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Final Geotechnical Report

David Evans and Associates, Inc.

Western Federal Lands Highway Division

ID DOT T 33(1), Idaho Teton Centennial Trail and WY DOT 22(1), Wyoming Teton Pass Centennial Trail Teton County, Idaho and Wyoming DTFH7015D00002, Task Order No. 69056718F000010

Prepared by

Oregon & Washington

June 9, 2020

Geotechnical Report GR08-20 i

TABLE OF CONTENTS

INTRODUCTION

PROJECT DESCRIPTION

SITE DESCRIPTION

GEOLOGIC SETTING

GEOLOGIC RECONNAISSANCE

SUBSURFACE CONDITIONS

General Materials Groundwater

CONCLUSIONS AND RECOMMENDATIONS

PROPOSED TRAIL ALIGNMENT

General Analyses Mike Harris Rock Cut Slope Earthwork Recommendations Retaining Wall Recommendations Pavement Recommendations

PROPOSED MOOSE CREEK REPLACEMENT BRIDGE

Bridge Foundation Support Recommendations Seismic Considerations Pavement Recommendations

PROPOSED MOOSE CREEK PARKING LOT

Earthwork Recommendations Pavement Recommendations

PROPOSED MIKE HARRIS HIGHWAY CROSSING

General Earthwork, Excavation, and Shoring Recommendations Foundation Support Recommendations Lateral Earth Pressure Recommendations Subdrainage Recommendations

PROPOSED TRAIL CREEK HIGHWAY CROSSING

General Earthwork, Excavation, and Shoring Recommendations Foundation Support Recommendations Lateral Earth Pressures ii

Subdrainage Recommendations

PROPOSED TRAIL CREEK CAMPGROUND PARKING LOT

Earthwork Recommendations Pavement Recommendations

DESIGN REVIEW AND CONSTRUCTION SERVICES

LIMITATIONS

FIGURES

Figure 1 Vicinity Map Figure 2 Trail Alignment Map Figures 3-50 Site Plans Figures 51-60 Trail Alignment Field Photographs Figure 61 Drainage Detail Figure 62 Typical Detail for Filling on Slopes Figure 63 Surcharge-Induced Lateral Pressure Figure 64 Earth Pressure for Cantilever Shoring

APPENDICES

Appendix A Field Explorations and Laboratory Testing Appendix B Global Slope-Stability Studies Appendix C Rockfall Trajectory Modeling Memo Appendix D Specific Cut and Fill Slope and Retaining Wall Recommendations

INTRODUCTION

At the request of David Evans and Associates, Inc. (DEA), GRI is providing geotechnical services to the Western Federal Lands Highway Division (WFLHD) for the Idaho Teton Centennial Trail project that starts in Teton County, Idaho, and extends into Teton County, Wyoming. The Vicinity Map, Figure 1, shows the general location of the proposed trail. The purpose of this updated final geotechnical report was to evaluate subsurface conditions along the proposed trail alignment and develop geotechnical recommendations for use in supporting the plans, specifications, and estimate (PS&E) phase of the project.

GRI completed a geotechnical investigation for the 70%-level phase of the project, the results of which are summarized in our November 22, 2019, draft report titled, “70%-Level Geotechnical Report, Idaho Teton Centennial Trail, ID DOT T 33(1) and WY DOT 22(1), Teton County, Idaho and Wyoming, DTFH7015D00002, Task Order No. 69056718F000010, Modification No. P00001,” prepared for DEA and WFLHD. The geotechnical scope of work for the PS&E sign-off phase of the project is described in the April 15, 2020, WFLHD document titled “Revised Statement of Work, Contract No. DTFH7015D00002, Task Order No. 69056718F000010, Modification No. P00008, ID DOT T 33(1) and WY DOT 22(1), Idaho Teton Pass Centennial Trail, Geotechnical Investigation and Environmental Assessment,” with modifications by Task Order No. 7, dated May 7, 2020.

Unless otherwise noted, elevations in this report reference the WFLHD project datum.

PROJECT DESCRIPTION

The overall Teton Pass Centennial Trail project will extend a distance of about 17 miles along the Teton Pass Highway from Victor, Idaho, to Wilson, Wyoming. This phase of the project involves an approximately 2.3-mile-long section of the Teton Centennial Trail that starts near Moose Creek in Teton County, Idaho, and extends past the Idaho/Wyoming state border to the Trail Creek Campground in Teton County, Wyoming.

The Trail Alignment Map, Figure 2, shows the general location of the proposed trail alignment included in this phase of the project. The trail will be constructed along the north side of the Teton Pass Highway (ID Highway 33 and WY Highway 22) and will include design and construction of a new bicycle/pedestrian path using portions of the Old Jackson Highway and an existing unimproved dirt trail.

Final plan and profile sheets provided by WFLHD indicate the trail will begin immediately north of Moose Creek at Station 10+00 and extend to the Trail Creek Campground at Station 133+80. A spur route will connect to the main trail alignment at Station 33+04 and will extend from Station 433+04 to the Mike Harris Campground at Station 442+91. As currently planned, the project elements will include the following:

A 10-ft-wide asphalt concrete (AC)-surfaced path (Proposed Trail Alignment) between Stations 10+00 and 133+80, and 433+04 and 442+91.

Replacement of the Moose Creek Bridge using 21-in., pre-stressed voided slabs (Proposed Moose Creek Replacement Bridge) near Station 13+00.

A new AC-paved parking area immediately adjacent the trail near Moose Creek (Proposed Moose Creek Parking Lot) with driveways near Station 14+40 and 16+90.

An underpass crossing of the Teton Pass Highway near the Mike Harris Campground in Teton County, Idaho, using a box culvert that provides a minimum-8-ft horizontal and vertical clearance (Proposed Mike Harris Highway Crossing). The Proposed Mike Harris Highway Crossing is generally located between Stations 437+00 and 438+00.

A mechanically stabilized earth (MSE) retaining wall from Station 34+77 to 37+07 and a soil nail retaining wall from Station 129+82 to 131+57.

An underpass crossing of the Teton Pass Highway near the Trail Creek Campground in Teton County, Wyoming, using a box culvert that provides a minimum-8-ft horizontal and vertical clearance (Proposed Trail Creek Highway Crossing). The Proposed Trail Creek Highway Crossing is generally located between Stations 131+00 and 132+00.

A new AC-paved parking area at the Trail Creek Campground (Proposed Trail Creek Campground Parking Lot) with a driveway near Station 133+80.

The Site Plans, Figures 3 through 50, show sections of the final trail alignment and stationing developed by WFLHD, and the current location of the planned project elements listed above. Photographs of the existing ground surface conditions along the Proposed Trail Alignment and at the location of the planned project elements are provided on the Trail Alignment Field Photographs, Figures 51 through 60.

SITE DESCRIPTION

The Proposed Trail Alignment runs parallel to the north side of the Teton Pass Highway and is located on the southwest-facing slope of the Taylor Mountain Upland, which rises to an elevation of about 10,352 ft (World Geodetic System [WGS] 84). The trail will begin near Moose Creek at about elevation 6,480 ft (WGS

84) and extend southeast to the Trail Creek Campground at about elevation 6,680 ft (WGS 84). The trail will traverse the hillside with grades typically ranging from 2% to 3% and 10% along relatively flat and rolling areas, respectively. As shown on Figures 51 through 60, the majority of the ground surface along the Proposed Trail Alignment is vegetated with wild grass, brush, and trees. Several drainages fed from the Taylor Mountain Upland cross the Proposed Trail Alignment and drain into Trail Creek.

Portions of the trail will be located at the base and top of rock cut slopes that were likely created during construction of the Teton Pass Highway. The rock slopes were cut at about 1H:1V to 1.5H:1V (Horizontal to Vertical). Additional discussion of the rock cut slope near the Mike Harris Campground entrance is provided in the Geologic Reconnaissance section below.

Portions of the trail will be located along the Old Jackson Highway that was abandoned following construction of the Teton Pass Highway. The Old Jackson Highway was built in 1913 and the portions remaining along the Proposed Trail Alignment consist of embankments and/or benches cut into the hillside.

The embankments and benches slope downward towards the Teton Pass Highway at about 3H:1V to 1H:1V, and benches into the hillside have ascending cut slopes of about 1.5H:1V.

Obvious surficial indications of large-scale, deep-seated slope instability were not observed along the Proposed Trail Alignment during our site reconnaissance in October 2016.

GEOLOGIC SETTING

The project is located in the Idaho-Wyoming thrust belt geologic province, which is a segment of the Cordilleran thrust belt comprising folded and faulted Paleozoic- and Mesozoic-age sedimentary rocks. The path is located on the west flank of Taylor Mountain, which is a prominent southern peak of the Teton Range.

The path will cross over several geologic units. Near Moose Creek, the path is located on Quaternary-age alluvial deposits consisting of unconsolidated silt, sand, and gravel. As the path gains elevation to the southeast, the upland rock units consist of Quaternary-age colluvium, Mesozoic-age sedimentary rocks, and Neogene-age volcanic rocks (Mitchell and Bennett, 1979). Quaternary-age colluvium consists of partially consolidated or cemented silt, sand, and gravel that accumulate on sloping ground due to weathering and erosional processes. The sedimentary rocks are mapped as Gannett Group and consist of sandstone, limestone, and dolomite. The volcanic rocks are mapped as Kirkham Hollow Volcanics and consist of tuff and rhyolite.

GEOLOGIC RECONNAISSANCE

A certified engineering geologist and professional engineer from GRI conducted a ground-level reconnaissance of the of the rock cut slope near the Mike Harris Campground entrance on September 25, 2019. GRI personnel met with Shawn Enright, a geologist with the Idaho Transportation Department (ITD), at the cut slope during our reconnaissance. The reconnaissance was completed to visually assess the condition of the existing rock cut slope, geologic and geomorphologic conditions exposed along the cut slope, indications of slope instability, site drainages, the potential for rockfall hazard, and other conditions that might affect the design and construction of the trail.

The existing cut has an overall slope of about 40° from horizontal (about 1.14H:1V), ascending from an approximately 20-ft-wide ditch that serves as a rockfall catchment area. The rock exposed in the cut slope is slightly to moderately weathered, extremely soft (R0) to hard (R4), light green to gray, limestone and dolomitic limestone of the Gannett Group. Alternating beds of soft limestone and hard dolomitic limestone with closely to moderately spaced fractures outcrop in the cut face of the slope. Bedding strikes an azimuth of about 298° and dips at an angle of about 64° adversely into the roadway. Two 10-ft scanline sections of exposed thinly bedded dolomitic limestone were observed to have a field rock quality designation (RQD) of 37.5% and 54%. RQD data could not be collected from the softer limestone beds as raveling from the upper portions of the slope, and a slightly overhanging brow of colluvium concealed the limestone discontinuities.

Interbedded soft limestone appeared to weather out of the slope in fragments less than about 3 in. in size.

Cobble- and boulder-sized rock fragments up to 30 in. in maximum size were observed at the base of the slope. Catchment of rockfall debris from the existing cut slope is provided by a zone about 20 ft wide between the bottom of the cut slope and the roadway shoulder. Localized rock fragments up to about 10 ft in size that could potentially topple to the base of the cut were observed along the cut slope.

Observations of the existing rock cut slope and discussions with the ITD geologist indicate the existing rock cut slope has performed satisfactorily since construction of the Teton Pass Highway. The cut slope exhibits no obvious surficial indications of large-scale, deep-seated slope instability and has not required significant maintenance or repair commonly related to slope instability.

SUBSURFACE CONDITIONS

General Subsurface materials and conditions for this phase of the Teton Pass Centennial Trail project were investigated between October 4 and 8, 2016, November 14 and 15, 2017, and September 24 and 25, 2019, with 11 drilled borings, designated B-1 through B-11, 10 hand-augered borings, designated HA-1 through HA-10, and 20 test pits, designated TP-1 through TP-20. Borings B-1 and B-2 were completed in the Proposed Trail Creek Campground Parking Lot; test pit TP-1 and boring B-3 were completed on the north and south sides, respectively, of the Teton Pass Highway near the Proposed Trail Creek underpass crossing location; borings B-6 through B-9, hand-augered borings HA-1 through HA-10, and test pits TP-2 through TP- 11 and TP-15 through TP-20 were completed at select locations along the Proposed Trail Alignment; borings B-10 and B-11 were completed on the east and west sides, respectively, of the Teton Pass Highway near the Proposed Mike Harris Highway Crossing location; test pits TP-13 and TP-14 were completed in the Proposed Moose Creek Parking Lot; and borings B-4 and B-5 were completed on the south and north sides, respectively, of Moose Creek near the abutments of the Proposed Moose Creek Replacement Bridge. The borings were advanced to depths of 6.5 ft to 26.5 ft, the hand-augered borings to practical refusal at depths of about 1 ft to 3.7 ft, and the test pits to depths of about 4.5 ft to 8 ft. The approximate locations of the explorations completed for this project are shown on the Site Plans, Figures 3 through 12. A discussion of the field exploration and laboratory testing programs conducted for this investigation is provided in Appendix A. Logs of the borings, hand-augered borings, and test pits are provided on Figures 1A through 19A. The terms and symbols used to describe the soils encountered in the explorations are defined in Tables 1A and 2A, and the attached legend.

It should be noted that utilities consisting of power, telephone, and fiber optic are located in the Teton Pass Highway right-of-way (ROW) on the north side of the road. The exploration locations were adjusted in the field to avoid utility conflicts.

Materials For the purpose of discussion, the materials disclosed by the explorations have been grouped into the following units based on their physical characteristics and engineering properties:

1. PAVEMENT

2. FILL

Clay and Silty Sand (Trail Creek Campground) Clay, Silt, Sand, Gravel, and Gravel and Cobbles (Teton Pass Highway Embankment) Sandy Gravel (Moose Creek Bridge Abutments) Silt, Sand, and Gravel and Cobbles (Old Jackson Highway Embankment)

3. SILT, SAND, and GRAVEL and COBBLES (Colluvium)

4. SILT, SAND, GRAVEL, and GRAVEL and COBBLES (Alluvium)

5. SILT and CLAY (Residual Soil)

6. LIMESTONE and Dolomitic LIMESTONE (Gannett Group)

7. DOLOMITE (Gannett Group)

8. TUFF (Kirkham Hollow Volcanics)

The following paragraphs provide a description of these units and a discussion of the groundwater conditions at the site.

1. PAVEMENT. Borings B-3 and B-5 were advanced in existing paved areas and encountered approximately

2.5 in. to 3 in. of AC pavement at the ground surface. The pavement is underlain by about 6 in. of crushed-rock base (CRB) course.

2. FILL

Clay and Silty Sand (Trail Creek Campground). Fill consisting of clay and silty sand was encountered at the ground surface in boring B-2. The fill extends to a depth of about 2.5 ft and is likely associated with construction of the campground entrance. The fill soils are generally brown to dark brown or red-brown with varying degrees of gray mottling. The clay fill contains some silt and fine- to coarse-grained sand, a trace of subangular gravel, wood debris, and organics. The silty sand fill is generally fine to coarse grained and contains a variable amount of clay ranging from trace to some clay, a trace of subangular gravel, and organics. The relative consistency of the clay fill is medium stiff and the relative density of the silty sand fill is loose based on a standard penetration test (SPT) N-value of 7 blows/ft.

The natural moisture content of the clay and silty sand fill is about 12%.

Clay, Silt, Sand, Gravel, and Gravel and Cobbles (Teton Pass Highway Embankment). Fill consisting of clay, silt, sand, gravel, and gravel and cobbles was encountered at the ground surface in borings B-6, B-7, and B-11 and beneath the pavement in boring B-3. The fill extends to depths of about 5.8 ft to 20 ft and is likely associated with construction of the Teton Pass Highway. The fill soils are generally red-brown, brown-gray, or light gray to gray with varying degrees of gray, rust, yellow, green, and red mottling. The clay fill has a variable silt content ranging from some silt to silty, a variable amount of fine- to coarse-grained sand ranging from trace to some sand, and a trace of subangular gravel. The clay fill also contains wood debris, organics, and shell fragments. The sand fill is generally fine to coarse grained, silty, and contains organics and a variable amount of clay and subrounded to subangular gravel ranging from trace to some clay and gravel. The gravel fill is generally subrounded to subangular, sandy with fine- to coarse-grained sand, and contains a trace of silt. The gravel and cobbles fill is generally subangular to angular, occurs in a matrix of gray-brown clay, silt, and fine- to coarse-grained sand, and contains boulders.

The relative consistency of the clay fill is stiff to hard based on SPT N-values of 9 to 32 blows/ft, and is typically stiff. The relative density of the sand, gravel, and gravel and cobbles fill is loose to dense based on SPT N-values of 4 to 42 blows/ft, N*-values of 41 to 46 blows/ft and observations during excavation, and is typically medium dense. The natural moisture content of the clay and sand fill ranges from 14% to 21% and is about 7%, respectively.

Atterberg-limits testing indicates the clay fill has a liquid limit of 46% and plasticity index of 27%, see Figure 27A.

Sandy Gravel (Moose Creek Bridge Abutments). Fill consisting of sandy gravel was encountered at the ground surface and beneath the pavement in borings B-4 and B-5, respectively. The sandy gravel fill extends to depths of about 3 ft and 5 ft in borings B-4 and B-5, respectively. The sandy gravel fill is generally subangular to angular with fine- to coarse-grained sand and contains a variable amount of silt ranging from trace to some silt. The relative density of the sandy gravel fill is medium dense based on an SPT N-value of 23 blows/ft.

Silt, Sand, and Gravel and Cobbles (Old Jackson Highway Embankment). Fill consisting of silt, sand, gravel, and cobbles was encountered at the ground surface in borings B-8 and B- 9 and test pits TP-2, TP-3, TP-4, TP-6, TP-8, TP-9, TP-11, TP-17, and TP-18. The fill extends to depths of about 0.5 ft to 4 ft and is generally brown to dark brown with varying degrees of rust mottling. The silt fill has a variable clay content ranging from trace clay to clayey, and contains a variable amount of fine- to coarse-grained sand and subrounded to angular gravel ranging from a trace of sand to sandy and trace to some gravel. The sand fill is generally fine to coarse grained and silty, and contains a variable amount of subrounded to angular gravel ranging from trace to some gravel. The gravel and cobbles fill is generally subangular to subrounded and in a matrix of brown mottled gray and rust silt and fine- to coarse-grained sand. Boulders and varying amounts of organics were present throughout the fill soils.

The relative consistency of the silt fill is soft to stiff based on an SPT N-value of 7 blows/ft, Torvane shear-strength values of 0.20 tsf to 0.50 tsf, and observations during excavation and is typically medium stiff. The relative density of the sand and gravel and cobbles fill is loose to medium dense based on observations made during excavation. The natural moisture content of the silt and sand fill ranges from 13% to 20% and 7% to 16%, respectively.

3. SILT, SAND, and GRAVEL and COBBLES (Colluvium). Colluvium consisting of silt, sand, and gravel and cobbles was encountered at the ground surface in hand-augered borings HA-6 and HA-7 and test pits TP-10, TP-15, and TP-16, and beneath the fill in borings B-8 and B-9 and test pits TP-2, TP-3, TP-8 through TP-11, TP-17, and TP-18. The colluvium extends to the maximum depths explored of about 2 ft to 11.5 ft in explorations HA-6, HA-7, B-8, TP-3, and TP-8 through TP-11, and to a depth of about 12 ft in boring B-9 and 2 ft to 4.5 ft in test pits TP-2, TP-15, and TP-16. The colluvium is generally decomposed to silt or silty sand near the ground surface. The silt is generally light brown to dark brown or red to red-brown, has a variable clay and fine- to coarse-grained sand content ranging from a trace of sand to sandy and trace to some clay, and contains up to a trace of angular to subangular gravel. The silty sand is generally red, fine to coarse grained, and contains some clay and subrounded to rounded gravel and cobbles. The granular content of the soil increases with depth as the degree of weathering decreases, and the soil transitions to subrounded to subangular gravel and cobbles in a matrix of brown to gray-brown clay, silt, and fine- to coarse-grained sand. Boulders were encountered in the gravel and cobbles colluvium in borings B-8 and B-9 and test pits TP-15, TP-16, and TP-18. The colluvium soil contains organics to depths of about 1.5 ft to 5 ft in hand-augered borings HA-6 and HA-7, and test pit TP-18. Borings HA-6, HA-7, and B-8, and test pits TP-3, TP-8 through 11, TP-15, TP-17, and TP-18 were terminated in colluvium.

The relative consistency of the silt colluvium is medium stiff to hard based on SPT N-values of 8 to 17 blows/ft, Torvane shear-strength values of 0.35 tsf to 1.0 tsf, and observations made during excavation and is typically medium stiff to stiff. The relative density of the gravel and cobbles colluvium is loose to dense based on SPT N-values of 7 to 45 blows/ft and observations made during excavation and is typically medium dense to dense . The natural moisture content of the silt colluvium is 10% to 25%.

4. SILT, SAND, GRAVEL, and GRAVEL and COBBLES (Alluvium). Alluvium consisting of silt, sand, gravel, and gravel and cobbles was encountered at the ground surface in borings B-1 and B-10, and test pits TP-19 and TP-20; beneath fill in borings B-2, B-4 through B-7, and B-11, and test pits 12 through TP-14; and beneath colluvium in boring B-9. The alluvium extends to the maximum depths explored of 5 ft to 26.3 ft in borings B-1, B-2, B-5, B-9, and B-11, and test pits TP-12 through TP-14, TP-19, and TP-20, and to depths of about

10.5 to 26.3 ft in borings B-4, B-6, B-7, and B-10. The alluvium is generally light brown to dark brown with varying degrees of rust mottling. The silt alluvium has a variable clay content ranging from trace clay to clayey, and contains a variable amount of fine- to coarse-grained sand ranging from trace of sand to sandy and up to a trace of subrounded to subangular gravel and cobbles. The sand alluvium is generally fine- to coarse-grained and contains a trace of silt and subrounded to rounded gravel. The gravel to gravel and cobbles alluvium is generally rounded to subangular and either contains a variable amount of fine- to coarse-grained sand ranging from some sand to sandy and a trace of silt, or occurs in a matrix of brown, with varying degrees of orange and red mottling, silt and fine- to coarse-grained sand. Organics were present in the silt alluvium to depths of about 1.5 to 14.5 ft in borings B-1, B-4 B-10, and B-11, and test pits TP-12 through TP- 14, TP-19, and TP-20. The silt alluvium is interbedded with 1-in.-thick lenses of sand at a depth of 5.5 ft in boring B-4 and contains wood debris below a depth of 5.5 ft in boring B-4. Boulders were encountered at various depths throughout the gravel and cobbles alluvium in borings B-4, B-5, B-10, and B-11, and test pits TP-19 and TP-20. Borings B-1, B-2, B-5, B-9, and B-11 and test pits TP-12 through TP-14, TP-19, and TP-20 were terminated in alluvium.

The relative consistency of the silt alluvium is very soft to stiff based on SPT N-values of 2 to 12 blows/ft, Torvane shear strength values of 0.60 tsf to 0.80 tsf, and observations made during excavation, and is typically medium stiff. The relative density of the sand, gravel and cobbles alluvium is loose to very dense based on SPT N-values of 8 blows/ft to 50 blows for less than 6 in. of sampler penetration (defined as practical refusal), SPT N*-values of 32 blows/ft to 50 blows for less than 6 in. of sampler penetration (defined as practical refusal), and observations made during excavation, and is typically medium dense to dense. It should be noted the relative density of very coarse, granular material such as gravel and cobbles tends to be overestimated using the standard split-spoon and CMS samplers. Caving conditions were encountered at a depth of 5 ft in test pits TP-19 and TP-20 during excavation, which is an indication that the gravel and cobbles alluvium may be “open-work.” Open-work gravel and cobbles is a deposit with limited fine material, a high permeability, and commonly capable of accepting large volumes of drilling fluid or grout. This is an important consideration for cast-in-place concrete piles extending into the gravel and cobbles alluvium.

One-dimensional consolidation testing was performed on a sample of silt alluvium obtained at a depth of 6 ft in boring B-4. Test results indicate the silt alluvium is overconsolidated and exhibits a relatively low compressibility in the preconsolidated range of pressures and high compressibility in the normally consolidated range of pressures, see Figure 21A. The natural moisture content of the silt and sand alluvium ranges from 13% to 43% and 3% to 7%, respectively. Atterberg-limits testing indicates the clayey silt to silty clay alluvium has a liquid limit of 49% and plasticity index of 23%, see Figure 27A.

5. SILT and CLAY (Residual Soil). Residual soil consisting of silt and clay were encountered at the ground surface in borings HA-1 through HA-5 and HA-8 through HA-10, and test pits TP-1, TP-5, and TP-7; beneath colluvium in TP-2; and beneath fill in test pits TP-4. The residual soil is derived from the weathering of the underlying Gannett Group Limestone and Kirkham Hollow Volcanic Tuff, and extends to the maximum depth of 7 ft in test pit TP-2 and depths of 0.8 ft to 4.5 ft in borings HA-1 through HA-5 and HA-8 through

HA-10, and test pits TP-1, TP-4, TP-5, and TP-7. The residual soil is generally dark brown at the ground surface and grades to brown, brown-gray, or red-brown with varying degrees of red and gray mottling with depth. The silt residual soil has a variable clay content ranging from trace clay to clayey, and the clay residual soil is typically silty. The residual soil contains a variable amount of fine- to coarse-grained sand and subangular to subrounded gravel ranging from trace to some sand and up to a trace of gravel to some gravel, and organics to depths of about 1 ft to 2 ft. Test pit TP-2 was terminated in residual soil.

The relative consistency of the residual soils is soft to very stiff based on Torvane shear strength values of

0.15 tsf to 1.5 tsf, and is typically soft to medium stiff at the ground surface and medium stiff to stiff below a depth of about 1 ft. The natural moisture content of the residual soils is 15% to 35%; however, a sample of residual soil from a depth of 0.5 ft in boring HA-8 has a moisture content of 125%, which is likely associated with the presence of organic material in the sample. Atterberg-limits testing indicates the residual soil has a liquid limit of 35% to 67% and a plasticity index of 17% to 39%, see Figure 20A.

6. LIMESTONE and Dolomitic LIMESTONE (Gannett Group). Extremely soft (R0) to hard (R4), green, gray, brown, orange, and red limestone and dolomitic limestone of the Gannett Group was encountered beneath fill in boring B-3 and test pit TP-6; beneath colluvium in test pit TP-16; beneath alluvium in borings B-4, B-6, B-7, and B-10; and beneath residual soil in borings HA-1 through HA-3, and HA-8 through HA-10, and test pits TP-1, TP-4, TP-5, and TP-7. The limestone is decomposed to slightly weathered, and extends to the maximum depths explored of 1.5 ft to 26.5 ft in borings B-4, B-6, B-7, and B-10, borings HA-1 through HA- 3 and HA-8 through HA-10, and test pits TP-4 through TP-7 and TP16; and to a depth of 5 ft to 25 ft in test pit TP-1 and boring B-3, respectively. Thin to very thin beds were observed in the limestone. SPT N-values of 31 blows/ft to 50 blows for less than 6 in. of sampler penetration (defined as practical refusal) and SPT N*-values of 47 to 50 blows for less than 6 in. of sampler penetration (defined as practical refusal) were recorded in the limestone. Borings B-4, B-6, B-7, B-10, HA-1 through HA-3, HA-8 through HA-10, and test pits TP-1, TP-4, TP-5, TP-7, and TP-16 were terminated in limestone.

A uniaxial compressive strength test was performed on a sample of limestone rock obtained at a depth of about 21 ft from boring B-7 to measure strength characteristics. The test results indicate the selected rock core sample has a uniaxial compressive strength of about 5,425 psi, which correlates to a rock hardness of medium hard (R3). The test results are tabulated in Appendix A.

7. DOLOMITE (Gannett Group). Extremely soft (R0) to very soft (R1), gray dolomite of the Gannett Group was encountered beneath limestone in test pit TP-1 and boring B-3 at depths of 5 ft and 25 ft, respectively.

The dolomite is slightly weathered to predominantly decomposed and extends to the maximum depths explored of about 8 ft and 26.5 ft in test pit TP-1 and boring B-3, respectively. Thin to very thin beds were observed in the dolomite. An SPT N*-value of 80 blows/ft was recorded in the dolomite. Boring B-3 and test pit TP-5 were terminated in dolomite.

8. TUFF (Kirkham Hollow Volcanics). Extremely soft (R0) to soft (R2), gray tuff of the Kirkland Hollow Volcanics was encountered beneath residual soil in borings HA-4 and HA-5 at a depth of 1.5 ft. The tuff is moderately weathered to predominantly decomposed and extends to the maximum depths explored of 1.6 ft and 1.8 ft in borings HA-5 and HA-4, respectively. The tuff has some vesicles, and joints and fractures are stained rust and filled with black secondary mineralization. Borings HA-4 and HA-5 were terminated in tuff.

Groundwater Groundwater was encountered at depths of about 4 ft to 18 ft in explorations B-4, B-5, B-10, B-11, TP-19, and TP-20, and groundwater seepage was encountered at a depth of about 7.5 ft in test pit TP-1.

Groundwater was not encountered in the remaining borings or test pits at the time of exploration. Due to the close proximity of Trail and Moose creeks to the Proposed Trail Alignment, the regional groundwater surface will rise and fall in response to fluctuations of the river level and rainfall. We anticipate the regional groundwater surface occurs at an elevation consistent with the surface water levels of the creeks, and may approach the ground surface near Trail and Moose creeks with a sustained creek level corresponding to a 50-year flood elevation of about 6,478 ft. In addition, perched groundwater can develop within the upper fine-grained soils during the wet spring months or following periods of prolonged or intense precipitation.

CONCLUSIONS AND RECOMMENDATIONS

The explorations indicate the Proposed Trail Alignment area is mantled with fill soils, colluvium, or residual soils; the Proposed Moose Creek Replacement Bridge and Parking Lot areas are mantled with alluvium; the Proposed Mike Harris Highway Crossing area is mantled with alluvium and fill soils; the Proposed Trail Creek Highway Crossing area is mantled with residual and fill soils; and the Proposed Trail Creek Campground Parking Lot area is mantled with alluvium. Shallow limestone and tuff outcrops are present throughout the majority of the Proposed Trail Alignment. As a result of the shallow rock outcrops, it should be anticipated that excavation along the Proposed Trail Alignment may be difficult. Groundwater was encountered at depths of about 9 and 4 ft to 18 ft in the explorations completed in the vicinity of Moose and Trail creeks, respectively. Regional groundwater was not encountered in the explorations completed east of the Proposed Mike Harris Highway Crossing; however, perched groundwater was encountered at a depth of

7.5 ft in test pit TP-1. We anticipate the regional groundwater level along the majority of the Proposed Trail Alignment will closely follow the surface water elevation of Trail Creek throughout the year; however, shallow perched groundwater conditions is likely to develop in the upper fine-grained soils and less-permeable zones of rock during the wet spring months or following periods of prolonged or intense precipitation.

The remaining sections of this report have been divided into six main sections based on the primary design elements. These sections include the Proposed Trail Alignment, the Proposed Moose Creek Bridge Replacement, the Proposed Moose Creek Parking Lot, the Proposed Mike Harris Highway Crossing, the Proposed Trail Creek Highway Crossing, and the Proposed Trail Creek Campground Parking Lot. The conclusions and recommendations are based on the Standard Specifications for the Construction of Roads and Bridges on Federal Highway Projects (FP-14) document. The following sections of this report provide our conclusions and recommendations concerning earthwork and design and construction of the trail, retaining walls, bridge foundations, underpass crossings, and pavements.

PROPOSED TRAIL ALIGNMENT

General The Proposed Trail Alignment traverses five primary topographic features: (1) floodplains associated with the adjacent creeks, (2) base of the Mike Harris rock cut slope, (3) hillsides above rock cut slopes, (4) drainages between hillsides, and (5) the Old Jackson Highway. Final trail grades will be achieved using a combination of cut and fill slopes, and MSE or soil nail retaining structures. Our review of the PS&E design indicates the height of cuts and fills to establish final grades along the Proposed Trail Alignment will typically be less than about 5 ft and the maximum height of cuts and fills will be on the order of 10 ft and 17 ft, respectively. All permanent cuts and embankment fills will be sloped at 2H:1V or flatter. The maximum height of the MSE and soil nail retaining walls will be on the order of 15 ft and 20 ft, respectively.

Portions of the trail alignment will traverse hillsides above rock cut slopes. The overall rock cut slopes appear to be grossly stable; however, there is a risk of localized rockfall, which could potentially undermine the trail. To reduce the long-term risk of rockfall undermining the trail, we recommend the trail alignment be located a minimum of 15 ft away from the top of rock cut slopes. Our review of the PS&E design indicates the Proposed Trail Alignment is generally located at least 15 ft away from the top of the rock cut slopes. GRI should be notified if areas of the Proposed Trail Alignment are closer to the rock cut slopes than the design files indicate, and further analysis may be required.

The primary geotechnical considerations associated with construction of the proposed trail include the presence of near-surface fine-grained soils that are extremely sensitive to moisture content and frost heave, variations in subsurface conditions between soil and rock, the potential for perched groundwater conditions following periods of extended precipitation, and the constructability of the walls.

Analyses General. In accordance with our Statement of Work, GRI completed global stability analyses to provide evaluations for five cut slope areas, two embankment fill areas, and three retaining wall sections along the Proposed Trail Alignment. In general, the areas with the proposed largest cuts, embankment fills, and retaining walls by height were selected for analyses. The following paragraphs discuss information used in our analyses and provide a brief summary of the results of our analyses.

Global Slope Stability Studies. Global stability of the Proposed Trail Alignment slopes and retaining walls were evaluated using infinite slope equations, or Spencer’s and Morgenstern-Price’s method of slices, which satisfy both force and moment equilibrium. The method of slices computation was completed with the aid of the software program Slide2, developed by Rocscience, Inc., of Toronto, Canada. The output of the analyses is a factor of safety, which is defined as the ratio of the forces and moments resisting movement to the forces and moments driving movement of the soil mass. This calculated result is used as a relative measure of the overall stability of the slope. As the factor of safety decreases and approaches 1.0, the relative stability of the slope is considered to decrease. A factor of safety less than 1.0 infers the soil mass is not in equilibrium and movement is likely to occur.

The Proposed Trail Alignment topography and generalized subsurface conditions were used to evaluate the global stability of the proposed cut and fill slopes, and retaining walls. The models used for global stability analyses were developed using cross sections provided by WFLHD and generalized subsurface profiles based on the results of the nearest subsurface exploration. The soil and rock parameters used in the analyses were determined based on the laboratory test results, our experience with similar soils, and our review of published literature. For our analysis, we assumed the groundwater level occurs at the interface between the overburden soils and underlying rock or gravel and cobbles. A distributed load of 200 psf was used to model the pavement section and pedestrian load on the trail in accordance with Section 11.6.2.3 of the 2020 American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO LRFD) manual.

The following table provides the engineering properties of the soil layers used for our Slide2 analysis.

SOIL PROFILE AND PROPERTIES FOR EVALUATION OF STABILITY

Material

Soil Properties

Unit Weight, pcf Cohesion, psf Phi, degrees

Embankment Fill 125 0 32

Silt/Clay 105 50 26

Sandy Silt/Silty Sand 115 0 34

Gravel and Cobbles 130 0 40

Tuff 130 Infinite Strength

Limestone 150 Infinite Strength

Dolomite 150 Infinite Strength

Drained strengths of the soils were used to evaluate slope stability. The fine-grained soils mantling the trail alignment where significant fills are planned are overconsolidated; therefore, long-term drained strengths will control the global slope stability.

In accordance with Section 11.6.3.7 of the AASHTO LRFD manual, a minimum global factor of safety of 1.3 to 1.5 is required for earth slopes and retaining walls. The global factor of safety required depends on the certainty of the geotechnical parameters and the improvements being supported by the earth slopes and retaining walls.

An infinite slope analysis was completed to evaluate the stability of permanent cut slopes in silt and clay soils; and a method of slices analysis was completed to evaluate the stability of the trail in embankment fill areas. Our slope stability analyses indicate the static global factor of safety exceeds 1.3 for permanent cuts and embankment fills slopes at 2H:1V or flatter. The embankment fill slope models used for our global stability analyses are provided in Appendix B.

A method of slices analysis was completed to evaluate the stability of the trail in retaining wall areas. The MSE wall was modeled using a unit weight of 130 pcf and infinite strength for the reinforced zone to drive the failure surface below the base of the wall. The soil nail wall was modeled assuming a rectangular, 4-ft grid pattern; an inclination of 15° from horizontal; an ultimate bond strength of 5 psi; a diameter of 6 in.;

and total lengths of 20 ft to 25 ft. Our slope analyses indicate the static global factor of safety meets or exceeds 1.3 for all of the retaining wall systems analyzed. The retaining wall slope models used for our global stability analyses are provided in Appendix B.

Mike Harris Rock Cut Slope General. The final trail alignment will be located at the base of the rock cut slope near the Mike Harris Campground entrance. A geologic reconnaissance of the rock cut slope was completed by GRI and is discussed in the Geologic Reconnaissance section of this report. In our opinion, based on observations during our reconnaissance and discussions with the ITD geologist, the risk of large-scale, deep-seated instability of the slope is low. However, accumulation of cobbles and boulders up to about 30 in. were observed at the base of the slope, which suggests there is a rockfall hazard. Catchment of rockfall debris from the existing cut slope is provided by a zone about 20 ft wide between the bottom of the cut slope and the roadway shoulder. The new Proposed Trail Alignment will practically eliminate the existing rockfall catchment ditch at the base of the cut; consequently, maintaining recommended catchment ditch widths to protect rockfall from rolling into the road or trail will not be feasible without steeper cuts into the slope face to provide adequate catchment accommodation space for rockfall debris or constructed barrier to reduce the risk of rock from reaching the trail. To protect the trail from rockfall, potential constructed barrier alternatives include an anchored mesh system, flexible rockfall barrier, flexible rockfall barrier, or pre-cast concrete barriers. The pre-cast concrete barrier alternative was preferred by project stakeholders and recommended for further evaluation with numerical rockfall modeling.

Rockfall Modeling. The rockfall behavior of the Mike Harris Cut Slope was evaluated by a numerical analysis to evaluate the feasibility of a 32-in.-tall ITD standard pre-cast concrete “jersey” barrier located about 7 ft from the base of the slope to provide protection to the Idaho Teton Centennial Trail surface and adjacent highway from potential rockfall. Three topographic cross sections were selected at Stations 245+84, 246+46, and 247+64. The material inputs, analysis methods, and modeling results are summarized in Appendix C. The modeling shows that 99% of rockfall is contained by the 32-in.-tall barrier located about 7 ft from the base of the slope. This includes rocks weighing up to 1,000 lbs and maximum model kinetic energy of 8.44 kilojoules (kJ). In general, pre-cast barriers have a capacity of 24 kJ from a single impact. The maximum bounce height recorded in the model is about 5 ft.

Recommended Rockfall Mitigation. Based on numerical modeling of rockfall behavior at the Mike Harris Cut Slope, suitable containment of the rock face could be provided by a 32-in.-tall, ITD standard pre-cast concrete “jersey” barrier about 7 ft from the base of the slope. Regular periodic maintenance to remove rockfall debris build up uphill of the barrier must be conducted to provide long-term effectiveness of rockfall mitigation. Barriers should be repaired or replace as soon as damage is observed. In addition to the long-term maintenance, we recommend scaling of the existing slope prior to installation of the permanent rockfall barrier to improve long-term trail safety. Scaling of the slope prior to construction of the trail will be required to remove loose rock and debris to improve safety conditions for trail construction. Scaling should be completed using boom-lift or rope-access and hand-tool methods. Based on our observations, we anticipate hand scaling of loose rock on the cut slope and recontouring of the brow of the slope will reduce the quantity of rockfall events in the short term.

Earthwork Recommendations General. The majority of the Proposed Trail Alignment is mantled with fine-grained soils with a 3- to 6-in.-thick heavily rooted zone at the ground surface. Fine-grained soils, such as the soil that mantles portions of the Proposed Trail Alignment, are sensitive to moisture, and perched groundwater may approach or inundate the ground surface during the wet spring months or following periods of prolonged or intense precipitation. Due to snow accumulation during the winter and wet ground conditions in the spring, it is our opinion that earthwork can most practically be completed during the dry summer months, typically extending from June to September. The contractor must use construction equipment and procedures that prevent disturbance and softening of the fine-grained subgrade soils. To minimize disturbance of the moisture-sensitive, fine-grained soils, site grading can be completed using track-mounted hydraulic excavators or bulldozers. The excavation should be finished using a smooth-edged bucket to produce a firm, undisturbed surface. It may also be necessary to construct granular haul roads and work pads concurrently with excavation to minimize subgrade disturbance. If the subgrade is disturbed during construction, soft, disturbed soils should be overexcavated to firm soil and backfilled with structural fill. The soils most susceptible to disturbance during construction are generally located near the creeks and in the drainages between the hillsides. We anticipate a lower risk of subgrade disturbance in rock cut areas and in the Old Jackson Highway roadbed where an existing rock section exists. The risk of subgrade disturbance can be significantly reduced if earthwork activities are completed during the dry summer months.

If construction occurs during wet ground conditions, granular work pads will be required to protect the underlying silt subgrade and provide a firm working surface for construction activities. In our opinion, a 12-to 18-in.-thick granular work pad should be sufficient to reduce disturbance of the subgrade by lighter construction equipment, such as pickup trucks and excavators, and limited traffic by dump trucks. Haul roads and other high-density traffic areas will require a minimum-18- to 24-in.-thick granular work pad to reduce the risk of subgrade deterioration. The actual thickness of the rock section should be determined based on the subsurface conditions encountered during construction. We recommend the granular work pads consist of material conforming to Subsection 704.07 of the FP-14. The use of a geotextile fabric over the subgrade prior to placement and compaction of the granular work pad may reduce maintenance during construction. Haul roads can also be constructed by placing a thickened section of pavement base course and subsequently spreading and grading the excess CRB after earthwork is complete; however, constructing the entire trail as a haul road may not be cost-effective. Constructing haul roads directly between the trail and highway and subsequently transporting the material to different areas along the trail using lighter equipment, such as tracked dumpers, can be considered to reduce the amount of material required to construct the haul roads.

Trail Preparation. All existing pavement, debris, trees, brush, and surficial organic material within the limits of the Proposed Trail Alignment should be removed. We anticipate stripping up to a depth of 6 in. will likely be required along the majority of the Proposed Trail Alignment; however, deeper grubbing may be required to remove brush and tree roots. Excavations required to remove existing improvements and trees should be backfilled with structural fill. Organic strippings should be disposed of off site or stockpiled on site for use in landscaped areas.

Upon completion of site stripping or excavation to the required subgrade level, the exposed subgrade should be evaluated by a qualified geotechnical engineer. Any soft soils or areas of unsuitable material should be overexcavated to firm, undisturbed soil and replaced with structural fill. See the Structural Fill section below for additional details. Due to the presence of fill soils across the Proposed Trail Alignment, it should be anticipated that overexcavation of subgrade may be required in localized areas. Based on our experience with similar projects, field observations, and subsurface conditions, we estimate about 5% of the trail may encounter unsuitable material at subgrade elevation that requires overexcavation and replacement. This information is provided to assist the design team with project planning and should only be used for planning purposes. It should be understood subsurface information along the entire trail alignment is limited, and the actual quantity of overexcavation will depend on the subsurface and groundwater conditions encountered at the time of excavation and the contractor’s means and methods.

Trail Grading. Final grading across the project should provide for positive drainage of surface water away from exposed slopes to minimize erosion. Prior to placing pavement base course aggregate, subgrade should be sloped to a minimum-0.5% slope to aid in drainage. Permanent cut and embankment fill slopes should be constructed no steeper than 2H:1V. In general, structural fills should extend a minimum horizontal distance of 1 ft beyond the limits of the improvements such as the edge of new pavements. The permanent cut and embankment fill slopes should be protected with vegetation to reduce to risk of surface erosion due to spring thaw and rainfall.

Site grading recommendations presented are based on the subsurface conditions disclosed by the explorations and the results of our stability analyses. Recommendations for five specific cut slopes and two specific embankment fill areas are summarized in Appendix D for reference.

Trail Drainage. Portions of the trail will be located at the base of existing slopes, along hillsides, or in low areas such as drainages.