additionally, fire damage was observed on the large slope immediately to the east and southeast of the bridge. Debris flows are a major hazard if intense rainfall follows a forest fire. The risk of a debris flow increases greatly due to vegetation loss, soil exposure, and deposition of new, loose debris. The area to the east and northeast of the easterly end of the Salt Gulch Bridge was previously identified as a landslide according to a March 1975 geotechnical report completed by CH2M Hill. This report is attached in Appendix E.

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SECTION THREE - Subsurface Investigation

3.1 SUBSURFACE EXPLORATION PROGRAM

A subsurface investigation targeting the bridge and suspected slide area was performed by a Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) geotechnical engineer on April 8 through 17, 2019. The geotechnical subsurface exploration program consisted of drilling a total of six borings, or three pairs: two near the southeast corner of the bridge abutment, two near the bottom of the presumed landslide area, and two near the top of the presumed landslide area on Forest Road 47N69. One boring of each pair was logged and sampled. Standard penetration testing (SPT) and sample collection was performed at 5-foot intervals in these borings. One boring of each pair was installed with inclinometer casing while the other was installed as a temporary groundwater monitoring point. Boring locations are shown on Plate 3. Subsurface 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 in Table 3.1.

TABLE 3.1 - Summary of Field Exploration Program

EXPLORATION

DESIGNATION LOCATION

APPROXIMATE

GROUND ELEVATION

(ft.)

TERMINATION

DEPTH (ft.)

DEPTH TO

GROUNDWATER

(ft.)

SI19-01 &

MW19-01

SE abutment corner 2,169 55.25 16

SI19-02 &

MW19-02

Top of presumed landslide 2,294 90.17 19.5

SI19-03 &

MW19-03

Bottom of presumed landslide

2,170 50.25 Groundwater not encountered.

Note: The exploration locations were estimated relative to existing features. Ground elevations were estimated from Google Earth and data acquired from the CFL Survey Team.

3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the borings by SPT 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), specific gravity (AASHTO T 100), sieve analysis (AASHTO T 11 and T27), classification (AASHTO M145), Atterberg limits (AASHTO T89 and T90), and soil pH (AASHTO T 289). Strength testing, such as triaxial or direct shear, was not performed. When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system.

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Visual classification and soil test results indicated a relatively uniform soil profile consisting of silty sands and gravels, classifying as A-1 to A-2-4 by AASHTO and SM and GM by USCS.

The test data and detailed descriptions of the laboratory testing program are presented in Appendix B.

3.3 SUMMARY OF SITE CONDITIONS

Salt Gulch Bridge (South Abutment)

The ground surface at the boring nearest the Salt Gulch Bridge (SI19-01 & MW19-01) consisted of an unpaved road surfaced with gravel. Boulders ranging in size from approximately 12 to 36 inches were observed on the ground surface and within the adjacent stream channel. Beneath the ground surface, a layer of assumed fill material consisting of silty sand with gravel, trace organics, and cobbles extended to a depth of about 9 feet. Underlying this material was a layer of silty gravel with sand and increased cobbles and rock fragments consisting of crushed schist and weathered diorite approximately 13.5 feet thick. Another layer of silty sand with gravel was encountered beneath the previously mentioned gravel layer. This layer extended to the maximum depth explored, approximately 55.25 feet, and an increase in weathered schist was observed.

The material ranged from loose to dense from approximately 0 to 35 feet beneath the ground surface. From about 35 feet to the bottom of the borehole the material transitioned to very dense.

Groundwater was measured during the subsurface investigation at 16 feet below the ground surface. Fluctuations in the ground water level due to seasonal and climatic effects are expected.

Presumed Landslide Area

The presumed landslide area was investigated by drilling one pair of boreholes near the top and bottom of the hillside. The borings located near the bottom of the hill, SI19-03 & MW19-03, consisted of the same unpaved road surface with scattered cobbles and boulders on the ground surface, between approximately 12 to 36 inches, as previously mentioned. A layer of assumed fill material consisting of low plasticity silt with gravel and trace organics extended to a depth of about 6 feet. Underlying this material was a layer of silty gravel with sand extending to a depth of about

21.5 feet. Around 15.5 feet beneath the ground surface increased rock fragments consisting of greenstone schist with minor quartz lenses were encountered. A layer of silty sand with gravel and rock fragments approximately 24.5 feet thick was encountered beneath the previously mentioned gravel layer. Increased green schist and quartz fragments were observed around 26 feet. A 3 feet thick layer of medium plastic, gravelly lean clay began around 46 feet. This layer was underlain by a thin layer of silty sand with gravel which extended to the maximum depth of the exploration.

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The material density ranged from very loose to dense from approximately 0 to 35 feet beneath the ground surface. From about 35 feet to the bottom of the borehole the material transitioned to very dense. Groundwater was not encountered during the exploration, although moisture was noted in the borehole around 41 feet.

The pair of borings located near the top of the hill on Forest Road 47N69, SI19-02 & MW19-02, consisted of the same gravel roadway surface and observed cobbles and boulders as the other borings. A layer of fill, topsoil-type, silty sand with gravel and organics extended to a depth of 5 feet beneath the ground surface. The silty sand with gravel continued past the fill material to a depth of approximately 30 feet. Underlying the sand layer was gravelly lean clay about 4 feet thick followed by silty sand with gravel extending to the termination depth of the borehole. A “flowout” of wet silty sand and completely weathered schist was observed around 41 feet. Increased green, completely weathered, schist and few quartz fragments were observed throughout the bottom 56 feet of the hole.

The material density ranged from loose to medium dense from approximately 0 to 60 feet beneath the ground surface. From about 60 feet to the bottom of the borehole the material transitioned to very dense. Groundwater was measured during the subsurface investigation at 19.5 feet below the ground surface.

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the bridge foundation, GRS abutment, culvert wing walls, landslide evaluation, and construction considerations for the design and construction of the Salt Gulch project. Based on discussions with the project team, a repair of the existing bridge is the preferred alternative. Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 FOUNDATIONS - SALT GULCH BRIDGE REPAIR

The proposed repair will be for the south abutment only. The existing concrete abutment cap and cutoff abutment piles will be removed and a new concrete abutment cap and deep foundation system will be placed outside of the existing pile footing. Based on the 95 percent design plans, the proposed abutment centerline station and cap elevation are shown in Table 4.1.

TABLE 4.1 - Proposed Bridge Foundation Location

FOUNDATION APPROXIMATE STATION TOP OF SHAFT ELEVATION*

Abutment 1 2+31 2,155.00 FT *Estimated from information presented on the 95% design plans.

4.1.1 Bridge Foundation Selection

A drilled shaft foundation system is proposed for the new south abutment due to the high axial and lateral loads. A driven pile foundation was considered but determined unfeasible due to limited space available and accessibility to the site.

A drilled shaft foundation is feasible from a geotechnical perspective. Drilled shafts are able to provide a small footprint, support large foundation loads, and provide earth retention and slope stabilization. Based solely on hydraulic recommendations and assumptions stated in the Final Hydraulics Report, scour potential was determined to be negligible due to the considerable distance between the south abutment and creek bank. This topic will be discussed in detail in the Final Hydraulics Report dated April 2020. Design recommendations for the drilled shafts are provided in this report.

The following preliminary bridge load factored for the strength limit state was provided by the bridge engineer on December 20, 2019:

• South abutment = 280 kips (per shaft)

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4.1.2 Site Characterization

The subsurface profile was assumed to be entirely sand for analysis purposes. Although gravel, silt, clay, and weathered schist and quartz were encountered in the designated boreholes, the subsurface materials, in general, were of a sand-like nature and composition. Groundwater was also assumed to be at 16 feet as was observed at boring MW19-01.

4.1.3 Drilled Shaft Axial Resistance

The drilled shaft axial resistance analysis was performed for 36 inch diameters at the abutment using the beta method for cohesionless soils presented in section 10.8.3.5.2b of AASHTO. The abutment foundations will obtain their resistance through side friction and tip resistance. The factored resistance for the strength limit state was calculated by applying 0.55 and 0.50 resistance factors for side and tip resistance, respectively. Based on the chart in Plate 4 and the selected shaft diameter of 3 feet, a minimum length of 32 feet is required to meet axial resistance.

4.1.4 Group Effects on Axial Resistance

The resistance of a shaft group to the applied axial loads is not necessarily the sum of the axial resistance of individual piles within the group. The zone of influence from an individual shaft in a shaft group may overlap with other shafts, depending on spacing. Historically the axial efficiency of groups of shafts has not been a concern if the center-to-center spacing between shafts is greater than 3 times the shaft diameter. Based on the geometry of the chosen shaft system for the Salt Gulch Bridge, group effects are not anticipated.

4.1.5 Lateral Loads on Deep Foundations

Lateral load analysis will be performed by the bridge engineer using the software program Lpile developed by Ensoft, Inc. This program analyzes a single pile or shaft considering deflection as a function of design loads, foundation construction, and subsurface conditions. Table 4.2 provides preliminary Lpile input parameters for the foundation soils based on available subsurface information and presumptive engineering correlations. It is also recommended that lateral support within five feet of the final road grade be neglected due to the potential loss of support from frost penetration or other shallow ground disturbance.

TABLE 4.2 - LPILE Parameters

APPROXIMATE

ELEVATION

(FT)

DEPTH BELOW

EXISTING

GROUND

SURFACE (FT)

LPILE P-Y

MODEL

EFFECTIVE

UNIT WEIGHT

(PCF)

FRICTION

ANGLE

(DEG)

SOIL

MODULUS

(PCI)

MODEL No. 1 (ALL SAND) 2,155 to 2,139 0 to 16 Sand (Reese) 118.0 32 225 2,139 to 2,100 16 to 55 Sand (Reese) 55.6 32 125

MODEL No. 2 (SAND & WEAK ROCK)

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2,155 to 2,139 0 to 16 Sand (Reese) 118.0 32 225 2,139 to 2,120 16 to 35 Sand (Reese) 55.6 32 125

TRANSITION TO ROCK PROPERTIES LPILE P-Y

MODEL

EFFECTIVE

UNIT WEIGHT

(PCF)

UNIAXIAL COMPRESSIVE

STRENGTH (PSI)

2,120 to 2,100 35 to 55 Weak Rock (Reese) 140.0 100

Material properties provided are for single shafts and do not account for the reduced lateral resistance of shafts in a group. P-multipliers are a function of the number of rows of shafts and center-to-center shaft spacing in the direction of loading. P-multipliers are required even for a single row of shafts if the center-to-center spacing is less than 5 shaft diameters. P-multipliers are specified in Table 10.7.2.4-1 in AASHTO. When this analysis method is used, the resistances at the strength limit state as represented by the P-y curves should not be factored since they already represent the nominal conditions.

4.1.6 Settlement

A resistance factor of 1.0 for the service limit state is recommended to assess the ability of the foundation to meet the specified deflection criteria. Elastic settlements will occur immediately and be essentially complete at the end of construction and are estimated to be less than one inch at all locations based on the loads provided.

4.1.7 Scour Potential and Erosion

Contraction scour and local abutment scour depths were not estimated by the hydraulics engineer.

It is believed that the abutment is set back far enough from the creek bank that scour is insignificant.

Riprap protection is not necessary in the area below the south abutment per guidance from the hydraulics engineer. For additional information please refer to the Final Hydraulic Report dated April 2020.

4.2 GRS BRIDGE ABUTMENT & CULVERT WING WALL DESIGN

The implementation of geosynthetic reinforcement of the bridge abutment backfill and headwalls and wing walls at six culvert locations has been considered for this project. Abutments and culvert wing walls should be designed to resist lateral earth pressures and other applicable lateral loads in accordance with AASHTO. Lateral earth pressure is influenced by the strength of the abutment backfill, the presence or absence of water, and the ability of the abutment or wall to move in response to lateral loads. Other loads, such as live loads, construction loads, and soil compaction loads should also be considered in the design.

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Unbalanced water behind an abutment or wall adds significant lateral pressure and should be avoided by using free draining gravity outlets for water. Abutment and wing wall backfill should consist of structural backfill as specified in Section 704.04 of FP-14.

The coefficient of at-rest earth pressure should be used for design if the abutment is so restrained that it cannot be expected to rotate (deflect at the top) 0.002 times the wall height. Where deflection of the abutment can be expected, a coefficient of active earth pressure should be used for wall design. Active and at-rest lateral earth pressures of native materials, properly placed and compacted structural backfill, and unclassified borrow above the water table are presented in Table

4.3. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

TABLE 4.3 - Lateral Earth Pressures for Bridge Abutments

Backfill Type Assumed Backfill Properties Case

Unfactored Equivalent Fluid

Density (pcf)

Nominal Friction Factor

Structural Backfill c = 0 psf φ = 34 deg.

γ = 125 pcf

Active 32 0.28

At-Rest 55 0.44

Native Soil c = 0 psf φ = 32 deg γ = 118 pcf

Active 32 0.31

At-Rest 55 0.47

4.2.1 Bearing Resistance for Abutment and Wing Wall Design

It is anticipated that the abutment and wing walls will be founded on granular soil with adequate bearing resistance. Based on the subsurface materials encountered at the project site and the methods used for determining soil strengths parameters, bearing resistance factors for long-term stability condition were selected from the LRFD AASHTO Manual (2014), Table 10.5.5.2.2-1 and are presented in Table 4.4.

TABLE 4.4 - Resistance Factors for Abutment and Wall Foundation 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

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Ultimate bearing resistance of the wing wall foundations is dependent on both the length and width of the foundation elements. For the purpose of these calculations, a foundation length of 20 feet was assumed based on the maximum corrugated metal pipe arch (CMPA) size (103-inches by 71-inches) proposed by the hydraulics engineer. Plate 4 presents the ultimate bearing capacity of the foundation soils for various effective foundation widths. The appropriate resistance factors shown in Table 4.4 above should be applied to the ultimate capacity to determine the factored capacity.

4.2.2 Settlement and Consolidation

Soils typically experience two types of volume change related to loading: short-term (elastic) settlement and long-term (consolidation) settlement. The classification and index testing of the site soils indicate that settlement due to structural loading will primarily be elastic settlement.

Elastic settlement can be evaluated using the Schmertmann method and are expected to be less than 1-inch. Similar to bearing resistance, elastic settlement depends on the foundation dimensions and loading. Therefore, the potential elastic settlement should be evaluated for a combination of foundation widths and loadings based on the calculated bearing resistance. Plate 5 shows the bearing resistance of various effective foundation widths assuming the soil properties of native soil as shown in Table 4.3 and a foundation length of 20 feet. A presumptive bearing resistance of 3 ksf was assumed based on the description of “fine-loose sand” as shown in Table C10.6.2.6.1-1 of the AASHTO LRFD Manual (2014).

4.2.3 Foundation Preparation

Prepare the foundation soils for the retaining walls in accordance with FP-14 Section 209. For foundations within the active channel, prepare the foundation soil as directed for culverts. Outside of the active channel, replace soft, yielding, or otherwise unsuitable materials in accordance with FP-14 Subsection 209.07.

4.2.4 Lateral Loads

The culverts should be designed to resist lateral loads based on the parameters reported in Table

4.3. The equivalent fluid densities do not include any surcharge for sloping backfill surfaces or other loads. These equivalent fluid densities do not include load factors or factors of safety; the designer should apply appropriate factors based on their design methodology. Below the mean water level, design the culvert wall for hydrostatic loading.

Lateral loads imposed on the structures are resisted by the development of friction between the base of the structure footing and the supporting soils. The nominal friction factors presented in Table 4.3 above should be used for the design of the culverts.

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4.3 PERMANENT EARTHWORKS

4.3.1 Embankment Construction

Embankment construction could be necessary with regards to the proposed culvert wing walls.

Construct permanent long-term embankments with a maximum slope ratio of 1V:2H to maintain slope stability and promote slope vegetation.

4.3.2 Shrink/Swell Recommendations

On-site soils expected to be encountered within the project limits generally consist of silty sand or silt with varying amounts of gravel. It is estimated that these soils will have a 10 percent shrink percentage, corresponding to a shrink/swell factor of 0.90. The recommended shrink/swell factor is based on a combination of standard tabled values for common materials in the FLH Technical Guidance Manual (2006) and experience with other CFLHD projects in similar materials.

4.3.3 GRS Backfill for Abutment

It is understood that geosynthetic reinforcement of the abutment backfill is being considered at the Salt Gulch Bridge. Based on a review of the borings and the elevations obtained from the draft bridge plans, the bottom of the abutment wall is expected to be founded on dense silty gravel with sand. A maximum 1-foot spacing of geosynthetic material should be maintained within the backfill wedge. Earth pressure at the face of a GRS structure is considerably lower than the active earth pressure. Based on Equation 2 in FHWA (2011a), an active coefficient (Ka) of 0.3, vertical reinforcement spacing of 1-ft, the lateral earth force per unit width of wall will be less than 0.1 times the soil unit weight. Therefore, use of an equivalent active coefficient of 0.1 for design of the wall is recommended to account for lateral earth pressures.

The CO should inspect the bottom of the excavation and consult with a geotechnical engineer to assure the suitability of the foundation materials for sustaining the imposed loads. GRS backfill material should consist of AASHTO M-43 gradation size No. 89. Additional notes regarding the GRS backfill can be found on Sheet S8 of the plan set.

4.4 PRESUMED LANDSLIDE AREA

The presumed landslide area located to the southeast of the bridge site was monitored over a five-month period from April 2019 to August 2019. Inclinometers and piezometers were installed at both the top and bottom of the hillside as discussed in Section 3.1 of this report. Regular measurements of the installed instrumentation were recorded and collected by a CFL geotechnical engineer. Due to snowmelt and increased precipitation, it was critical to capture information on the slide area during the spring and early summer months. The largest cumulative displacement measured was less than roughly 1/16th of an inch. Correspondingly, the inclinometer equipment provides a system accuracy of ±0.01 inch per readings or, ±0.3 inches accumulated over 50

Page | 15 readings (Durham Geo-Enterprises, 2019). Based on this data, the landslide is likely suspended.

Furthermore, surface expressions related to landslides, such as scarp, creep, or cracks, although present at select locations, do not appear to be prevalent in the area surrounding the bridge foundation. Additional information and displacement plots related to the inclinometer data is presented in Appendix D of this report.

4.5 CONSTRUCTION CONSIDERATIONS

Roadway Excavation: Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits.

Based on the subsurface investigation and surface geologic mapping, the rock encountered, primarily weathered schist, is expected to be rippable in the area surrounding the south abutment of the Salt Gulch Bridge; however, final determination of bedrock rippability is the responsibility of the contractor. Shear wave velocity of the geologic layers was not evaluated by geophysical methods. However, based on Table F.1 in Appendix F, the rock encountered can be presumed to be between the soft and hard rock descriptions. Additional information regarding the excavation characteristics of rock and rippability charts can also be found in Appendix F.

Evidence of definitive in-place bedrock was not encountered from the subsurface investigation detailed in this report. However, appropriate construction equipment should be mobilized to the site based on the rock information detailed above. The contractor is responsible for safety of excavations and shoring design.

GRS Abutment: Foundations should be prepared in accordance with Section 208.08 of FP-14. The bottom of the GRS should not be founded directly on soft, loose organic soils. Refer to Section

4.3.3 of this report for further information.

Drilled Shafts: The contractor will be required to submit a drilled shaft construction plan as required in Section 565.04 of the FP-14 which includes the outlining of proposed methods to maintain borehole stability, manage the excavation of rock, concrete placement, and dewatering.

Weathered schist and quartz rock were encountered at all the exploration boreholes. Be prepared for cobbles and boulders ranging in size from approximately 12 to 36 inches. Loose subsurface materials were also encountered at shallow depths, which may warrant the use of temporary casing.

It is anticipated that groundwater will be encountered at the foundation excavations. The likelihood of flowing sands could also present a challenge for the driller as the mixture of fine sand and water could infiltrate the drilling equipment. The contractor should be prepared for wet construction methods as groundwater can infiltrate even when temporary casing is used.

Page | 16

4.6 SPECIFICATIONS

Special provisions were developed to be consistent with geotechnical recommendations stated above and were incorporated into the special contract requirements (SCR) to amend the FHWA Standard Specification for Construction of Roads and Bridges on Federal Highway Projects;

known as FP-14.

Page | 17

4.7 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations in this report are based on the data obtained from exploratory borings, field review, measurements from installed inclinometers and piezometers, 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.

Page | 18

SECTION FIVE - References

American Association of State Highway and Transportation Officials (AASHTO), 2014, LFRD Bridge Design Specifications, 7th Edition.

Brown, Dan A., Turner, John P., and Castelli, Raymond J., 2010, “Drilled Shafts: Construction Procedures and LRFD Design Methods,” NHI Course No. 132014, Geotechnical Engineering Circular No. 10, Report No. FHWA NHI-10-016, dated May.

Donato, M.M., Coleman, R.G., and Kays, M.A., 1980, Geology of the Condrey Mountain Schist, northern Klamath Mountains, California and Oregon: Oregon Department of Geology and Mineral Industries, Oregon Geology v. 42, no. 7, scale 1: 211,200.

Durham Geo-Enterprises, Inc., 2019, “Inclinometer Accuracy,” DGSI Slope Indicator, <https://durhamgeo.com/resources/tech-notes/inclinometers/inclinometer-accuracy/>.

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), 2011, “Geosynthetic Reinforced Soil Integrated

Bridge System Synthesis Report,” Publication No. FHWA-HRT-11-027.

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), 2019, “Technical Memorandum - Rehabilitation of

Salt Gulch Bridge Over West Horse Creek,” dated December.

Federal Highway Administration (FHWA), 2020, “Salt Gulch Bridge Final Hydraulic Report,” dated April 2020.

Harned, C.H., 1975, “Geotechnical Report for Salt Gulch Bridge and Slide Located on Klamath

National Forest Near Horse Creek, California,” CH2MHill, dated March.

Hawthorne Cat, 2018, “Caterpillar Performance Handbook,” Edition 48, dated June.

Hotz, P.E., 1979, Regional metamorphism in the Condrey Mountain quadrangle, north-central

Klamath Mountains, California: U.S. Geological Survey, Professional Paper 1086, scale 1:

631,177.

Irwin, W.P., 1972, Terranes of the western Paleozoic and Triassic belt in the southern Klamath Mountains, California: U.S. Geological Survey, Professional Paper 800-C, scale 1:

511,000.

Page | 19

Irwin, W.P., 1994, Geologic map of the Klamath Mountains, California and Oregon: U.S.

Geological Survey, Miscellaneous Investigations Series Map I-2148, scale 1: 500,000.

Isenhower, William M., Wang, Shin-Tower, and Vasquez, Gonzalo L., 2016, “Technical Manual for LPile 2016 (Using Data Format Version 9),” Ensoft, Inc., dated January.

Kimmerling, Robert E., 2002, “Geotechnical Engineering Circular No. 6 - Shallow Foundations,” Report No. FHWA-SA-02-054, dated September.

Reese, Lymon C., Wang, Shin-Tower, Arrellaga, Jose A., and Vasquez, Luis, 2017, “SHAFT v2017 - Technical Manual,” Ensoft, Inc., dated June.

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

Turner, Keith A. and Robert L. Schuster, 1996, “Landslides: Investigation and Mitigation,” Transportation Research Board Special Report 247, National Academy Press.

U.S. Department of Agriculture (USDA), 2012, “National Engineering Handbook (NEH), Part 631 Geology,” Chapter 4: Engineering Classification of Rock Materials, dated January.

U.S. Forest Service (USFS), Region 5, Bridge Condition Inspection Reports, Bridge 46N50.0-067, compilation dated 1974-2012.

U.S. Geological Survey (USGS), 2019, “Quaternary Fault and Fold Database of the United States.”

USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/hazards/qfaults/>.

U.S. Geologic Survey (USGS), 2019, “U.S. Seismic Design Maps,” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/designmaps/us/application.php>.

Weaver, J.M., (1975), “Geologic Factors Significant in the Assessment of Rippability,” The Civil Engineer in South Africa (Die siviele ilngenieur in Suid-Afrika), Volume 17, Issue 12, pgs. 313-316.

502105

0 50 100 150 200 250 300 350 400 450 500

El ev at io n (ft

Factored Axial Resistance (kips)

CA FS 1038(1)

Salt Gulch Bridge

Factored Axial Resistance Strength Limit State

Qb, Ultimate Base Resistance Qu, Ultimate Pile Capacity Qs, Ultimate Side Resistance

From Bridge:

280k Factored (Strength I)/shaft

PLATE 4 - Factored Axial Resistance for Drilled Shafts

0 1 2 3 4 5 6 7 8 9 10

Be ar in g

Re si st an ce , R n (K

SF

Effective Foundation Width B' (FT)

EFFECTIVE WIDTH vs. BEARING RESISTANCE (LRFD) Culvert Wing Walls - FS Salt Gulch (CA FS 1038(1))

B'

Q

Nominal (unfactored) strength and extreme event limit state -Strength limit state resistance factor = 0.45 -Extreme event limit states resistance factor = 1.0

Presumptive (factored) service limit state for 1-in settlement AASHTO LRFD Bridge Design SpecificationsTable C10.6.2.6.1-1

For cast-in-place concrete footing use sliding factor = 0.80 AASHTO LRFD Bridge Design SpecificationsTable 10.5.5.2.2-1

PLATE 5 - Bearing Resistance for Concrete Wing Walls

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 CA FS 1038(1), Salt Gulch Bridge, from April 4th through April 18th, 2019. The scope of work for the geotechnical field exploration program included drilling a total of six (6) borings, or three pairs: two near the southeast corner of the bridge abutment, two near the bottom of the presumed landslide area, and two near the top of the presumed landslide area on Forest Road 47N69. One boring of each pair was installed with inclinometer casing while the other was installed as a temporary groundwater monitoring point. Inclinometer data is attached in Appendix D. The field exploration program was coordinated and observed by CFLHD Geotechnical personnel. Field exploration locations are illustrated on Plate 3. Individual boring logs are attached in this appendix. These logs represent a compilation of field and laboratory data and description of the soil and rock by CFLHD Geotechnical personnel. The methods used to conduct the field exploration program are described below. Photos of drilling equipment and field exploration activities are included in Appendix C. All 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

Borings

Woodward Drilling of Rio Vista, California provided the drilling services for the soil borings.

Borings were completed using a Mobile B57 track mounted rig. Borings were advanced through overburden using hollow stem augers with drive sampling until practical auger refusal or desired depth was reached. Following drilling activities, field personnel backfilled the borings with cuttings generated during the drilling in accordance with applicable local, state, and federal regulations.

If water was encountered at the time of drilling, field personnel measured water levels in the borings. Fluctuations in the ground water level due to seasonal and climatic effects are expected.

The GPS coordinate location and elevation of individual borings were estimated using Google Earth and data acquired from the CFL Survey Team. Boring locations are listed on individual boring logs and are shown below in Table A-1.

TABLE A-1

FIELD EXPLORATION LOCATIONS

Exploration Designation Instrumentation Location

Approx.

Latitude

Approx.

Longitude (°)

Approx.

Elevation

(ft)

Approx.

Depth

(ft) MW19-01 Piezometer South Abutment 41.872453 -123.085167 2,169 55 SI19-01 Inclinometer South Abutment 41.872453 -123.085167 2,169 55.25

Exploration Designation Instrumentation Location

Approx.

Latitude

Approx.

Longitude (°)

Approx.

Elevation

(ft)

Approx.

Depth

(ft) MW19-02 Piezometer Top of Slope 41.871978 -123.084097 2,294 90 SI19-02 Inclinometer Top of Slope 41.871978 -123.084097 2,294 90

MW19-03 Piezometer Toe of Slope 41.872528 -123.084914 2,170 50 SI19-03 Inclinometer Toe of Slope 41.872528 -123.084914 2,170 50

A.3 SOIL AND ROCK SAMPLING

Borings

Disturbed samples were obtained from the borings in accordance with the Standard Penetration Test (SPT), the procedures of which are detailed in AASHTO T-206. The SPT involves driving a 2-inch outside diameter, 1.375-inch inside diameter split spoon sampler a depth of 18 inches with a 140-pound hammer falling a distance of 30 inches. The number of blows required to advance the split-spoon sampler through each of the 6-inch increments was recorded. The SPT resistance, or N-value, is defined as the number of blows required to drive the sampler over the second and third 6-inch increments. The N-value provides a means for evaluating the relative density or compactness of cohesionless (granular) soils and consistency or stiffness of cohesive (fine-grained) soils. An automatic hammer was used to drive the samplers for this project.

Representative portions of the split-spoon sample obtained in conjunction with the SPT were placed in plastic baggies and transported to the CFLHD Materials Laboratory for testing.

A.4 SOIL AND ROCK CLASSIFICATION SYSTEM

During the completion of borings, 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.

A.5 INSTRUMENTATION

Upon completion, selected borings were instrumented for purposes of measuring groundwater levels and ground movement. Piezometers were installed in Borings MW19-01, MW19-02, and MW19-03. Inclinometers were installed in Borings SI19-01, SI19-02, and SI19-03. More details on the employed inclinometer system and the results of inclinometer readings are presented in Appendix D.

A piezometer is used to measure piezometric head levels to determine the stability of slopes and embankments. Piezometers are also used to monitor ground improvement techniques, dewatering schemes, and seepage or groundwater movement.