01_CA_FLAP_03S11(1)_GeotechRpt_Body.pdf

SUBMITTED TO:

Jacobs 2485 Natomas Park Dr, Ste 600 Sacramento, CA 95833

BY:

Shannon & Wilson, Inc.

664 West Broadway Glendale, CA 91204

(818) 543-4560 www.shannonwilson.com

REVISED FINAL GEOTECHNICAL REPORT

Reds Meadow Road Improvements

MADERA COUNTY, CALIFORNIA

January 22, 2021 Shannon & Wilson No: 100062-003

Reds Meadow Road Improvements Revised Final Geotechnical Report

100062-003 January 22, 2021

PAGE INTENTIONALLY LEFT BLANK FOR DOUBLE-SIDED PRINTING

Revised Final Geotechnical Report Revised Final Geotechnical Report

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Submitted To: Jacobs 2485 Natomas Park Dr, Ste 600 Sacramento, CA 95833 Attn: Ed Henderson

Subject: REVISED FINAL GEOTECHNICAL REPORT, REDS MEADOW ROAD

IMPROVEMENTS, MADERA COUNTY, CALIFORNIA

Shannon & Wilson prepared this report and participated in this project as a subconsultant to Jacobs. Our scope of services was specified in Revision 5 to Agreement Number 101001707 with CH2M Hill, Inc. This report presents our geotechnical design recommendations and was prepared by the undersigned.

We appreciate the opportunity to be of service to you on this project. If you have questions concerning this report, or we may be of further service, please contact us.

Sincerely, SHANNON & WILSON, INC.

Stephen D. Diem, PE, GE, PG, CEG Associate

Gregory R. Fischer, PhD President

SDD:GRF/bpt n D. Diem, PE, GE, PG, CEG

01/22/2021

Associate

G R Fi h Ph

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CONTENTS

1 Introduction

2 Scope of Work

3 Site and Project Description

4 Field Explorations and Laboratory Testing

4.1 Subsurface Exploration Program

4.2 Geological Mapping

4.3 Rock Slope Geological Mapping

4.4 Seismic Refraction Survey

4.5 Laboratory Testing

5 Regional Geology and Subsurface Conditions

5.1 Regional Geology

5.1.1 Surficial Units

5.1.2 Bedrock Units

5.2 Geographic Setting

5.3 Subsurface Conditions

5.3.1 Existing Pavement

5.3.2 Surficial Units

5.3.3 Bedrock Units

5.4 Groundwater

6 Geologic Hazards

6.1 Slope Stability

6.1.1 Rockslides

6.1.2 Rockfall

6.1.3 Debris Flows

6.1.4 Erosion

6.2 Corrosion Potential

6.3 Seismic Hazards

6.3.1 Fault Rupture

6.3.2 Seismic Site Classification

6.3.3 Seismic Ground Motion

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6.3.4 Seismic Settlement

6.3.5 Seismically Induced Slope Stability

6.4 Volcanic Eruption

7 Geotechnical Recommendations

7.1 Pumice

7.2 Engineered Slopes

7.2.1 Fill Slopes

7.2.2 Rock Cut Slopes

7.2.2.1 Geomechanical Rock Mass Classification

7.2.2.2 Rockfall Catchment

7.2.2.3 Kinematic Stability

7.3 Reinforced Soil Slopes

7.3.1 Stability Evaluation

7.3.2 Stability Evaluation Results

7.4 Soil Nail Walls

7.4.1 Design Recommendations

7.4.2 Stability Evaluation

7.4.3 Stability Evaluation Results

7.4.4 Drainage Considerations

7.4.5 Corrosion Protection

7.5 Subexcavation – Unsuitable Pavement Subgrade

7.6 Reinforced Foundation - Unsuitable Fill Slope and RSS Subgrade

7.7 Drainage

7.8 Shrink and Swell Factors

7.9 Pavement Design Recommendations

7.9.1 Traffic Loading

7.9.2 Design Subgrade Resilient Modulus

7.9.3 Full Depth Reclamation

7.9.4 Recommended Pavement Sections

8 Construction Considerations

8.1 Site Preparation

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8.2 Earthwork and Grading

8.2.1 Excavation

8.2.2 Temporary Excavations

8.2.3 Subgrade Preparation and Proof Rolling

8.2.4 Fill Placement and Compaction

8.3 Rock Scaling

8.4 Culverts

8.5 Soil Nail Walls

8.5.1 Construction

8.5.2 Testing

8.6 Pavement Materials

9 Construction Observation

10 Limitations

11 References

Exhibits Exhibit 4-1: Rock Slopes Exhibit 5-1: Pumice-Rich and Non-Pumice-Rich Surficial Soils Exhibit 6-1: Seismic Design Ground Motion Parameter Exhibit 7-1: Summary of Ditch Width Values Exhibit 7-2: Summary of Cut Slope Angles Evaluated Exhibit 7-3: Reinforced Soil Slope Recommendations Exhibit 7-4: Soil Nail Wall Design Parameter Exhibit 7-5: Soil Nail Wall Recommendations Exhibit 7-6: Subexcavation Locations Exhibit 7-7: Shrink/Swell Factors for Common Materials Exhibit 7-8: Flexible Pavement Design Parameters Exhibit 7-9: Flexible Pavement Section Alternatives Exhibit 8-1: D8R Ripper Performance and Seismic Wave Velocity Exhibit 8-2: Summary of Rippability Velocities and Estimated Depths1 Exhibit 8-3: Scaling Existing Slopes to Remain Exhibit 8-4: Number of Verification Tests Recommended Exhibit 8-5: Recommended Paving Materials

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Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Regional Geologic Map Figure 4: Geologic Cross Sections

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Calculation Packages’ Summaries and Results Appendix D: Exploration Photographs Appendix E: Pavement Condition Assessment and Photographs Appendix F: Existing Rock Slope Photographs Important Information

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1 INTRODUCTION

This report provides geotechnical engineering recommendations for the proposed improvements to California Forest Service Road 03S11 (Reds Meadow Road), which is located west of Mammoth Lakes in Madera County, California. The project includes design of the two-lane widening for the upper 2.5 miles of Reds Meadow Road; pavement resurfacing, rehabilitation, and restoration (3R) of the lower 5.8 miles of Reds Meadow Road; and rehabilitation of the Minaret Vista lookout and parking area.

The purpose of this report is to present our geotechnical, pavement, and materials recommendations for the project. The report summarizes our subsurface explorations, laboratory testing, and geotechnical engineering studies, and presents conclusions and recommendations for design and construction of the roadway features based on the 80-percent plan (Jacobs, 2019a), revised 80-percent plan sheets (Jacobs, 2019b, 2020), review comments and resolutions (U.S. Department of Transportation, Federal Highway Administration [FHWA], Central Federal Lands Highway Division [CFL], 2019a,b), and our conversations with Jacobs and CFL.

This report supersedes our previous reports (Shannon & Wilson, 2016, 2018, 2019, 2020).

2 SCOPE OF WORK

We conducted our services in general accordance with our Purchase Order No. 101001707 with Jacobs dated April 26, 2018 and subsequent agreement revision. For the project we:

managed field preparation, including coordinating with the Forest Service and subcontractors, and completing utility locates observed, logged, and collected soil and rock samples from borings and test pits mapped surficial geologic materials and rock slopes performed a seismic refraction survey completed laboratory testing on selected soil and rock samples evaluated geotechnical data and completed geotechnical engineering analyses completed pavement design resolved design team comments prepared this report

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3 SITE AND PROJECT DESCRIPTION

Reds Meadow Road is in Inyo National Forest, approximately 4 miles west of Mammoth Lakes, California. The project is divided into two parts, the upper 2.5 miles and the lower

5.8 miles. As indicated on Figure 1, the upper 2.5 miles begins on top of the ridge at Minaret Vista Entrance Station and traverses down the mountainside to Agnew Meadow. The lower

5.8 miles winds through the valley floor, beginning at Agnew Meadow and ending at Reds Meadow Resort and Pack Station. We have also included the Minaret Vista lookout and parking lot in the lower 5.8 miles. The existing topography for the upper 2.5 miles is mountainous with steep grade changes on either side of the roadway, while the lower 5.8 miles is generally gentle to hummocky with localized steep grades.

The existing roadway is currently paved with asphalt concrete (AC). The age of the existing AC is unknown. The condition of the pavement is variable, but generally deteriorated with abundant cracking and asphalt patches. Within the upper 2.5 miles, there is typically a longitudinal crack approximately 3 feet from the downslope edge of the pavement. This crack is typically open, vertically offset (outside edge down), and frequently patched (see Figure E-2, Photograph 3). In Appendix E, we provide a summary of our pavement conditions assessment and photographs illustrating the observed conditions.

In the upper 2.5 miles, the project includes realignment and widening of the roadway from the single lane with turnouts that currently exists to two lanes. The widening and realignment are being accomplished using soil nail walls, reinforced soil slopes, and cut and fill slopes. The roadway typical section consists of two 11-foot wide lanes, 1-foot wide shoulders on each side, a 2-foot wide paved gutter and curb on the upslope side, and a 3-foot clear zone on the downslope side (Jacobs, 2019a).

In the lower 5.8 miles, the proposed pavement improvements consist of full-depth reclamation (FDR) and constructing a new hot-asphalt concrete pavement (HACP) on top.

Geotechnical mitigation as part of the improvements may also include stabilization of shoulders using reinforced soil slopes (RSS; height 5 feet) or subexcavation in localized areas to improve poor subgrade conditions.

4 FIELD EXPLORATIONS AND LABORATORY TESTING

We performed our field services in two phases, the first from May 18 through 27, 2018 and the second from October 15 to November 5, 2018. Shannon & Wilson representatives and subcontractors conducted geologic mapping of the surficial units and select rock slopes, completed a pavement condition survey, excavated test pits, completed geotechnical

A-3 exploration borings, and performed seismic refraction lines. We performed geotechnical laboratory testing on select samples collected during our field services. The following sections describe each of these activities.

4.1 Subsurface Exploration Program

Our subsurface exploration program included excavating 8 test pits, drilling and sampling 51 borings, and performing a seismic refraction survey at 16 locations. We excavated the test pits into the toe of the existing cut or natural slope on the upslope side of the roadway.

In the upper 2.5 miles, we drilled the borings approximately 3.5 feet from the outside edge of the existing roadway. In the lower 5.8 miles and at the Minaret Vista lookout and parking area, we drilled the pavement borings in the center of one of the travel lanes.

The subsurface exploration locations are shown on Figure 2. Appendix A discusses the exploration program in further detail, describing the excavation procedure used to complete the test pits, the drilling and sampling procedures used to complete the borings, and presents the seismic refraction survey report. In addition, Appendix A presents the individual test pit and boring logs. Appendix D shows photographs of the test pit and boring locations.

4.2 Geological Mapping

We performed geologic mapping of the upper 2.5 miles of Reds Meadow Road. We observed rock outcrops in both cut slopes and natural slopes along the alignment, typically on the upslope side of the roadway. On the downslope side of the roadway, we primarily observed fill slopes and natural slopes, and occasionally observed a rock outcrop. We mapped the limits of the geologic units and collected descriptions of each unit. We did not perform geologic mapping of the lower 5.8 miles or Minaret Vista lookout and parking area and instead relied on our borings for our interpretation of the subsurface conditions. See Figure 2 for the distribution of the geologic materials along the upper 2.5 miles.

4.3 Rock Slope Geological Mapping

The 30-percent plan field review redline markup included 8 proposed cut slopes, which we designated RW-1 through RW-8 (Jacobs, 2018). Slopes RW-3, RW-5, RW-6, and RW-7 were subsequently removed from the proposed project based on roadway realignments of the design. The 80-percent design submittal plan includes the previously proposed cut slopes RW-1, RW-2 (parts A and B), RW-4, and RW-8 and adds new cut slopes from Stations 34+20 to 34+50 and 95+25 to 98+80 (Jacobs, 2019a). We have designated these new slopes as RW-9 and RW-10.

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A Shannon & Wilson representative performed geologic mapping of cells of the existing rock outcrop within or near the limits of the proposed rock cut slopes RW-1, RW-2 (parts A and B), RW-4, and RW-8, which included observation of geologic features and collection of discontinuity data and rock mass information. We did not perform geologic mapping of cells within or near RW-9 and RW-10, because these cut slopes originated after our field work was completed.

We have summarized the rock cut slope designations, mapped cells, and limits of the existing rock outcrops in the following exhibit.

Exhibit 4-1: Rock Slopes

Proposed Rock Cut Slope Mapped Cells

(stations)

Existing Rock Outcrop

Designation Limits

(stations) Limits

(stations) Inclination (degrees

RW-1 15+15 to 16+50 14+50 to 15+10

12+00 to 13+25

6314+30 to 16+80

28+20 to 28+90

RW-9 34+20 to 34+50 Not Performed - -

RW-2A and RW-2B 38+10 to 40+50 41+40 to 47+00

38+00 to 38+40 37+80 to 40+80

70 to 8441+70 to 42+30 41+50 to 47+20

43+05 to 43+50

RW-4

80+20 to 84+45 81+50 to 83+50 84+00 to 84+50 79+50 to 84+50 60 to 78

86+45 to 90+10 86+50 to 86+80 88+00 to 88+25 88+50 to 89+05

85+20 to 89+80 60 to 78

RW-10 95+25 to 96+80 Not Performed - -

RW-8 132+00 to 135+00 133+00 to 134+50 132+00 to 135+50 70 to 85

4.4 Seismic Refraction Survey

We performed a seismic refraction survey to assist in determining the rippability of the bedrock and to assist in determining the depth of the bedrock in select locations along the length of the proposed road improvements. The locations of the seismic lines were limited by the steepness of the existing topography, obstructions (e.g. fallen trees), and existing pavement. The seismic line locations are shown on Figure 2 and the seismic refraction survey report is included in Appendix A.

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4.5 Laboratory Testing

We completed geotechnical laboratory tests on select samples retrieved from the test pits and borings to estimate soil and rock index properties and engineering properties. Tests included natural water content determination, grain size distribution analysis, Atterberg limits determination, moisture-density relationship (i.e. compaction characteristics), California Bearing Ratio (CBR), and corrosion potential. Appendix B provides discussion of the laboratory test methods performed and provides the laboratory test results. The natural water content, fines content, and Atterberg limits are also shown on the individual boring logs included in Appendix A.

5 REGIONAL GEOLOGY AND SUBSURFACE

CONDITIONS

We developed our understanding of the geology and subsurface conditions at the project site based on a review of regional geologic maps and data collected from the surficial mapping and subsurface exploration program.

5.1 Regional Geology

Reds Meadow Road is located on the northwestern side of Mammoth Mountain, a lava-dome complex within California’s Sierra Nevada mountain range. Published regional geologic mapping by Huber and Rinehart (1965) indicates the geologic units underlying the Reds Meadow Road consist of surficial deposits, volcanic rocks, granitic rocks, and metamorphic rocks as shown in Figure 3. The hillside terrain in the upper 2.5 miles of roadway is composed of surficial talus and slope wash, andesite volcanic rocks, granitic rocks that include felsic dikes and masses, and metamorphic units that include undifferentiated metavolcanics rocks and calcareous metasedimentary rocks. The lower 5.8 miles of Reds Meadow Road is underlain by surficial deposits that include valley fill and talus and slope wash, andesite volcanic rocks, granitic rocks, and metamorphic rocks that include undifferentiated metavolcanics rocks and undifferentiated metasedimentary rocks.

5.1.1 Surficial Units

The surficial deposits are Quaternary in age and previously mapped as valley fill (Qal) and talus and slope wash (Qts). Valley fill consists of alluvial deposits that vary from fine-grained soil to gravels, including localized zones of appreciable pumice. Talus and slope wash consist of unsorted clay, silt, and gravel, including pumice, with scattered cobbles and boulders. Talus and slope wash appear to originate from volcanic and metamorphic outcrops overlying the roadway.

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Fill material is not mapped on published geologic maps, but it is present along the entire alignment in the roadway and is comprised mainly of poorly graded sand with subrounded to subangular gravel.

5.1.2 Bedrock Units

The bedrock geology along the upper 2.5 miles of the Reds Meadow Road is comprised of Triassic-Jurassic age undifferentiated metavolcanics (JTru) and undifferentiated metasedimentary (JTrc) rocks. Undifferentiated metasedimentary rocks consist of shale, marble, siltstone, quartzite, phyllite, and pelite. Metasedimentary fabric consists of all or some of the following: relic bedding to non-bedded and massive, foliated, schistose, crenulated, locally sheared, and glacially polished. Undifferentiated metavolcanics rocks consist of rhyolite-dacite and welded rhyolite tuff. Metavolcanic fabric consists of all or some of the following: phaneritic, porphyritic, foliated, porphyroblastic, and vesicular.

Bedrock in the lower 5.8 miles of the Reds Meadow Road are comprised of Triassic-Jurassic undifferentiated metasedimentary rocks (JTrc), Cretaceous age coarse-grained granodiorite (Kcp), and Quaternary age porphyritic andesite (Qad).

5.2 Geographic Setting

The project site is located approximately 4 miles west of the town of Mammoth Lakes, California, in the Inyo National Forest of Madera and Mono Counties. Mammoth Lakes received an average annual precipitation of about 21 inches over the past 20 years (National Oceanic & Atmospheric Administration, 2016). Mammoth Lakes and Reds Meadow Road are considered a snow area with frost depth reported to be approximately 8 inches (U.S.

Frost Depth Map, 2018). Mammoth Lakes design guidelines recommends designing for a frost depth of 24 inches (Mammoth Lakes, California 2016).

5.3 Subsurface Conditions

The subsurface conditions at the site can be grouped into three general categories: existing pavement, soil units, and bedrock units. For the soil and bedrock, we correlated the regional geologic units discussed above with the soil and rock samples collected during our exploration program. The following sections provide additional information on the geologic units encountered.

Subsurface explorations indicate that these units are variable in composition and origin, and to some degree, have differing engineering characteristics. We developed 21 cross sections to illustrate the subsurface conditions and to perform engineering analyses for the proposed slopes, RSS, and soil nail walls. These cross sections are presented on Figure 4.

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5.3.1 Existing Pavement

We drilled borings within the existing roadway and encountered AC underlain by soil, with no aggregate base course present. The existing AC thickness varied from 1.8 to 4.6 inches.

The observed pavement subgrade materials consisted of poorly graded sand with variable amounts of clay, silt, and gravel; silty sand with gravel; and poorly graded gravel with sand, with localized appreciation of pumice. Along the lower 5.8 miles of Reds Meadow Road, our borings occasionally encountered shallow refusal on fresh, high strength granodiorite or andesite beneath the existing pavement.

5.3.2 Surficial Units

The soils we encountered during subsurface exploration included artificial fill (af), valley fill (Qal), and talus and slope wash (Qts). These soils were generally classified as loose to dense, clayey/silty sand with gravel, sand with gravel, clayey gravel with sand, and gravel with sand; and medium stiff to very stiff lean clay with sand and gravel. Along the upper

2.5 miles, the observed thickness of soil extended to 4.5 to 17.5 feet below the existing ground surface (bgs), except for boring SW-B-46 (located at Station 112+50) where soil was encountered to the maximum depth of the boring at 31.5 feet bgs. Along the lower 5.8 miles of road, where the borings were a maximum of 6.5 feet bgs, the observed thickness of soil varied from 2.0 to the bottom of the borings (greater than 6.5 feet). We observed fill thicknesses in the borings that ranged from 0 to 5.0 feet.

The surficial soils we encountered during geologic mapping and subsurface exploration included pumice-rich soil. This soil is lightweight, relative to the non-pumice-rich soil, and has poor strength properties. We have summarized the approximate station ranges of pumice-rich, non-pumice-rich, and zones of interfingering of the two materials (both) in Exhibit 5-1, shown below.

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Exhibit 5-1: Pumice-Rich and Non-Pumice-Rich Surficial Soils

Station Range SoilsStart End

10+00 30+00 non-pumice-rich

30+00 50+00 both 50+00 67+00 pumice-rich

67+00 77+00 non-pumice-rich

77+00 87+00 pumice-rich

87+00 95+00 non-pumice-rich

95+00 98+00 pumice-rich

98+00 111+00 both

111+00 129+00 pumice-rich 129+00 136+00 both

5.3.3 Bedrock Units

We observed bedrock in outcrops during our geologic mapping and our subsurface explorations. Rock outcrops were observed in both cut slopes and natural slopes along the alignment. The rock types observed were dark gray non-calcareous relic-bedded shale, gray to white fissile marble, yellow-brown to reddish brown and dark blue relic-bedded non-calcareous quartzite, light gray crenulated and schistose pelite and phyllite, red-brown to light brown porphyritic rhyolite-dacite, and red-brown porphyritic welded rhyolite tuff.

The rock ranged from moderately strong to medium high strength, was moderately weathered, and contained very close to wide spaced discontinuities. Observed rock block sizes varied from 1-foot to 5 feet in maximum dimension.

The bedrock we encountered during the subsurface exploration consisted of andesite (Qad), granodiorite (Kcp), undifferentiated metavolcanic (JTru) rocks, and undifferentiated metasedimentary (JTrc) rocks. Along the upper 2.5 miles, we observed bedrock in the borings at depths that ranged from 4.5 feet to greater than the total depth explored of 31.5 feet bgs. Auger refusal was encountered in 23 of the 25 borings along the upper 2.5 miles of road at depths that ranged from 7.7 to 31.5 feet bgs. In most cases, bedrock was identifiable by rock chips recovered during sampling. In borings where we did not obtain a sample after reaching refusal and an outcrop was present at a proximal distance, we assumed bedrock caused the refusal. In borings where we did not obtain a sample and no outcrop was within a proximal distance, refusal may have occurred in these borings on a boulder, cobble, or bedrock. In borings along the lower 5.8 miles of Reds Meadow Road, we observed bedrock at depths that ranged from 2 to 2.5 feet, when encountered. Nearby

A-9 outcrops were used to quantitatively estimate rock strength which ranged between strong to very strong.

Where we show bedrock in Figure 2, it is overlain by approximately 4 feet or less of surficial soil. This material may consist of weathered bedrock, talus and slopewash, and/or existing artificial fill.

5.4 Groundwater

We observed groundwater within 16 of the 51 borings, which was likely seepage that will fluctuate with snowmelt and rainfall. Within the upper 2.5 miles, we observed groundwater in 4 of the borings, at depths ranging between 4.5 and 17 feet bgs. We observed groundwater along the lower 5.8 miles of the alignment within 12 of the borings, at depths ranging from 1.7 to 5.5 feet bgs. In addition, we observed groundwater at the surficial soil-bedrock contact in 2 test pits along the upper 2.5 miles of the alignment, approximately 2 to 4 feet bgs. We also observed groundwater seeps on the existing surface during geologic mapping of the upper 2.5 miles. The locations of observed seepage on the existing surface are presented on Figure 2.

An affiliate of the Reds Meadow Resort and Pack Station indicated that portions within the lower 5.8 miles of the road alignment overflows with several inches of water each April.

This overflow reportedly occurs between stations 613+20 and 615+90, and between stations 757+00 and 760+50.

6 GEOLOGIC HAZARDS

This section identifies potential geologic hazards that exist along Reds Meadow Road, discusses the potential adverse impacts of the geologic hazards, and provides recommendation measures to mitigate these impacts, where required.

Geologic hazards that could potentially impact the project include slope stability (including rockslides, rockfall, debris flows, erosion), corrosion potential, seismic hazards (including fault rupture, seismic ground motion, liquefaction, and seismic compression), and volcanic eruption. We understand CFL’s performance expectation for the project is to widen the roadway to address traffic safety concerns while maintaining the existing risk level of geologic hazards. As such, several of the geologic hazards discussed in the following sections do not include mitigation measures. Along the same lines, we did not evaluate geologic hazards for the existing natural, cut, and fill slopes along the road alignment that were not being modified by the proposed design.

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6.1 Slope Stability

Potential slope stability hazards include rockslides, rockfall, debris flows, and erosion.

These various potential hazards are discussed in the following sections.

It is apparent that rockslides, rockfall, and/or debris flows have occurred above portions of the upper section of the roadway, based on our geologic mapping, borings, oblique view of images in Google Earth, and observations provided by the Forest Service. These events appear to have originated from the volcanic rocks that form the steep topography of the overlying ridge. The mostly treeless, bowl-shaped terrain between the ridge and roadway funnels the debris into a channel that crosses Reds Meadow Road from Stations 104+00 to 106+60 and, to a lesser degree, from Stations 96+60 to 99+40, 114+00 to 116+00, and 127+60 to 128+60. Treeless channels or areas with new growth trees and saplings suggest previous slope failures from above the roadway stripped the trees and transported them downslope.

Mitigation options for slope instability from above the road could include using debris flow and/or rockfall fences, oversizing culverts for debris passage, creating flow deflection or areas for debris deposition, or improved maintenance options for quick recovery following an event. These measures could be implemented later if the performance criteria for the roadway changes.

6.1.1 Rockslides

Rockslides consist of a relatively large mass of rock (or rock and soil) mobilized along a discontinuity in the bedrock. These differ from rockfall events, which consists of individual blocks that originate from outcrops or over-steepened slopes. We are not aware of any recent rockslides that have affected the roadway and, as such, the risk of a large rockslide appears to be low at this time.

6.1.2 Rockfall

Reds Meadow Road is located beneath natural cliffs and steep slopes with a rockfall hazard.

There are indications of rockfall events as evidenced by the topographic expression of the source areas and boulders resting on the slope above and below the roadway in the upper

2.5 miles. However, we did not observe scars from rockfall within the roadway. In addition, based on conversations with Forest Service maintenance personnel, rockfall has not impacted the road. While we cannot dismiss the potential for rockfall hazard to impact the roadway, it does not appear to be a frequent occurrence.

In addition to naturally occurring rockfall hazards, rockfall may also originate from new rock cut slopes. Rockfall hazards are increased where cuts expose weak or unfavorably

A-11 jointed rock or undercut surficial soils such as slope wash and talus. While the slope excavation angles can be designed to reduce rockfall hazard, weathering action on slopes may increase the rockfall hazards with time. Appropriate maintenance operations may be required to manage these hazards, as discussed in the Rock Cut Slopes section below.

6.1.3 Debris Flows

A debris flow is a viscous, flowing mixture of water, mud, rock fragments, and organic debris transported down slope during heavy rainfall or snowmelt runoff events. Forest fires can be a contributing factor to an increased frequency of debris flows. Debris-flow deposition areas, or debris fans, form through repeated debris flows primarily at the base of drainages or debris flow channels. Large flow volumes and boulders incorporated into a flow can bury a roadway and cause damage to structures.

Evidence of previous debris flows crossing the roadway exist between Stations 104+00 and 106+60 and extending to the lower section of the roadway between Stations 548+25 and 552+00. Also, two debris flows occurred on October 16, 2016 during heavy rains near Stations 105+00 and 503+00. The events consisted of sediment and storm water runoff concentrating in natural drainage channels, overwhelming the existing drainage system, and covering the roadway. Similar debris flow hazard areas are present through the Reds Meadow Road alignment where it crosses natural drainages. Potential mitigation measures for debris flows are discussed above.

6.1.4 Erosion

Typically, sandy soil on steep slopes subject to high velocity water flow or non-vegetated areas are susceptible to erosion. Existing cut slopes (except for pumice) appear to be performing well, although raveling is present in some locations where surficial units appear to be eroding during periods of inclement weather. Slopes that expose surficial units are susceptible to shallow slumps where over-steepened, which leads to sedimentation within the drainage ditches and clogging or blockage of culverts. Forest Service maintenance personnel indicated several culverts need to be cleaned and drainage ditches re-established along the upslope side of the roadway at the beginning of each season.

6.2 Corrosion Potential

Soil can be corrosive when in contact with concrete or steel pipes and other buried metal elements. To assist in estimating the corrosion potential across the project, we tested 11 boring samples and 4 test pit samples for chloride concentration, water soluble sulfate concentration, resistivity, and pH. Discussion of these parameters, the testing procedures performed, and results are provided in Appendix B.

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The concentration of water-soluble sulfates and chlorides measured in the samples was 31 to 58 parts per million. Based on classifications as defined by ACI-318-19 (ACI, 2019), these test results suggest an exposure class of S0 for concrete exposed to site soils.

The resistivity measured in the samples ranged from 4,988 to 46,994 ohm-centimeters. Based on correlations developed by Roberge (2012), these values suggest essentially noncorrosive to corrosive subsurface conditions for metal in contact with subsurface materials across the site.

The pH test results indicated a normal range for soils in terms of corrosion potential.

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion. As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

6.3 Seismic Hazards

The project is located in a seismically active region of California and is expected to experience the effects of future earthquakes on active faults located within the region.

Seismic hazards that may potentially impact the proposed improvements include fault rupture, seismic ground motion, seismic settlement (which includes liquefaction and seismic compression), and seismically induced slope stability. We provide a discussion of each of these hazards in the following sections. However, consistent with CFL’s decision to maintain the current level of risk, we understand these geologic hazards will not be mitigated as a part of this project.

6.3.1 Fault Rupture

Surface fault rupture hazard evaluation is based upon criteria developed by California Division of Mines and Geology (CDMG), now known as the California Geologic Survey (CGS), for the Alquist-Priolo Zone Act program (CGS, 2018). Based on the CGS criteria, an active fault is a fault that has ruptured within the Holocene geologic time period (about the last 11,700 years). According to the United States Geological Survey (USGS) online mapping tool, active faults do not cross the project alignment. However, there are mapped faults near the project that include the Hartley Springs Fault Zone and the Hilton Creek Fault Zone (USGS, 2006). According to USGS reports, these fault zones have experienced Quaternary movement.

The Hartley Springs Fault Zone is mapped approximately 1 to 3 miles east of the Minaret Vista entrance station (Sawyer and Bryant, 1995). It is approximately 14 miles long with

A-13 escarpment heights of 2,000 feet and is comprised of mostly northwest trending faults (Bailey, 1989). The Hilton Creek fault zone is mapped at least 7½ miles to the east and was responsible for several events in 1980 with moment-magnitudes greater than 6.0 (Sawyer and Bryant, 1995; Taylor and Bryant 1980). It is approximately 18.5 miles long with escarpment heights of nearly 5,000 feet and is comprised of mostly northwest trending faults (USGS, 2016). In the past 80 years the region has experienced 33 earthquakes of moment-magnitude 5.0 or greater (USGS, 2016).

Considering there are no documented faults with Quaternary movement that cross Reds Meadow Road, it is our opinion the potential for surface rupture is low.

6.3.2 Seismic Site Classification

Site class depends on the type of rock and the thickness of overburden deposits. Based on the results of our subsurface exploration, we recommend using Site Class B for the earth retaining structures located in the upper 2.5 miles.

6.3.3 Seismic Ground Motion

The road alignment will likely experience seismic ground shaking during an earthquake along faults in the region. Earthquakes along several active faults in the region could cause moderate to strong ground shaking. The intensity of earthquake motion will depend on the characteristics of the generating fault, distance to the earthquake fault, earthquake magnitude, earthquake duration, and site-specific geologic conditions. Ground motions may be amplified or attenuated by soil deposits at the site depending on the level of ground shaking in the underlying bedrock, soil type/density, depth to bedrock, and other factors.

According to the Federal Lands Highway Project Development and Design Manual (PDDM) (U.S. Department of Transportation [USDOT] and FHWA, 2018) and supplemental Technical Guidance Manual (TGM; USDOT and FHWA, 2007), geotechnical seismic design should be consistent with the philosophy for structure design, which state that loss of life and serious injury due to structure collapse are minimized to the extent possible and economically feasible.

Seismic hazards are evaluated considering two different design events:

10 percent probability of exceedance in 50 years, or 475-year event (USDOT and FHWA, 2018)

7 percent probability of exceedance in 75 years, or 1,000-year event (AASHTO, 2017)

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We determined ground motion parameters for the project site using the USGS Unified Hazard Tool (USGS, 2017). The following exhibit presents recommended seismic design ground motion parameters for the upper 2.5 miles of Reds Meadow Road.

Exhibit 6-1: Seismic Design Ground Motion Parameter

Ground Motion Parameters / Coefficients Return Period

475-Year 1,000-Year Site Class B B

Peak Ground Acceleration (PGA) (g) 0.26 0.34 Short-period Spectral Acceleration, SS (g) 0.55 0.74

Long-period Spectral Acceleration, S1 (g) 0.12 0.17

Site Factor, Fpga 1.00 1.00

Site Factor, Fa 1.00 1.00

Site Factor, Fv 1.00 1.00

Peak Design Spectral Acceleration, AS (g) 0.26 0.34 Short-period Spectral Acceleration Adjusted for Site Class Effects, SMS (g) 0.55 0.74

Long-period Spectral Acceleration Adjusted for Site Class Effects, SM1 (g) 0.12 0.17

Short-period Design Spectral Acceleration, SDS (g) 0.37 0.49

Long-period Design Spectral Acceleration, SD1 (g) 0.08 0.11 Notes:

g = gravity

6.3.4 Seismic Settlement

Seismically induced settlement is divided into two parts, soil above and below the groundwater table. Settlement below the water table is associated with liquefaction of loose, saturated, granular soil due to the seismic loading. Settlement above the water table is associated with seismic compression of loose, granular soil during seismic ground shaking.

Based on the site geologic conditions and the design Peak Ground Acceleration (PGA), it is our opinion there is a negligible potential for liquefaction and seismic compression to impact the upper 2.5 miles of the roadway and a low to moderate potential for the lower 5.8 miles.

6.3.5 Seismically Induced Slope Stability

The risk for slope instability increases during earthquakes, which may result in seismically induced landsliding, surficial failures, rockfalls, and rockslides. Considering the estimated design PGA for the site, steep nature of the overlying slopes and numerous rock outcrops, it is our opinion there is the potential for seismically induced slope instability to impact the

A-15 proposed project. Portions of the existing slopes may have a factor of safety (FS) near 1.0 in the present condition. As a result, minor earthquake induced shaking is assumed to produce ground accelerations that may mobilize some material. See Section 7.0 for additional discussion.

6.4 Volcanic Eruption

According to the USGS (2016), Reds Meadow Road is in an area of past volcanic activity. It is at least ¾ of a mile west of the Long Valley Caldera, a basin-shaped volcanic depression that was formed when it erupted approximately 760,000 years ago. Smaller eruptions and uplift of the magma chamber occurred periodically over the next several hundred thousand years. The Devil’s Postpile eruptions and flows occurred approximately 100,000 years ago and repeated eruptions between 110,000 and 56,000 years ago formed Mammoth Mountain.

The area has experienced increased volcanic related activities in the past few decades including earthquakes, ground uplift, and gas emissions.

Volcanic eruption and the consequences are beyond the standard of practice for civil engineering projects at this time and location. As such, and in line with CFL’s desire to maintain the existing hazard level, we have not provided potential mitigation measures.

7 GEOTECHNICAL RECOMMENDATIONS

Engineered slopes, RSS, and soil nail walls are planned to accommodate the proposed roadway widening and realignment of the upper 2.5 miles. The appropriate type of system at any location depends on the existing topography, geology, and project constraints. We understand that a 13-foot wide travel lane must be maintained during construction, with limited periods of full roadway closures.

7.1 Pumice

We observed areas that were dominated by gravel-sized particles of pumice (i.e., pumice-rich soil) while performing geologic mapping and subsurface exploration. These locations are summarized in Exhibit 5-1 presented in Section 5.3.2.

Pumice is a cellular volcanic rock with a relatively low unit weight and poor strength characteristics. We observed severe levels of pavement distress, including subgrade failures and fatigue cracking and rutting, in areas where pumice was present. SPT blow counts in pumice-rich materials ranged from 2 to 4 blows per foot.

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We recommend pumice-rich soil not be used for direct support of fill slopes, RSS, or improvements where dense and unyielding subgrade cannot be achieved. In these areas, we recommend a subexcavation.

We understand CFL has had success breaking down the gravel-sized particles of pumice into a well graded soil, by crushing it with a bulldozer, that can be used for fill material. If this method is used, we recommend performing field and laboratory testing of the pumice-derived fill soil to evaluate its engineering properties.

7.2 Engineered Slopes

Fill and soil cut slopes are proposed at gradients of 1V:2H to a maximum height of approximately 30 feet and 1.5 feet, respectively (Jacobs, 2019a,b, 2020). Rock cut slopes are proposed at a gradient of 1V:0.5H to a maximum height of approximately 22 feet.

We evaluated the proposed fill slopes to determine if they exceed the CFL-required static FS of 1.3 (USDOT and FHWA, 2018). The PDDM indicates standard practice is that seismic analysis is not performed on slopes, except where ground anchors or stabilization structures are used. Considering this, we did not perform seismic slope stability analyses of the proposed cut or fill slopes.

The proposed cut slopes in soil are located on the upslope side of the roadway, behind the proposed curb. Considering their exposed height is 1.5 feet or less, we did not evaluate the global stability of these soil slopes.

7.2.1 Fill Slopes

Fill slopes are planned at a gradient of 1V:2H to heights of 30 feet of less. For our analyses, we assumed a fill material with an angle of internal friction of 34 degrees and placement is in accordance with FP-14 U.S. Customary Units (USDOT and FHWA, 2014) standards (FP- 14). Based on our analyses, the proposed fill slopes meet the CFL-required static FS with a minimum key width of 15 feet. For fill slope heights of 20 feet, or less, the key width may be reduced to 10 feet.

The base of the fill slope should be keyed into the existing slope surface and the fill benched into existing native soils during filling, in accordance with the requirements provided in FP- 14 (USDOT and FHWA, 2014). We recommend the key be tilted into the slope at 2 percent and placing a subsurface drain in the heel of the key to avoid building up hydrostatic pressure. Details regarding subsurface drain are provided below in the Drainage section.

We anticipate fill slopes will be difficult to vegetate and likely be subject to surficial deterioration and ongoing maintenance. We recommend considering temporary erosion

A-17 control blankets or permanent turf reinforcement mats and hydroseed to vegetate the fill slopes.

7.2.2 Rock Cut Slopes

We evaluated rock cut slopes for kinematic stability at the five proposed rock cut locations (RW-1, RW-2A, RW-2B, RW-4, and RW-8) shown in the 30-percent design plans. We evaluated rockfall catchment for five cut slope inclinations provided in the Rockfall Catchment Area Design Guide published by FHWA (Pierson and others, 2001) and three catchment ditch configurations.

The 80-percent design plans have changed the extents of rock cut slopes RW-1, RW-2A, RW- 2B, RW-4, and RW-8, and added two new slopes, RW-9 and RW-10. The extents of slopes RW-1, RW-2A, RW-2B, and RW-4 have been reduced, while the extents of RW-8 been expanded on the southeastern end by 23 feet. The extents of the rock cut slopes are summarized in Exhibit 4-1.

The rock slopes that we did not previously evaluate, which include the expanded southeastern end of RW-8, RW-9, and RW-10, may expose surficial soil or bedrock, depending on the thickness of the surficial soil and depth of cut. We should observe these locations during construction to confirm bedrock is exposed at the finished slope face and perform the cell mapping. We will use the data collected to perform the kinematic analyses described below and provide mitigation measures, if needed.

Where temporary cuts for soil nail walls expose bedrock, these walls may potentially be changed to rock cut slopes. As above, we should observe these locations to perform cell mapping and kinematic analyses so that we could recommend the appropriate slope inclination and ditch width.

We did not evaluate the existing cut or natural rock slopes that are not being modified by the proposed construction. This is consistent with CFL’s performance expectation for the project which is to maintain the existing risk level.

7.2.2.1 Geomechanical Rock Mass Classification

We completed geomechanical rock mass classifications using the field data collected following the Rock Mass Rating System (RMR) by Bieniawski (1989) and the Geological Strength Index from Hoek and Bray (1997).

The RMR, also referred to as the geomechanics classification system, is based on the algebraic sum of six rock mass property ratings: strength of intact rock material, rock quality designation, spacing of discontinuities, condition of discontinuities, groundwater

A-18 conditions, and orientation of discontinuities relative to the excavation or rock slope. To estimate the RMR, we compared field data to published tables by Bieniawski (1989). Values for RMR can range from 0 to 100.

In Appendix C, we provide summaries of the geomechanical rock mass information collected during our field activities as part of the Rock Slopes calculation package summaries and results.

7.2.2.2 Rockfall Catchment

We used the rockfall slope charts presented in the Rockfall Catchment Area Design Guide published by FHWA (Pierson and others, 2001) to evaluate ditch widths and configurations assuming 80-percent of the rocks will be retained (pers. comm., CFL, 2017). We considered a 40-foot high slope with cut slope inclinations of near vertical (80 degrees), 1V:0.25H, 1V:0.5H, 1V:0.75H, and 1V:1H. The slope charts provide ditch configurations at 1V:4H, 1V:6H, and flat. The shortest slope chart slope height is 40 feet, while most of the cut slopes along the alignment are less than 10 feet high. To relate the 40-foot high slope to the 10-foot high slope, we reduced the slope chart recommended ditch width by 75-percent (to 25-percent of the chart width). For a 20-foot high slope, we reduced the slope chart recommended ditch width by 50 percent to obtain 80-percent retention of rockfall. We provide the rock slope charts with 80-percent catchment, varying ditch configurations, and slope inclinations as part of the Rock Slopes calculation package summary and results in Appendix C. We provide ditch width alternatives for 1V:0.5H rock cut slopes in Exhibit 7-1.

Exhibit 7-1: Summary of Ditch Width Values

Slope Inclination Ditch Configuration Width for 80% Catchment (feet)

1V:4H 1V:6H Flat 1V:0.5H 2.3 3.8 7.5

Maintenance operations should routinely remove rocks deposited in the ditches to sustain the ditch functionality.

7.2.2.3 Kinematic Stability

To evaluate potential rock cut slope inclinations, we used the discontinuity data we collected to construct pole plots on equal area stereonets using the program Dips (Rocscience, 2018a). Where the stability of a rock cut is controlled by the structure of the rock mass, a kinematic analysis is used to estimate the kinematic potential for rock blocks to move out of the slopes. We analyzed three types of failure modes: planar, wedge, and toppling. The information required to perform an analysis are the design slope dip and dip

A-19 direction, the orientation of the discontinuities within the rock mass, and the friction angle of the discontinuities.

A kinematically admissible planar failure occurs when the dip of the slope exceeds the dip of the potential slip plane, the potential slip plane daylights the slope, the dip of the plane exceeds the friction angle of the rock, and the dip direction of the sliding plane falls within ± 20 of the dip direction of the slope face.

A wedge failure is similar to a planar failure except the sliding occurs on two planes. A wedge failure is identified when the criteria for a planar failure is met; however, wedge failures may fall outside of the ± 20 of the variation defined within a planar failure since the sliding direction is defined by the line of intersection.

The third type of failure is toppling, which are generally associated with blocks of rock formed by discontinuities with a high angle joint at the slope face. For the purposes of this analysis, we evaluated toppling. Toppling is considered feasible when planes can slide with respect to one another and the direction of the slip planes lies within ± 30 of the dip direction of the slope (Goodman, 1980).

A kinematic analysis does not consider a cohesion intercept when modeling the strength of discontinuities. This method also assumes that the discontinuities are continuous and through going with no “bridging” within the discontinuity. The effect of “bridging” would allow a tensional component (or cohesion intercept) of discontinuity strength. We assumed the rock discontinuity friction angle based on the geomechanical information that we collected in the field, experience with similar rock types, and guidance from the Rock Slopes Reference Manual (Munfakh and others, 1998). We assumed a friction angle on discontinuities of 34 degrees for slopes RW-1, RW-2A, RW-4 and RW-8. For slope RW-2B, we assumed a friction angle on discontinuities of 27 degrees to account for the different rock type.

Our recommended cut slope angles are summarized in Exhibit 7-2. Appendix C provides the calculation for Rock Slopes and presentation of the kinematic analysis.

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Exhibit 7-2: Summary of Cut Slope Angles Evaluated

Proposed Rock Cut Slope Recommended Slope

InclinationSlope Number Station Range Maximum Height (feet)

Dip Direction (degrees)

RW-1 15+15 to 16+50 6 157 1V:0.5H RW-2A 38+10 to 40+50 7 207 1V:0.5H

RW-2B 41+40 to 47+00 9 Varies over length 1V:0.5H

RW-4

80+20 to 84+45

22 Varies over length 1V:0.5H

86+45 to 90+10

RW-8 132+00 to 135+00 18 Varies over length 1V:0.5H

The rock cut slope inclinations presented in Exhibit 7-2 do not remove all kinematically admissible failure types. The inclinations were selected if a failure mode was observed as having the same percentage of dip vectors fall within a critical zone at a lower inclination (i.e., 53 degrees) value as the steeper inclination value (i.e., 63 degrees). The cut slope inclination of 1V:0.5H is generally flatter than the existing rock slope inclinations observed in the field. The existing rock slope inclinations vary from about 60 degrees to near vertical and the reduction in cut slope inclination reduces the kinematically admissible failures observed during the stereonet analysis.

For all slopes there is potential for planar failures to occur.