UT.FLAP.2500(1).Taylor Mountain Road.Geotech Rpt FINAL.March 2021.pdf

TAYLOR MOUNTAIN ROAD

UT FLAP 2500(1)

UINTAH COUNTY, UT

FINAL Geotechnical Report Report No. UT-CX-10044-20-01

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch March 2021 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

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

Report reviewed by: ____________________________________________________________

James M. Arthurs, P.E., Ph.D., Geotechnical Engineer

Approved for distribution by: ____________________________________________________

Marilyn D. Dodson, P.E., Lead Geotechnical Engineer

Distribution

Electronic:

N:\UT\ut2500(1)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Designer

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TABLE OF CONTENTS

SECTION ONE - INTRODUCTION

1.1 BACKGROUND AND LOCATION

1.2 SCOPE AND PURPOSE

SECTION TWO - GEOLOGY AND SEISMICITY

2.1 REGIONAL GEOLOGY

2.2 SITE GEOLOGY

2.3 REGIONAL SEISMICITY

2.4 SEISMIC DESIGN PARAMETERS

2.5 GEOLOGIC HAZARDS

SECTION THREE - SUBSURFACE INVESTIGATION

3.1 SUBSURFACE EXPLORATION PROGRAM

3.2 LABORATORY TESTING PROGRAM

3.3 SUMMARY OF SITE CONDITIONS

3.3.1 General Topography

3.3.2 Surface Reconnaissance

3.3.3 Subsurface Conditions

3.3.4 Groundwater

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 EARTHWORK AND DRAINAGE

4.1.1 Subexcavation

4.1.2 Underdrains

4.1.3 Subgrade Soil Support Recommendations for Concrete Headwalls

4.1.4 Ditch Recommendations

4.1.5 Shrink/Swell Recommendations

4.1.6 Central Drainage Improvements

4.2 EMBANKMENT STABILIZATION

4.2.1 Deep Patch Design

4.2.2 Roadway Maintenance

4.3 GEOSYNTHETICS

4.4 CONSTRUCTION CONSIDERATIONS

4.5 SPECIFICATIONS

4.6 DISCLAIMER/LIMITATIONS CLAUSE

SECTION FIVE - REFERENCES

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TABLES

TABLE 2.1 Summary of Nearby Seismic Source Faults TABLE 2.2 Summary of Seismic Parameters Corrected for Site Class D TABLE 3.1 Summary of the Field Exploration Program TABLE 3.2 Summaries of Laboratory Index Test Results TABLE 3.3 Summaries of Laboratory Corrosivity Results TABLE 4.1 Static Lateral Earth Pressures for Concrete Headwalls TABLE 4.2 Estimated Material Properties of Native and Backfill Soils for Deep Patch

Design

FIGURES

FIGURE 1 Typical Roadway Conditions FIGURE 2 Approximate Location of Embankment Distress Near STA 185+00 FIGURE 3 Cross Section of Partial Width Deep Patch (Conceptual) FIGURE 4 Cross Section of Full Width Deep Patch (Conceptual)

PLATES

PLATE 1 Project Vicinity Map PLATE 2 Geology Map PLATE 3 Pavement and Geotechnical Boring Locations PLATE 4 Bearing Resistance for Concrete Headwalls at STA 185+11

APPENDICES

APPENDIX A Field Exploration Program APPENDIX B Laboratory Test Results APPENDIX C Deep Patch Analysis APPENDIX D Photographs (70 Percent Site Visit) APPENDIX E Excavation Characteristics of Rock & Rippability Charts

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Taylor Mountain Road, UT FLAP 2500(1) March 2021

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering assessment and provides recommendations for rehabilitation and improvements to Taylor Mountain Road, UT FLAP 2500(1) project. Located north of Vernal, UT, the project route begins at the intersection of W 3500 N and Taylor Mountain Road and proceeds north, terminating at a cattle guard at the USFS boundary. This route provides access to the Ashley National Forest, Bureau of Land Management, State Trust Lands, and various private properties. Typical roadway conditions are shown in Figure 1 and a project vicinity map is presented on Plate 1 of this report.

Figure 1 - Typical Roadway Conditions

Roadway, pavement, and drainage conditions of Taylor Mountain Road currently exist as poor to fair. The pavement shows a range of distress; exhibiting longitudinal, transverse, and fatigue cracking along the entire route, with the worst areas of distress within the last two miles of the project. Poor drainage conditions along the roadway have led to ditch erosion, head cutting at culverts, and insufficient ditches in multiple locations.

The proposed project involves rebuilding the existing roadway to a two-lane, two-way roadway with 11-foot lanes and 1-foot shoulders. Drainage improvements including updates to the existing culverts and the addition of new culverts are proposed by the hydraulics engineer. Geotechnical-related improvements will include subexcavation with stabilization geogrid, an underdrain system, and a deep patch embankment repair. These improvements have the potential to reduce roadway maintenance costs, improve surface drainage, and mitigate pavement cracking.

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1.2 SCOPE AND PURPOSE

The scope of work included a geotechnical investigation performed by the CFL Pavement and Materials group, analysis, and recommendations for subexcavation, underdrain, culvert foundations, roadway maintenance, and a deep patch embankment repair. This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, interpretation and correlation of field measurements, and geotechnical engineering analysis. Specifically, this investigation was conducted to explore the subsurface conditions in the areas of pavement cracking and to develop recommendations concerning surface/subsurface drainage, corrosion potential, geological hazards, material shrink/swell, and construction considerations for the design and construction of drainage features and embankment repairs within the alignment.

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SECTION TWO - Geology and Seismicity

2.1 REGIONAL GEOLOGY

Taylor Mountain Road is located in northeastern Utah within the Uinta Mountain Range. The Uinta Mountains are an east-west trending range of the Southern Rocky Mountains, occupying a large area of northern Utah and extending into southern Wyoming. Geology of the site has been published by the Utah Geologic Survey (Haddox et al., 2010). The alignment and surrounding area is largely underlain by sedimentary rocks of various origins, ranging in age from 360 Ma to present time. The oldest rocks in the area were deposited during the Paleozoic Era (542 to 251 Ma) and generally consist of dolostone, shale, and sandstone; representing marine, marginal marine, and fluvial environments. Progressing into the Mesozoic Era (252 to 66 Ma), in addition to marine and fluvial environments, eolian and lacustrine depositional settings become common. With this, geologic units generally consist of mudstone, limestone, sandstone, and windblown silts/sands.

Lastly, Quaternary deposits overlie a large portion of the mapped area. These deposits largely consist of eolian, alluvial, and colluvial deposits, but mass movement, spring, and mixed deposits exist to a lesser extent as well. These deposits can generally be described as unconsolidated to poorly consolidated, poorly to moderately sorted, with grain sizes ranging from silt to gravel, with some cobbles and boulders.

2.2 SITE GEOLOGY

The Taylor Mountain Road project area begins north of Vernal, UT, terminating further north at the Ashley National Forest. The alignment begins in a mix of alluvial and eolian sand deposits that may generally be described as unconsolidated, well-sorted, fine-grained sand deposits. These sands were deposited by alluvial processes and later reworked by wind in subdued drainages and small depressions. Continuing north, the alignment crosses in and out of eolian, colluvial, and mixed eolian/colluvial deposits. These deposits generally consist of unconsolidated, well-sorted, fine-grained sands or silts, although some coarser materials may exist. As the alignment continues further north towards Ashley National Forest and gains elevation, the quaternary deposits transition to general alluvium and piedmont alluvium deposits. Having similar qualities, these deposits may be described as unconsolidated to poorly consolidated, poorly- to moderately- sorted sand, gravel, cobbles, and boulders. Some areas may have poorly developed soil profiles with calcic soil (caliche) coatings on the undersides of clasts.

These overburden deposits are underlain by the Nugget Sandstone and Park City Formation. The Nugget Sandstone is found in the lower section of the alignment, and the Park City Formation exists further north towards the Ashley National Forest. The Nugget Sandstone is light brown to light pinkish gray and medium to fine grained. The sandstone forms cliffs and ledges, and weathers into modern eolian deposits. The Park City Formation is a dolostone interbedded with siltstone

Page | 4 and shale. Dolostone beds are light gray and brown, glauconitic, sandy, and form prominent ledges.

The siltstone is reddish-orange and forms slopes.

Other zones about halfway along project route are underlain by Triassic-age units including the Formation of Bell Springs, Upper Member of the Chinle Formation, Gartra Member of the Chinle Formation, Moenkopi Formation, and the Dinwoody Formation. The Formation of Bell Springs is composed of sandstone, siltstone, and mudstone and forms ledges and slopes beneath the aforementioned Nugget Sandstone. The Upper Member of Chinle Formation also includes sandstone, siltstone, and mudstone but is interbedded with conglomerate and carbonate lenses. The Gartra Member of Chinle Formation contains more conglomerate and sandstone and is fluvially deposited into the underlying Moenkopi Formation. The Moenkopi Formation is then comprised of siltstone and sandstone interbedded with gypsum and is considered marginal marine to fluvial.

Lastly, the Dinwoody Formation includes shale and gypsum interbedded with sandstone, minor amounts of limestone, and some chert. This formation is considered to be marine.

A geologic map of the project area is presented in Plate 2.

2.3 REGIONAL SEISMICITY

Numerous seismic source faults are mapped within 40 miles of the project site. The critical active faults to the project site may be found in Table 2.1 (U.S. Geological Survey, 2019). These fault systems are located northeast and northwest of the project site. All fault systems are normal faults with slip rates less than .008 inches per year.

Table 2.1 - Summary of Nearby Seismic Source Faults

FAULT ZONE

DISTANCE

FROM CENTER

OF PROJECT

SLIP-RATE

CATEGORY

FAULT

LENGTH AGE

(miles) (inch/year) (miles) (years) Diamond Gulch Faults 17 <0.00787 12.4 <1,600,000

Pot Creek Faults 23.5 <0.00787 8.1 <1,600,000 Faults on North Flank of Phil Pico Mountain 38.6 <0.00787 2.5 <130,000

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the Taylor Mountain Road site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 7th Edition, 2016, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 100-year return period). The 1000-year return period uniform hazard spectrum for the Taylor Mountain Road project site, located at 40.506469°N & -109.577272°W, was obtained in accordance with the AASHTO ground motion maps.

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Based on the material encountered and tested during drilling, the average Standard Penetration Test (SPT) blow count (blows per foot) (ASTM D1586) for the top 100 feet of the soil profile was determined to be between 15 and 50 blows per foot. Therefore, the site is classified as Class D according to the site class definitions specified in Table 3.10.3.1-1 of AASHTO. The recommended spectral acceleration coefficient values for probabilistic design are summarized in Table 2.2.

Table 2.1 - Summary of Seismic Parameters Corrected for Site Class D Horizontal Peak Ground Acceleration, (PGA) 0.098 g

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (Ss) 0.194 g Horizontal Response Spectral Acceleration at Period of 1.0 sec, (S1) 0.061 g

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.6 Site Factor at Short-Period Range of Acceleration Spectrum, (Fa) 1.6 Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 2.4

Factored Horizontal Peak Ground Acceleration, (As) 0.158 g Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDs) 0.311 g Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec, (SD1) 0.147 g

Seismic Zone Zone 1

Based on the long acceleration coefficient SD1 value of 0.147, the project site is assigned to seismic hazard “Zone 1” in accordance with Table 3.10.6-1 of AASHTO.

2.5 GEOLOGIC HAZARDS

Geologic hazards exist from both the natural environment of the project site and from existing and proposed construction of the roadway. The main geologic hazards that may exist within the vicinity of the project limits include: flash floods, debris flows, frost heave of silts and highly-erodible, loose granular soils. Debris flow and sediment deposits may be observed at existing culvert inlets, causing varying degrees of burial and blockage. The widespread loose, granular soils may be susceptible to erosion and should be stabilized as necessary during construction. Related information regarding the local climate can be found in Section II of the Final Pavement and Materials Report dated November 2020.

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SECTION THREE - Subsurface Investigation

3.1 SUBSURFACE EXPLORATION PROGRAM

A subsurface investigation was performed by a Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) Pavement and Materials engineer on August 8, 2019. The geotechnical subsurface exploration program consisted of drilling a total of sixteen (16) borings; consisting of fourteen (14) shallow (5-foot) pavement borings and four (4) deeper geotechnical borings (two borings served the purpose of both geotechnical and pavement borings). The purpose of the geotechnical borings was to investigate the subsurface along the project alignment and at proposed deep patch locations. Standard penetration testing (SPT) and sample collection were performed at 5-foot intervals in the geotechnical borings. Additionally, bulk grab samples were collected from the top 5 feet in the pavement borings. Boring locations are presented on Plate 3. Subsurface conditions were logged and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing. Logs of the explorations and laboratory test results are presented in Appendix A and B, respectively. Additional information and photographs related to this exploration can be found in the Final Pavement and Materials Report. A summary of the field exploration is provided below in Table 3.1.

Table 3.1 - Summary of Field Exploration Program

EXPLORATION

DESIGNATION LOCATION PURPOSE

APPROXIMATE

GROUND

ELEVATION (ft.)

TERMINATION

DEPTH (ft.)

P-1 STA 38+78,

4 ft. LT of centerline

Pavement Investigation 5,814 5

P-2 STA 59+90,

4 ft. RT of centerline

Pavement Investigation 5,901 5

P-3 STA 86+30,

4 ft. LT of centerline

Pavement Investigation 6,086 5

P-4 STA 112+70,

4 ft. RT of centerline

Pavement Investigation 6,157 5

P-5 STA 139+10,

4 ft. LT of centerline

Pavement Investigation 6,040 2

P-6 STA 165+50,

4 ft. RT of centerline

Pavement Investigation 6,016 5

P-7 STA 176+06,

4 ft. LT of centerline

Pavement Investigation 6,032 5

BH19-03 STA 184+84,

8 ft. LT of centerline

Culvert Area & Deep Patch

Investigation 6,030 35.08

BH19-02 STA 185+28,

2 ft. RT of centerline

Culvert Area & Deep Patch

Investigation 6,032 35.08

BH19-04/P-8 STA 191+90,

11 ft. LT of centerline

Pavement Distress & Severe Shoulder

Cracking 6,069 11.5

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EXPLORATION

DESIGNATION LOCATION PURPOSE

APPROXIMATE

GROUND

ELEVATION (ft.)

TERMINATION

DEPTH (ft.)

BH19-01/P-9 STA 223+58,

4 ft. LT of centerline

Pavement Distress & Severe Shoulder

Cracking 6,152 11.5

P-10 STA 239+42,

4 ft. RT of centerline

Pavement Investigation 6,195 5

P-11 STA 271+10,

4 ft. LT of centerline

Pavement Investigation 6,301 5

P-12 STA 297+50,

4 ft. RT of centerline

Pavement Investigation 6,386 4

P-13 STA 323+90,

4 ft. LT of centerline

Pavement Investigation 6,605 4

P-14

STA 383+00,

25 ft. LT of centerline (Parking Area)

Pavement Investigation 7,090 1

Note: The exploration locations were estimated relative to existing features. Ground elevations were estimated from Google Earth.

3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the borings by SPT were tested in the laboratory to support the field classifications and to provide an estimate of the engineering characteristics and mechanical properties of the soil. Laboratory tests included moisture content (AASHTO T255), sieve analysis (AASHTO T 11 and T27), classification (AASHTO M145), R-value (AASHTO T 190), Atterberg limits (AASHTO T89 and T90), resistivity (AASHTO T 288), and soil pH (AASHTO T 289).

When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. Results of the testing are summarized below in Table 3.2 and are presented in Appendix B.

Table 3.2 - Summaries of Laboratory Index Test Results

BORING

NUMBER

SAMPLE

DEPTH

PERCENT

GRAVEL

PERCENT

SAND

PERCENT

PASSING

#200

LIQUID

LIMIT

PLASTIC

LIMIT

USCS

CLASS.

AASHTO

CLASS.

(feet)

P-1 0 - 5 7 70 23 NV NP SM A-2-4(0)

P-2 0 - 5 3 73 24 NV NP SM A-2-4(0)

P-3 0 - 5 2 74 24 NV NP SM A-2-4(0)

P-4 0 - 5 1 77 22 NV NP SM A-2-4(0)

P-6 0 - 5 7 23 70 24 16 CL A-4(3)

P-7 0 - 5 11 38 51 NV NP ML A-4(0)

P-10 0 - 5 16 41 43 NV NP SM A-4(0)

P-11 0 - 5 5 48 47 28 21 SC-SM A-4(1)

P-12 0 - 4 5 77 18 21 15 SC-SM A-1-b(0)

P-13 0 - 4 32 36 32 21 19 SM A-2-4(0)

P-14 0 - 1 53 33 14 19 15 GC-GM A-1-a(0)

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BH19-01/P-9 2.5 - 4 3 12 85 39 29 ML A-4(10)

BH19-01/P-9 5 - 6.5 0 9 91 67 52 MH A-7-5(22)

BH19-01/P-9 10 - 11.5 0 24 76 63 35 MH A-7-5(24)

BH19-02

5 - 6.5 & 10 - 11.5

0 73 27 NV NP SM A-2-4(0)

BH19-02 15 - 16.5 38 43 19 NV NP SM A-1-b(0)

BH19-02

20 - 21.5 &

25 - 26.5

9 43 48 36 20 SC A-6(4)

BH19-03

2.5 - 4, 5 -

6.5, & 10 - 11.5

3 33 64 21 15 CL-ML A-4(1)

BH19-03 15 - 16.5 34 42 24 NV NP SM A-1-b(0)

BH19-03

20 - 21.5 &

25 - 26.5 8 32 60 43 21 CL A-7-6(11)

BH19-04/P-8

2.5 - 4, 5 -

6.5, & 10 - 11.5

1 37 62 21 15 CL-ML A-4(1)

Soil test results indicated a range of material types including silty to clayey gravels and sands to clays, classifying as A-1-a(0) to A-7-6(11) by AASHTO and GC-GM, SM, SC, SC-SM, ML, MH, CL-ML, and CL by USCS.

Corrosivity and chemistry testing of select samples were conducted for the pavements investigation and are summarized in Table 3.3.

Table 3.3 - Summaries of Laboratory Corrosivity Results

BORING

NUMBER

SAMPLE

DEPTH

RESISTIVITY PH SULFATE ION

CONTENT

CHLORIDE ION

CONTENT

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

P-3 0 - 5 862 8.7 1,570 145

P-7 0 - 5 862 8.6 7,080 57

P-11 0 - 5 729 8.4 13,300 151

P-14 0 - 5 1,591 8.6 270 26

BH19-02

20 - 21.5 &

25 - 26.5

- 8.2 - -

BH19-03

20 - 21.5 &

25 - 26.5 - 8.3 - -

- Tests not completed due to insufficient amount of material provided.

3.3 SUMMARY OF SITE CONDITIONS

A field review of the site conditions was performed in conjunction with the marking of the aforementioned boring locations by a Central Federal Lands Highway Division (CFLHD) geotechnical engineer on June 26, 2019. The existing roadway consists of oxidized asphalt

Page | 9 pavement ranging in width from 16 to 28 feet with high severity fatigue cracking through the project. The condition of existing drainage features ranges from fair to poor with substantial ditch erosion, head cutting at culverts, and insufficient ditches in select locations. This section presents the results of the surface reconnaissance and subsurface exploration.

3.3.1 General Topography

The project is located within the Middle Rocky Mountains physiographic province. The Utah portion of this province includes the north-south trending Wasatch and east-west trending Uinta mountain ranges. The general topography of the project site is characterized by features typical of this geographic region that include surrounding folded mountains and desert terrain (NPS, 2018).

The landscape leading through Taylor Mountain Road can be described as rolling. Rock outcroppings are present along the route. Two major streams are located within the project: Spring Creek and an unnamed tributary to Spring Creek. These streams cross Taylor Mountain Road near STA 185+11 and STA 176+90, respectively. The project area is vegetated in grasses and brush with few juniper trees. Land adjacent to the roadway is primarily undeveloped with a few isolated residences.

3.3.2 Surface Reconnaissance

The project begins near the intersection of Taylor Mountain Road and W 3500 N near the Ashley Valley Water Treatment Plant. The road proceeds uphill at approximately a 3 to 4 percent grade and then terminates at the cattle guard at the Forest Service boundary. Fatigue cracking was observed on the asphalt pavement throughout the length of the project. Hydraulic concerns including ditch erosion and head cutting near existing culverts was also observed. Saturated subgrade and minor embankment distress was noted near STA 245+00.

3.3.3 Subsurface Conditions

In general, silty and clayey sands with some gravel were encountered in the pavements borings labeled P-1 through P-7 and P-10 through P-14. These borings were drilled mostly at one-half mile increments along the roadway alignment and extended to depths of 1 to 5 feet below the ground surface. Drilling was noted to be relatively easy, however auger refusal was reached in four of the aforementioned borings.

The geotechnical borings labeled BH19-02 & BH19-03 were drilled in an area near an existing 60-inch corrugated metal pipe located at STA 185+11. Boring BH19-02 was drilled through existing asphalt roughly 19 feet north of the aforementioned pipe location and approximately 2 feet right of the centerline. This boring encountered very loose, moist, silty sand beneath the pavement section which extended to a depth of 16.5 feet beneath the ground surface. The material then transitioned to an approximately 11.5-foot thick medium dense, moist, clayey sand layer underlain by very hard, wet, sandstone which continued to the termination depth of roughly 35

Page | 10 feet. Boring BH19-03 was drilled through existing asphalt roughly 25 feet south of the previously mentioned pipe location and approximately 8 feet left of the centerline. This boring encountered soft, moist, lean clay with sand and silt beneath the pavement section to a depth of approximately 14 feet. The material then transitioned to medium dense, moist, silty sand with gravel to a depth of 18 feet which was then underlain by very stiff, moist, lean clay with sand. Very hard sandstone was encountered at 18 feet and extended to the termination depth of roughly 35 feet.

The geotechnical and pavements borings labeled BH19-01/P-9 & BH19-04/P-8 were drilled in areas of significant pavement cracking and embankment distress at STA 223+58 and STA 191+90, respectively. Boring BH19-01/P-9 was drilled through existing asphalt roughly 4 feet left of the centerline. The existing asphalt section was observed to be approximately 1-foot thick due to asphalt patching repairs. This boring encountered an approximately 4-foot thick layer of soft, moist, silt underlain by soft, moist elastic silt to the termination depth of the boring at 11.5 feet.

Boring BH19-04/P-8 was drilled through existing asphalt roughly 11 feet left of the centerline.

This boring encountered soft, moist, lean clay with silt beneath the structural section to the termination depth of the boring at 11.5 feet.

Cobbles up to 4 inches in diameter were encountered during drilling at depths below the structural section as noted in the Final Pavement and Materials Report. Boulders may also be encountered due to the mountainous terrain.

3.3.4 Groundwater

Groundwater was encountered during drilling in borings BH19-02 and BH19-03 at depths of 22.7 and 24.9 feet below the existing ground surface, respectively. These groundwater levels are noted on the boring logs included in Appendix A. Fluctuations in the groundwater level due to seasonal and climatic effects are expected.

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the management of subsurface water, slope stability, road embankment repair, and construction considerations for the design and construction of the Taylor Mountain Road project. Based on discussions with the project team, the critical components of work are to address the pavement condition and improve the drainage along the existing road. Apparent roadway cracking and subsidence, however, has been observed as a cause of roadway distress. Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 EARTHWORK AND DRAINAGE

Unstable subgrade materials and poor drainage are conditions that have contributed to the current damage along the Taylor Mountain Road. The following sections will discuss recommended earthwork and drainage solutions to address these geotechnical issues for the project.

4.1.1 Subexcavation

Subexcavation is recommended for this project from approximately STA 245+50 to STA 246+50 to stabilize areas of weak or unstable subgrade materials. This area was observed to have the most pavement cracking. Additional areas may be identified by the CFLHD Pavements Engineer. The subexcavation should be two feet deep with one layer of non-woven separation geotextile and one layer of stabilization geogrid, both placed at the bottom of the subexcavation. The bottom of the subexcavation should be inclined to drain outside of the roadway prism. Subexcavation should extend the full roadway width based on the conditions encountered at the judgement of the CO.

4.1.2 Underdrains

An underdrain system is recommended along the left side of the road from approximately STA 245+00 to STA 246+20. The subgrade should be crowned to drain appropriately. Additional areas of poor drainage could be identified during construction. Underdrains should be installed in this area to reduce the potential for water to saturate the pavement subgrade. Underdrain outlet pipes should be located at a maximum of 300-feet on-centers spacing. In addition to this identified area, an allowance of 500 linear feet should be included in the contract to be used at the discretion of the Contracting Officer.

4.1.3 Subgrade Soil Support Recommendations for Concrete Headwalls

A soil unit weight (γ) of 110 pounds per cubic foot, internal friction angle (φ) of 25, and cohesion intercept (c) of 50 pounds per square foot are the estimated design values for the subgrade soil.

These properties were estimated based on the testing and classification data provided from borings BH19-02 & BH19-03 and typical design values shown in the NAVFAC Design Manual 7.01

Page | 12 assuming silty to clayey sand and clay material. This material is expected to be underlain by a sequence of weathered sedimentary rocks including sandstone, siltstone, and shale. Based on this knowledge of the geologic conditions near the structure site, the presumptive bearing resistance for the assumed soil type can be estimated to be between 2,000-4,000 psf with a recommended value of use of 2,000 psf according to Table C10.6.2.6.1-1 of AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017. The bearing resistance values presented in this table are based on a maximum settlement of one inch and only apply at the service limit state (resistance factor of 1.0). Plate 4 presents the ultimate bearing resistance of the foundations soils for various foundation widths associated with the headwalls for the proposed culverts at STA 185+11. For strength limit state, a resistance factor of 0.45 should be applied to the ultimate values presented in Plate 4. It is recommended that the base of all proposed culverts and headwalls be constructed below the depth of frost potential, which is estimated at approximately 30 inches according to the state average frost depth and local practices (Hammerpedia, 2020).

Active and at-rest lateral earth pressures of properly placed and compacted select granular backfill and unclassified borrow (on-site silty to clayey sand and sandy clay) above the water table are presented in Table 4.1 below. The values are unfactored loads and assume that the surface of the soil slope behind the headwalls is horizontal. Unbalanced water behind a wall adds hydrostatic pressure and should be avoided by using structural backfill (FP-14, Section 704.04) against backfilled structures and assuring a free draining gravity outlet for captured water. Structural backfill should be used with the active zone, located within a 1V:1H plane projected from the back of the bottommost structural element. Unclassified borrow (FP-14, Section 704.06) may be used in the retained zone beyond the active zone behind the headwalls. Below the mean stream level, the walls must also be designed for the full hydrostatic condition. headwall design should consider surcharge loading due to traffic and construction equipment, and any sloping backfill.

Table 4.1 - Static Lateral Earth Pressures for Concrete Headwalls

BACKFILL TYPE ASSUMED BACKFILL

PARAMETERS

RANKINE EARTH

PRESSURE CASE

UNFACTORED

EQUIVALENT FLUID

PRESSURE, pcf

Structural Backfill c = 0 psf φ = 34 deg.

γ = 130 pcf

Active 35

At-Rest 55

On-Site Silty to Clayey Sand and Clay Soils c = 50 psf φ = 25 deg.

γ = 110 pcf

Active 45

At-Rest 65

Lateral loads imposed upon structures may be resisted by the development of friction between the base of the footings and the foundation soils. A nominal friction factor of 0.47 (based on assumed foundation friction angle of 25 degrees) may be used in design for footings established on sandy silt. A resistance friction factor of 0.8 or 0.9 for cast-in-place concrete or precast concrete, Page | 13 respectively, should be applied in the design for sliding. Further information regarding failure by sliding is discussed in Section 10.6.3.4, specifically equations 10.6.3.4-1 and 10.6.3.4-2, of the 8th edition of AASHTO.

Because the box culvert is located in a natural drainage channel, depending on the construction season, the surface and groundwater levels will vary but should be expected in the footing excavation and may requiring dewatering and/or water diversion. All foundation preparations should be in accordance with Section 209.

4.1.4 Ditch Recommendations

The existing roadway has insufficient ditches in locations. Debris filling appears to be the main source of this problem is impacting the roadway. As stated in the plans, the gradient and width of the roadway ditches and the excavation and embankment ratios may be adjusted by the CO to assure adequate drainage and stability. Typical section drawings shown in the plan set, Sheets A6 & A7, provide further details. Refer to the Final Hydraulics Report dated January, 2020.

4.1.5 Shrink/Swell Recommendations

On-site soils expected to be encountered within the project limits generally consist of silty to clayey sands and clays. It is estimated that these soils will have a 10 percent shrink percentage, corresponding to a shrink/swell factor of 0.90. Greater than average shrinkage may be encountered in areas of very loose, shallow, material as noted on the boring logs. The recommended shrink/swell factor is based on a combination of standard tabled values for common materials in the FLH Technical Guidance Manual (2006) and experience with other CFLHD projects in similar materials.

4.1.6 Central Drainage Improvements

Erosion of soils at culvert inlet and outlet locations are likely contributing to the poor performance and overflow of drainage systems. This, in turn, may be a cause of the pavement distress and could lead to subgrade piping. Improvements to existing culverts and the addition of proposed culverts are anticipated per guidance from the hydraulics engineer. For additional information please refer to the Final Hydraulics Report.

4.2 EMBANKMENT STABILIZATION

Settlement or consolidation due to either inadequately compacted fill material and/or downslope fill creep is a concern for the section of the project corridor between STA 182+00 to STA 184+00.

Worsening tension cracks and subsidence in the outer part of the road along with bulging within the slope were noted by the Project Manager during the 30 percent site visit in early June 2020.

Figure 2 shows eastward and southward facing views of the above-mentioned slope and roadway.

Observations of this site include the following:

Page | 14

• Spring Creek crosses the roadway at STA 185+11 and generally flows from north to south to the confluence with Ashley Creek.

• The corrugated metal pipe shown in the east facing photo in Figure 2 at STA 185+11 will be replaced with two 72-inch diameter culverts with headwalls and riprap per the plan set as detailed in the Final Hydraulics Report.

• The boring logs for BH19-02 & BH19-03 indicate very loose silty sand and soft lean clay for the first 14 to 16.5 feet beneath the structural section and groundwater was noted at

22.7 and 24.9 feet beneath the ground surface, respectively.

• The area is vegetated with desert brush, predominantly sagebrush, which can control water erosion and provide slope stability by improving the soil cohesion.

• At least two drainage paths carved on the slope can be seen in the east facing photo in Figure 2 to the north and south of the pipe.

In general, the loose, soft, shallow earth material and the hydrologic setting are the two main factors likely causing creep and pavement cracking at this location. Typical methods to repair slopes with settlement problems include, but are not limited to, reconstruction, realignment, or retaining structures. These options can be costly, excessive, and disproportionate to the small scale of shallow slope repairs. Therefore, a deep patch is considered to be a practical and effective repair for this project. Regular roadway maintenance should be performed in addition to the deep patch.

The following sections will discuss the proposed deep patch and roadway maintenance further.

Figure 2 - Approximate Location of Embankment Distress Near STA 185+00 (Left- Facing East & Right- Facing South)

4.2.1 Deep Patch Design

A deep patch is the recommended repair for the site between approximately STA 181+35 to STA 184+00. A deep patch is a shallow, road-fill slope repair that is constructed by excavating the upper 1.5 to 6 feet of the subsiding section of roadway, replacing the fill material with compacted

Page | 15 backfill, and reinforcing the fill material with layers of geosynthetic material, usually geogrid (Musser & Denning, 2005). Cross sections of both a typical FHWA partial width deep patch and full width deep patch are shown in Figures 3 and 4, respectively.

Figure 3 - Cross Section of Partial Width Deep Patch (Conceptual)

Figure 4 - Cross Section of Full Width Deep Patch (Conceptual)

Slope stability analysis was completed for the deep patch repair site. Back calculation analysis for slope conditions at failure were first conducted using the Slide 7.0 computer program developed by RocScience. Slide 7.0 uses limit equilibrium methods to determine the factors of safety for slope stability. The back-analyzed soil parameters were then used for the deep patch design calculations.

Presumptive soil properties were estimated based on laboratory testing, visual classification, and experience with similar soil materials for initial input in the back analysis. Back analysis of the assumed pre-existing slope included a water table elevation near the approximate high water elevation. The high water elevation was estimated from the water levels observed during drilling, as discussed in Section 3.3.4 of this report, and available water well data supplied by the Bureau of Land Management. An incipient static factor of safety of 1.1 was used to refine the estimated soil parameters. Conservative, presumptive properties for deep patch materials were assigned

Page | 16 based on experience with similar materials. Table 4.2 provides a summary of the estimated overburden soil properties used in the stability analysis models.

Table 4.2 - Estimated Material Properties of Native and Backfill Soils for Deep Patch Design

MATERIAL

DESCRIPTION

UNIT WEIGHT

ɣ, pcf

COHESION

c, psf

FRICTION ANGLE

φ, deg

Silty SAND (SM) 120 0 30 Lean CLAY with sand and silt (CL-ML) 110 50 25

Lean CLAY with sand (CL) 110 100 26 Backfill Material 130 0 32

Analyses of the deep patch was performed using ReSSA, Version 3.0, and SLIDE, Version 7.0:

two dimensional, limit equilibrium computer programs from ADAMA Engineering and RocScience, respectively. The analysis conducted for the deep patch design consisted of external, compound, and internal stability in accordance with AASHTO “LRFD Bridge Design Specifications” (8th Edition). The slope geometry was determined from cross sections developed by the designer.

A preliminary analysis was performed to develop minimum trial reinforcement lengths and spacings at the at the top and bottom of the proposed deep patch. The trial length and spacing were then modeled in ReSSA and SLIDE to determine the final embedment length, spacing, and tensile strength required of the geosynthetic reinforcement to meet a suitable factor of safety, according to AASHTO, of greater than or equal to 1.3. Due to the low probability (seismic hazard zone 1 for a seismic event to occur in this region) seismic effects were considered negligible with regards to deep patch design and were not considered in the analysis.

The back-analyzed and assumed soil parameters were used in further analyses to examine the effectiveness of the deep patch repair option. The slope stability modeling included an analysis of both a proposed half and full width deep patch. A 250 pound per square foot traffic surcharge was applied in both models. It was assumed that the proposed deep patch model was drained and would limit the maximum height of the groundwater table in the slope. Results of the slope stability analysis are presented in Appendix C.

The depth of the proposed deep patch is recommended to be 3 feet and the full roadway width.

The reinforcement spacing is recommended to be not more than 1 foot vertically and should daylight at the slope face at a maximum slope ratio of 1V:2H. A Type 1 reinforcement geogrid conforming to Section 714.04 in FP-14 should be used for the deep patch repair. The deep patch should also include a geocomposite sheet drain as shown in the plans. Backfill material should conform to Section 704.03(b) in the FP-14. On-site material may require processing to meet this specification.

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4.2.2 Roadway Maintenance

A regular slope maintenance program consisting of maintenance personnel that possess the necessary specialized skills and equipment would help to alleviate the concern related to these unstable slopes. In general, routine slope maintenance activities should prevent conditions that promote erosion, deteriorate soil shear strength, or induce adverse loading (Sindlinger, 2009).

Examples of maintenance activities would include, but are not limited to the following:

• Routine observation and monitoring of slope performance.

• Ditch cleaning and repair.

• Pavement crack filling to prevent water infiltration.

4.3 GEOSYNTHETICS

The geogrid for subexcavation and the deep patch repair should meet the requirements of stabilization and reinforcement geogrid, respectively (FP-14 and SCR sections 714.03 & 714.04).

Separation geotextile for subexcavation and underdrains should be Class 1 Type B non-woven geotextile (FP-14 section 714.01).

4.4 CONSTRUCTION CONSIDERATIONS

Roadway Excavation: Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits. Shallow rock was encountered during the pavement and geotechnical investigation. The Contractor should be prepared with appropriate equipment based on the local geology as discussed in Section 2.1 and 2.2.

Based on the subsurface investigation and surface geologic mapping, the rock encountered, primarily shallow sandstone and limestone, is expected to be rippable in the area surrounding the proposed deep patch repair; however, final determination of bedrock rippability is the responsibility of the contractor. Limited controlled blasting may be efficient for extraction of the sandstone bedrock. Shear wave velocity of the geologic layers was not evaluated by geophysical methods. Refer to Table E.1 in Appendix E for rock descriptions. Additional information regarding the excavation characteristics of rock and rippability charts can also be found in Appendix E.

Evidence of definitive in-place bedrock appeared to be encountered from the subsurface investigation detailed in this report. Appropriate construction equipment should be mobilized to the site based on the rock information detailed in this report. The contractor is responsible for safety of excavations and shoring design.

Corrosivity: Based on guidelines outlined in subsection 704.08(b) of FP-14, a pH below 5 or above 10, a resistivity below 3,000 ohm-cm, and sulfate and chloride concentrations above 200 and 100 parts per million, respectively, are representative of an aggressive soil environment. Six

Page | 18 samples for this project were tested for corrosivity and the results are summarized in Table 3.3.

The pH ranged from 8.2 to 8.7. In addition, three of the samples tested below 3,000 ohm-cm for resistivity, four tested above 200 parts per million for sulfate ion content, and two tested above 100 parts per million for chloride ion content. These test results are indicative of an aggressive soil environment and considerations need to be taken with regards to metal culverts. The elevated sulfate levels also show that sulfate resistant cement should be considered. Non-reactive fill material may also need to be used for the proposed deep patch and it should be verified that the geogrid material can remain durable in aggressive soils.

Seepage Concerns: Water may be encountered during construction activities. It is recommended that a combination of dewatering and stabilization methods of the subgrade be performed if seepage is noted. If the material at base of standard subexcavation is observed to be wet or soft, then it will need to be stabilized prior to backfilling. Refer to subsection 208.08(d) of FP-14. This process could include alternatives such as removing additional material and suitably replacing it, adding roughly 6-inches of rock material into the subgrade, or adding additional geotextile or geogrid stabilization. For the proposed underdrain systems, trench boxes or stabilizers may be needed if seepage and high water levels become a concern during construction.

Slope Instability: Slopes could become unstable during construction operations due to precipitation, flooding events, or heavy loading on soft subgrade soils. Precautions should be taken if movement or cracking is observed. Seepage, as discussed above, is a contributor to slope instability and similar dewatering and stabilization methods could be used to suspend slope movement.

4.5 SPECIFICATIONS

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

known as FP-14. Final versions of these SCR’s have been provided to the CFLHD Design Engineer to incorporate into the Plans, Specifications, and Estimate package for the 95% review milestone.

4.6 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations in this report are based on the data obtained from exploratory borings, field review, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the

Page | 19 process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Page | 20

SECTION FIVE - References

ADAMA Engineering Inc, Computer Program, ReSSA, Version 3.0, Copyright 2013.

American Association of State Highway and Transportation Officials (AASHTO), 2017, LFRD

Bridge Design Specifications, 8th Edition.

Federal Highway Administration (FHWA), 2007, Geotechnical Technical Guidance Manual, dated May.

Federal Highway Administration (FHWA), 2008, Federal Lands Highway Project Development and Design Manual (PDDM), dated March.

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of

Roads and Bridges on Federal Highway Projects FP-14.

Federal Highway Administration (FHWA), 2017, Soil Description and Identification Guidelines, dated December.

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

December.

Federal Highway Administration (FHWA), 2021, “UT FLAP 2500(1) Final Hydraulic Report,” dated January 14.

Federal Highway Administration (FHWA), 2020, “UT FLAP 2500(1) Final Pavement and

Materials Report,” dated November.

Haddox, D.A., Kowallis, B.J., and Sprinkel, D.A., 2010, “Geologic map of the Steinaker

Reservoir quadrangle, Uintah County, Utah,” Utah Geological Survey, Miscellaneous Publication 10-3, scale 1:24,000.

Hammerpedia, 2020, “Utah Frost Line,” Source: U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), <https://www.hammerpedia.com/utah-frost-line/>.

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

Musser, Sandra W. & Denning, Cliff, 2005, “Deep Patch Road Embankment Repair Application

Guide,” United States Department of Agriculture (USDA) Forest Service, dated October.

National Park Service (NPS), 2018, “Rocky Mountain System Provinces,” Series: Physiographic

Provinces, <https://www.nps.gov/articles/rockies.htm#:~:text=The%20Middle%20Rocky%20Mountai ns%20province,Colorado%2C%20Utah%2C%20and%20Idaho>, updated May 1.

RocScience Inc, Computer Program, Slide Version 7.0.

Sindlinger, Alicia, 2009, “Asset Management Highlights Slope Maintenance,” Public Road

Magazine, Publication No. FHWA-HRT-09-003, Issue No. Vol. 72 No. 5, dated March/April.

Page | 21

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

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

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

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

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

PLATES

PROJECT VICINITY MAP,

GEOLOGY MAP,

BORING LOCATIONS MAP, &

BEARING RESISTANCE FOR

CONCRETE HEADWALLS AT STA 185+11

0.5

1.5

2.5

3.5

4.5

0 1 2 3 4 5 6 7 8 9 10

Be ar in g

Re si st an ce , R n (K

SF

Effective Foundation Width B' (FT)

EFFECTIVE WIDTH vs. BEARING RESISTANCE (LRFD) Concrete Headwalls at STA 185+11 - Taylor Mountain Road (UT FLAP 2500(1))

B'

Q

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

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

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

*Assuming 7.50-ft. length of foundation.

APPENDIX A

FIELD EXPLORATION PROGRAM

APPENDIX A

FIELD EXPLORATION PROGRAM

A.1 INTRODUCTION

The Central Federal Lands Highway Division (CFLHD) Pavements & Materials Section completed a field exploration program on behalf of the Geotechnical Section for UT FLAP 2500(1), on August 8th, 2019. The scope of work for the field exploration program included drilling a total of 16 borings; consisting of 14 pavement borings and 4 geotechnical borings (2 borings served the purpose of both geotechnical and pavement borings). Each component of the field exploration program was coordinated and observed by a Pavements & Materials Engineer from CFLHD. Field exploration location plans are included in Plate 3 of this report and individual boring logs are attached in the following pages. These logs represent a compilation of field and laboratory data and description of the soil samples by CFLHD Pavements & Materials personnel. The methods used to conduct the field exploration program are described below.

Representative soil and rock samples collected during the field exploration program were transported to the CFLHD Materials Laboratory in Lakewood, Colorado for testing. A summary of the laboratory testing program is provided in Appendix B.

A.2 EXPLORATIONS

Borings

Haz-Tech Drilling Inc. of Nampa, ID provided the drilling services for the soil borings.

Borings were completed using a truck mounted BK-81 drill rig.

Pavement borings were drilled at roughly 0.5 mile increments along the project alignment and ranged from approximately 1 to 5 feet below the ground surface to explore roadway subgrade conditions. Geotechnical borings were drilled at proposed deep patch locations and ranged from approximately 11.5 to 35.1 feet below the ground surface. Borings were drilled and sampled using auger drilling and drive sampling.

If water was encountered at the time of drilling, field personnel measured water levels in the borings. Fluctuations in the ground water level due to seasonal and climatic effects are expected. The location and elevation of individual borings were estimated relative to features shown on the plan set and Google Earth. Boring locations are listed on individual boring logs and are show in Table 3.1 of this report.

Following drilling activities, field personnel backfilled the borings with cuttings generated during drilling in accordance with applicable local, state, and federal regulations.

A.3 SOIL…