Geotechnical Recommendations.pdf

Technical Memorandum

Date: July 25, 2022

To: Thomas Sohn, Project Manager, CFLHD, Lakewood CO Jeff Felling, Lead Design Engineer, CFLHD, Lakewood CO

From: Marilyn Dodson, Lead Geotechnical Engineer, CFLHD, Lakewood CO & Rebecca Borst, Geotechnical Engineer, CFLHD, Lakewood CO

Subject: Geotechnical Recommendations for WY FLAP 740(1) Skyline Drive and Fremont Lake Road Bridger-Teton National Forest, Sublette County, Wyoming

1 PROJECT BACKGROUND INFORMATION

This geotechnical memorandum presents the field evaluation results and recommendations for the reconstruction work to be performed on Skyline Drive and Lower Fremont Lake Road.

The project is located in Sublette County near Pinedale, Wyoming and provides visitor access to the Elkhart Park Trailhead along upper Skyline Drive and lake access along the Lower Fremont Lake Road and is considered a CFLHD 3R+ project for design.

Skyline Drive is a continuation of a previous CFLHD FLAP project that resumes work just north of the turnoff to the ski area and ends at the top where the route meets with the parking lots and camping loop, for a total length of approximately 4.30 miles. For Skyline Drive, proposed improvements include rehabilitation of the existing roadway to provide 22-foot width, uniform roadway width, drainage improvements, slope repair, slope stabilization, subgrade improvement, asphalt surfacing, paving, and safety improvements. Existing conditions include poor-condition asphalt surfacing and very little clear zone with a narrow and winding roadway corridor. There are multiple drainage issues which has resulted in deteriorating shoulders and full width damage.

Additionally, there are slope failures where recent windstorm damage and timber harvesting has left the terrain with minimal ground cover at one location. Geotechnical recommendations along Skyline Drive include subexcavation (full and half-width), deep path (full and half-width), mechanically placed rock buttress, and underdrains in additional to standard cut and fill slope angle recommendations, shrink-swell factor for on-site materials, and geosynthetic material selection. Figures 1 and 2 show typical existing roadway conditions along upper Skyline Drive.

Figure 1: Typical roadway conditions along upper Skyline Drive.

Figure 2: Typical roadway conditions along upper Skyline Drive.

Lower Fremont Lake Road begins at the intersection with Skyline Drive and leads to the boat ramp and parking area for Fremont Lake for a total length of approximately 1.54 miles. Proposed improvements for Lower Fremont Lake Road include rehabilitation of the existing roadway to provide 24’ uniform width with cross slope and crown corrections, drainage and sight distance improvements, asphalt surfacing, and permanent signing and striping. Existing conditions of Fremont Lake Road include a gravel surfacing with very little clear zone, poor drainage conditions, sight distance concerns, overhead and buried utilities, multiple driveways, and two approach roads require realignment. Geotechnical recommendations along Fremont Lake Road include standard cut and fill slope angle recommendations, shrink-swell factor for on-site materials, and geosynthetic material selection. Figure 3 shows typical existing roadway conditions along Lower Freemont Lake Road.

Figure 3: Typical roadway conditions along Fremont Lake Road.

2 FIELD EVALUATION

A subsurface investigation targeting the proposed improvements was not performed for this scope of work. A pavement and subgrade investigation was conducted on October 13, 2020, and is discussed in Section III of the Final Pavement and Materials Report. Observations of surficial conditions and reconnaissance mapping was performed during an independent geotechnical scoping site visit on September 28, 2020.

The geotechnical features that were examined during this visit included existing cut slopes and subgrade soil support for drainage improvements. Evaluations and recommendations presented in this memorandum are based on visual observations and limited measurements taken during the site visit. Drainage improvements will be discussed to a certain degree in this memorandum as they relate to geotechnical concerns. Further discussions are addressed in the Final Hydraulic Report.

The surficial material on site is interpreted from laboratory testing for the pavements investigation to be colluvial material and is classified as silty sands to sandy clays, primarily A- 2-4(0) and A-1-b(0). When handled, the material felt loose and non-plastic, and thus fell apart easily. The material making up the roadside slopes could be sluffed off by hand and showed obvious signs of sediment transport due to storm water erosion, characterizing it as highly erodible material. Because this project is in mountainous terrain, boulders and cobbles may be encountered during construction.

The existing Skyline Drive appears to have been constructed using cut and fill methods on native soil. Native slopes in the area are vegetated with trees and bushes, typical mountainous-terrain vegetation. Raveling was noted in the existing cut slopes, but, generally, appeared to be limited to a maintenance level issue, except one location noted below. The existing roadway surface, cuts, and fills appeared to be in mostly good condition.

One location, approximately Sta 520+95 to 522+95 right, exhibited cut-slope raveling and instability with lack of vegetation due to a combination of timber cutting and a windstorm that downed trees, as shown below in Figure 4.

Figure 4: Existing cut slope with filled ditch and pavement distress, Sta 520+95 to 522+95.

3 RECOMMENDATIONS

This section presents recommendations for the management of subsurface water, slope stability, and road embankment repair for the design and construction of the Skyline Drive and Freemont Lake Road project. Based on discussions with the project team, the critical components of work are to address the pavement condition and improve the drainage along the existing road. Apparent roadway cracking and subsidence, however, has been observed as a cause of roadway distress.

Generalized subsurface profiles were developed based on field reconnaissance and surficial visual evaluation.

3.1 Cut Slope Recommendations

In general, non-saturated coarse-grained granular soil slopes can be constructed at a safe slope ratio of 1V:3H or flatter; non-saturated slopes composed of fine-grained. 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 and raveling; flatter slopes should be utilized where possible.

3.2 Fill Slope Recommendations

The designer has indicated minor fill slope sections along the route, generally less than 2 to 3-feet or less. It is worth noting that sliver fills should be avoided to prevent differential settlement which could lead to continued raveling and destabilization. The fill slopes included in the project should be constructed at 1V:3H and at a maximum slope ratio of 1V:2H, to fit the existing grade as determined by the Contracting Officer.

3.3 Slope Reconditioning

Slope reconditioning is recommended to establish safe slopes for construction and to preserve the condition of the corridor. Significant cuts are not expected, although mitigation measures are still advisable. Standard earthwork equipment, such as an excavator, is anticipated to be used to round and shape hazardous slopes.

3.4 Ditch Recommendations

The existing roadway has variable ditch widths, and at some locations, little to no ditch width.

Therefore, it is recommended that both graded and paved ditches be constructed along the route to improve drainage, collect debris, and reduce impacts to the roadway. The ditches should be installed with 1V:3H slopes with a maximum of 1V:2H to fit the roadway. As stated in the plans, the gradient and width of the roadway ditches and the excavation and embankment ratios may be adjusted by the CO to assure adequate drainage and stability. Typical section drawings shown in the plan set, Sheets A8 through A10, and the Final Hydraulics Report provide further details.

3.5 Subexcavation

Unstable subgrade materials and poor drainage are conditions that have contributed to the current damage along Skyline Drive. Subexcavation is recommended for this project at several spot locations detailed in the final plans to stabilize areas of weak or unstable subgrade materials.

These areas were observed to have the most pavement cracking identified by the CFLHD

Pavements Engineer. Subexcavation is recommended to extend either one-lane width or full roadway width based on surface site conditions during design that may be altered during construction based on the site conditions encountered at the judgement of the CO.

Subexcavation should be a maximum of two feet deep with one layer of non-woven separation geotextile placed at the bottom of the subexcavation. The bottom of the subexcavation should be inclined to drain outside of the roadway prism so water can drain, as shown in the plans.

3.6 Underdrains

A standard underdrain system is recommended along the cut side of the road at various locations as shown in the plans to a depth of 5 feet. The subgrade should be crowned to drain appropriately. Additional areas of poor drainage could be identified during construction.

Underdrains should be installed in this area to reduce the potential for water to saturate the pavement subgrade. Underdrain outlet pipes should be located at a maximum of 300-feet on-centers spacing. In addition to this identified area, an allowance should be included in the contract to be used at the discretion of the Contracting Officer.

3.7 Ditch Recommendations

The existing roadway has insufficient ditches in locations. Debris filling appears to be the main source of this problem is impacting the roadway. As stated in the plans, the gradient and width of the roadway ditches and the excavation and embankment ratios may be adjusted by the CO to assure adequate drainage and stability. Typical section drawings shown in the plan set, Sheets A6 & A7, provide further details. Refer to the Final Hydraulics Report dated January 2022.

3.8 Shrink/Swell Recommendations

On-site soils expected to be encountered within the project limits generally consist of silty sand to sandy clay. It is estimated that these soils will have a 10 percent shrink percentage, corresponding to a shrink/swell factor of 0.90. Greater than average shrinkage may be encountered in areas of very loose, shallow, material. 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.

3.9 Central Drainage Improvements

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

3.10 Embankment Stabilization

Settlement or consolidation due to either inadequately compacted fill material, erosion of side slopes, and/or downslope fill creep is a concern for the section of the project corridor at spot locations. Tension cracks and subsidence in the outer part of the road were noted by the project team during design site visits.

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

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

3.10.1 Deep Patch Design

A deep patch is the recommended repair for various locations as shown in the plans. A deep patch is a shallow, road-fill slope repair that is constructed by excavating the upper 4 to 6 feet of the subsiding section of roadway, replacing the fill material with compacted backfill, and reinforcing the fill material with layers of geogrid (Musser & Denning, 2005). Deep patch locations may appear to similar to subexcavation but are intended to address deeper-seated subgrade stabilization issues and contain stabilization geogrid.

The depth of the proposed deep patch is recommended to be 4 feet minimum and 6 feet maximum and either full roadway width or partial roadway width as shown in the plans. The reinforcement spacing is recommended to be not more than 1 foot vertically and should daylight at the slope face at a maximum slope ratio of 1V:2H. A stabilization geogrid conforming to Section 714.04 in FP-14 should be used for the deep patch repair. The deep patch should also include a geocomposite sheet drain as shown in the plans. Backfill material should conform to Section 704.03(b) in the FP-14 and a separation geotextile (nonwoven, Class 1, Type C) placed below the bottom lift.

3.10.2 Rock Buttress Design

A buttress is the recommended repair for Sta 520+95 to 522+95, as shown in the final plans. A buttress is recommended as cut slope erosion protection against the unvegetated, oversteepened cut slope to reduce the impacts of material shedding onto the roadway at this location. A buttress is composed of mechanically placed rock embankment materials with specified gradation with maximum height of 20 feet, slope face angle of 1V:1.5H, and back slope temporary excavation slope angle approximately 1V:1H. Embedment depth of 4 feet vertical and base width of 5 feet minimum will provide a stable base of material to counter act any additional slope movement in this cut slope and stabilize the face against surficial erosion. An underdrain is recommended to accompany this buttress to assist with drainage and to keep the cutslope in an unsaturated condition that will contribute to improved stability.

3.10.3 Roadway Maintenance

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

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

Routine observation and monitoring of slope performance.

Ditch cleaning and repair.

Pavement crack filling to prevent water infiltration.

3.11 Geosynthetics

The geogrid for the deep patch repair should meet the requirements of stabilization geogrid (FP- 14 and Sections 714.03). Separation geotextile for subexcavation, deep patch, rock buttress and underdrains should be Class 1 Type C non-woven geotextile (FP-14 section 714.01).

4 DISCLAIMERS/LIMITATIONS CLAUSE

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 and surface mapping of soil and rock outcrops. The Recommendations section of this report includes 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 judgement of the Contractor.

5 REFERENCES

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

Bridge Design Specifications, 8th Edition.

Andrew, Richard D., Bartingale, Ryan, & Howard Hume, 2011, “Context Sensitive Rock Slope Design Solutions,” Federal Highway Administration, Report No. FHWA-CFL/TD-11- 002, dated January.

Department of the Navy Facilities Engineering Command, 1986, NAVFAC, DM-7.01, “Soil Mechanics,” Design Manual.

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

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

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14.

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

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

Federal Highway Administration (FHWA), 2022, “WY FLAP 740(1) Skyline Drive and Lower Fremont Lake Road, Final Hydraulic Report,” dated January.

Federal Highway Administration (FHWA), 2022, “WY FLAP 740(1) Skyline Drive and Lower Fremont Lake Road, Final Pavement and Materials Report,” dated January.

Musser, Sandra W. & Denning, Cliff, 2005, “Deep Patch Road Embankment Repair Application Guide,” United States Department of Agriculture (USDA) Forest Service, dated October.

Sindlinger, Alicia, 2009, “Asset Management Highlights Slope Maintenance,” Public Road Magazine, Publication No. FHWA-HRT-09-003, Issue No. Vol. 72 No. 5, dated March/April.