UT.FLAP.73(2).La Sal Mountain Loop FINAL.2020.pdf

LA SAL MOUNTAIN LOOP ROAD (PHASE 2)

MANTI-LA SAL NATIONAL FOREST

SAN JUAN COUNTY, UT

UT FLAP 73(2)

FINAL Geotechnical Report

Report No. UT-FX-0073-20-01

Prepared by:

Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch March 2020 ii

SIGNATURE SHEET

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

Report reviewed by: ____________________________________________________________ Devin T. Dixon, P.E., Geotechnical Engineer

Approved for distribution by: ____________________________________________________

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

Electronic Distribution N:\UT\ut73(2)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Designer i

La Sal Mountain Loop, Phase 2, UT FLAP 73 (2) March 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. SEISMIC DESIGN PARAMETERS

2.3. GEOLOGIC HAZARDS

SECTION THREE --FIELD EXPLORATION

3.1. FIELD EXPLORATION PROGRAM

3.2. LABORATORY TESTING PROGRAM

3.3. EXISTING CUT SLOPES

SECTION FOUR --ANALYSIS AND RECOMMENDATIONS

4.1. CUTSLOPE EVALUATION AND RECOMMENDATIONS

4.2. ROCKFALL CATCHMENT DITCH

4.3. ROCK SLOPE SCALING

4.4. RETAINING WALLS

4.5. ROCK EMBANKMENT

4.6. EMBANKMENT CONSTRUCTION

4.7. SHRINK/SWELL RECOMMENDATIONS

4.8. SUBEXCAVATION

4.9. UNDERDRAINS

4.10. GEOSYNTHETICS

4.11. CONSTRUCTION CONSIDERATIONS

SECTION FIVE --LIMITATIONS

SECTION SIX --REFERENCES

ii

La Sal Mountain Loop, Phase 2, UT FLAP 73 (2) March 2020 Page ii

TABLES

Table 2.1: Summary of Seismic Parameters 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 3.4: Soil and Rock Type Categories within the Cutslopes

Table 4.1: Summary of Cut Slope Recommendations

Table 4.2: Summary of Ditch Width Recommendations

Table 4.3: Summary of Shrink/Swell Factors

FIGURES

Figure 1: Typical Roadway Conditions Figure 2: Applicable Ditch Widths for Varying Slope Heights Figure 3: Range in boulder sizes at lower portion of project. Boulders in photo range from approximately 1-foot to 10-foot diameter. Photo is at approximate STA 120+00.

Figure 4: Range of boulder sizes at lower portion of project. Boulders in photo range from approximately 1-foot to 2-foot diameter. Photo is at approximate STA 160+00 Figure 5: Range of boulder sizes at middle portion of project. Boulders on native slope above cut slope range from approximately 1-foot to 3-foot diameter. Photo is at approximate STA 278+00 Figure 6: Range of boulder sizes at middle portion of project. Boulders range from approximately 1-foot to 10-foot diameter. Photo is at approximate STA 330+00 Figure 7: Range of boulder sizes in upper portion of project. Boulders range from approximately 1-foot to 6-foot diameter. Photo is at approximate STA 450+00 Figure 8: Range of boulder sizes in upper portion of project. Boulders range from approximately 1-foot to 4-foot diameter. Photo is at approximate STA 650+00

PLATES

Plate 1: Location Map Plate 2: Geology Map

APPENDICES

Appendix A: Summary of Nearby Active Faults Appendix B: Rock Scaling Areas Appendix C: Excavation Characteristics of Rock and Rippability Charts

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 1

SECTION ONE--INTRODUCTION

1.1. BACKGROUND AND LOCATION

This report presents the results of the geotechnical engineering study and provides recommendations for the reconstruction of the 12.15-mile long section of the La Sal Mountain Loop UT FLAP 73(2) project. The project corridor is located in the La Sal Mountains, within the Manti-La Sal National Forest, San Juan County, Utah. The project begins at the intersection of Old Airport Road at Sta.

10+00.00 at an approximate elevation of 4,868 feet. The road climbs to a high point of approximately 7,960 feet near the intersection with Geyser Pass Road (FR 50071) and then descends in a northerly direction, ending at an approximate elevation of 7,740 feet at Sta. 651+35, where the UT FLAP 73(1) La Sal Mountain Loop Road project began. The project is located in generally undeveloped desert, brush, and forested terrain between the aforementioned elevations with varying seasonal precipitation (see Figure 1 below for typical roadway conditions). Isolated private inholdings with residential development are present along the route. Due to the high elevations along the route, construction on portions of this route will be limited to spring, summer, and autumn months (April through October) depending on winter precipitation and temperatures. Lower elevation portions of the route may be open for construction year-round. A site map is presented on Plate 1.

Figure 1: Typical Roadway Conditions

The La Sal Mountain Loop serves as a Utah scenic backway, and provides access to recreational areas, including fishing, biking/hiking trails, hunting, skiing, snowmobiling, and camping. The overall goal of the La Sal Mountain Loop improvement project is to balance transportation requirements and roadway maintenance within an environmentally sensitive setting. The roadway

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 2 currently exists as a 20 to 22 feet wide asphalt paved road, with existing bench widths varying from 20 to 26 feet. The alignment is curvilinear with a few sharp turns and switchbacks. The route traverses desert, scrubland, and juniper forests in hilly to mountainous terrain. Cut slopes are excavated into surficial soils (colluvium or residual materials) or bedrock, with side cast embankments to provide the existing roadway width. Limited areas of through-cut and through-embankment are also present.

The current average daily traffic provided with the FLAP application is 380 vehicles per day with a seasonal average daily traffic of 457.

This project proposes to rehabilitate and reconstruct the existing road to provide two 11-foot lanes and 2-foot paved shoulders (24-foot total width) with minor realignment, superelevation correction, and ditch reconstruction. The existing pavement is in poor condition with significant rutting, cracking, and potholing apparent along the route. Minor realignment including curve widening will improve safety on the route. To complete the proposed widening and realignment, cuts and fills are expected. Culvert replacements are also proposed. Areas with noted seepage or soft subgrade will be overexcavated and stabilized using geotextile fabric.

1.2. SCOPE AND PURPOSE

The scope of work included geotechnical investigation, analysis, and recommendations for cuts and fills within the project limits for use in design and construction. This involved several tasks including field reconnaissance, interpretation and correlation of data, and geotechnical engineering analysis.

Specifically, this investigation is to determine the soil profiles at each cut and fill location and develop recommendations concerning embankments, cut slopes, surface/subsurface drainage, geological hazards, material shrink/swell, and construction considerations for the design and construction of slopes and retaining structures within the alignment. The stationing in this report is based on the 95 percent project plans, dated March 12, 2019.

1.3. PREVIOUS WORK

Two previous projects have been completed in this region in the past decade:

• La Sal Mountain Loop Road – Miners Basin and Mill Creek Segments, Project No. UT PFH 46-1(2)

• La Sal Mountain Loop Road, Project No. UT FLAP 73(1)

Geotechnical reports were prepared for both projects. A pavement report was completed for the Miners Basin and Mill Creek Segment project; however, this pavement report includes data and recommendations for the entire La Sal Mountain Loop corridor. Following is a summary of available reports:

• “Manti-La Sal National Forest, UT PFH 46-1(2) La Sal Mountain Loop Road, Pavement Report”, Report No. 11-02, dated April 2011.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 3

• “La Sal Mountain Loop Road, Miners Basin and Mill Creek Segments, UT PFH 46-1(2), Final Geotechnical Report”, Report No. UT-FX-0046-11-01, dated March 2012.

• “La Sal Mountain Loop Road, Manti-La Sal National Forest, Grand County, San Juan County, UT FLAP 73(1), Final Geotechnical Design Report”, Report No. UT-FX-0073-16- 01, dated June 2016.

The subject reports were reviewed for content related to the general geologic setting and hazards in the project region. The scope of the geotechnical reports was limited to the proposed project scopes, while the scope of the pavement report included the entire La Sal Mountain Loop corridor. Relevant data from the pavement report is discussed in Section 3 of this report.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 4

SECTION TWO--GEOLOGY AND SEISMICITY

2.1. REGIONAL GEOLOGY

The project is located within the La Sal Mountains, an isolated group of mountains in southeast Utah, near the Colorado border. The La Sal Mountains are located within the Canyonlands Section of the Colorado Plateau Province. The core of the range is composed of trachytic intrusive rocks, which were emplaced into older sedimentary rocks. The flanks of the La Sal mountains are draped in Jurassic and Cretaceous sedimentary rocks, primarily sandstone and mudstone, with minor limestone and conglomerate. The principle mapped deposits within the project alignment consist of recent alluvial gravel (Qagy); Quaternary Pediment-mantle deposits (Qap), mixed eolian-alluvial deposits (Qea), and slumps and landslides (Qms); Cretaceous Dakota Sandstone (Kd) and Burro Canyon Formation (Kbc); and Jurassic Morrison, Tidwell, and Summerville Formations (Jmb, Jsms, Jsmt, Jms) (Doelling, 2004). A geology map of the area is shown on Plate 2.

No seismic hazard faults are located within 40 miles of the project. Numerous known Quaternary faults are mapped within approximately 40 miles of the roadway alignment. These faults are considered inactive and are summarized on Appendix A. The Moab Fault and Deformation Zone crosses the project alignment (U.S. Geological Survey, (2018).

2.2. SEISMIC DESIGN PARAMETERS

Based on the preliminary 30% plans, no major structures are proposed for the project. Rock embankment is proposed to retain the roadway prism at limited locations. The materials in the project area are assumed to include a thick mantle of recent stream bed (alluvial) 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 La Sal Mountain Loop Road (38.458103º N latitude and -109.417346º W longitudes) was obtained in accordance with the AASHTO ground motion maps.

The site was classified as “Class D” according to site class definitions specified in Table 3.10.3.1-1 of AASHTO based on the assumed soil profile. The recommended spectral acceleration coefficient values for probabilistic design are summarized in the following table.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 5

Table 2.1: Summary of Seismic Parameters for Site Class D

Peak Seismic Ground Acceleration Coefficient, (As) 0.094

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.209

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.093

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.6

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 1.6

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 2.4

Based on the long acceleration coefficient SD1 value of 0.093, the site is assigned to seismic hazard Zone 1 in accordance with Table 3.10.6-1 of AASHTO. Based on this classification, seismic loading is assumed to not control design of the proposed retaining structures, and is not included in the design.

2.3. GEOLOGIC HAZARDS

The La Sal Mountain Loop is located in mountainous terrain with portions at high elevation with rugged topography and adverse climatic conditions. Due to the steep terrain, rock fall and landslides could potentially occur within the project area. Some fine-grained soils in the project area are highly erodible, presenting concerns for performance of the roadway prism. Sandstone, siltstone, claystone, and shale rock units are frequently susceptible to slaking and deterioration when exposed to alternating wetting and drying cycles in rock cuts. Differential erosion of cuts in sedimentary rocks, where weaker shale is eroded away faster than more resistant sandstone units, is a common cause of rockfall in terrain similar to this project. Debris flows and flash floods are also possible.

Some shale units likely contain smectite clays, are sensitive to changes in moisture content (shrink / swell), and may provide poor roadway subgrade support. Shale units also expand during weathering and are potentially subject to collapse upon subsequent wetting. An irrigation canal is present adjacent to portions of the roadway, and is a potential source of subsurface water that may reduce subgrade stability. Many of the shale units are marine derived and are likely corrosive and have higher potential concentrations of sulfates.

The Salt Wash Member of the Morrison Formation (noted as Jms on Plate 2) is described as “upper sandstone lens is generally mineralized with uranium and vanadium.” Material produced from cuts in Jms (generally from Sta. 330+00 to Sta. 525+00) should be observed. Areas with evidence of a reducing environment, such as greenish coloration, are more likely to contain deposits of uranium.

If such materials are encountered during construction, then they should be tested and handled according to applicable local, state, and federal regulations.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 6

These hazards can generally be mitigated by constructing slopes at appropriate angles and close control of moisture content and density of subgrade materials, as discussed in Section 4 of this report.

Tight moisture and density control of native soils below the base of embankments should be followed. Underdrains are recommended for areas with identified seepage as discussed in Section 4 of this report. Surface drainage should also be controlled with appropriate hydraulic countermeasures, as directed by the CFLHD Hydraulics Engineer.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 7

SECTION THREE--FIELD EXPLORATION

3.1. FIELD EXPLORATION PROGRAM

A subsurface exploration targeting proposed cut and fill slopes and other proposed improvements was not performed for this scope of work. Surficial evaluation and mapping was performed based on areas identified during office review of the preliminary 30% plans and the associated site visit. A pavement investigation was previously performed as part of the UT PFH 46-1(2) La Sal Mountain Loop Road project, which includes the current project segment. The relevant results of this investigation are presented in Pavements Report #11-02, dated April, 2011, and are summarized in Table 3.1.

Table 3.1: Summary of the Field Exploration Program

BORING

NUMBER STATION OFFSET EXPLORATION

DEPTH

(inches)

B-4 140+65.68 RT 59.5

B-8 193+45.68 RT 58

B-12 246+25.68 RT 55

B-16 299+05.68 RT 25

B-20 351+35.68 RT 23.5

B-24 404+15.68 LT 18

B-28 456+95.68 LT 28

B-32 509+75.68 LT 46.5

B-36 562+55.68 LT 44

B-40 615+35.68 RT 44

Note: The exploration locations were presented in the Pavements Report based on Milepost with note that MP 2.0 is located at the intersection of La Sal Loop Road and Ken’s Lake Road. This point was used to estimate the boring station based on the 95% project plans.

3.2. LABORATORY TESTING PROGRAM

No samples were collected for evaluation for this scope of work. As discussed previously, a pavement investigation that included collection of samples for laboratory testing was performed for the UT PFH 46-1(2) project. Laboratory tests included gradation (AASHTO T88), classification (AASHTO M145) and Atterberg limits (AASHTO T89 and T90). 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 presented in full in Pavements Report #11-02.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 8

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.

(inches)

B-4 - - - - - - - -

B-8 16 - 58 8 50 42 20 10 SC A-4(1)

B-12 7 - 55 12 40 48 33 23 SC A-6(6)

B-16 9 - 25 23 49 28 21 12 SC A-2-6(0)

B-20 11.5 - 23.5 24 43 33 26 13 SC A-2-6(1)

B-24 8 - 18 9 61 30 18 6 SC-SM A-2-4(0)

B-28 12 - 28 16 38 46 24 12 SC A-6(2)

B-32 6.5 - 46.5 15 29 56 32 20 CL A-6(7)

B-36 12 - 44 27 21 52 45 31 CL A-7-6(12)

B-40 8 - 44 10 31 59 38 25 CL A-6(11)

- Tests not completed.

Soil test results indicated a range of material types including silty to clayey sand with gravel to sandy clay materials, classifying as A-2-4 to A-7-6 by AASHTO and SM, SC 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

(inches) (ohm-cm) (ppm) (ppm)

B-12 7 - 55 1130 7.1 152 140 B-36 12 - 44 1130 7.7 139 50

3.3. EXISTING CUT SLOPES

Existing cut slope conditions were evaluated along the roadway alignment. The evaluation was conducted to assess the stability of existing slopes and to design cut ratios for various slopes along the route. Cut and fill slopes were observed to be of variable slope ratios and heights, with evidence of raveling and rockfall in select cut slopes.

Visual inspection of slopes, along with geological knowledge, provided the means for evaluating slope stability. In general, slopes with smooth contours indicate that surficial erosion processes are

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 9 dominant, whereas irregular slopes with stair-stepping contours, scarp lines, sloping trees, or changes in vegetation, typically indicate slope movement. Due to the interbedded nature of the bedrock at the project site, many slopes present a series of slopes and ledges. Evaluation of the stability of these slopes requires additional evidence beyond overall topographic expression, such as presence of seeps, disrupted bedrock layers, and tilted vegetation. Although the majority of slopes appeared to meet an acceptable factor of safety for low volume roads, isolated areas of slumps, landslides, and rockfall are present. For cut slope design purposes and evaluation of shrink/bulk factors, the materials encountered were categorized into five different types as show in Table 3.4.

Table 3.4: Soil and Rock Type Categories within the Cutslopes

GENERALIZED SLOPE MATERIAL DESCRIPTION

Soil Type I - SM-SC

Alluvial silty to clayey SAND with gravel, cobble, and boulder, light brown to light reddish brown, dry to slightly moist, fine sand to boulders, sub-rounded to rounded, low plastic, non-cemented. Boulders are approximately 25 to 35 percent of total deposit.

Soil Type II – SM-SC

Alluvial/colluvial silty to clayey SAND with gravel, cobble, and boulder, light brown to light reddish brown, dry to slightly moist, fine sand to boulders, sub-rounded to rounded, low plastic, non-cemented. Boulders are approximately 35 to 50 percent of total deposit. Highly fractured, moderately weathered sandstone bedrock present in slope and forms most of the boulders.

Soil Type III – SM-SC

Alluvial silty to clayey SAND with gravel, cobble, and boulder, light brown to light reddish brown, dry to slightly moist, fine sand to boulders, sub-rounded to rounded, low plastic, non-cemented. Boulders are approximately 10 to 30 percent of total deposit.

Rock Type IV – SS/SH

Interbedded SANDSTONE and SHALE. Sandstone is white, green, light orange, and light brown, fine to medium grained, slightly to moderately weathered, R2 to R3 strength. Bedding, moderately spaced, good condition, frequently dipping out of slope. Fractures, moderately to widely spaced, fair to poor condition, near vertical. Shale is fine grained, moderately weathered, R1 to R2 strength. Bedding and fractures similar orientation to sandstone, very closely spaced, poor condition.

Rock Type V - SH SHALE, pale reddish brown, light brown, dark reddish brown, and white, fine grained, moderately to highly weathered, R0 strength. Rock fabric frequently disrupted by weathering processes.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 10

SECTION FOUR--ANALYSIS AND RECOMMENDATIONS

This section provides analysis and recommendations for the cut slopes, rockfall catchment ditches, embankment fills, shrink/swell factors, subexcavation, underdrains, and construction considerations.

4.1. CUTSLOPE EVALUATION AND RECOMMENDATIONS

Expansion of the existing cutslopes is proposed to accommodate roadway design concepts. Table 4.1 summarizes existing cutslope materials and geometry and provides new cutslope ratios for design.

Recommended slope ratios are based on observed conditions at the existing cut face, investigation data, and anticipated slope height. In general, non-saturated coarse-grained granular soil slopes can be constructed at a safe slope ratio of 1V:1.5H or flatter; non-saturated slopes composed of fine-grained, cohesive soils can be constructed at a safe slope ratio of 1V:2H; weathered bedrock slopes can be constructed at a slope ratio of 1V:1H or flatter depending on fracturing and degree of weathering; and combination soil and rock slopes can be cut at 1V:1.5H. Cutslopes should not exceed slope ratios presented in Table 4.1. Areas of observed springs and seeps encountered during construction may require flattening or additional support and should be evaluated by the CFLHD geotechnical engineer on a case by case basis. Steeper slopes decrease the likelihood of re-vegetation and increase the likelihood and frequency of slumping, raveling, and rock fall events; flatter slopes should be utilized where possible.

On-site soils encountered within the anticipated excavation and construction limits generally consist of colluvial and residual silty to clayey sands with varying amounts of gravels and silt and clay/silt soils. Weathered bedrock encountered within the anticipated excavation and construction limits largely consists of interbedded sandstones and shales. The sandstones are generally hard and resistant with varying joint spacing from a few feet to approximately ten feet. Intact sandstone is marginally rippable, and may require use of hydraulic rams to break it up into pieces that can be handled by conventional construction equipment. Blasting may be more efficient for excavation of sandstone.

Blasting should follow the provisions of SCR Section 205, including requirements for a test blast.

Location of the test blast should be selected by the Contractor in a location near areas of proposed blasting and in similar ground conditions. The shales are generally firm and weathered with closely spaced bedding and joints. Intact shale is likely rippable with convention heavy-duty construction equipment. Additional discussions of rock rippability are presented in Appendix C.

Table 4.1: Summary of Cut Slope Recommendations

Begin Sta.

End Sta. Offset

Existing Cut

Slope Height

Existing Cut

Slope Angle

Cut Slope

Material (Table

3.3)

Max.

Rec.

Slope Ratio (V:H)

Max.

Rec.

Slope

Height

Notes and Recommendations

(feet) (degrees) (feet)

95+00 122+00 RT 0 - 5 20 Soil I 1 : 2 15 122+00 128+00 RT 5 - 10 31 Soil II 1 : 1.5 60 133+00 141+00 RT 5 - 10 13 - 26 Soil I 1 : 2 20

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Begin Sta.

End Sta. Offset

Existing Cut

Slope Height

Existing Cut

Slope Angle

Cut Slope

Material (Table

3.3)

Max.

Rec.

Slope Ratio (V:H)

Max.

Rec.

Slope

Height

Notes and Recommendations

(feet) (degrees) (feet)

164+00 184+00 LT - - Soil II 1 : 2 10 Do not impact or undercut irrigation canal.

190+00 215+00 LT 0 - 5 12 - 16 Soil III 1 : 3 10 251+00 265+00 LT 10-25 31 - 35 Soil I 1 : 1.5 25 265+00 271+00 LT 10-25 31 - 35 Soil II 1 : 2 20

274+00 282+00 LT 25 38 Rock IV 1 :

1.5** 20

Bedrock is highly weathered, with thick soil mantel.

319+00 335+50 RT 20 - 48 38 - 65 Rock IV 1 : 1** 10

Limit cuts to reduce impacts to native slopes. Scale sandstone blocks.

335+50 340+00 RT 20 - 48 38 - 65 Rock IV 1 : 2 25 Transition to shallower cut in alluvial materials.

372+50 374+50 RT 48 34 Rock V 1 : 1.5 10 444+50 454+50 LT 30 45 Rock IV 1 : 1** 10 Scale gully area.

456+50 462+50 LT 45 45 Rock IV 1 : 1** 10

468+50 472+00 LT 30 45 Rock IV 1 : 1** 5 Clean ditches and scale slopes.

487+50 491+00 RT 22 45 Rock IV 1 : 2 5 506+00 508+50 LT 30 36 Rock IV 1 : 1 15

511+00 515+50 LT 23 45 Rock IV 1 : 1.5 50 Re-cut slope to improve stability.

537+50 643+50 RT - - * See Notes

Obscured by snow.

Slopes likely wet and quasi-stable. Do not cut.

586+50 590+00 LT - - * See Notes Match existing slopes, max. 1V : 2H.

649+00 649+50 RT 25 43 Rock IV See Notes Ditch clean only.

* Geology not apparent due to snow on slope.

** Maximum slope ratio presented is approximate average for the entire slope. For short (less than 10-feet high) rock cuts, hard, intact sandstone layers may be cut to match existing slopes, typically 1V:0.5H. Weak, friable sandstone should be cut to 1V:1H. Shale should be cut to 1:1.5.

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4.2. ROCKFALL CATCHMENT DITCH

Based on observations in the field and discussions with maintenance personnel, rockfall has not typically impacted the roadway in the past. Much of the existing roadway has relatively wide ditches, which frequently contain significant quantities of rockfall and other slope debris. We assume that these ditches have performed well in the past at retaining rockfall and preventing it from impacting the roadway. Therefore, we recommend effort be made to reestablish ditches in areas where the existing cut slopes will not be modified. In addition, the native slopes above the existing roadway cut slopes are commonly very high and steep, and would require significant impacts to reduce slope angles and improve overall slope stability. Reestablishment of the roadside ditches in these areas helps to reduce the potential of rockfall impacting the roadway. Reestablishment of the existing ditches should be limited to cleaning out loose debris and should not modify the cut slope.

Where new rock cut slopes are proposed and where existing cut slopes will be modified, rockfall catchment ditches should be installed. Catch ditches should be installed where the final cut slope angle will be steeper than 1V:1.5H. A rockfall catchment area is defined as the area between the roadway edge of shoulder and the base of a cut slope used to restrict a high percentage of rockfalls from reaching the roadway. The use of catchment areas to contain and restrict rockfall from reaching the roadway is one of the best and most cost effective rockfall protective measures. Oregon Department of Transportation (ODOT) guidance on rockfall catchment was used to analyze the ditch catchment effectiveness of various slope heights (Pierson et al., 2001). The intent of the study was to determine what ditch width would yield 80 percent catchment for various slope heights. The recommended catchment ditch widths for 1V:1H slopes for 80% catchment are presented in Figure

2. The solid dots in the figure represent statistical data points presented in the ODOT report. The dashed line presents a best fit trend for extrapolation beyond the measured data.

The catchment ditches should be installed with 1V:4H slopes with widths varying depending on the height of the cut slope as shown in Figure 2 and Table 4.2. At either end of the cut slope section, the rockfall catchment ditch width should transition to the standard roadside ditch width.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 13

Figure 2: Applicable Ditch Widths for Varying Slope Heights

Table 4.2: Summary of Ditch Width Recommendations

Begin Sta.

End Sta. Offset

Maximum Cut Slope

Height

Minimum Catchment

Ditch Width

(feet) (feet)

274+00 282+00 LT 25 n/a

319+00 337+50 RT 48 11.5

372+50 374+50 RT 48 11.5

444+50 454+50 LT 30 8.5

456+50 462+50 LT 45 11

468+50 472+00 LT 30 8.5

Due to the narrow nature of the existing roadway bench, the recommended ditch widths may not be attainable without significant cut and fill construction and associated environmental impacts. Where the minimum catchment ditch widths in Table 4.2 above cannot be included, there is an increased risk that rockfall will reach the roadway. The consequences of this risk varies from nuisance raveling of small rocks reaching the roadway that require clearing by maintenance crews, to large rocks reaching the travel lanes in areas where drivers may not have sufficient reaction time to avoid them.

0 10 20 30 40 50 60 70 80 90

M in

. D itc h W id th

(f ee t)

Slope Height (feet)

Catchment Ditch Width for 1V:1H Slopes

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 14

Based on the ODOT design charts, we estimate that the reduction in catchment in reducing an 11.5-foot ditch to a 6-foot ditch is approximately 30 percent (reduced from 80 percent to 50 percent).

However, this remains an improvement over the existing conditions at the site.

4.3. ROCK SLOPE SCALING

Slope scaling is the removal of loose, broken, or partially detached rock from the slope using manual methods, or excavation equipment. Manual, or “hand-scaling”, methods commonly involve a crew consisting of 3 experienced scalers skilled at systematically removing loose, isolated rockfall hazards with a variety of specialized hand tools, as well as manual and pneumatic jacks. Scaling crews typically work on ropes, but may occasionally work from crane baskets or lifts, and always require spotters on the roadway to continually survey the slope for unsafe working conditions. Slope excavation, or “machine-scaling,” methods commonly involve excavators using buckets to pry rock from the slope, hydraulic hammers to bring down larger unstable rock masses, and/or heavy chains to brush large quantities of loose debris from the slope. Machine scaling uses similar crew sizes and clean-up/haul equipment as hand-scaling, and also relies on roadway spotters to ensure operation safety. Hand-scaling and machine-scaling may be done in tandem on raveling slopes where high, localized rockfall exists outside the reach of common excavators (30-40 ft).

Scaling of existing rock slopes is recommended at select locations not mitigated by new permanent cut slopes. Scaling is conducted to reduce the amount and frequency of rock fall debris on the roadway by removal of loose and unstable material from existing slopes. Scaling is anticipated to be achieved by mechanical means, using a standard excavator equipped with a flat tooth (clean up) bucket. Approximately 40 crew hours (120 man hours) are recommended to be included in the contact documents. Scaling areas identified are located between STA 329+00 and STA 336+00.

Photographs of areas of slope scaling are presented in Appendix B. The highlighted areas may be adjusted as they are general locations for the proposed scaling operation. Other locations may be identified during construction. The primary goal of the scaling operations is to remove unstable blocks of sandstone within the existing cut slope, especially blocks that are undercut due to weathering and erosion of shale beds. To the maximum extent possible, the scaling should be limited to the lowest band of sandstone adjacent to the roadway. If unstable material higher on the slope is identified by the scaling contractor, the geotechnical engineer should be notified to evaluate the condition and provide a recommended course of action. All proposed scaling locations should be discussed on-site during the scaling pre-work meeting with a member of the CFLHD Geotechnical team and the scaling contractor.

It is recommended that the scaling operation be accomplished by means of hand tools only.

Unless the contractor can successfully demonstrate the use of machinery (long-reach excavators) to the satisfaction of the CFLHD Geotechnical engineer, scaling should not incorporate machinery as their use could easily loosen additional rock within the slopes that was not intended to be removed.

Equipment used during the scaling operation and ditch clean-up work should be sufficiently armored for the operator’s safety. As with any construction operation, it is the contractor’s responsibility to independently access the safety of the scaling operation and provide necessary measures to protect all construction personal, traveling public and roadway elements from harm.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 15

Assumptions for the slope scaling recommendations include:

• For slopes with moderate to low volumes of loose rock, scaling crews can quickly dress slopes with minimal delays to traffic. For slopes with large rockfall volumes, large localized rock masses to be removed, dangerous rockfall exposures high on the slope, or requiring slope crest excavation, roadway closures may be required.

• Scaling cannot be expected to remove the entire loose rock hazard from the slope. Hand-scalers will generally do a better job of removing small, loose rock to ensure safe overhead conditions as work progresses downslope. Machine-scaling operations typically remove larger volumes of larger rock from the slope, but tend to leave small slope debris behind. “Chaining” or “brushing” by dragging chains over the slope can be effective in removing small rock debris.

• Hand scaling should not progress downslope if stable conditions cannot be obtained within the upper reaches of the slope. If unstable conditions cannot be remedied, the scaling operation should be abandoned or machine scaled after discussions with a member of the CFLHD Geotechnical staff.

• Scaling results are proportional to the skill and experience of the scalers. Experienced crews cost more, but typically work faster and produce higher quality results (buying years of slope performance in many cases).

• Unsupervised scaling may result in substantial over-excavation and significant overruns in scaling quantities if expected results are not well communicated and closely monitored.

• Scaling is not a permanent solution to rockfall. Scaling frequency is dependent on many factors, including the erosional/weathering durability of the slope, extent and persistence of structural features producing loose rock, and presence of other effective mitigation measures such as catchment ditches or barriers.

Effective scaling programs require clearly communicated expectations during contracting (pre-bid on-site meetings and special contact specifications), and supervised operations by knowledgeable construction management staff. Scaling measures for each slope identified should be evaluated early in the scaling program for continuity of work products.

Scaling is only a temporary or short-term rock fall mitigation technique to remove any loose material that could potentially dislodge in the near future. A regular scaling program every 5 to 10 years should be included in the county’s long-term maintenance plan.

4.4. RETAINING WALLS

The DRAFT version of this report and discussion during the 30% field review included recommendations for a retaining wall between STA 161+50 and STA 165+50. The purpose of this retaining wall was to reduce impacts from the proposed roadway fill slope into the adjacent channel.

Based on subsequent discussions, this impact is of lower concern than originally anticipated.

Therefore, the retaining wall is no longer required at this location.

4.5. ROCK EMBANKMENT

To limit impacts of the roadway embankment fill slopes, rock embankment consisting of hand or

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 16 mechanically placed rock (FP-14 705.04) can be used. The shoulder stabilization should be constructed with a maximum 8-foot height and 1V:1H outboard slope. At the maximum height, the base width should not be less than 6 feet. Details for shoulder stabilization are included in the Plans.

4.6. EMBANKMENT CONSTRUCTION

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

Construct traditional embankments with a maximum slope ratio of 1V:2H 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).

Between approximate STA 561+00 and STA 563+00, construction of the roadway embankment will be relocated to an existing agricultural canal. To reduce potential for influx of water from the canal into the toe of the embankment, the canal should be lined with a geosynthetic clay liner meeting the requirements of Subsection 714.06 of the SCR.

4.7. 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, when the boulder content of the soils is ignored.

Where boulders are removed, the overall shrinkage will increase to approximately 20-25% (shrink/swell factor of 0.80-0.75) if materials are processed for use as unclassified borrow. If materials are proposed for use with a smaller maximum particle size, additional shrink can be expected. Igneous cobbles and boulders found in the alluvial deposits are generally durable. The durability of sandstone boulders found in the colluvial deposits varies throughout the project, and should be tested at each location where reuse is considered. Igneous cobbles and boulders of alluvial origin were typically 2 to 4 feet in diameter. Sandstone boulders are typically larger, generally ranging from 5 to 10 feet in diameter. Photos depicting typical boulder sizes throughout the project are presented in Figures 3 though 8.

Sandstone boulders that are durable enough to survive excavation, handlings, transportation, and placement may be incorporated into shoulder stabilization. These boulders should be evaluated in the field by the Project Engineer or the Geotechnical Engineer.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 17

Figure 3: Range in boulder sizes at lower portion of project. Boulders in photo range from approximately 1-foot to 10-foot diameter. Photo is at approximate STA 120+00.

Figure 4: Range of boulder sizes at lower portion of project. Boulders in photo range from approximately 1-foot to 2-foot diameter. Photo is at approximate STA 160+00.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 18

Figure 5: Range of boulder sizes at middle portion of project. Boulders on native slope above cut slope range from approximately 1-foot to 3-foot diameter. Photo is at approximate STA 278+00.

Figure 6: Range of boulder sizes at middle portion of project. Boulders range from approximately 1-foot to 10-foot diameter. Photo is at approximate STA 330+00.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 19

Figure 7: Range of boulder sizes in upper portion of project. Boulders range from approximately 1-foot to 6-foot diameter. Photo is at approximate STA 450+00.

Figure 8: Range of boulder sizes in upper portion of project. Boulders range from approximately 1-foot to 4-foot diameter. Photo is at approximate STA 650+00.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 20

If the sandstone bedrock is processed for use as fill material, we estimate that is will have a 20-30% bulk percentage, corresponding to a shrink/swell factor of 1.20-1.30. The shale bedrock is not recommended for use as fill within the project. Table 4.7 below presents the average shrink/swell factors that should be used for various station ranges on the project. 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.

Table 4.3: Summary of Shrink/Swell Factors

Begin Sta.

End Sta. Anticipated Materials Shrink/Swell

Percentage Shrink/Swell

Factor

(feet) (feet)

10+00 300+00 Soil I, II, or III, Alluvial silty sand and gravel with cobble and boulder -20% 0.80

300+00 350+00 Rock IV, interbedded sandstone and shale 20% 1.20

350+00 445+00 Rock V, shale * 15% 1.15

445+00 651+37 Rock IV, interbedded sandstone and shale 20% 1.20

* Not recommended for use as fill material.

4.8. SUBEXCAVATION

Two types of subexcavation are recommended for this project: Type I and Type II.

Due to the age and condition of the existing pavement section, no specific areas for subexcavation, type I were identified by the Geotechnical Engineer. An allowance of at least 1,000 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. Additional areas may be identified by the CFLHD Pavements Engineer. Subexcavation, Type I should be two feet deep with one layer of non-woven separation geotextile placed at the bottom of the subexcavation.

Subexcavation, Type II is proposed for the portion of the roadway between STA 375+00 and STA 400+00. The purpose of this subexcavation is to stabilize portions of the roadway that will be constructed predominantly on clay and weathered shale subgrade materials. Subexcavation, Type II 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 subexcation.

In both cases, 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.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 21

4.9. UNDERDRAINS

One area of poor drainage was observed during the field investigation between STA 568+00 to STA 573+00. Additional areas of poor drainage and cut slope seepage are likely to be identified during construction. Underdrains should be installed in these areas to reduce the potential for water to saturate the pavement subgrade. In addition to the 500 linear feet of underdrain identified, an allowance of 1,000 linear feet should be included in the contract to be used at the discretion of the Contracting Officer. Underdrain outlet pipes should be located at a maximum of 100-feet on-centers spacing.

4.10. GEOSYNTHETICS

The geogrid for subexcavation should meet the requirements of stabilization geogrid (FP-14 and SCR section 714.03). Separation geotextile for subexcavation and underdrains should be Class 2 Type C non-woven geotextile (FP-14 section 714.01). Geocomposite underdrains are not recommended for use on this project due to the high proportion of fines in the on-site soils and potential for clogging.

Separation geotextile for riprap, rock embankment, and similar applications should be Class 1, Type C non-woven geotextile (FP-14 section 714.01).

4.11. 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.

Based on the subsurface investigation and surface geologic mapping, the bedrock is expected to be rippable near the surface in areas identified for cut slope construction; however, final determination of bedrock rippability is the responsibility of the contractor. Limited controlled blasting may be efficient for exaction of the sandstone bedrock. Blasting is not allowed between STA 545+00 and STA 606+00 due to known nesting sites of Peregrine Falcon and potential habitat for Mexican Spotted Owl. Restrictions are discussed in detail in Subsection 107.10 of the SCR.

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 22

SECTION FIVE--LIMITATIONS

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

La Sal Mountain Loop, UT FLAP 73 (2) March 2020 Page 23

SECTION SIX--REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2018, “AASHTO LFRD Bridge Design Specifications”, 7th Edition.

Doelling, H.H. (2004). “Geologic Map of the La Sal 30’ x 60’ Quadrangle, San Juan, Wayne, and Garfield Counties, Utah, and Montrose and San Miguel Counties, Colorado.” 1:100,000.

1:125,000. Map 205. Utah Geologic Survey.

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.

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

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

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

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

http://earthquake.usgs.gov/research/hazmap Pierson, L.A., C.F. Gullixson, and R.G. Chassie, 2001, Rockfall Catchment Area Design Guide –

Final Report, Oregon Dept. of Transportation Research Group pub. No. SPR-3(032), November 2001.

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

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.

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

La Sal Mountain Loop, UT FLAP 73 (2) March 2020

APPENDIX A

Summary of Nearby Active Faults

NAME DISTANCE

FROM PROJECT SLIP RATE FAULT

LENGTH

TIME OF LAST

DEFORMATION

(miles) (inches/year) (miles) (years ago)

Moab Fault and Deformation Zone 0.00 <0.008 42.25 1,600,000

Castle Valley Faults 4.08 <0.008 7.46 1,600,000

Sinbad Valley Graben 8.95 <0.008 19.88 1,600,000

Lisbon Valley Fault Zone 9.20 <0.008 22.99 1,600,000

Lockhart 9.76 <0.008 9.94 15,000

Paradox Valley Graben 11.32 <0.008 34.80 1,600,000

Pine Ridge 12.05 <0.008 3.73 1,600,000

Fisher Valley Faults 13.38 <0.008 10.56 1,600,000

Salt and Cach Valleys 15.41 <0.008 36.04 1,600,000

Unnamed Fault of Lost Horse Basin 26.22 <0.008 4.97 1,600,000

Unnamed Fault near Pine Mountain 26.71 <0.008 19.26 1,600,000

Shay Graben Faults 27.96 <0.008 24.85 1,600,000

Ryan Creek Fault Zone 28.30 <0.008 24.23 1,600,000

Granite Creek Fault 28.89 <0.008 14.29 1,600,000

Sand Flat Graben Faults 29.04 <0.008 14.29 1,600,000

Big Gypsum Valley Graben 29.59 <0.008 20.51 1,600,000

Needles Fault Zone 29.98 <0.008 19.88 15,000

Unnamed Fault near Wolf Hill 30.38 <0.008 9.32 1,600,000

Ten Mile Graben 30.84 <0.008 21.75 1,600,000

Dolores Fault Zone 31.56 <0.008 9.32 1,600,000

Unnamed Fault East of Atkinson Mesa 32.16 <0.008 25.48 1,600,000

Little Dolores River Fault 38.49 <0.008 9.32 1,600,000

APPENDIX B

Rock Scaling Area Photos

Photo 1:- Scaling areas near Sta. 330+00.

Photo 2:- Scaling Areas near Sta. 330+50.

Photo 3:- Scaling areas near Sta. 331+50. Note near vertical fractures in sandstone bed.

Photo 4:- Scaling areas near Sta. 332+00, with vertical fractures in sandstone.

Photo 5:- Scaling areas near Sta. 332+00.

Photo 6:- Scaling areas near Sta. 332+50.

Photo 7:- Scaling areas near Sta. 333+00.

Photo 8:- Scaling areas near Sta. 334+00.

Photo 9:- Scaling areas near Sta. 334+50.

Photo 10:- Scaling areas near Sta. 335+00.

Photo 11:- Scaling area near Sta. 335+00.

Photo 12:- Scaling areas near Sta. 336+00.

APPENDIX C

Excavation Characteristics of Rock and Rippability Charts

Table C.1: Rock Hardness and Excavation Characteristics1

Rock Hardness

Description

Identification Criteria

Unconfined Compressive Strength

Seismic Compression (P-Wave) Velocity Excavation

Characteristics MPa psi m/s f/s

Very Soft Rock

Material crumbles under firm blows with sharp end of geological pick; can be peeled with a knife; too hard to cut a triaxial sample by hand. SPT will refuse. Pieces up to 3-c, thick can be broken by finger pressure.

1.7-3.0 246-435 450-1,200 1,475- 3,935 Easy Ripping

Soft Rock

Can just be scraped with a knife;

indentations 1-mm to 3-mm show in specimen with firm blows of the pick point; has dull sound under hammer.

3.0-10.0 435-1,450 1,200- 1,500

3,935- 4,920 Hard Ripping

Hard Rock

Cannot be scraped with a knife; hand specimen can be broken with a pick with a single firm blow; rock rings under hammer.

10.0-20.0 1,450- 2,900

1,500- 1,850

4,920- 6,070

Very Hard Ripping

Very Hard Rock

Hand specimen…