FINAL GEOTECHNICAL REPORT JANUARY 2021.pdf

SANTAQUIN CANYON ROAD

UINTA-WASATCH CACHE

NATIONAL FOREST

UTAH COUNTY, UT

UT ERFO 70014(1)

FINAL Geotechnical Report Report No. UT-FE-70014-20-01

Prepared by:

Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch January 2021 ii

SIGNATURE SHEET

Report prepared by: ____________________________________________________________ James M. Arthurs, P.E., Ph.D., Geotechnical Engineer

Report reviewed by: ____________________________________________________________ Rebecca L. Borst, E.I., Geotechnical Engineer

Approved for distribution by: ____________________________________________________

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

Electronic Distribution N:\UT\ERFO 70014(1)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Designer i

Willow Beach Road, AZ FTNP LAKE 113(2) January 2020 Page i

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. SITE SEISMICITY

2.3.1. Nearby Faults

2.3.2. Seismic Design Parameters

2.4. GEOLOGIC HAZARDS

SECTION THREE --FIELD EXPLORATION

3.1. FIELD EXPLORATION PROGRAM

3.1.1. Site 1

3.1.2. Site 2

3.1.3. Sites 3 and 3A

3.1.4. Site 4

3.1.5. Site 5

3.1.6. Site 5A

3.1.7. Site 6

3.1.8. Site 7

3.1.9. Site 7A

3.2. LABORATORY TESTING PROGRAM

SECTION FOUR --ANALYSIS AND RECOMMENDATIONS

4.1. EARTHWORK AND EMBANKMENTS

4.1.1. Temporary Cut Slopes and Shoring

4.1.2. Embankment Fill

4.1.3. Rock Embankments

4.2. RETAINING WALLS

4.2.1. Retaining Wall Material Design Parameters

4.2.2. MSE Wall Design

4.2.3. Gravity Wall Design

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4.2.4. Retaining Wall Design Considerations

4.3. SHRINK/SWELL RECOMMENDATIONS

4.4. SUBEXCAVATION

4.5. GEOSYNTHETICS

4.6. CULVERTS AND WINGWALLS

4.7. CONSTRUCTION CONSIDERATIONS

SECTION FIVE --LIMITATIONS

SECTION SIX --REFERENCES

TABLES

Table 2.1: Seismic Hazard Faults

Table 2.2: Summary of Seismic Parameters for Site Class C

Table 3.1: Summary of Laboratory Test Results

Table 4.1: Recommendations Summary

Table 4.2: Results of Slope Stability Back-analysis and Material Properties Used in Design

Table 4.3: MSE Wall Design Minimum Acceptable Factors of Safety

Table 4.4: Analyzed Wall Geometries

Table 4.5: Calculated Global Stability Factors of Safety

Table 4.6: Gravity Wall Design Minimum Factors of Safety

Table 4.7: Analyzed Wall Geometries

Table 4.8: Calculated Global Stability Factors of Safety for Gravity Walls

FIGURES

Figure 1.1: Typical Roadway Conditions

Figure 1.2: Project Location Map. Repair sites exist along road closure area indicated in yellow.

Figure 2.1: Regional Geologic Map.

Figure 3.1: Site 1. Scoured embankment area.

Figure 3.2: Site 2. Scour area. Bedrock exposed near stream level.

Figure 3.3: Site 3. Scour area with undercut pavement. Large boulder in stream creating waterfall.

Figure 3.4: Site 3A. Scour area adjacent to roadway. Large boulder in stream (with trees) at left side of photo.

Figure 3.5: Site 4. Culvert and roadway embankment damage.

Figure 3.6: Site 5. Culvert damaged during high flows and woody debris on roadway.

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Figure 3.7: Site 5. Extensive roadway damaged caused by water flow in cut-side ditch and woody debris on roadway.

Figure 3.8: Site 5A. Embankment erosion.

Figure 3.9: Site 6. Culvert and roadway damage due to high flows and debris clogging.

Figure 3.10: Site 6. Avalanche chute on opposite valley wall.

Figure 3.11: Site 7. Damage to roadway due to side channel flow and erosion.

Figure 3.12: Site 7A. Culvert outlet with bedrock in slope.

APPENDICES

Appendix A: Field Exploration Appendix B: Laboratory Testing Appendix C: Field Exploration Photos Appendix D: Excavation Characteristics of Rock and Ripability Charts Appendix E: Preliminary Retaining Wall Design Calculations

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 1

SECTION ONE--INTRODUCTION

1.1. BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering study and provides recommendations for roadway repairs across eight (8) sites along Santaquin Canyon Road (Forest Road 70014). The project corridor is in the Uinta-Wasatch Cache National Forest in Utah County, Utah. The Santaquin Canyon Road starts at the intersection of Highland Drive and South Center Street in Santaquin, UT.

The road proceeds through a residential neighborhood before entering the narrow canyon cut by Summit Creek. The damaged sites are located starting 6 miles southeast of the City of Santaquin.

The roadway follows a southeast trend along Summit Creek (Figure 1.1). No traffic or daily usage counts are available. A project location map is presented in Figure 1.2.

The roadway was damaged due to excess runoff from larger than normal snow pack and subsequent avalanching during the Spring of 2019. High runoff and debris flows have caused severe undercutting of the roadway, blocked culverts and roadway sections with debris, and in addition, completely obliterated multiple roadway sections and culverts. Repairs are planned as part of the Emergency Relief for Federally Owned Roads (ERFO) program. The road is currently closed between the Trumbolt Day Use Area and the Tinney Flat Campground.

The Santaquin Canyon Road provides access to numerous recreational activities including camping and picnic areas. It also connects to the Mt. Nebo Scenic Byway.

Figure 1.1: Typical Roadway Conditions

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 2

Retrieved from https://www.fs.usda.gov/detail/uwcnf/news-events/?cid=FSEPRD627938 on July 20, 2020.

Figure 1.2: Project Location Map. Repair sites exist along road closure area indicated in yellow.

1.2. SCOPE AND PURPOSE

The scope of work included geotechnical investigation, analysis, and recommendations for retaining walls, fills, and other geotechnical aspects of the roadway repair. This involved several tasks including field reconnaissance, interpretation and correlation of data, and geotechnical engineering analysis. Specifically, this investigation is to evaluate soil conditions at proposed retaining wall and fill locations, and develop recommendations concerning retaining wall design, embankments, surface/subsurface drainage, and construction considerations for the design and construction of slopes and retaining structures within the alignment.

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SECTION TWO--GEOLOGY AND SEISMICITY

2.1. REGIONAL GEOLOGY

The project site is near the boundaries of the Colorado Plateaus, Basin and Range, and Middle Rocky Mountains provinces. The nearby Wasatch fault zone serves as an indicator of a transitional zone between the provinces. The site is likely located within the small wedge of the Middle Rocky Mountains province that extends into the southern Wasatch Range. The southern Wasatch Range is a 30-mile segment of the Wasatch Range that extends from Spanish Fork Canyon southwest towards Nephi, UT. The Wasatch fault zone truncates the west flank, appearing as a straight, steep, north-trending mountain front, while the eastern edge of the range appears to be less impressive as it has been deeply eroded.

The Wasatch fault is the major high-angle normal fault in the area. It is north trending and currently active. Additionally, a system of high-angle normal faults (also north trending) break the crest and west flank of the entire Wasatch Plateau, creating horst and graben topography. The origin of these faults and grabens is uncertain. Likely, the faults reflect widespread crustal spreading that has dominated the western interior of the United States since Miocene time (Witkind & Weiss, 1991).

Near the project area, bedrock consists of Paleozoic beds that are right side up and dip moderately southeastward. The site is mapped as underlain by formations ranging in age across the entirety of the Paleozoic, from Cambrian to Permian. In order of occurrence from site 1 to 7, bedrock consists of the Oquirrh Formation (PPo), undivided Middle Cambrian Rocks (Cmu), undivided Upper Cambrian Rocks (Cu), and the Fitchville Formation (MDf). The Oquirrh Formation consists of interbedded limestone, sandstone, and quartzite, with carbonate rocks dominating the lower part and clastics dominating the upper. The undivided Middle Cambrian and Upper Cambrian Rocks both consist of mainly dolomites and limestones. Lastly, the Fitchville Formation is made up of medium bedded to massive dolomite and minor interbedded black shale. At the canyon bottom, the bedrock units are mapped as overlain by Quaternary deposits, as described in the next section. A map of the regional geology is presented in Figure 2.1.

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Figure 2.1: Regional Geologic Map.

2.2. SITE GEOLOGY

Mapping of surficial, unconsolidated deposits in the project area was published by the Utah Geologic Survey (Solomon, 2010). As the name suggests, Santaquin Canyon road resides within a canyon and parallels Summit Creek. This environment, with steep cliffs and flowing streams, has led to an abundance of rock fall, stream, and alluvial fan deposits.

Sites 1 and 2 are mapped as underlain by young stream deposits (Qaly), ranging in age from Holocene to upper Pleistocene. These deposits are moderately sorted pebbles, cobbles, and gravel within a matrix of sand and minor silt and clay. Locally, areas of alluvial-fan and colluvial deposits may be encountered. This could include debris flow, debris flood, and canyon mouth deposits (Qafy). The thickness of this unit varies, but is likely less than 15 feet.

Sites 3 and 4 are mapped as underlain by talus deposits (Qmt), ranging in age from Holocene to middle Pleistocene. This is escribed as very poorly sorted, angular debris, ranging in size from gravel to boulders, with minor amounts of fine-grained sediment. Typically deposited by rock fall on and at

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 5 the base of steep slopes, the talus is derived from the nearby Paleozoic bedrock. Generally, this unit is 20 feet or less in thickness.

Surficial, unconsolidated deposits were not mapped within the vicinity of sites 5, 6, and 7. Slopes and cliff sides in these areas appear to be more stable and comprised largely of competent rock.

Although, the environment remains consistent with that of canyon walls incised by Summit Creek, so the aforementioned deposits may still be present, but not at map scale.

2.3. SITE SEISMICITY

2.3.1. Nearby Faults

Numerous seismic source faults are mapped within 40 miles of the project area. The critical active faults to the project site may be found in Table 2.1 (U.S. Geological Survey, 2020). The entire project area is surrounded by faults, all of which could generally be described as normal faults exhibiting a northern strike and western dip. Slip rates vary between approximately 0.005 to 0.10 inches per year, with the Nephi and Provo sections of the Wasatch Fault displaying the largest slip rates at 0.0874 and 0.1028 inches per year, respectively.

Table 2.1: Seismic Hazard Faults

FAULT NAME

DISTANCE

FROM CENTER

OF PROJECT

FAULT

PARRALLEL

SLIP RATE

FAULT

LENGTH

LAST

ACTIVITY

(miles) (in/yr) (miles) (years ago) Wasatch Fault – Nephi Section 2.0 0.0874 28.6 < 15,000

Wasatch Fault – Provo Section 4.5 0.1028 47.8 < 15,000

Wasatch Fault – Levan Section 20.0 0.0154 19.9 < 15,000

Strawberry Fault 32.5 0.0051 24.2 < 15,000 Oquirrh – Southern Oquirrh Mountains

Fault Zone

32.5 0.0102 39.1 < 15,000

Joes Valley Fault Zone 36.0 0.0102 29.2 < 15,000

2.3.2. Seismic Design Parameters

Wingwalls associated with culvert replacements are proposed for this project. Retaining walls are also proposed to retain fill slopes. The materials at the project site include a mantle of recent stream

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 6 bed (alluvial) and talus material, with sedimentary bedrock at depth. Seismic design parameters for analysis of these structures are presented in this section.

The American Association of State Highway and Transportation Officials (AASHTO) criteria for recommended seismic response parameters are based on the (AASHTO) LRFD Bridge Design Specifications, 7th Edition, 2014, and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1000-year return period). The 1000-year return period uniform hazard spectrum for Santaquin Canyon Road (39.912611º N latitude and 111.745237º W longitudes) was obtained in accordance with the AASHTO ground motion maps.

The site was classified as “Class C” according to site class definitions specified in Table 3.10.3.1-1 of AASHTO based on assumptions of the soil profile and geologic mapping in the project vicinity.

The recommended spectral acceleration coefficient values for probabilistic design are summarized in the following table.

Table 2.2: Summary of Seismic Parameters for Site Class C

Peak Seismic Ground Acceleration Coefficient, (As) 0.338

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.788

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.392

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.09

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 1.118

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.546

Based on the long acceleration coefficient SD1 value of 0.392, the site is assigned to seismic hazard Zone 3 in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 in AASHTO.

2.4. GEOLOGIC HAZARDS

The Santaquin Canyon Road is located in a narrow, steep-sided canyon with seasonal weather conditions that can include significant snowfall and rain. The primary geologic hazards that may exist at the project site are landslides (including rockfall and debris flow), avalanches, and flash flooding. The proposed repairs can be made more resilient to these hazards by founding retaining walls either below scour depth or on bedrock. Embankment slopes and retaining walls should include scour countermeasures as recommended by the CFLHD Hydraulics Engineer. Additional hazard mitigation is beyond the scope of the repairs.

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SECTION THREE--FIELD EXPLORATION

3.1. FIELD EXPLORATION PROGRAM

Field conditions were initially evaluated on June 25 and 26, 2020 by the CFLHD Geotechnical Engineer. We understand that additional, separate site visits were conducted by CFLHD Environmental Protection and Hydraulics personnel. A 70% Design milestone site review was conducted with CFLHD personnel (Project Manager, Justin Henwood; Hydraulics Engineer, Cassidy Cote; and Geotechnical Engineer, James Arthurs) and U.S. Forest Service personnel.

Field observations focused on identifying soil properties and documenting existing conditions at the documented Damage Survey Report (DSR) sites, as well as evaluating additional site for inclusion in this repair project. Evaluation and recommendations presented in this memorandum are based on visual observations, limited measurements taken during the site visits, and ground survey performed by CFLHD. Due to ERFO project time and budget constraints to expedite project delivery, no subsurface investigation was conducted.

3.1.1. Site 1

Site 1, located at mile post 2.4, consists of a large erosional failure along the roadway on the creek side (Figure 3.1). The failure is not currently impacting the roadway, as about 3 feet of shoulder remains. The erosional cut is approximately 60 feet long, 20 feet wide, and 25 feet in height. Large boulders, up to approximately 10-foot diameter, are found both within the failure and at the base, along with those deposited by the stream channel. Large granular soil appears to be consistent throughout the entire cut. Damage was likely caused by excess runoff from larger than normal snowpack and subsequent avalanches, causing high flows to erode beneath the roadway shoulder and deposit large debris.

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Figure 3.1: Site 1. Scoured embankment area.

3.1.2. Site 2

Site 2, located at mile post 2.7, consists of a large erosional cut beneath the shoulder on the creek side (Figure 3.2). The eroded area is approximately 31 feet long, 12 feet wide, and 12 feet in height.

It undercuts the pavement, leaving about 1 foot of overhang beneath the asphalt shoulder. Soil consists of loose, unconsolidated material appearing to range in size from silt to small gravel.

Outcroppings of weathered bedrock, a highly-fractured quartzite, are also present beneath the roadway near the stream elevation. Boulders in the stream up to about 6-foot diameter were observed.

Above the roadway, the slopes consist of scree and quartzite outcrops. Similar to Site 1, damage was likely caused by excess runoff from larger than normal snow pack and subsequent avalanches, causing high flows to erode beneath the roadway shoulder and deposit large debris.

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Figure 3.2: Site 2. Scour area. Bedrock exposed near stream level.

3.1.3. Sites 3 and 3A

Sites 3 and 3a, located from mile point 3.27 to 3.37, consist of multiple roadway sections that have been undercut due to excess stream flow (Figures 5 & 6). Scoured areas range in length from approximately 15 to 60 feet, and up to 15 feet in depth. Some areas have suffered significant pavement undercutting, displaying up to 2 feet of overhang. Soil consists of silty sand with gravel, cobbles, and boulders. The cobbles and boulders are sub-angular to sub-rounded. Boulders as large as 25 feet in diameter can be found at the west end of the site. Boulders and other debris are found both at stream level and in the roadway. Bedrock is visible in the cut slope near the western portion of these sites, but not apparent at stream level. Like the previous sites, damage was likely caused by excess runoff from larger than normal snowpack and subsequent avalanching.

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Figure 3.3: Site 3. Scour area with undercut pavement. Large boulder in stream creating waterfall.

Figure 3.4: Site 3A. Scour area adjacent to roadway. Large boulder in stream (with trees) at left side of photo.

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3.1.4. Site 4

Site 4, located at mile point 3.47, consists of complete destruction of approximately 50 feet of roadway and a 5-foot diameter metal pipe culvert (Figure 3.5). Additionally, scour damage and undercut pavement exists north and south of the destroyed roadway section, for 100 feet to the south, and 200 feet to the north. Large amounts of debris, boulders, and sediment can be found blocking the culvert, at stream level, and along the roadway. Like the pervious sites, soil consists of a loose, unconsolidated silty sand with gravel, cobbles, and boulders. Damage caused due to blockage of the culvert with debris from an upstream avalanche, with the resulting high flow runoff destroying the roadway.

Figure 3.5: Site 4. Culvert and roadway embankment damage.

3.1.5. Site 5

Site 5, located at mile point 3.75, consists of a destroyed box culvert and roadway section, along with extensive roadway damage and undercutting of pavement along the shoulder in other areas (Figure

3.6 and Figure 3.7). Minor damage to the southwest wingwall of the bridge, along with loss of fill behind the northeast wingwall, has also occurred. Asphalt is about 12 inches thick on the left shoulder, and 6 inches thick on the right shoulder, with a paving fabric underneath. Soil consists of sandy gravel with cobbles. Large boulders, up to 10 feet in diameter, can be found at stream level and along the embankment. Additionally, boulders, trees, and other debris lie on the roadway.

Bedrock (quartzite) follows the right shoulder, displaying joint sets orthogonal to bedding planes.

The bedding planes generally dip into slope. Damage likely caused by an upstream avalanche causing blockage of the culvert and stream with debris, resulting in high flows along the road obliterating the roadway and culvert.

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Figure 3.6: Site 5. Culvert damaged during high flows and woody debris on roadway.

Figure 3.7: Site 5. Extensive roadway damaged caused by water flow in cut-side ditch and woody debris on roadway.

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3.1.6. Site 5A

This site is located approximately 0.1 miles southeast of Site 5. Erosion damage at this location is approximately 45 feet long and 15 feet high. Soil conditions are generally similar to the other sites.

Figure 3.8: Site 5A. Embankment erosion.

3.1.7. Site 6

Site 6, located approximately 0.35 miles southeast of site 5, consists of a culvert blowout, loss of roadway embankment, and extensive pavement damage (Figure 3.9). Part of a squash pipe culvert, with an estimated cross section of 6’ x 10’, failed and was washed downstream. Approximately 75 feet of embankment loss, and 100 feet of pavement damage, may also be found downstream.

Pavement section is about 8 inches thick, with no base, and a 3-inch asphalt layer above a fabric. Soil conditions are similar to that of site 5, consisting of sandy gravel with angular cobbles and boulders.

Boulders in excess of 10 feet in diameter, or possibly bedrock, exist in the stream near the crossing.

Damage was caused by avalanche debris blocking the stream and culvert (Figure 3.10), creating excess runoff and high flows, leading to the subsequent destruction of the roadway and culvert.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 14

Figure 3.9: Site 6. Culvert and roadway damage due to high flows and debris clogging.

Figure 3.10: Site 6. Avalanche chute on opposite valley wall.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 15

3.1.8. Site 7

Site 7, located at mile post 4.5, exhibits roadway damage and severe scour along the shoulder. Scour damage is approximately 125 feet long and 5 to 8 feet high (Figure 12). The scour occurred along a side channel that diverges from the main stream channel further upstream. Pavement section and soil conditions are similar to that of site 6, with the addition of a large number of roots underneath the roadway. Boulders, trees, and other debris reside in the stream channel and along the roadway. The largest boulder in the area is approximately 75 feet in diameter.

Figure 3.11: Site 7. Damage to roadway due to side channel flow and erosion.

3.1.9. Site 7A

Site 7A was identified during the 70% field review by the CFLHD Hydraulics Engineer. Details of damage observed and recommendations are presented in the Hydraulics Report. Soil conditions are similar to the other sites: silty sands with gravel, cobble, and boulder. Bedrock is exposed near the outlet of the culvert and along the left side of the road.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 16

Figure 3.12: Site 7A. Culvert outlet with bedrock in slope.

3.2. LABORATORY TESTING PROGRAM

Laboratory testing was performed on a single surficial sample obtained from the roadway embankment at Site 1. Results of the testing are summarized on Table 3.1.

Table 3.1: Summary of Laboratory Test Results

Grain size distribution

L iq ui d lim it

Pl as tic ity in de x

R es is tiv ity , o hm c m pH

R -v al ue

L A a br as io n, lo ss

So di um su lfa te so un dn es s, Soil classification

G ra ve l, Sa nd

Fi ne s, U

SC

S

A A

SH

T

O

73 16 11 24 2 4,770 8.4 75 27 0 GP-GM A-1-a(0)

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SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

The Santaquin Canyon Road project includes reconstruction of roadway embankments including retaining walls and rock embankments, and surface drainage improvements. This section provides analysis and recommendations for retaining walls, embankment fills, and construction considerations. The recommendations are summarized on Table 4.1 and detailed in the following sections. Recommendations for repairs related to surface water management, including culverts, are presented in the CFLHD Hydraulics Report.

For the analysis and design of the proposed embankments, geotechnical profiles were developed from surface observations. Published geologic mapping was also considered where relevant.

Retaining wall design is in accordance with the American Associate of State Highway Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD), 8th Edition.

Table 4.1: Recommendations Summary

SITE RECOMMENDATIONS SUMMARY

Site 1 Rock Embankment Site 2 Retaining Wall Site 3 Retaining Wall / Rock Embankment

Site 3A Retaining Wall / Rock Embankment Site 4 Culvert Replacement Site 5 Culvert Replacement

Site 5A Retaining Wall Site 6 Culvert Replacement Site 7 Divert Side Channel / Embankment

Reconstruction Site 7A Culvert Replacement

4.1. EARTHWORK AND EMBANKMENTS

The Santaquin Canyon Road project requires embankment work including conventional embankments, rock embankments, retaining walls, and drainage improvements. In addition, temporary cut slopes and shoring may be required to construct permanent structures.

4.1.1. Temporary Cut Slopes and Shoring

Temporary cut slopes and shoring are anticipated for construction of the culverts and retaining walls.

Design and safety of this work is the responsibility of the contractor. The work shall be performed in a manner to minimize hazards and exposure to the public, construction personnel, and equipment.

The contract should either include a temporary shoring pay item or include the costs of temporary shoring as subsidiary to the other pay items.

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4.1.2. Embankment Fill

The embankment areas should be constructed with traditional embankment construction methods.

Construct traditional embankments with a maximum slope ratio of 1V:1.5H to maintain slope stability and promote revegetation of the slope. An erosion control mat or other erosion mitigation devices should be placed on the completed slopes to assist with revegetation requirements. Construct embankments in accordance with Section 204 of FP-14 and the project special contract requirements (SCR). Embankments slopes should include scour protection as recommended by the CFLHD Hydraulics engineer.

4.1.3. Rock Embankments

To limit impact to Summit Creek and reduce the potential for embankment erosion, rock embankments may be considered. Construct rock embankments with a maximum slope ratio of 1V:1.5H to maintain stability. Construct rock embankments according to Section 252 of the FP-14 and project SCR.

4.2. RETAINING WALLS

Retaining walls are proposed at Sites 2, 3, 3A, and 5A to limit impacts from embankments to Summit Creek. Including embedment for scour protection, the maximum design height is anticipated to be 25 feet. The maximum wall height is anticipated to be 18 feet, without considering additional embedment for scour protection. Several retaining wall types are technically feasible for this application, but the proposed type of construction (fill slopes) and maximum wall height indicate that certain wall types will be more economical. In addition, proximity to Summit Creek also poses a risk of scour and damage to the wall face. The following wall types were considered in this evaluation:

• Stone fill for gabions is readily available from commercial pits near the project; however, gabion walls are most economical at heights below 10 feet, which is less than the 18-foot height of the proposed wall. Gabion baskets are also potentially susceptible to abrasion during high stream flows. Gabion walls are not likely suitable for this application.

• Conventional cast-in-place concrete retaining walls are technically feasible for the project, but will also require significant temporary excavation and shoring. The cost of concrete walls is anticipated to be higher than the other options evaluated.

• Geosynthetic Reinforced Soil (GRS) or Mechanically Stabilized Earth (MSE) walls are technically feasible for this project; however, facing treatments should be carefully selected to reduce risk of abrasion or scour of the facing material. Welded-wire basket or gabion faced walls are therefore not likely suitable for this application. Dry-cast retaining wall blocks are more resistant to abrasion and scour, and are considered marginally suitable. Wet-cast concrete blocks or concrete panels offer the highest resistance to abrasion and scour, with the blocks likely having a lower cost and easier construction in confined areas.

• Sheet pile or soldier pile and lagging walls (with or without ground anchors) would likely require pre-drilling to install vertical elements due to dense, coarse-grained soils. The cost of these wall systems is typically higher than other alternatives.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 19

• For shorter wall segments (heights less than 10 feet), gravity walls using similar wet-cast concrete blocks as discussed above could be considered.

• Soil nail walls are not technically feasible due to the loss of embankment and need to re-establish the roadway width.

Based on the evaluation above and discussions with the Project Team, wet-cast concrete block MSE and gravity walls are the preferred alternative. Additional discussions about wall face aesthetics will be conducted with FS personnel as the design progresses. Wall design and construction consideration are presented in the following section.

The damaged sites presented in this report will likely continue to degrade prior to the start of construction. The design presented in this report represents the best information available at the time of preparation, including ground survey collected by CFLHD and the 70% plans.

4.2.1. Retaining Wall Material Design Parameters

Slope stability analysis was completed for the failed embankments across all sites except Sites 4 and 7A, which did not involve an embankment failure. Back calculation analyses for slope conditions at failure were 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.

Presumptive soil properties were estimated based upon visual classification during the site visit and experience with similar soil materials for initial input in the back analysis. Soil properties were kept consistent across all sites. Reports from Forest Service staff indicate that the failures occurred during a high flow event. Larger than normal snowpack and subsequent avalanching created excess run off and a large amount of debris, leading to extensive scouring of embankments and blockage of culverts.

Therefore, back analysis of the assumed pre-existing slopes targeted a factor of safety between 1.1 and 1.2, as no indication of slope instability had previously been observed, and a large, catastrophic event was required to cause failure. Conservative, presumptive properties for bedrock materials were assigned based on visual classification and experience with similar materials.

Bedrock was observed both above the roadway and below the embankment at select sites. Global stability analysis across all sites included bedrock depths ranging from 0 feet to greater than 20 feet below the existing road surface. Depth to bedrock was assumed based on apparent dip of bedrock exposed at the surface and the angle of the slope above the roadway. Therefore, this assumption for slope stability analysis does not necessarily reflect the actual subsurface profile. Excavation for the retaining walls should be expected to encounter cobbles and boulders (sizes as noted in Section 3), trees and other vegetal debris, tree roots, and bedrock. Recommendations regarding excavation in bedrock are presented below.

Slope geometry was based on cross sections prepared by the CFLHD Roadway Engineer from ground survey performed by CFLHD.

The back analyzed and assumed soil and rock parameters were used in further analyses to examine the effectiveness of repair options across the project sites. Material properties used in the design analyses are summarized below in Table 3, and the results of the slope stability analysis are attached

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 20 to this memorandum.

Table 4.2: Results of Slope Stability Back-analysis and Material Properties Used in Design

MATERIAL UNIT WEIGHT COHESION FRICTION ANGLE

(pcf) (psf) (degrees)

AC/Base/Subgrade 130 0 30 On-site Soil & Fill 125 20 40 Weathered Bedrock 125 500 36 Granular Backfill 130 0 34 Concrete Block 150 2000 35

1) Cohesion used in global stability only.

2) Cohesion of concrete block was artificially inflated to simulate assumed rigidity of a properly constructed face.

4.2.2. MSE Wall Design

Global stability analysis was performed during preliminary external design to evaluate the proposed wall geometry. The final MSE wall bearing pressures and internal stability will be designed and submitted by the contractor. Walls were designed in general accordance with the AASHTO LRFD Bridge Design Specifications 8th Edition, and FHWA Publication NHI-10-024 and NHI-10-025, Volumes 1 and 2, respectively, entitled “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes” dated November 2009.

The recommended factors of safety for global stability for static and seismic design are shown below in Table 4.3.

Table 4.3: MSE Wall Design Minimum Acceptable Factors of Safety

MODE OF

FAILURE

FACTOR OF

SAFETY

FACTOR OF

SAFETY

(STATIC) (SEISMIC)

Global Stability 1.3 1.1

MSE walls are earth retaining structures constructed of soil reinforcing elements (either steel or geosynthetic) placed in a high quality backfill material. These structures rely on the soil reinforcement to create a coherent mass to resist the lateral pressures imposed by materials behind the reinforced fill. MSE walls have been utilized on many projects as context sensitive solutions preserving scenic, aesthetic, historic, and environmental resources.

The MSE walls are designed in accordance with the guidelines provided in FHWA (2009) and AASHTO LRFD. Table 4.4 presents a summary of the analyzed wall geometries. Slide 7.0, published by Rocscience, was used to check global stability. Slide 7.0 uses the conservative Bishop simplified method to calculate overall factors of safety for slope or retaining wall stability. Static design

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 21 included a 250 lb/ft2 distributed load and seismic design used 50% of As for the horizontal acceleration in accordance with AASHTO. The MSE wall at Site 3 was analyzed under two scenarios; with bedrock and without. The assumed subsurface profile has the MSE wall founded on bedrock, but due to uncertainty in depth to bedrock and to be more conservative, the wall was also analyzed under no bedrock conditions. The preliminary analysis assumed a reinforcement spacing of 18-inch based on commonly available wall systems. The Contractor design should be based on the actual reinforcement spacing of the select wall system.

Table 4.4: Analyzed Wall Geometries

WALL

SITE

MAXIMUM

DESIGN

HEIGHT

(feet)

MINIMUM

REINFORCEMENT

LENGTH

(feet)

REINFORCEMENT

TYPE

(FP-14, 714.04)

Site 2 15 10.5 Type 1

Site 3 25 17.5 Type 3

Site 3a 20 14 Type 3

The minimum reinforcement lengths presented in Table 4.4 above are based on the limited preliminary analysis as described in this section. Specifically, these minimum lengths represent the values required to provide an acceptable global factor of safety. External stability (sliding, bearing, and overturning) and internal stability (facing connection, reinforcement strength, pullout) were not checked and are the responsibility of the Contractor as specified in Section 257 of the FP-14 and project SCR. Longer reinforcement lengths may be required to satisfy other design criteria or for ease of construction.

Results of the global stability modeling are attached to this memo and summarized in Table 4.5.

The design values exceed the minimum values in Table 4.3.

Table 4.5: Calculated Global Stability Factors of Safety

WALL SITE

FACTOR OF SAFETY

STATIC SEISMIC

Site 2 1.67 1.28

Site 3 (With bedrock) 1.78 1.40 Site 3 (No bedrock) 1.53 1.10

Site 3a 1.51 1.10

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 22

4.2.3. Gravity Wall Design

Global stability analysis was performed during preliminary external design to evaluate the proposed wall geometry. The final gravity wall external and internal stability will be designed and submitted by the contractor. Walls geometries analyzed were based on typical gravity wall layouts and geometries..

The recommended factors of safety for global stability for static and seismic design are shown below in Table 4.6.

Table 4.6: Gravity Wall Design Minimum Factors of Safety

MODE OF

FAILURE

FACTOR OF

SAFETY

FACTOR OF

SAFETY

(STATIC) (SEISMIC)

Global Stability 1.3 1.1

Gravity block walls are earth retaining structures constructed of large interlocking, dry-stacked concrete blocks, typically placed without mortar or reinforcement. These structures rely on the weight, size, shape, and interface friction of the individual blocks to resist earth pressures and provide overall stability. Most wet-cast concrete block units include pins, knobs, or other connection devices to maintain alignment during construction and improve connection strength between blocks. Gravity block walls have been utilized on many projects as context sensitive solutions preserving scenic, aesthetic, historic, and environmental resources.

The gravity walls are designed in accordance with the guidelines provided in FHWA Geotechnical Technical Guidance Manual (2007) and AASHTO LRFD. Design was based on the proposed typical roadway section and cross sections prepared by the CFLHD Roadway Engineer. Table 4.7 presents a summary of the analyzed wall geometries. Slide 7.0, published by Rocscience, was used to check global stability. Slide 7.0 uses the conservative Bishop simplified method to calculate overall factors of safety for slope or retaining wall stability. Static design included a 250 lb/ft2 distributed load and seismic design used 50% of As for the horizontal acceleration.

Table 4.7: Analyzed Wall Geometries

WALL

SITE

HEIGHT

H, ft

BASE

WIDTH

B, ft

EMBEDMENT

DEPTH AT

TOE

D, ft

BACKSLOPE

ANGLE

β, degrees

WALL

FACE

BATTER

degrees Site 3a 10.5 2 2 26.6 5 Site 7 7.5 2 2 26.6 5

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 23

Results of the global stability modeling are attached to this memo and summarized in Table 4.8.

The design values exceed the minimum values in Table 4.6.

Table 4.8: Calculated Global Stability Factors of Safety for Gravity Walls

WALL SITE

FACTOR OF SAFETY

STATIC SEISMIC

Site 3a 1.63 1.38 Site 7 2.64 2.23

4.2.4. Retaining Wall Design Considerations

MSE and gravity retaining walls should be designed according to Section 257 of the FP-14 and Project SCR’s. Contractor design should include evaluation of external stability (sliding, overturning, bearing capacity, etc.) and internal stability (block to block connection, geogrid strength, etc.). Use of shored or stable feature MSE wall (SMSE) can be considered based on bedrock conditions exposed during construction. Prior to wall design, the contractor should excavate test pits at the retaining wall locations to evaluate subsurface conditions. Specifically, they should verify the presence, location, and quality of bedrock within or adjacent to the proposed retaining wall excavation. The CFLHD geotechnical engineer should be present to observe excavation of test pits and the wall excavation to confirm that design assumptions are as anticipated in the design. The wall design should consider the presence of bedrock, if encountered, and provide a truncated base wall design. In any case, excavation of bedrock should be limited to the maximum extent possible.

Contractor designed retaining walls should be designed using material properties as listed in Table 3, and conform to Section 257 of the FP-14 and project SCR’s. MSE walls should be constructed in accordance with Section 255 of FP-14. Gravity walls should be constructed in accordance with Section 264 of the project SCR’s. Based on the global stability analysis presented in the memorandum, the minimum reinforcement length should meet the AASHTO recommended minimums (70 percent of design height to a minimum of 8 feet). Selection of MSE or gravity walls for each site is left to the discretion of the Contractor. At minimum, the Contractor should consider cost and constructability in their wall selection.

4.3. SHRINK/SWELL RECOMMENDATIONS

We estimate that alluvial soils within the project area will have a 5-10% shrink percentage, corresponding to a shrink/swell factor of 0.95-0.90. The recommended shrink/swell factor is based on a combination of standard tabled values for common materials in the FLH Technical Guidance Manual (2006) and experience with other CFLHD projects in similar materials.

4.4. SUBEXCAVATION

No subsurface investigation was performed for this scope of work and no areas of soft, wet soils were

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 24 identified during site reconnaissance. Therefore, no specific areas for subexcavation were identified by the Geotechnical Engineer. We anticipate that areas of unstable subgrade are likely to be limited to areas of poor drainage or saturated materials, which may occur seasonally following periods of precipitation. An allowance of 100 cubic feet of subexcavation should be included in the project to be used at the discretion of the Contract Officer to stabilize areas of soft or unstable subgrade materials, if encountered. The subexcavation should be two feet deep with one layer of non-woven separation geotextile (Class 1, Type B) placed at the bottom of the subexcavation. The bottom of subexcavation should be inclined to drain outside of the roadway prism. Subexcavation should extend either to centerline or the full roadway width based on the conditions encountered at the judgement of the CO. Because the requirement for subexcavation is likely to be caused primarily by excess moisture in the on-site materials, adjustment of the moisture content of these materials will likely produce suitable fill material to replace in the subexcavation area.

4.5. GEOSYNTHETICS

Separation geotextile for retaining walls should be Class 1 Type A non-woven geotextile (FP-14 section 714.01). Separation geotextile for riprap applications should be Class 1, Type A non-woven geotextile (FP-14 section 714.01).

4.6. CULVERTS AND WINGWALLS

Design of culverts and wingwalls should follow the material properties used in the retaining wall analysis above (Table 4.2). Debris racks should be included on large (greater than 48-inch diameter, or equivalent) culverts to capture debris and ease maintenance of the culverts.

4.7. CONSTRUCTION CONSIDERATIONS

Specifications: All geotechnical recommendations stated above are incorporated into the SCRs to amend or are already incorporated within the FHWA Standard Specification for Construction of Roads and Bridges on Federal Highway Projects; known as FP-14.

Roadway Excavation: Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits.

Hard bedrock is anticipated to be encountered at some locations in excavations for the roadway, retaining walls, and culverts. Based on the surface geologic reconnaissance, the bedrock is expected to be rippable near the surface in areas identified for retaining wall construction; however, final determination of bedrock rippability is the responsibility of the contractor. Limited controlled blasting may be efficient for excavation of the bedrock or to break up larger boulders.

Retaining Walls: Considerations for roadway excavation also apply to excavation for the retaining walls. Due to no subsurface investigation, depth to bedrock is assumed, and may be encountered during excavation at all retaining wall sites. Notably, bedrock appears to be present near the base of the Summit Creek channel at Site 2. The foundation conditions should be evaluated by the CFLHD Geotechnical Engineer to verify the materials encountered. The contractor should document the

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 25 location of bedrock encountered during the retaining wall excavation for review by the CFLHD Geotechnical Engineer. The contractor should work with CFLHD to document bedrock locations early in the construction process, so that design revision may be completed in a timely manner. This should include excavation of test pits at wall locations to provide subsurface information in advance of other construction activities. The encountered bedrock should be evaluated by the CFLHD Geotechnical Engineer. If hard, intact bedrock is encountered, revision of the retaining wall design may be required. This design revision may include truncation of the lower portion of the MSE wall.

As previously stated, we recommend that the contractor perform test excavations at the wall locations to evaluate subsurface conditions prior to the wall design work to limit the need for wall design revisions.

The toe of all retaining walls are anticipated to be below the high-water mark and should be protected against scour as recommended by the CFLHD Hydraulics Engineer. From the bottom of the wall to one foot above the design high-water mark, the Select Granular Backfill should be replaced with Granular Rock Backdrain. This will allow water pressures induced in the wall during high flow events to dissipate rapidly, reducing the risk of wall failure. The upstream end of the walls should be finished with a return to the full width of the embankment or to a stable bedrock surface. This will reduce the reduce the risk of future floods flanking the retaining wall and causing a wall failure.

All walls should be founded either below scour depth (as directed by the CFLHD Hydraulics Engineer), on intact bedrock (as evaluated during construction by the CFLHD Geotechnical Engineer), or the ground below the wall should be protected with scour countermeasures (as directed by the CFLHD Hydraulics Engineer). Where walls are to be founded on rock, the leveling pad should be constructed by performing limited excavation into the bedrock, constructing appropriate formwork, and placing concrete.

Import materials for MSE and gravity walls should be evaluated according to relevant sections of the FP-14 and project SRC’s before delivery to the project site.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 26

SECTION FIVE--LIMITATIONS

The recommendations in this report are based on the data obtained from limited 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. Subsurface exploration was not performed as a part of this scope of work.

Interpretation of surface and subsurface conditions is based on limited field reconnaissance of soil and rock outcrops. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Santaquin Canyon Road, UT EFRO 70014(1) January 2021 Page 27

SECTION SIX--REFERENCES

American Association of State Highway and Transportation Officials (AASHTO). (2018).

“AASHTO LFRD Bridge Design Specifications”, 7th 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), 2017, Soil Description and Identification Guidelines, dated December.

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

Solomon, B.J. (2010). “Interim geologic map of unconsolidated deposits in the Payson Lakes quadrangle, Utah and Juab Counties, Utah,” 1:24,000. Open-File Report 571. Utah Geological Survey.

“National Seismic Hazard Map”, United States Geological Survey (USGS), 2019, website:

http://earthquake.usgs.gov/research/hazmap

U.S. Geological Survey (2019). “Quaternary Fault and Fold Database of the United States.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/hazards/qfaults/>

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2009). “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes.”

Publication No. FHWA-NHI-10-024.

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. p 313- 316.

Witkind, I.J., and Weiss, M.P. (1991). “Geologic map of the Nephi 30’ X 60’ quadrangle, Carbon, Emery, Juab, Sanpete, Utah, and Wasatch Counties, Utah,” 1:100,000. Miscellaneous Investigations Series Map I-1937. U.S. Geological Survey.

http://earthquake.usgs.gov/research/hazmap

Santaquin Canyon Road, UT EFRO 70014(1) January 2021

APPENDIX A

Field Exploration

APPENDIX A

FIELD EXPLORATION PROGRAM

A.1 INTRODUCTION

The Central Federal Lands Highway Division (CFLHD) Geotechnical Section completed a field exploration program for UT ERFO 70014(1), Santaquin Canyon Road, from June 25 through 26, 2020. The scope of work for the geotechnical field exploration program included limited geological and geotechnical reconnaissance. The methods used to conduct the field exploration program are described below. Photos of the damaged sites are included in Appendix C.