Upper Mill Creek Canyon Road Improvements_Geotech_Final_Rev 1_20240715 - signed v3 reduced.pdf

Report Cover Page

Upper Mill Creek Canyon Road Improvements

UT FLAP SLA 10(1)

Final Geotechnical Engineering Report

July 15, 2024 | Terracon Project No. 61215174A

Prepared for:

DJ&A, P.C.

2000 Maple Street

Missoula, MT 59808

6952 S. High Tech Drive, Suite B

Midvale, UT 84047

P (801) 545-8500

Terracon.com

Facilities | Environmental | Geotechnical | Materials

Report Cover Letter to Sign

July 15, 2024

DJ&A, P.C.

2000 Maple Street Missoula, MT 59808

Attn: Mr. Jon Pederson, P.E.

P: (406) 549-6371 E: jonathan@djanda.com

Re: Final Geotechnical Engineering Report Upper Mill Creek Canyon Road Improvements

UT FLAP SLA 10(1)

Salt Lake County, Utah Terracon Project No. 61215174A

Dear Mr. Pederson:

We have completed the scope of Geotechnical Engineering services for the above-referenced project in general accordance with the Mill Creek Canyon Road UT FLAP SLA 10(1) MOD P00003: Final Design Scope of Work (SOW), dated July 15, 2022, and this report is being submitted as a deliverable for activity G4. This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerning earthwork and the design and construction of foundations and retaining walls for the proposed project. Design changes resulting from the 70% design have been incorporated.

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

Sincerely, Terracon

Jeff Gilbert, P.E. Rick L. Chesnut, P.E., P.G.

Senior Engineer Senior Principal cerning this report or if we may

P.E. Ric eer Se

Geotechnical Engineering Report

Upper Mill Creek Canyon Road Improvements UT FLAP SLA 10(1) | Salt Lake County, Utah

July 15, 2024 | Terracon Project No. 61215174A

Facilities | Environmental | Geotechnical | Materials i

Table of Contents

Report Summary .............................................................................................. i Introduction Project Description Site Conditions Geology Geotechnical Characterization

Bedrock Geophysical Groundwater

Geologic Hazards Active Faults Landslides / Debris Flows Rockfall

Seismic Overview Liquefaction

Corrosivity Geotechnical Overview Earthwork

Site and Subgrade Preparation Excavation Soil Stabilization Fills Fill Material Types Fill Placement and Compaction Requirements Shrink/Swell Factors Grading and Drainage Dewatering Earthwork Construction Considerations Construction Observation and Testing

White Bridge, Elbow Fork Culverts, & Culvert Extensions Design Parameters Foundation Construction Considerations

Lateral Earth Pressures Design Parameters Subsurface Drainage for Retaining Walls

Geotechnical Soil Design Parameters Soil Nail Wall Construction Considerations Rockery Walls Lateral Earth Pressures

Fills Slope Stability

Surficial Slope Stability

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General Comments

Appendix A Field Exploration Exploration and Testing Procedures Vicinity Map Geology Exploration Plan (16) Exploration Logs (28) Geophysical Report (9) General Notes Unified Soil Classification System Rock Classification AASHTO Classification System

Appendix B Laboratory Results Laboratory Summary Table (5) Grain Size Distribution (16) Corrosivity (5) Direct Shear

Appendix C Analysis Results Geotechnical Design Parameters (19) Global Stability Results (114) General Limit Equilibrium Results (2)

Appendix D Supporting Information Photography Log Rock Cores (2) Drill Rig Hammer Calibration Report

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Report Summary

Topic1 Overview statement2

Project Description

Reconstruct and widen approximately 4.5 miles of the roadway from the Winter Gate to the Big Water Trailhead to improve safety natural resources. This section of roadway is closed to vehicular traffic from November 1 to early July. Widening will include replacing White Bridge, constructing rockery and soil nail retaining walls, constructing cut slopes and embankments, and extending box culverts.

Geotechnical Characterization

The site soils consist primarily of sand and gravel, with varying amounts of silt and clay, and varying amounts of cobbles and boulders. Shallow bedrock was encountered in some borings and geophysical arrays, and rock outcrops were observed at multiple locations along the canyon side slopes. Bedrock outcrops and subsurface conditions appear to have a high degree of variability along the project. Groundwater ranged from 4 to 25 feet below existing grade when encountered and was not observed in several borings. Seeps or springs were observed at multiple locations along the edge of the existing roadway and coming down the canyon slopes.

Earthwork

Complete earthwork in accordance with FP-14.

Design soil cut slopes range from 1V:1.5H to 1V:3H depending on subsurface conditions and existing slopes.

Design rock cut slopes estimated to be 2V:1H.

Fill slopes recommended to be 1V:3H or flatter. Steeper slopes require reinforcement or constructing a stabilizing rock berm as part of the slope.

Global stability results for cuts are provided in the Slope Stability section.

Variable rock and soil conditions are anticipated in excavations and cut slopes. Specialized construction equipment will be required. Cut slopes may need to be adjusted during construction based on slope conditions.

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Drainage of existing and encountered springs/seeps must be incorporated into the roadway, retaining wall, and cut slope design.

White Bridge & Culverts, Elbow Fork Parking Box Culverts

Boxes to be supported on 12 inches of Foundation Fill. Bearing resistances are provided in the White Bridge & Culvert Extensions and Elbow Fork Box Culverts section of this report.

Earth pressures are provided in the Lateral Earth Pressure section.

Dewatering will be necessary to complete box construction and extensions.

Retaining Walls

Global stability results for rockery and soil nail walls are provided in the Slope Stability section. Retaining wall geotechnical design parameters are also provided.

Embankments

Embankment induced settlements are estimated to be 1 to 2 inches. Geotechnical soil design parameters for the steepened slope at 81+80 to 82+20 are provided.

General Comments

This section contains important information about the limitations of this geotechnical engineering report.

1. If the reader is reviewing this report as a pdf, the topics above can be used to access the appropriate section of the report by simply clicking on the topic itself.

2. This summary is for convenience only. It should be used in conjunction with the entire report for design purposes.

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Facilities | Environmental | Geotechnical | Materials 1

Introduction

This report presents the results of our subsurface exploration and Geotechnical Engineering services performed for the proposed Mill Creek Canyon Road improvements from Winter Gate to Big Water Trailhead located along the upper portion of UT FLAP SLA 10(1) in Salt Lake County, Utah. The purpose of these services was to provide information and geotechnical engineering recommendations relative to:

subsurface soil and rock conditions groundwater conditions

Seismic Site Class at White Bridge per AASHTO LRFD Bridge Design Specifications site preparation and earthwork recommendations dewatering and drainage considerations foundation design and construction lateral earth pressure retaining wall design and construction

The geotechnical engineering Scope of Services for this project included the advancement of 28 borings, completion of eight geophysical arrays, laboratory testing, engineering analysis, design, and preparation of this report. This report was initially submitted April 24, 2023, as a draft and has been updated per design changes to the 70% plans. We also previously provided a Pavement Report, dated January 6, 2023, as a deliverable for the V2 activity. We have also submitted a Summary of Interim Findings, dated November 7, 2022, as part of the G2 activity.

Drawings showing the site and exploration locations are provided in the Vicinity Map and Exploration Plan, respectively. The results of the laboratory testing performed on soil samples obtained from the site during our field exploration are included in the boring logs and as separate graphs in Appendix B. Geophysical array results are provided in Appendix A.

Project Description

Our understanding of the project is based on communications with the design team and the project plans, and is as follows:

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Item Description

Information provided

Scope of Work (SOW), Mill Creek Canyon Road, UT FLAP SLA 10(1), Mod P00002, dated August 11, 2021

Structure Selection Report for White Bridge Replacement, dated April 22, 2022

Draft Hydraulics Report, 30% Preliminary Design, dated April 1, 2022

50% Plans and Cross Sections, dated November 2022

50% Design Review/Trip Report, dated 12/9/2022

70% working plans and cross sections received February and April 2023.

70% Design Review/Trip Report, dated 7/10 7/13/2023

Project description

Reconstruct and widen approximately 4.5 miles of the roadway from the Winter Gate to the Big Water Trailhead to improve preserving natural resources. This section of roadway is closed to vehicular traffic from November 1 to early July. Widening will include replacing White Bridge, constructing retaining walls and cut slopes, reconstructing some embankments, and extending some box culverts.

Proposed structures

Project structures consist of three box culverts, box culvert extensions, and retaining walls. Structures are summarized in the following tables.

Embankments

Fills on the order of 10 feet tall or less are proposed at various locations along the alignment with slopes anticipated to be 1V:1H to 1V:3H. The steepened slopes (1V:1H) are located where the widening fills encroach on project limits or stream channels. Embankment locations are summarized in one of the following tables.

Cuts Cuts in soil and rock are anticipated. Soil cut slopes are anticipated to range from 1V:1.5H to 1V:4H. Cut locations are summarized in one of the following tables.

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Element Description Approximate stationing Notes

Culverts

Box culvert

White Bridge 18+00 Construct new box culvert

Elbow Fork

Vehicular1 74+20 New box culvert for vehicular access at proposed parking lot

Elbow Fork

Vehicular1 76+00 New box culvert for pedestrian access at proposed parking lot

Culvert extension

-- 69+75 Extend existing

-- 116+50 Extend existing

-- 219+00 Extend existing

-- 236+00 Extend existing

1. Added during development of 95% plans.

Element Description Approximate stationing

Max height

(ft)1

Top of slope

Bottom of slope

Walls

Soil nail2,4 53+00 to 55+75 22 1V:1.6H Level

Soil nail3 219+25 to 220+00 11 1V:1.7H Level

Rockery2,4 180+00 to 182+00 (Alexander Basin

Parking Lot) 9 1V:1.7H Level

Soil Nail 224+75 to 226+75 16 1V:1.5H Level

Rockery 0+20 (Big Water Trailhead Parking

Lot) 4 1V:2H Level

1. Height of the exposed wall face.

2. Added during development of 70% plans.

3. Added during development of 95% plans.

4. Geotechnical subsurface exploration not completed directly at this location due to design changes following completion of the geotechnical exploration phase.

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Element Approximate stationing Max height1 (ft) Top of slope

Bottom of slope

Cuts

23+502 14 1V:2H Level

25+00 to 26+252 19 1V:1.7H Level

35+00 to 35+252,3 10 1V:1.2H Level

40+00 to 40+502,3 3 1V:0.8H Level

47+25 to 48+752,3 16 1V:3H Level

56+50 to 59+002 23 1V:2.5H Level

82+25 to 82+75 7 1V:3H Level

85+00 to 85+40 13 1V:1.7H Level

101+00 to 103+252 24 1V:1.25H Level

105+75 to 107+252,3 7 1V:1.5H to

2H Level

110+00 to 112+002 29 1V:1.5H Level

121+50 to 122+00 11 1V:2H Level

126+00 to 129+502 17 1V:3H Level

136+00 to 138+002,3 14 1V:5H Level

152+75 to 155+752 10 1V:5H Level

170+00 to 171+752 14 1V:5H Level

182+50 to 186+502 21 1V:5H Level

215+25 to 218+502 30 1V:1.7H Level

220+00 to 221+752 26 1V:2.5H Level

222+75 to 224+50 11 1V:1.6H Level

229+00 to 233+50 19 1V:4H Level

237+75 to 251+00 28 1V:2.5H Level

251+00 to 253+002 37 1V:2H Level

256+00 to 257+75 12 1V:5H Level

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Element Approximate stationing Max height1 (ft) Top of slope

Bottom of slope

1. Height of the exposed wall face.

2. Added or revised during development of 95% plans.

3. Geotechnical subsurface not completed directly at this location due to design changes following completion of the geotechnical exploration phase.

Element Description Approximate stationing

Max height

(ft)4

Top of slope

Bottom of slope

Fills1

Armored slope2 24+80 to 25+23 10 Level Stream

Armored slope2 26+05 to 27+05 5 Level 1V:1.3H

Armored slope2,3 35+75 7 Level Stream

Armored slope2,3 37+00 to 37+25 4.5 Level 1V:2.5H

Armored slope2 75+80 to 76+00 4.5 Level 1V:1.6H

Armored slope

82+00 to 82+25 14.5 Level 1V:2H

Armored slope2 83+75 7.5 Level 1V:7H

Armored slope2 112+25 to 113+75 7.5 Level 1V:2H

Armored slope2 115+80 to 116+30 7.5 Level Level

Armored slope2,3 118+25 to 119+00 7 Level 1V:3H

Armored slope2,3 159+00 to 159+25 6.5 Level 1V:4H

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Element Description Approximate stationing

Max height

(ft)4

Top of slope

Bottom of slope

Armored slope2 161+00 to 162+25 6.5 Level 1V:2H

Armored slope2 164+75 to 166+50 7.5 Level Stream

Armored slope2 174+25 to 175+00 7 Level Stream

Armored slope2 177+68 to 177+85 6 Level 1V:2.5H

Armored slope2 178+50 to 178+73 7 Level 1V:1.3H

Armored slope2,3 191+00 to 191+25 5.5 Level Stream

Armored slope2,3 194+00 to 194+75 6.5 Level Stream

Armored slope2,3 211+50 to 212+25 5.5 Level Stream

Armored slope2 252+25 to 252+50 10.5 Level Level

Fill 234+00 to 237+50 8 -- Level to 1V:5H

Fill 250+75 to 254+50 9 Level Level

1. Fills less than 3 feet tall not included.

2. Added during design progression after completion of the field exploration phase.

3. Geotechnical subsurface exploration not completed directly at this location due to design changes following completion of the geotechnical exploration phase.

4. Vertical height, not including embedment.

Some walls, cut, and armored slopes were added during development of the 95% plans, after the geotechnical field program had been completed. In some instances, subsurface information from either the geotechnical or pavement field exploration programs was not available at these locations. Geotechnical subsurface parameters were estimated based on the nearest available exploration, geophysical array, existing site topography, visual observations, and engineering judgement. If additional explorations are not completed

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Facilities | Environmental | Geotechnical | Materials 7 during the design phase of the project, modification of geotechnical recommendations may be necessary once construction begins and subsurface conditions are exposed in these areas.

Terracon should be notified if any of the above information is inconsistent with the planned design or construction as modifications to our recommendations may be necessary.

Site Conditions

The following description of site conditions is derived from our site visits in association with the field explorations, site walks, and our review of publicly available geologic and topographic maps.

Item Description

Alignment information

Mill Creek Canyon Road is in, and maintained by, Salt Lake County. It provides access to the Uinta-Wasatch-Cache National Forest System (NFS) lands from 3800 South Wasatch Boulevard.

The project limits extend from the Winter Gate (near Maple Grove picnic area), approximately 4 miles up canyon from the Mill Creek Canyon fee booth to the end of the roadway at Big Water Trailhead, approximately 8.5 miles from the fee booth.

See Vicinity Map

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Item Description

Existing improvements

Two-lane (narrowing to one-lane in some sections) asphaltic concrete road with no shoulder to variable shoulder width traveling through the relatively narrow canyon located within the National Forest.

White Bridge is a single span open-bottom concrete culvert structure that crosses Mill Creek and was constructed around the 1920s. The structure provides an 11-foot-wide opening with a 30-degree skew to the roadway and a top width of around 20 feet. Other culvert and pipe crossings are located along the length of the alignment for crossing over Mill Creek and other tributary streams that flow from the surrounding canyon drainages. Several culverts have stone headwalls that may have historical significance. All the existing box culvert crossings were completed recently as part of a Forest Service project. Existing cut slopes are found along the alignment.

There are several trailheads with associated parking lots or pull-off areas (gravel and asphalt covered) and restroom facilities along the corridor alignment. Cobbles and boulders have been placed to delineate parking at multiple parking areas and along sections of the road shoulder.

Seasonal private residences are located approximately 2.7 miles up canyon from the Winter Gate.

Current ground cover

Asphaltic concrete pavement Canyon slope vegetation ranges from brush and grass to heavily vegetated with mature trees.

Existing topography

(from Mt. Aire, Utah Topographical map, 1998)

Beginning at the Winter Gate, the roadway climbs from an elevation of approximately 6,080 feet to 7,640 feet over a distance of 4.5 miles, resulting in an average gradient of 6.5 percent. Grades approach 10 percent on some stretches of the alignment. The surrounding canyon side slopes range from 25 to 90 percent.

Geology

The site is located in the Wasatch Mountains at the eastern margin of the Basin and Range physiographic province. The northeast portion of the Basin and Range province is situated north of the Colorado Plateau and is bounded by the Wasatch Mountains to the east. Formed during middle and late Tertiary time (1 million to 23 million years ago), the Basin and Range province is dominated by fault-controlled topography. The topography

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Facilities | Environmental | Geotechnical | Materials 9 consists of mountain ranges and relatively flat, broad alluvial valleys. These mountain ranges and valleys have evolved from generally complex movements and associated erosional and depositional processes. Structurally, the site lies within the Wasatch Front Mountain Range, immediately east of the Salt Lake Valley and the Wasatch Fault Zone.

The topography of the lower section of the canyon is the result of incisement from stream flow. The upper section of the canyon reflects the u-shape from glacial advancement.

Geological units mapped at the surface in the vicinity of the project alignment are summarized in the following table and shown on the Geology Exhibit.

Geological unit Description

Qm Morainal embankments consisting of glacial till found in the upper canyon.

TRt

Thaynes Formation (Lower Triassic), Brown stained fine-grained limy sandstone, interbedded with olive green to red shale and gray fine-grained limestone. Contains some massive limestone beds.

TRW

Woodside Shale (Lower Triassic) Dark red or purplish red shale

Ppc Park City Formation (Permian) Pale gray cherty limestone with some shale.

1. Crittenden, M.D. (1965). Geology of the Mount Aire quadrangle, Salt Lake County, Utah.

Utah Geological Survey, Geologic Quadrangle Map GQ-379.

Geotechnical Characterization

We have developed a general characterization of the subsurface conditions based on our review of the subsurface explorations, laboratory data, geologic setting, and our understanding of the project. A generalized summary of subsurface conditions encountered is presented below.

Layer name General description

Fill Sand and gravel with varying amounts of silt and clay and possible cobbles and boulders. Some asphalt or organics were encountered in some of the borings.

Granular

Sand, gravel, possible cobbles, and boulders, with varying amounts of silt and clay. Relative density ranging from very loose to very dense. Minimal minor clay layers with varying amounts of sand and gravel were encountered in a limited number of explorations.

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Bedrock

Variably weathered bedrock consisting of sandstone, and shale were encountered in cores completed in select explorations.

Limestone and shale bedrock outcrops were observed in several areas along the alignment.

Conditions observed at each exploration point are indicated in the individual logs and interpreted design profiles are provided in the global stability cross sections. The individual logs can be found in Appendix A.

Drilling exploration locations were limited to the existing roadway and shoulder and could not be completed in the retaining zone of proposed retaining walls or the cut zone of proposed cuts due to very steep slopes, thick undergrowth, heavily wooded areas, and the owner s direction not to disturb the canyon slopes with drill access roads.

Terracon utilized geophysical arrays to explore subsurface conditions on slopes above the roadways. The geophysical testing will be addressed in a subsequent section of this report. Additionally, we used site observations and analyses of existing slopes that appeared to be stable to infer subsurface conditions along the canyon slopes. Given these limitations on intrusive exploration within the cut zones and behind planned walls, variations in subsurface conditions from what we used may occur. If changes are encountered, modification to slope geometry and our recommendations may be necessary.

Bedrock

Estimated bedrock depths and observation notes are summarized in the table below.

Exploration1

Estimated bedrock depth

(elevations)2

(ft)

Description

SB-02 15

Inferred from sampler refusals and drill rig performance.

WB-02, A-01

Boring: 9.5 (6,551)

Array: 10 (6,555)

Inferred from sampler refusals and drill rig performance. Trace of limestone rock outcrop observed in adjacent slope below thick vegetation. Geophysical array A-01 located on the slope above the roadway suggests bedrock is 10 feet below ground surface along the survey line upslope from WB-02.

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Exploration1

Estimated bedrock depth

(elevations)2

(ft)

Description

WB-05, A-02

Boring: NE

Array: 10 (6,690)

Not encountered in the boring WB-05. Limestone rock outcrop observed upslope of exploration.

Geophysical array A-02 located on the slope above the roadway suggests bedrock is less than 10 feet below ground surface along the survey line upslope from WB-05.

WB-06, A-03

Boring: 7 (6,692)

Array: 15 to 20 (6,725 to 6,720)

Inferred from sampler refusals. Rock outcrop observed upslope of exploration. Geophysical array A-03 located on the slope above the roadway suggests bedrock is 15 to 20 feet below ground surface along the survey line upslope from WB-06.

WB-07, A-03

Boring: 20 (6,686)

Array: 15 to 20 (6,705 to 6,710)

Inferred from sampler refusals and drill rig performance. Geophysical array A-03 located on the slope above the roadway suggests bedrock is 15 to 20 feet below ground surface along the survey line upslope from WB-07.

WB-08 19 (6,701)

Inferred from sampler refusals and drill rig performance.

EB-01 18 (6,761)

Inferred from sampler refusals and drill rig performance.

WB-09 5 (7,007)

Inferred from sampler refusals and drill rig performance. Variably weathered siltstone rock outcrop observed nearby.

A-04 10 to 40 (7,070 to 7,115) Inferred from array wave speed measurements.

Rock outcrops not observed.

EB-04, A-05

Boring: 9 (7,322)

Array: NE

Inferred from drilling and confirmed with core. No outcrops observed on the south side of creek.

Limestone outcrops observed on the north side of the creek. Geophysical array A-05 located on the slope above the roadway did not encounter apparent bedrock along the entire array.

SB-05 8 (7,343)

Inferred from sampler refusals and drill rig performance.

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Exploration1

Estimated bedrock depth

(elevations)2

(ft)

Description

WB-12, A-06

Boring: 17 (7,347)

Array: NE

Inferred from sampler refusals and drill rig performance. Geophysical array A-06 located on the slope above the roadway did not encounter apparent bedrock along the survey line upslope of

WB-12.

WB-13, A-07

Boring: 7 (7,401)

Array: 15 to 40 (7,385 to 7,410)

Inferred from drilling and confirmed with core. No outcrops observed. Geophysical array A-07 located on the slope above the roadway encountered apparent bedrock along the survey line at depths ranging from 15 to 40 feet below the ground surface on the slope above WB-13.

Rock depth appeared to decrease east along the survey line.

EB-06 12 (7,520)

Inferred from sampler refusals and drill rig performance. No outcrops observed.

WB-14, A-08

Boring: 7 (7,574)

Array: 20 to 35 (7,550 to 7,600)

Inferred from sampler refusals and drill rig performance. Bedrock outcrop observed near toe of slope. Geophysical array A-08 located on the slope above the roadway encountered apparent bedrock along the survey line at depths ranging from 20 to 35 feet below the ground surface on the slope above WB-14.

1. Explorations shown based on project stationing. Explorations not encountering or adjacent to observed bedrock are not included.

2. The elevations are approximate. Bedrock elevations differ at locations away from nearby borings.

3. NE Not encountered.

A photo log of the rock cores collected is included in Appendix D.

Bedrock depth was observed to be variable between borings and along array lines, and difficult to predict based on surface conditions; explorations completed adjacent to an exposed bedrock outcrop sometimes did not encounter bedrock, while other explorations away from exposed outcrops encountered rock. The bedrock surface appears to be highly variable, dipping steeply in some locations or ending abruptly. Additionally, it was difficult to differentiate bedrock due to the cobbles and boulders present within the subsurface profile, the potential zone of variably weathered rock transitioning into more

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Facilities | Environmental | Geotechnical | Materials 13 competent rock at depth, and the downhole hammer drilling method used to advance the borings.

Geophysical

Terracon oversaw the completion of eight seismic P-wave (Vp) refraction profiles for the purpose of exploring subsurface conditions to estimate depth to bedrock on steep canyon slopes in sometimes heavily wooded areas. P-wave two-dimensional survey results are included Appendix A.

The observed P-wave velocities were used to group subsurface conditions into three general velocity zones:

Vp < 3,500 feet per second (fps) Overburden Sediment

VP between 3,500 fps to 5,000 fps Overburden Sediment and Potentially Weathered Rock

Vp > 5,000 fps Variably Weathered to Competent Bedrock

The geophysical surveys suggest the weathered rock and bedrock surface can be highly irregular along the survey lines located on the canyon slopes. The rock also appears to be highly irregular as it approaches the roadway or to be steeply dipping in various locations; shallow rock was inferred below the geophysical array on the slope but was not encountered in the boring completed at the toe of the slope.

Groundwater

The borings were advanced in the dry using an air rotary drilling technique that allows short-term groundwater observations to be made while drilling. Groundwater depths were noted at the time of drilling. The table below summarizes encountered groundwater at exploration locations.

Boring ID Groundwater depth (ft)/ elevation (ft) Boring ID

Groundwater depth (ft)

EB-01 7 / 6,772 WB-01A 13 / 6,175

EB-02 20 / 6,964 WB-02 4 / 6,556

EB-03 NE WB-03 9 / 6,564

EB-04 12 / 7319 WB-04 25 / 6,612

EB-05 25 / 7,456 WB-05 NE

EB-06 20 / 7,512 WB-06 NE

EB-07 12 / 7,638 WB-07 4 / 6,702

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Boring ID Groundwater depth (ft)/ elevation (ft) Boring ID

Groundwater depth (ft)

SB-01 15 / 6,127 WB-08 5 / 6,715

SB-02 7 / 6,510 WB-09 13 / 6999

SB-03 5 / 6,543 WB-10 7 / 7,059

SB-04 7 / 7627 WB-11 9 / 7,063

SB-05 7 / 7344 WB-12 7 / 7,357

SB-06 12 / 7,466 WB-13 5 / 7,403

WB-01 NE WB-14 5 / 7,576

Some springs or seeps were observed during our site visits at the following approximate locations:

Station 24+50, RT

Station 113+50, LT

Station 163+00, RT

Station 233+50 to 236+00, LT

Other springs or seeps may be found during additional site visits or during earthwork and wall construction. Groundwater may be encountered within cut slopes, particularly at the top of bedrock.

Groundwater level fluctuations and the presence and flow rate of springs may vary due to seasonal variations in the amount of rainfall and snowpack, runoff, creek water level, and other factors not evident at the time of our field visits or field exploration program.

Therefore, groundwater levels and the location and flow rate of springs during construction or at other times in the life of the project elements may vary from the levels indicated in the boring logs and observed during our field visits. Long-term groundwater monitoring was outside the scope of services for this project.

Geologic Hazards

Active Faults

The mapped segment of the Salt Lake City Segment of the Wasatch Fault Zone is approximately 5.2 miles (8.4 kilometers) west of the west terminus of the project alignment. Based on mapped fault, fault surface rupture along the project alignment is not expected; however, strong ground shaking and surface deformations will occur during a design seismic event.

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Landslides / Debris Flows

Based on a review of landslide and debris flow special study area maps (Christenson and Shaw, 20081,2), landslide and debris flow are not mapped along the alignment. Most of the alignment is shown to have slopes of 30% or greater. The Salt Lake County ordinance states that special landslide and debris flow studies are required for commercial or residential developments where slopes are 30% or greater. Although we did not visually observe any obvious evidence of active movement at the structure and cut locations, landslides or debris flows could occur and impact the project alignment.

Modifications to the ground surface such as slope cuts can increase these risks.

Rockfall

Existing rockfall hazards are present along the existing alignment in the canyon.

Documentation of significant rockfalls was not found during our review of public sources.

Evidence of raveling of soil and smaller cobble (i.e., up to 6-inch diameter rock) was observed during our project site visits. Rockfall analysis was not a part of this scope.

Terracon can provide a proposal for these services if requested.

Seismic Overview

Strong ground shaking is anticipated along the alignment during a seismic event.

Based on the subsurface information at White Bridge, Elbow Fork box culverts, and proposed wall locations, the Site Class according to AASHTO guidelines, defined by the upper 100 feet of the subsurface profile, was determined. Where exploration depths did not extend to 100 feet, engineering judgement was used to select the Site Class. The Seismic Site Class at each location is summarized below.

1 Christenson, G. E., & Shaw, L. M. (2008). Landslide special study areas, Wasatch Front and nearby areas, Utah. Utah Geological Survey, Circular 106, scale 1:200,000.

2 Christenson, G. E., & Shaw, L. M. (2008). Debris-flow/alluvial-fan special study areas, Wasatch Front and nearby areas, Utah. Utah Geological Survey, Circular 106, scale 1:200,000.

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Location AASHTO Seismic Site Class

White Bridge: 18+00 C

Soil Nail: 53+00 to 55+75 D

Rock Armored Slope: 81+80 to 82+20 D

Soil Nail: 219+30 to 220+00 C

Soil Nail: 224+75 to 226+75 C

Rockery Wall: 0+20 (Big Water Trailhead Parking Lot) E

Peak ground acceleration (PGA) values were also determined for these structure locations for a design seismic event with a 7 percent probability of exceedance (PE) in 75 years using the Unified Hazard Tool.3 Factored PGA values (As) are provided in the geotechnical design parameters table for each structure.

Liquefaction

A liquefaction analysis was completed on boring SB-01 at White Bridge using deterministic calculation spreadsheet and considering a design seismic event with 7% PE in 75 years. The groundwater level was based on measurements during drilling.

Based on our analysis, liquefaction triggering of site soils did not occur. We estimate liquefaction or seismic induced settlements manifested at the ground surface to be on the order of 1 inch or less during the design seismic event.

Corrosivity

Analytical tests are summarized in Appendix B. Test results completed on site soils suggest soils are corrosive to soil nails (FHWA GEC #7 Soil Nail Wall Reference Manual, FHWA-NHI-14-007). Corrosion to concrete appears to be minimal.

3 https://earthquake.usgs.gov/hazards/interactive/

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Geotechnical Overview

Variable subsurface conditions are anticipated along the alignment. Subsurface conditions at explored structure, cut, and embankment locations ranged from very loose to very dense sands and gravels with varying amounts of silt and clay, as well as varying amounts of cobbles and boulders. Bedrock was also encountered in some of the borings as shallow as 5 feet below existing site grade. Variability in the amount of cobbles and boulders encountered was observed vertically within individual borings, as well as between adjacent borings. The presence and depth of bedrock appeared to vary significantly between and away from the exploration locations. Rock weathering and strength also appeared to vary as observed in the cores, exposed rock outcrops, and

Bedrock outcrops were observed at several locations along the project alignment where cuts are planned. These existing outcrops, and other rock outcrops exposed during earthwork activities, should be mapped to determine joints and discontinuities, and rock slope stability analyses should be completed. The rock slope stability analyses may revise cut slope geometry or the use of rock bolts or other mitigation measures.

Terracon should observe and document cut slopes in rock during construction so additional recommendations can be provided as necessary.

Explorations were not completed in the soil nail zone or retaining zone behind proposed walls and in proposed cuts due to access limitations as discussed earlier. However, it is anticipated that variable subsurface conditions may be encountered such as changes in soil type and density, boulders and cobbles randomly occurring in the soil matrix, and irregular bedrock surface. Cuts and wall excavations may encounter differing materials, including soil and rock within the same excavation or wall limits.

Difficult excavating conditions through variable subsurface conditions such as interchanging soil, cobbles, boulders, and bedrock may be encountered during construction. Excavation into or through boulders or into bedrock may require heavy-duty or specialized equipment or potentially drilling and blasting. Some additional effort may be necessary to extract boulder sized materials, particularly in deep narrow excavations such as culvert or drainage trenches. Boulders or bedrock may extend into the cut limits and require partial removal. Equipment, drilling methods, and bit types should be selected for the subsurface conditions; however, these conditions may change along the length of individual soil nails, cuts, walls, or other project elements.

Groundwater was encountered in the borings at various depths. Several springs or seeps were noted along the toe of the existing slope and along the edge of the existing road during our field visits. Groundwater and spring flowrates and locations may vary with time, site precipitation levels, and with construction along the corridor. We anticipate that additional seeps or springs will be encountered during earthwork grading or wall construction. Zones of perched groundwater may be encountered at times in the

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Facilities | Environmental | Geotechnical | Materials 18 subsurface soils overlying bedrock, on top of the bedrock surface, or within permeable fractures in the bedrock materials.

Saturated soils may result in unstable subgrade, difficulty in compacting embankment or pavement section material, or difficulty maintaining stable temporary or permanent cuts at wall or cut slope locations. Collecting spring/seep water in drainage pipes, supporting temporary excavations, and flattening cut slopes may be required. Collecting and removing water will need to be incorporated into the design and construction activities.

Culvert extensions may require dewatering or stream diversion to complete the widening activities. Additionally, stream diversion and rerouting may be necessary at embankment slope armoring or repair locations where the creek has eroded the roadway embankment.

Varying amounts of undocumented fill were encountered in the explorations and additional fill may be discovered during construction along the project alignment. The fill may have been placed in an uncontrolled manner and be susceptible to settlement, become unstable and pump under construction traffic, or provide limited shear strength.

Stabilization or bridging of fill using angular rock and geotextiles may be required, or the fill should be fully removed and replaced.

Earthwork

Earthwork activities, including site preparation, fill material types, compaction requirements, and grading and drainage, should be completed in accordance with the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-14). The following presents general recommendations for site subgrade preparation and placement of fill considering these guidelines.

Site and Subgrade Preparation

Follow FP-14. We recommend that the geotechnical engineer should be retained to observe subgrade soils.

Excavation

Permanent and temporary excavations will encounter variable conditions ranging from loose saturated granular soils to very dense soils with cobbles and boulders, to variably weathered to competent bedrock. Varying conditions may be encountered within the same excavation (cuts transition from soil to rock to soil, etc.). We anticipate that excavations for the proposed project will require specialized heavy-duty construction equipment suited for working with cobbles, boulders, and bedrock. Bedrock p-wave

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Recommended cut slopes are presented in the following table.

Cut type Recommended slopes Description

Soil 1V:1.5H

Recommended cut slope in soil except as noted below:

1V:1.75H o 121+50 to 122+40

1V:2H o 222+75 to 224+50 o 229+00 to 233+50 o 237+50 to 250+50

1V:3H o Where seepage or sloughing soils are encountered o 229+00 to 234+00

Rock 2V:1H

Rock cut slopes proposed at:

101+50 to 103+00

110+40 to 112+00

251+00 to 253+00

Recommended rock cut slopes are valid for intact competent rock that has been reviewed and mapped by Terracon, confirming joints and discontinuities and the resulting failure mechanisms and factors of safety. Additional areas should be surveyed if rock outcrops are observed or encountered during construction.

Highly weathered rock, bedrock with soil layers/infill, rock with unfavorable joints and discontinuities, or saturated zones may require flatter slopes or other mitigative measures.

Soil Stabilization

Soft, deflecting, or otherwise disturbed subgrade should be removed and replaced with embankment material as indicated in FP-14 Section 204. If pumping or unstable conditions persist, then the use of Stabilization Fill (crushed angular rock or recycled crushed concrete) or Crushed Aggregate (703.06) in combination with a geotextile may be necessary. Placement of geosynthetic fabrics should follow Section 207 of FP-14.

Chemical stabilization as presented in FP-14 Section 213 is not recommended due to project subgrade soils containing variable amounts of gravel, cobbles, and boulders.

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Further evaluation of the need and recommendations for subgrade stabilization can be provided during construction as the subgrade conditions are exposed.

Fills

Design fill slopes should be placed at 1V:3H or flatter, however slopes may be constructed up to 1V:2H if using material that meets the requirements of Unclassified Borrow (704.06). Slopes steeper than 1V:2H will require reinforcement or that a portion of the fill slope be constructed with select material. Rock armored slopes constructed with 1V:1H slopes should be constructed using select material and designed to resist sliding, overturning, and lateral pressures, similar to a rockery wall.

Fill Material Types

Material used should meet FP-14 Section 704. Stabilization Fill, if required, should meet the following requirements.

Fill type Application Requirements

Stabilization Fill Fill used to stabilize loose, soft, saturated subgrade

4-inch max

Less than 5% passing No. 200 Sieve

Angular

Fill Placement and Compaction Requirements

Place and compact fill in accordance with FP-14 Sections 204 and 209. This includes placement of fill on existing slopes steeper than 1V:3H.

Shrink/Swell Factors

Approximate shrink/swell factors indicating the amount of estimated volume change occurring in a material from its in-situ condition to its placed and compacted condition in an embankment are presented below. Values were selected from the 2007 FHWA Geotechnical Technical Guidance Manual, Exhibit 4.6-F, and soil and rock types encountered along the project alignment in the completed explorations. The shrink/swell values vary considerably depending on the material type, moisture content, and construction methods used to excavate and place the material. The referenced FHWA table indicates factors are subject to +33% variation. Other factors that can influence the apparent shrink/swell factors include poor survey control (pre- and post-earthwork), improper section construction, poor cut/fill slope control, improper compaction, missed design quantities, and haulage losses. The factors presented below are to estimate the difference in density between cut and embankment of the listed materials, and do not

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Facilities | Environmental | Geotechnical | Materials 21 attempt to account for the other factors listed (e.g., survey, waste, or construction practices).

Material Shrink/Swell1,2

Granular soils with variable amounts of silts and clays

-20 to 5

Weathered/decomposed rock and soil mixtures -10 to 15

Bedrock: limestone 25 to 50

Bedrock: shale 35 to 65

Bedrock: sandstone 25 to 50

1. Shrink/Swell factors taken from FHWA Geotechnical Technical Guidance Manual, 2007, Exhibit 4.6-F. Values are based on average in-situ densities. Factors presented represent approximate ranges.

2. A negative number represents shrinkage.

Grading and Drainage

Water from surface runoff and springs/seeps should be captured and diverted away from project structures, cut and fill slopes, and subgrade during construction and over the design life of the project. The near-surface soils could become unstable with typical earthwork and construction traffic, especially when wet. If possible, the grading should be performed during the warmer and drier times of the year. If grading is performed during the times of precipitation, an increased risk for needing possible undercutting and replacement of unstable subgrade will persist.

Saturated soils should be removed prior to placing fill, structural elements, pavement section materials, or proceeding with construction.

Pavement subgrade surface should be sloped appropriately to the outside edges to facilitate drainage of water from below the pavement section. Ditches or other drainage features should be constructed adjacent to the roadway on each side to prevent surface water runoff from migrating below pavements. The pavement surface should have adequate crown or cross slope to allow water to rapidly run off to adjacent drainage features.

Underdrains or pavement edge drains based on Eastern Federal Lands (EFL) Standard Detail E605-01 should be placed wherever seeps or springs are encountered adjacent to the roadway. At a minimum, pavement edge drains should be planned in the following areas:

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Station 22+00 to 27+00, RT

Station 113+00 to 114+00, LT

Station 162+50 to 163+50, RT

Station 220+00 to 221+00, LT

Station 231+20 to 236+60, LT

Additional underdrain areas may be required outside of these limits. The plans should be referenced for design underdrain locations and extents.

The locations and extents of pavement edge drains should be verified in the field during earthwork and grading activities. The limits of the underdrains may change, and additional drains may be required based on encountered site conditions.

Seeps or springs may be encountered in soil and rock cuts, particularly at the soil to bedrock interface. Underdrains on cut slopes (EFL Standard Detail E605-03) should be installed to collect and divert this water away from slopes.

Retaining wall drainage is addressed in the Lateral Earth Pressures section.

Dewatering

Dewatering or stream diversions will be required at White Bridge, Elbow Fork box culverts, and the proposed culvert extensions. The water level should be maintained a minimum of 12 inches below the working surface during construction. Dewatering plans and specifications should be developed and stamped by a Professional Engineer registered in the State of Utah.

Earthwork Construction Considerations

Upon completion of filling and grading, care should be taken to maintain the subgrade water content prior to the construction of embankments, pavements, or retaining walls bearing on subgrade soils. Construction traffic over the completed subgrades should be avoided or the subgrade should be protected using temporary overbuilt haul roads. The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over or adjacent to construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the affected material should be removed or scarified, moisture conditioned, and recompacted prior to continued construction.

As a minimum, excavations should be performed in accordance with OSHA 29 CFR, Part local and/or state regulations.

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Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations. Under no circumstances shall the information provided herein be interpreted to mean Terracon is assuming responsibility for construction site safety or the contractor s activities; such responsibility shall neither be implied nor inferred.

Excavations or grading activities have the potential to affect surrounding slopes.

Excavation or grading activities should be monitored or instrumented for potential ground movements. Terracon can assist in developing a geotechnical instrumentation monitoring plan if requested.

Construction Observation and Testing

The grading efforts should be observed by the Geotechnical Engineer (or others under their direction). Observation should include documentation of adequate removal of surficial materials (vegetation, topsoil, and pavements) and evaluation and remediation of existing fill materials and subgrade soils. Soil and rock cuts should be observed and documented to verify design assumptions and corresponding stability calculations. Rock outcrops should be mapped and surveyed so a rockfall slope stability analysis can be completed if necessary. Additionally, observations should include documenting seeps and springs, and locating or revising underdrains.

In areas of foundation excavations, the bearing subgrade should be evaluated by the Geotechnical Engineer. Terracon should be notified if soil nail, cut slope, or rock armored slope subsurface conditions vary from those used during design. If unanticipated conditions are observed, the Geotechnical Engineer and/or wall or slope designer should assist to develop mitigation options.

In addition to the documentation of the essential parameters necessary for construction, the continuation of the Geotechnical Engineer into the construction phase of subsurface conditions, including assessing variations and associated design changes.

White Bridge, Elbow Fork Culverts, & Culvert Extensions

If the site has been prepared in accordance with the requirements noted in Earthwork, the following design parameters are applicable for shallow foundations.

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Design Parameters

Item Description

Location White Bridge Elbow Fork Wing Walls

Required bearing stratum1 Minimum of 12 inches of Foundation Fill (704.01)

Nominal & extreme limit bearing…