CA.ERFO.FS PLMAS.2021-02.North Fork Feather River Bridge.Geotech Rpt FINAL.rev.pdf

NORTH FORK FEATHER RIVER BRIDGE

PLUMAS NATIONAL FOREST

PLUMAS COUNTY, CA

CA ERFO FS PLMAS 2021-02(1)

FINAL Geotechnical Report

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch January 2023 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

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

Aaron A. Baukus, Geotechnical Engineer Trainee

Report reviewed by: ____________________________________________________________

James M. Arthurs, P.E., PhD., Acting Lead Geotechnical Engineer

Approved for distribution by: ____________________________________________________

James M. Arthurs, P.E., PhD., Acting Lead Geotechnical Engineer

Distribution

Electronic:

N:\CA\erfofs2021-02(1)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Designer Bridge Hydraulics

JAMES M

ARTHURS

Digitally signed by JAMES M

ARTHURS

Date: 2023.01.19 13:29:27 -07'00'

JAMES M

ARTHURS

Digitally signed by JAMES

M ARTHURS

Date: 2023.01.19 13:49:51 -07'00'

AARON ALGIRDAS BAUKUS Digitally signed by AARON ALGIRDAS BAUKUS Date: 2023.01.19 14:55:34 -07'00'

DEVIN T DIXON

Digitally signed by DEVIN T

DIXON

Date: 2023.01.19 15:19:09 -07'00'

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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

3.3.1 General Topography

3.3.2 Average Temperatures and Precipitation

3.3.3 Surface Reconnaissance

3.3.4 Scour Potential and Erosion

3.3.5 Subsurface Conditions

3.3.6 Groundwater

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 FOUNDATIONS

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 Recommended Shaft Lengths

4.1.8 Field Testing

4.1.9 Scour Potential and Erosion

4.2 ABUTMENT & WINGWALL DESIGN

4.3 PERMANENT EARTHWORKS

4.3.1 Embankment Construction

4.3.2 Shrink/Swell Recommendations

4.4 CONSTRUCTION CONSIDERATIONS

4.5 SPECIFICATIONS

4.6 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 Field Exploration Program Table 3.2 - Summaries of Laboratory Index Test Results Table 3.3 - Summaries of R-Value Results Table 3.4 - Summaries of Laboratory Corrosivity Results Table 3.5 - Summary of Uniaxial Compressive Strength Results Table 3.6 - Climate Data Summary Table 4.1 - Proposed Bridge Foundation Location Table 4.2 - Bridge Loading Table 4.3 - LPILE Parameters Table 4.4 - Shaft Lengths Required for Axial and Lateral Loading Table 4.5 - Lateral Earth Pressures for Bridge Abutments & Wingwalls

PLATES

Plate 1 Project Vicinity Map Plate 2 Geology Map Plate 3 Geology Map Legend Plate 4 North Fork Feather River Bridge Geotechnical Boring Locations Plate 5 Plumas County Geologic Hazards Map Plate 6 Areas More Likely to Contain Naturally Occurring Asbestos: Plumas N.F.

Plate 7 Access to West Abutment of N. Fork Feather River Bridge

APPENDICES

APPENDIX A Field Exploration Program APPENDIX B Laboratory Test Results APPENDIX C Photographs APPENDIX D Axial Loading Analysis APPENDIX E Excavation Characteristics of Rock & Rippability Charts

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N. Fork Feather River Bridge, CA ERFO 2021-02(1) January 2023

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering study and provides recommendations for the replacement of the North Fork Feather River Bridge in Plumas County, California. The project proposes to replace the damaged bridge crossing over the North Fork of the Feather River, which is located on Forest Route 26N26 within the Plumas National Forest. A project vicinity map is presented on Plate 1 of this report.

The North Fork Feather River Bridge was severely damaged by fire during the 2021 fire season.

The timber deck, beams, rails, and timber cribbing abutments were burned, and their components have collapsed.

The former North Fork Feather River Bridge was constructed as a single lane, two span, 101-foot-long structure in 1957 and designed to H20-44 load-bearing criteria. The superstructure consisted of two steel I-girders per span with timber ledger planks on the top flanges, a treated timber deck with timber running planks, timber curbs, and timber railings. The substructure abutments, pier, and wingwalls were timber cribs founded on concrete footings. Rock bearing of foundations is unknown. Original bridge plans do not exist, however, partial bridge plans from a railing replacement project in 1991 and an inspection report from July 2019 were available from the Forest Service.

The North Fork Feather River Bridge will be replaced to meet HL-93 legal load standards. The proposed replacement structure type is assumed to be a steel plate girder bridge with a concrete deck. The existing timber abutments and pier will be removed, and the embankments will be constructed with a new configuration of spill-through abutments. The new abutments will be simple cast-in-place concrete caps and wingwalls supported by deep foundations. Deep foundations are proposed as spread footers likely cannot be founded on competent soils or rock at a reasonable cost. The potential for scour also exists at the bridge location and deep foundations bearing on suitable materials below such susceptible soils would provide protection against this problem. Considerations related to the abutments for the North Fork Feather River Bridge are presented in Section 4 of this report.

1.2 SCOPE AND PURPOSE

The scope of work included a geotechnical investigation, analysis, and recommendations for bridge foundations for use in design and construction. This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, interpretation and correlation of field measurements, and geotechnical engineering analysis. Specifically, this investigation was

Page | 2 conducted to determine the soil profile at the bridge location and develop recommendations concerning bridge foundations, retaining structures, geologic hazards, and construction considerations.

We understand that a hydraulics report has been 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 project is located within the Sierra Nevada Section of the Cascade-Sierra Mountains Province.

This province is part of the Pacific Mountain System physiographic region. The Cascade-Sierra Mountains Province extends from Alaska in the north to the southern tip of South America. This province is divided into two regions: the Cascade Mountains and the Sierra Nevada Mountains (NPS, 2018). Features and landscapes within the Sierra Nevada Section are characterized by rugged and steep mountains, Mesozoic aged granitic rock, and glacial landforms.

Quaternary sedimentary and metasedimentary rocks and deposits that exist within the region include alluvium (Qal), Quaternary lake deposits (Ql), Quaternary nonmarine terrace deposits (Qt), and Pleistocene nonmarine (Qc), as well as igneous and meta-igneous rocks including recent to Pleistocene basalt (Qrvb, Qpvb). Tertiary sedimentary and metasedimentary rocks and deposits include Eocene and Tertiary nonmarine (Ec, Tc) as well as igneous and meta-igneous rocks including Pliocene andesite and basalt (Pva,b), Miocene andesite (Mva), and Tertiary andesite and basalt (Tva,b). Mesozoic sedimentary and metasedimentary rocks and deposits include Upper to Lower Jurassic marine (Ju, Jml) and Triassic marine (TR) as well as igneous and meta-igneous rocks including Mesozoic granite rocks (gr), Mesozoic ultrabasic intrusive rocks (ub), Jurassic- Triassic metavolcanic rocks (JTRV), and Pre-Cretaceous metavolcanic rocks (mv). Paleozoic sedimentary and metasedimentary rocks and deposits include Paleozoic, Permian, and Mississippian marine (IP, Pm, CM) as well as igneous and meta-igneous rocks including Paleozoic metavolcanic rocks (IPv) (Lydon et al., 1960).

2.2 SITE GEOLOGY

Although no USGS mapping is available for the overburden material, the soil conditions at the site are assumed to be alluvium based on the project setting and the riverbed deposits observed at the bridge site. The project site is mapped as underlain by Mesozoic ultrabasic intrusive rocks (ub), namely of the serpentine subgroup. Triassic and Paleozoic marine deposits (TR, IP) as well as Tertiary basalt (Tvb) are also mapped in the vicinity of the site. These observations are consistent with the borings as will be discussed in Section 3 of this report. The deposits and landforms are a result of volcanic activity, plate tectonics, and glacial movement and the terrain can be described as mountainous and rugged. A geology map of the project area and corresponding map legend are presented in Plates 2 & 3.

2.3 REGIONAL SEISMICITY

Nine seismic hazard faults are located within 50 miles of the project. These faults are summarized in Table 2.1 and are located northeast, northwest, southeast, and southwest of the project site.

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Three of these faults have experienced deformation in the last 15,000 years or less. Known slip rates range from less than 0.00787 to 0.19685 inches per year (USGS, 2020a). The site-source distance using the Caltrans ARS Online (v3.0.2) tool was calculated to be approximately 40 miles with a mean magnitude for PGA to be approximately 6 g. Based on the time of most recent deformation and distance to these faults, surface rupture at the bridge site is not anticipated.

Table 2.1 - Summary of Nearby Seismic Source Faults

FAULT OR

FAULT ZONE

DISTANCE

FROM

CENTER OF

PROJECT

SLIP-RATE

CATEGORY

DIP

DIRECTION

AVERAGE

STRIKE

FAULT

LENGTH

TIME OF MOST

RECENT

DEFORMATION

(miles) (inch/year) (miles) (years) Skinner Flat Fault, (Class A) No. 192

10.8 Not listed Not listed Not listed 9.9 <130,000

Almanor Fault Zone, (Class A) No. 21

12.3 0.03937 to

0.19685 W N14°W 24.9 <130,000

Keddie Ridge Fault, (Class A) No. 191

16.2 Unspecified Not listed Not listed Not listed <1,600,000

Walker Spring Fault, (Class A) No. 190

16.9 Unspecified Not listed Not listed 24.9 <130,000

Mohawk Valley Fault Zone, Mohawk Valley Section, (Class A) No. 25a

25.1 0.00787 to

0.03937 55° E to 90° N42°W

(for section)

14.9 (for section) <15,000

Honey Lake Fault Zone, (Class A) No. 22

35.6 0.03937 to 0.19685

52° SW to vertical N47°W 60.9 <15,000

McArthur-Hat Creek Fault Zones, (Class A) No. 8 & 9

36.4

0.00787 to 0.03937

(McArthur)

0.03937 to

0.19685 (Hat Creek)

W

N17°W (McArthur)

N15°W (Hat Creek)

36.7 <15,000

Foothills Fault System, (Class A) No. 26

37.3 Insufficient

data Not listed Not listed Not listed <1,600,000

Battle Creek Fault Zone

(Class A) No. 20

44.6 0.00787 to

0.03937 Not listed Not listed Not listed <130,000

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2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the North Fork Feather River Bridge project site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 8th Edition, 2017, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1,000-year return period). The 1,000-year return period uniform hazard spectrum for the North Fork Feather River Bridge project site, located at 40.03799°N latitude and 121.22072°W longitude, was obtained in accordance with the AASHTO ground motion maps.

Based on subsurface conditions encountered during drilling and the USGS Vs30 online map viewer, the onsite soils can be assumed to be reasonably dense. 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.

Table 2. - Summary of Seismic Parameters Corrected for Site Class C Horizontal Peak Ground Acceleration, (PGA) 0.277 g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 0.66 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.206 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.1 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.1 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.6

Factored Horizontal Peak Ground Acceleration, (As) 0.311 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.749 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.328 g

Seismic Zone Zone 3

Based on the long acceleration coefficient SD1 value of 0.328, the project 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 of AASHTO.

2.5 GEOLOGIC HAZARDS

Potential geologic hazards at the bridge site include those related to earthquakes, mass movements, and mineral hazards as shown in the geologic hazards map for Plumas County, California presented in Plate 5. Earthquakes, as discussed in the previous section, could pose a threat to the North Fork Feather River Bridge. Northern California and the project site are located in an area of moderate to high potential for damaging earthquakes, although it is impossible to accurately predict the timing or location of future earthquakes. Liquefaction potential is low since the site soils are relatively dense and mostly unsaturated. Seismic hazards can be mitigated by appropriate design analysis as specified in AASHTO.

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Mass movements are also common in California due to major wildfires, heavy rains, the fines content of the soils, steep terrain, and fracturing and weathering of bedrock materials. These rock and soil movements include seismically triggered landslides, debris flows, mud flows, and rock falls. The project is located in terrain susceptible to mass movement and these events could impact the bridge site. No specific investigation was conducted to evaluate slope stability in the project area. Mitigation of landslides is outside the scope of this emergency repair project.

Minerals-related hazards, such as asbestos, mercury, and radon, are present in northern California.

Serpentinite and ultramafic bedrock often contain naturally occurring asbestos (NOA), but not always. The Plumas National Forest has mapped areas more likely to contain NOA as shown in Plate 6. Asbestos materials were not identified during routine description and engineering classification of the site materials. No microscopy or other detailed testing was performed and, thus, a contingency plan for managing NOA encountered during construction should be considered. CFLHD does not take responsibility for development, implementation, or enforcement of any sort of management or abatement plan for NOA.

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SECTION THREE - Subsurface Investigation

3.1 SUBSURFACE EXPLORATION PROGRAM

A subsurface investigation targeting the bridge site was performed by a Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) geotechnical engineer on July 26 and 27, 2022. The geotechnical subsurface exploration program consisted of drilling a total of two borings, one to approximately 26 feet deep and another to approximately 35 feet deep. One boring was drilled near each bridge abutment. Hollow-stem augers and mud rotary were used to drill through the overburden soils, and rock samples were recovered using NQ diamond core drilling. ODEX drilling, or air-operated down-the-hole hammer drilling, was initially considered the preferred geotechnical investigation method given the expected rock strata and boulders in the subsurface. However, the selected drilling company had previously contracted out their sole ODEX system without Central Federal Lands knowledge, and, thus, it was decided to move forward with traditional hollow-stem auger drilling and NQ rock core drilling to meet the project schedule.

Standard penetration testing (SPT) and sample collection was performed at 5-foot intervals for each boring when possible. Two grab samples of the upper subsurface were also collected.

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 boring locations are presented in Appendix A and Plate 4, respectively.

Photographs related to this exploration can be found in Appendix C. 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.)

APPROXIMATE

TERMINATION

DEPTH (ft.)

APPROXIMATE

DEPTH TO

GROUNDWATER

(ft.)

BH22-01 Abutment 1;

STA 10+55, 3 ft. RT 2,414.7 25.5 Not encountered

BH22-02 Abutment 2;

STA 12+56, 8 ft. LT 2,416.5 35.0 Not encountered

Note: The exploration locations were estimated relative to existing features. Ground elevations were acquired from CFL survey data.

3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the boring 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), sieve analysis (AASHTO T 11 and T 27), classification (AASHTO M 145 and ASTM D 2487), Atterberg limits

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(AASHTO T 89 and T 90), R-value (AASHTO T 190), resistivity (AASHTO T 288), 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. Results of the testing are summarized below in Table 3.2 and are presented in Appendix B.

Table 3.2 - Summaries of Laboratory Index Test Results

BORING

NUMBER

SAMPLE

DEPTH

PERCENT

GRAVEL

PERCENT

SAND

PERCENT

PASSING

#200

LIQUID

LIMIT

PLASTIC

LIMIT

USCS

CLASS.

AASHTO

CLASS.

(feet)

BH22-01 0 - 5 35 46 19 22 19 SM A-1-b(0)

BH22-01 5 - 6.5 52 36 12 28 22 GW-GC A-1-a(0)

BH22-01 10 - 11.5 34 52 14 20 19 SM A-1-b(0)

BH22-02 0 - 5 57 31 12 25 20 GP-GC A-1-a(0)

BH22-02 5 - 6.5 32 52 16 28 22 SC-SM A-1-b(0)

Soil test results indicated a range of material types composed of well and poorly graded, clayey gravel to clayey and silty sand classifying as A-1-a to A-1-b by AASHTO and GW-GC, GP-GC, SM, and SC-SM by USCS.

R-value testing of two samples taken directly beneath the asphalt are summarized in Table 3.3.

Table 3.3 - Summaries of R-Value Results

BORING NUMBER SAMPLE DEPTH R-VALUE, SUBGRADE

(feet)

BH22-01 0 - 5 65

BH22-02 0 - 5 70

Note: Bridge approaches are not paved.

Corrosivity and chemistry testing of select samples were sent to an outside laboratory for testing and are summarized in Table 3.4.

Table 3.4 - Summaries of Laboratory Corrosivity Results

BORING

NUMBER

SAMPLE

DEPTH

RESISTIVITY PH SULFATE ION

CONTENT

CHLORIDE ION

CONTENT

(feet) (ohm-cm) (ppm) (ppm)

BH22-01 0 - 5 5,040 8.0 - -

- Tests not completed.

Unconfined compressive strength (UCS) of rock material was conducted on intact samples of rock core recovered from the borings. Results of the UCS testing are presented in Table 3.5.

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Table 3.5 - Summary of Uniaxial Compressive Strength Results

BORING NUMBER SAMPLE DEPTH RQD1 LENGTH / DIAMETER2 UCS

(feet) (%) (psi)

BH22-01 24.2 44 2.4 21,000

BH 22-02 17.7 19 2.3 12,153

BH22-02 26.3 43 2.4 18,010

BH22-02 29.0 62 2.4 24,330

BH22-02 30.8 62 2.4 35,260

1Rock Quality Designation for core run from which sample was taken in unweathered rock and boulders, not bedrock surface.

2Values less than 2.0 do not meet the requirements of ASTM D 7012. Not included in analysis if encountered.

3.3 SUMMARY OF SITE CONDITIONS

This section presents the results of the surface reconnaissance and subsurface exploration.

3.3.1 General Topography

The general topography of the project site is characterized by the steep Sierra Nevada Mountains that lie primarily in central and eastern California and reaches its northern terminus in the Plumas National Forest. Metamorphic boulders were observed to be intermixed within sandy soil matrices at both bridge abutments measuring up to 3 to 4 feet in diameter and larger. Portions of boulders were buried; therefore, the actual sizes may be greater than what was observed. Rock outcroppings were not visible within the vicinity of either bridge abutments. Burnt, dead standing, trees typical of mixed conifer forests containing Ponderosa pine intermingled with strands of old growth, scattered lodgepole pine, and red fir are abundant at the project site. Signs of revegetation are noticeable in the surrounding grass and shrubs. The North Fork Feather River generally flows southwards from its headwaters near Lassen Peak to Lake Oroville approximately 45 to 50 miles south of the Forest Road 26N26 bridge crossing.

3.3.2 Average Temperatures and Precipitation

Table 3.6 shows the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data for the period from 1895 to 2016 for the Quincy, California (047195) weather station (Western Region Climate Center, 2022).

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Table 3.6 - Climate Data Summary

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ANNUAL

Avg. Max. Temp. (°F) 45.3 51.1 56.8 63.7 72.6 81.4 89.5 88.4 82.1 71.1 55.4 46.1 67.0

Avg. Min. Temp. (°F) 23.5 26.2 29.0 32.2 37.6 42.2 44.0 41.6 36.8 31.9 28.5 25.1 33.2

Avg. Total Precip. (in.) 7.38 6.47 5.53 2.74 1.74 0.79 0.14 0.22 0.87 2.60 4.83 6.84 40.15

Avg. Total Snowfall (in.) 16.9 11.1 10.2 2.8 0.5 0.0 0.0 0.0 0.0 0.5 3.2 9.9 55.1

Avg. Snow Depth (in.) 4 3 1 0 0 0 0 0 0 0 0 1 1

Note:

1) Period of Record: 04/01/1895 to 06/10/2016.

2) Percent of possible observations for period of record: Max. Temp.: 80.9% Min. Temp.: 81% Precipitation: 89%

Snowfall: 84.5% Snow Depth: 84.6%.

3.3.3 Surface Reconnaissance

Superstructure timber elements were incinerated by the fires and partial remaining elements will be removed. Remaining steel elements are stockpiled adjacent to the site and will also be removed.

Anchor rods protruding from the ground at the pier location are presumed to remain in place along with concrete footings. Access to the east abutment is available from Caribou Road, however access to the west abutment requires a lengthy detour on gravel-surfaced Forest Service roads.

Plate 7, courtesy of the Plumas National Forest, shows the recommended route to access the west abutment.

The forested area around the bridge site is severely burned. The significant loss of native vegetation has exposed the soil to erosion and increased water runoff. Regrowth of grass and small bushes has begun to take place; however, many exposed areas remain. Increased sediment transport was observed and is likely to continue until vegetation is well reestablished. Due to the weakened surface conditions and the steepness of the surrounding slopes, sediments, debris, loose rocks and boulders, and tree parts have the potential to slide downslope towards North Fork Feather River Bridge. In general, however, any movements related to the slopes adjacent to the project sites appear to be surficial and slow moving.

The approaches leading to the bridge site consist of an aggregate surfacing. Alluvial soils and sediments encompass the bridge site. Mostly subangular to rounded boulders ranging in size from approximately to 3 to 4 feet in diameter were observed on both the upstream and downstream sides of the bridge abutments and embankments. These boulders could have been placed around the previous bridge as scour protection and may have been locally sourced during the original construction. Rock outcroppings were not observed.

3.3.4 Scour Potential and Erosion

Long-term degradation, contraction scour, and local (abutment) scour depths were estimated by the assigned CFLHD hydraulics engineer. Scour elevations were provided by the CFLHD

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Hydraulics Group in the Final Hydraulics Report dated December 9, 2022 (CFLHD, 2022). The recommended scour elevation for the bridge abutments was estimated to be 2,384.80 feet for the scour design flood and 2,384.61 feet for the scour check flood. This is approximately 14.4 feet below the assumed channel thalweg for the scour design flood. Scour countermeasures consisting of Class 5 riprap are proposed to provide additional protection along the wingwalls and abutments.

Please refer to the previously mentioned Final Hydraulics Report for more information.

3.3.5 Subsurface Conditions

The subsurface conditions at Abutment 1 of the North Fork Feather River Bridge were investigated by drilling boring BH22-01. Hollow stem auger drilling was used from approximately 0 to 17 feet below the ground surface and standard triple-tube HQ wireline rock coring methods were used from approximately 17 feet below the ground surface to the termination depth of approximately 26 feet. The ground surface surrounding the boring location consisted of silty sand and gravel alluvial soils. The boring encountered dense to very dense silty sand with gravel and varying amounts of cobbles and rock fragments to the termination depth. Boulders were encountered within this soil matrix from approximately 17 feet to the termination depth. The boulders were observed to be up to approximately 3 to 4 feet in diameter or larger. The borehole was terminated at approximately 26 feet, roughly 4.4 feet above the proposed scour elevation, due to slipping of the drill casing caused by an uneven or irregular surface. Neither groundwater nor hard, intact bedrock were encountered to the depths explored.

The subsurface conditions at Abutment 2 of the North Fork Feather River Bridge were investigated by drilling boring BH22-02. Hollow stem auger and mud rotary drilling were used from approximately 0 to 10 feet and 10 to 12 feet below the ground surface, respectively. Standard triple-tube HQ wireline rock coring methods were used from approximately 12 feet below the ground surface to the termination depth of approximately 35 feet. The ground surface surrounding the boring location consisted of silty sand and gravel alluvial soils. The boring encountered loose poorly graded gravel with clay, rock fragments, and cobbles to approximately 6 feet. The material then transitioned to medium dense clayey sand with gravel and rock fragments from approximately 6 feet to the termination depth. Boulders were encountered within this soil matrix from approximately 12 feet to the termination depth. The boulders were observed to be up to approximately 3 to 4 feet in diameter or larger. The borehole was terminated at approximately 35 feet, roughly 3.3 feet below the proposed scour elevation, due to borehole instability. Neither groundwater nor hard, intact bedrock was encountered to the depths explored.

3.3.6 Groundwater

Groundwater was not encountered during drilling in either borings BH22-01 or BH22-02.

However, fluctuations in the groundwater level due to seasonal and climatic effects are expected and will likely follow water levels in the adjacent river.

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the bridge foundations, abutment wingwalls, permanent earthworks, and construction considerations for the design and construction of the North Fork Feather River Bridge project. Based on discussions with the project team and the severe damage to all bridge structure components, the bridge will be replaced. Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 FOUNDATIONS

The existing single span bridge will be replaced with a steel plate girder bridge with a concrete deck. The remaining existing bridge elements will be removed. Based on the 95% bridge plan sheets dated for distribution in November 2022, 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 ELEVATION1

Abutment 1 10+90.25 2,409.40 FT Abutment 2 12+14.75 2,410.02 FT

1 As presented on the 95% foundation plan.

4.1.1 Bridge Foundation Selection

A drilled shaft foundation system is proposed for the new abutments due to the relatively high axial and lateral loads. Micropiles were considered but determined impractical due to the large lateral loads. A driven pile foundation was also considered but determined impractical due to the presence of cobbles and boulders that could cause pile damage during driving, as well as the limited space available and accessibility to the site being difficult for standard pile driving equipment.

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 lateral resistance at a shallower elevation than driven piles. Based solely on hydraulic recommendations and assumptions stated in the Final Hydraulics Report (CFLHD, 2022), scour potential as measured from the thalweg was determined to be present at the bridge abutments. Based on available data, it was determined by the combined team of Bridge, Hydraulics, and Geotechnical Engineers that drilled shafts would provide the best foundation system for this bridge.

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Table 4.2 presents the final bridge loads factored for the strength limit state as provided by the CFL bridge engineer in October 2022:

Table 4.2 - Bridge Loading1

FOUNDATION

LOCATION

REQUIRED FACTORED

SHAFT RESISTANCE

(kips)

STRENGTH I

BRIDGE LOAD

PER SHAFT

(kips)

SERVICE I

BRIDGE LOAD

PER SHAFT

(kips) STRENGTH I

( = 0.99)2 Abutment 1 & 2 303 300 200

1 Bridge loading assumes 3-30-inch drilled shafts per abutment.

2 Reduction factor for group effects per Section 4.1.4.

4.1.2 Site Characterization

The subsurface profile was assumed to be entirely sand for analysis purposes. Although gravel, silt, cobbles, rock fragments and boulders were encountered in the borings, the subsurface materials, in general, were of a sand-like nature and composition. Groundwater was also assumed to be at the channel thalweg elevation of approximately 2,399 feet. These assumptions are conservative, or, in other words, purposefully broad to account for the limited subsurface data collected below the scour depth.

4.1.3 Drilled Shaft Axial Resistance

The drilled shaft axial resistance analysis was performed for 24, 30, and 36-inch diameter shafts at the abutments using the beta method for cohesionless soils presented in section 10.8.3.5.2b of AASHTO. The selected drilled shaft diameter of 30 inches was chosen to meet both structural considerations and loading requirements. 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.

Results of the calculations for 30-inch diameter drilled shafts are presented in Appendix D.

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 shafts within the group. The zone of influence from an individual shaft in a shaft group may intersect with other shafts, depending on the shaft spacing. Historically, the efficiency of groups of drilled shafts has not been a concern as long as the center-to-center spacing between shafts is greater than three times the shaft diameter (3D) to avoid interference between adjacent shafts, assuming a single row shaft group configuration be applied for spacing less than 3D as shown in Table 10.8.3.6.3-1 of AASHTO.

Besides the effect of overlapping zones of influence, effects of construction on ground conditions in and around the group can be significant. Excavated deep foundation elements in cohesionless

Page | 14 soils tend to decrease the effective stress of the surrounding soils. Poorly controlled shaft construction methods can result in soil loosening during drilling and adversely reduce the lateral stress around other shafts within the group. Casing driven in advance of excavation may increase the relative density and effective stress of the surrounding soil and prevents caving of overburden material.

4.1.5 Lateral Loads on Deep Foundations

Lateral load analysis was 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 4.3 provides LPILE input parameters for the foundation soils based on available subsurface information and presumptive engineering correlations. We recommend that lateral support above the scour elevation defined in the CFLHD Hydraulics Report be neglected due to potential loss of material during the design flood event. 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 4.3 - LPILE Parameters

APPROXIMATE

ELEVATION

(ft)

DEPTH BELOW

EXISTING

GROUND (ft)

LPILE P-Y

MODEL

EFFECTIVE

UNIT WEIGHT,

’ (pcf)

FRICTION

ANGLE,

(deg)

HORIZONTAL

SUBGRADE

MODULUS, Ks

(pci)

NORTH FORK FEATHER RIVER BRIDGE

2,415 to 2,402 (Abutment 1) 2,417 to 2,405 (Abutment 2)

0 to 13 (Abutment 1)

0 to 12 (Abutment 2)

Sand (Reese)

127.0 38

(Abutment 1)

(Abutment 2)

2,402 to 2,399 (Abutment 1) 2,405 to 2,399 (Abutment 2)

13 to 16 (Abutment 1)

12 to 18 (Abutment 2)

Sand (Reese)

127 38 225

2,399 to 2,389 (Abutment 1) 2,399 to 2,382 (Abutment 2)

16 to 26 (Abutment 1)

18 to 35 (Abutment 2)

Sand (Reese)

64.6 38 125

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

Page | 15 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. Differential settlements are not anticipated. Shaft group settlement is not anticipated.

4.1.7 Recommended Shaft Lengths

The final lengths of the drilled shafts are based on both axial and lateral loading. Table 4.4 below shows the required shaft lengths needed for both axial and lateral capacities. This is based on the selected shaft diameter of 30 inches. The lateral capacities needed to meet loading requirements were determined to control the design shaft tip elevations for the North Fork Feather River Bridge.

Table 4.4 - Shaft Lengths Required for Axial and Lateral Loading

STRUCTURE

ELEMENT

REQUIRED SHAFT LENGTH

FOR AXIAL CAPACITY

(feet)

REQUIRED SHAFT LENGTH

FOR LATERAL CAPACITY

ESTIMATED TIP

ELEVATION

Abutment 1 41 48 2,361.40 Abutment 2 41 48 2,362.02

4.1.8 Field Testing

Steel crosshole sonic logging tubes (1.5-inch diameter steel tubes) should be installed in all drilled shafts prior to concrete placement for integrity testing. The recommended number of access tubes and tube spacing are dependent on the selected shaft diameter, typically one access tube per 12-inches of shaft diameter, with a minimum of three access tubes per shaft. The crosshole sonic logging tests should be conducted in accordance with ASTM D-6760 and FP-14 for quality assurance/quality control of the drilled shafts.

4.1.9 Scour Potential and Erosion

Contraction scour and local abutment scour depths were estimated by the hydraulics engineer.

Sections 5.3 and 5.4 in the Final Hydraulics Report details the scour depth estimates.

Provision of scour countermeasures can be used to mitigate scour effects and reduce maintenance.

For additional scour and countermeasure information please refer to the previously mentioned Final Hydraulics Report dated December 2022.

Page | 16

4.2 ABUTMENT & WINGWALL DESIGN

Abutments and wingwalls 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.

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 wingwall 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 and properly placed and compacted structural backfill above the water table are presented in Table 4.5. The values are unfactored loads and assume that the surface of the soil slope behind the wall is horizontal.

Table 4.5 - Lateral Earth Pressures for Bridge Abutments & Wingwalls

BACKFILL TYPE

ASSUMED

BACKFILL

PROPERTIES

CASE

UNFACTORED

EQUIVALENT

FLUID DENSITY

(PCF)

NOMINAL

FRICTION

FACTOR

Structural Backfill c = 0 psf = 34 deg.

Active 35 0.67

At-Rest 55 0.67

Native Soil c = 0 psf = 38 deg

7 pcf

Active 30 0.78

At-Rest 49 0.78

4.3 PERMANENT EARTHWORKS

4.3.1 Embankment Construction

Embankment construction will be necessary with regards to the concrete wingwalls. 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 and clayey sands with varying amounts of gravel. It is estimated that these soils will have a 10 percent

Page | 17 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.

4.4 CONSTRUCTION CONSIDERATIONS

Traffic and Access: A temporary bridge or diversion for traffic is not required. However, access to the west abutment is limited due to a lengthy detour, shown on Plate 7, and the road condition.

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, intact bedrock was not encountered to the depths explored at the site. Conventional heavy-duty excavation and ripping are assumed to be capable of completing the shallow excavations in the areas surrounding the abutments of the North Fork Feather River Bridge; however, final determination of rock rippability is the responsibility of the contractor. Shear wave velocity of the geologic layers was not evaluated by geophysical methods. Excavation considerations for drilled shafts are presented later in this section. Additional information regarding the excavation characteristics of rock and rippability charts can also be found in Appendix D.

Evidence of intact bedrock was not encountered during the subsurface investigation as detailed in this report. However, appropriate construction equipment should be mobilized to the site based on the excavation information detailed above. The handling of boulders of at least 4 feet in diameter will be required for excavations in the project area. The contractor is responsible for the safety of excavations and shoring design.

Existing Bridge Elements: Existing timber abutments and pier will be removed and a new configuration of spill-through abutments with 1:1.75 or 1:2 embankment slopes will be constructed.

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 boulders, concrete placement, and dewatering.

Boulders were encountered at both borings BH22-01 and BH22-02. Be prepared for cobbles and boulders ranging in size from approximately 12 to 48 inches and larger. The boulders are composed of very hard to extremely hard rock, and will likely require specialized tooling and drilling methods to maintain the shaft alignment. Loose subsurface materials were also encountered at shallow

Page | 18 depths, which may warrant the use of temporary casing. The contractor should review the geotechnical borings to determine the need and extents of temporary casings.

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. Pockets of sandy material were encountered to the maximum depth explored during the subsurface excavation. The contractor should be prepared to maintain the excavation through flowing sand conditions.

Dewatering: Dewatering may be needed due to water infiltration from the river. Perform dewatering according to Section 208.07 of the FP-14.

Slope Instability: Slopes could become unstable during construction operations due to precipitation, flooding events, or heavy loading on soft subgrade soils. Precautions should be taken if movement or cracking is observed. Seepage is a contributor to slope instability and similar dewatering and stabilization methods could be used to suspend slope movement. The contractor is responsible for temporary working platforms including those needed for cranes and shaft drilling rigs.

Hard Layers: Hard layers, such as boulders, were encountered during the geotechnical investigation. The Contractor should be prepared with appropriate equipment based on the local geology as discussed in Sections 2.1 and 2.2, and Appendix D (Rippability).

4.5 SPECIFICATIONS

Special provisions were provided prior to the 95% design milestone 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.

4.6 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations in this report are based on the data obtained from exploratory borings, 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.

Page | 19

SECTION FIVE - References

American Association of State Highway and Transportation Officials (AASHTO), 2017, LFRD Bridge Design Specifications, 8th 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.

Central Federal Lands Highway Division (CFLHD), 2022, “CA ERFO FS 2021-02(1) N. Fork Feather River Bridge Final Hydraulics Report,” dated December.

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(a), Soil Description and Identification Guidelines, dated December.

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

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

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.

Lydon, P.A., Gay, T.E., and Jennings, C.W., 1960, “Geologic map of California: Westwood sheet,” California Division of Mines and Geology, scale 1:250,000.

National Park Service (NPS), 2018, “Series: Physiographic Provinces, Cascade-Sierra Mountains

Province,” updated April 30, <https://www.nps.gov/articles/cascadesierra.htm>.

State of California (CA.gov), 2019, “Geologic Hazards,”

< https://www.conservation.ca.gov/cgs/geohazards>, Accessed March 3, 2021.

State of California (CA.gov), 2020, Caltrans ARS Online (v3.0.2), <https://arsonline.dot.ca.gov/index.php?>, Accessed August 18, 2022.

U.S. Geological Survey (USGS), 2020(a), “Quaternary Fault and Fold Database of the United States,” Interactive fault map & database search, USGS Earthquake Hazards Program, <https://www.usgs.gov/natural-hazards/earthquake-hazards/faults?qt-science_support_page_related_con=4#qt-science_support_page_related_con>.

U.S. Geological Survey (USGS), 2020(b), “Vs30 Map Viewer,” online map, USGS Earthquake Hazards Program, <https://earthquake.usgs.gov/data/vs30/>.

U.S. Geologic Survey (USGS), 2020(c), “U.S. Seismic Design Maps,” USGS Earthquake Hazards

Page | 20

Program, <http://earthquake.usgs.gov/designmaps/us/application.php> & <https://earthquake.usgs.gov/ws/designmaps/aashto-2009.json?latitude=34&longitude=- 118&siteClass=C&title=Example>.

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.

PLATES

PROJECT VICINITY MAP,

GEOLOGY MAP & LEGEND,

GEOTECHNICAL BORING LOCATIONS MAP,

GEOLOGIC HAZARDS MAP,

AREAS MORE LIKELY TO CONTAIN NATURALLY

OCCURRING ASBESTOS: PLUMAS N.F.,

AND ACCESS TO WEST ABUTMENT OF N. FORK FEATHER

RIVER BRIDGE MAP

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 ERFO FS PLMAS 2021-02(1) N. Fork Feather River Bridge, from July 26th through 27th, 2022. The scope of work for the field exploration program included drilling a total of two borings to depths ranging from approximately 26 to 35 feet. One boring was drilled near each of the bridge abutments. The field exploration program was coordinated and observed by a Geotechnical Engineer from CFLHD. Field exploration locations are illustrated on the “Geotechnical Boring Locations” sheet in Plates. Individual boring 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 drilling equipment and field exploration activities are included in Appendix C. All soil and rock 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

Taber Drilling of West Sacramento, California provided the drilling services for the soil borings. Borings were completed using a CME-55 HD truck mounted drill rig. Borings were advanced through overburden using hollow stem augers with drive sampling until practical augur refusal was encountered. After refusal, the borings were advanced using a rock coring and continuous sampling system. 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.

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.