WY_FTFS_23(5)_Geotech_Memo_FINAL.pdf

Cut Slopes 1-3: MP 11.16, 11.59, & 11.82

In general, these three cut slopes present similar conditions in that they are colluvial slopes composed of cobble and boulder-sized rock in a matrix of silty sand with light vegetation. The boulder-sized rock was observed to have a typical size of roughly 3-5 feet, although larger rocks were noticed intermittently along the slopes. At the time of the site visit these slopes were moderately saturated due to recent snowfall activity. The height of these slopes ranged between 18 feet and 55 feet while the slope ratios ranged from roughly 1:1 (~45°) and steeper (~67°). The ditch widths were fairly consistent at approximately 3 feet and much of the ditch was either vegetated or overlaid with loose silt material.

Some raveling of the fine material was noted on these slopes which, in turn, could lead to future rock fall.

Large pine trees lined the crests of the slopes at the sites and smaller pine-like trees were sparsely observed along the mid-slope and near the ditch line. A degree of marginal creep could be seen in isolated trees along the slope crests, indicating possible surficial erosion.

Cut Slope 4, MP 14.56

This slope was composed almost entirely of gray quartz diorite from the Louis Lake batholith formation with closely to widely spaced (2” to 6 feet) discontinuities with daylighting sliding surfaces oriented in unfavorable directions with respect to the roadway. The term “daylighting” essentially means that the rock mass joint angle is less than the cut angle. This situation is particularly dangerous when the joint angle is then greater than the friction angle which will then lead to sliding of the rock mass. Evidence of previous rock fall was noted near the ditch where large boulders rested in a pile (observed as approximately 15-20 rocks in sizes varying from 4-6 feet) near the toe of the slope. Roughly halfway up the slope, a ledge of loose rock could also be seen which may have the potential to fall due to seasonal freeze-thaw changes and the erosion of fine material. The current proposed alignment (70 percent plan set) does not require this slope to be cut. However, from a geotechnical perspective this slope has the potential for rock fall hazards in the future.

Cut Slope 5 & 6, MP 14.66 & MP 14.75

Cut Slopes 5 & 6 are also composed of gray quartz diorite with very closely to moderately spaced discontinuities that are mostly oriented in unfavorable directions with respect to the roadway. The existing ditch width was minimal, roughly 3 feet, and the presence of vegetation increased between Cut Slopes 5 & 6. At Cut Slope 5, visible rock joints were noted as were pockets of what appeared to be loose, unsupported rock near the middle of the slope. It is understood that no rock cut will be required in the area of Cut Slope 5, roughly STA 776+00. Cut Slope 5 (and likely Cut Slope 6) have major discontinuities that are daylighting at the cut face and present an adverse condition for a sliding failure.

Trees lined the crest of the slope and much of the rock was exposed and free of overburden material. At Cut Slope 6, a number of small pine trees were scattered among pockets in the rock slope and more overburden and silty sand material overlaid the rock outcroppings. A cut of roughly 6-8 feet into the vicinity of Cut Slope 6 is possible to accommodate for a vehicle pullout area according to the designer.

Recommendations

Cutting into the aforementioned slopes presents unique challenges. The recommendations below are preliminary and based on visual observation alone. Further field measurements and analysis would need to be conducted on the larger rock cuts (“greater than 15 feet high or greater than 10 feet deep,” according to the PDDM) if construction funds become available. In addition, the acquisition of field survey data would be advised in order to better understand the overall dimensions and shape of the cut slope areas. A summary of the preliminary cut slope recommendations can be found after the location and geology maps at the back of this report. The below text details those recommendations.

Cut Slopes 1-3

In general, Cut Slopes 1-3 would likely require minor rock scaling to remove any loose, unsupported boulders. It is recommended that scaling be accomplished primarily by means of hand tools, with the exception of possibly using a mini-excavator if deemed necessary. The goal of the scaling operation should be to remove only the problematic rocks without loosening additional rocks not intended to be removed within the colluvial slopes. The vegetation present on the slopes is beneficial to stability and if cuts are to be made in these slopes, then it would be advisable to cut the entire slope section at the same angle where vegetation appears to be flourishing. Generally speaking, it may be difficult to establish vegetation on slopes with inclinations greater than 1V:2H without the use of erosion mats or other stabilization methods, so it would be advisable to keep slope angles less than a 1V:2H. Ditch reconditioning is also recommended and would greatly benefit the structure of these slopes and help preserve the condition of the roadway.

Cut Slope 4

Rock scaling is recommended at this location. Similar to the previously discussed cut slopes, hand scaling would be the preferred method of rock removal as to avoid large impacts to the slope. Evidence of previous rock fall and loose, unsupported rocks were noted during the site visit. Although this slope will likely not intrude on the roadway alignment and excavation will not be required, slope scaling as well as ditch reconditioning would still be recommended as a precautionary measure.

Cut Slopes 5 & 6

These two slopes present the most difficult conditions along the route. It would be recommended to not excavate into Cut Slope 5 due to the adversely oriented discontinuities daylighting out of the slope. If excavation is entirely unavoidable at this location, then alternative and likely costly solutions can be evaluated.

At Cut Slope 6, it is to the geotechnical engineer’s understanding that the Forest Service would like to construct a vehicle pullout area at this location into the cut slope. Compared to Cut Slope 5, Cut Slope 6 presents more inviting conditions for a vehicle pullout area due to the presence of more overburden material at the toe of the slope and a less steep cut angle. If a cut is made at this location, then it is critical that an adequate catchment ditch be constructed that is capable of restricting rockfall from reaching the roadway. Using the Rockfall Catchment Area Design program (RCAD) Version 1.1, ditch catchment effectiveness can be evaluated for various slope heights. Figure 1 below shows the general slope geometry used in the RCAD analysis and, assuming the desired percentage of catchment is 80 percent, Figure 2 shows the applicable catchment widths for varying slope heights.

Figure 1: General Slope Geometry Used in RCAD Analysis

Figure 2: Applicable Ditch Widths for Varying Slope Heights

In addition to rockfall catchment, an earth retaining structure may be needed to resist the earth pressures generated from cutting into Cut Slope 6 for the desired vehicle pullout area. One option to provide overall stability in the cut slope would be to construct a rockery. Rockeries are earth retaining structures constructed of large interlocking, dry-stacked rocks typically placed without mortar or reinforcement.

These structures rely on the weight, size, shape, and interface friction of the individual rocks to resist earth pressures and provide overall stability. Rockeries have been recently utilized on many projects as context sensitive solutions preserving scenic, aesthetic, historic and environmental resources, while maintaining safety and mobility.

40 60 80

D itc h W id th

(f t)

Slope Height (ft)

Slope Height vs. Ditch Width (based on RCAD data and assuming 80% catchment)

Linear Interpolation

Average of 3 ft.

3 ft.

~20 ft.

KEY

The environmental setting at the location of Cut Slope 6, as discussed in the “Site Observations” section above, shows steep slope conditions with exposed rock surfaces near the middle and top of the slope with unconsolidated overburden material mixed with what appears to be roughly 3-4 foot boulders near the bottom 10-15 feet of the slope. It is possible and very likely that larger rocks and outcroppings may be exposed during cutting operations near the toe of this slope. These conditions could be difficult for the construction of a rockery depending on what exactly is exposed behind the cut. However, a proposed rockery at this location would provide the retaining capability needed for the proposed cut and it would blend into the natural aesthetics of the environment. The rockery would also be backfilled with free-draining, 6 inch minus granular material, which would serve as the drainage system for the wall and reduce the buildup of pore water pressures behind the wall. A 4-inch-diameter perforated PVC collector pipe would be installed within the drainage layer near the base of the wall, and a separation geotextile would be placed along the backslope prior to placement of the drainage layer. Figure 3 below shows an illustration of a cut-side rockery and the required parameters needed for its design.

Figure 3: Illustration of the Cut-Side Rockery Parameters Used in Design

Conclusions

The recommendations in this technical memorandum are based on the data obtained from the 70 percent field review. The observations collected from this field review represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The “Recommendations” section in this memo include interpretations and suggestions 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.

References

Bayley, Richard W. (1965). “Geologic Map of the Louis Lake Quadrangle, Fremont County, Wyoming.”

Scale 1:24000. GQ-461, Department of the Interior, United States Geological 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), 2017, Rock Characterization Guidelines, dated December.

Mack, Darren A. et al (2006). “Rockery Design and Construction Guidelines.” Report No. FHWA-

CFL/TD-06-006, dated November.

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.

Portland State University, Computer Program, RCAD-RS VISUAL, Version 1.1, Copyright 2004.

PROJECT LOCATION MAP

GEOLOGY MAP

PRELIMINARY CUT SLOPE

RECOMMENDATIONS

Native Slope Ratio (V:H)

Existing Slope Ratio

(V:H)

Existing Cut Height (ft)

Existing Ditch Width

(ft)

Seepage or Drainage Issues

Maximum Slope Ratio

(V:H)

Maximum Slope Height

(ft)

Est. Ditch Width for 80%

Catchment (rock/colluvium/t alus)

Shrink (-)/ Swell(+)

598+00 592+00 RT 1 : 1 1 : 1.5 20 3 N/A 1 : 1.5 - 1 : 2 Varies 3 -10 Colluvial slope with occasional 3-5 feet boulders

611+00 614+00 RT > 1 : 1 1 : 1.5 55 3 N/A 1 : 1.5 - 1 : 2 Varies 3 -10 " "

623+00 626+00 RT > 1 : 1 1 : 1.5 18 3

Culvert near STA 626+50 should be cleaned out 1 : 1.5 - 1 : 2 Varies 3 -10 " "

770+00 772+00 LT N/A N/A N/A 4 N/A N/A Varies Varies 43 Rock slope composed of gray quartz diorite

775+00 778+00 LT N/A N/A N/A 2-3 N/A N/A Varies Varies 43 " "

780+00 782+00 LT N/A N/A N/A 2-3 N/A N/A Varies Varies 43 " "

Recommendations/Notes Offset

LT- Left RT- Right

Preliminary Cut Slope Recommendations Field Worksheet

Preliminary Cut Slope Recommendations

Approximate Begin Station

(feet)

Approximate End Station

(feet)

Cut Slope Conditions

SITE VISIT PHOTOGRAPHS

OCTOBER 24, 2018

CUT SLOPE 1 PHOTOS

Approximately STA 590+00 (R)

Figure D-1-1: Looking Downstation (top) & Looking Upstation (bottom)

Figure D-1-2: Looking Upstation (top) & Looking Downstation (bottom)

Figure D-1-3: Silty Sand Material Along Slope (top) & Direct View of Slope (bottom) {*Note large boulders evident in bottom photograph.}

Figure D-1-4: Ditch Width, ~3 Feet (top) & Downslope, Looking Downstation (bottom) {*Note silt material and vegetation within the ditch in the top photograph.}

Figure D-1-5: Looking Downstation (top) & Looking Downstation (bottom)

Figure D-1-6: Looking Upstation (top) & Looking Downstation (bottom) {*Note poor condition of existing ditch in top photograph.}

Figure D-1-7: Bend in Tree, Possible Soil Creep (top & bottom)

Effect of downhill creep. Likely erosion related.

Figure D-1-8: Loose Boulders Along Slope

CUT SLOPE 2 PHOTOS

Approximately STA 613+00 (R)

Figure D-2-1: Looking Upstation (top & bottom)

Figure D-2-2: Direct View of Slope (top) & Ditch Width, ~3 Feet (bottom) {*Note poor condition of ditch in bottom photograph.}

Figure D-2-3: Looking Downstation (top & bottom)

Figure D-2-4: Direct View of Slope

CUT SLOPE 3 PHOTOS

Approximately STA 625+00 (R)

Figure D-3-1: Looking Upstation (top & bottom)

Figure D-3-2: Ditch Width, < 3 Feet (top) & Looking Downstation (bottom)

Figure D-3-3: Culvert Inlet on Upslope Side (top) & Culvert Outlet on Downslope Side (bottom)

Figure D-3-4: Looking Downstation

CUT SLOPE 4 PHOTOS

Approximately STA 770+00 (L)

Figure D-4-1: Looking Upstation (top & bottom)

Evidence of previous rock fall.

Figure D-4-2: Direct View of Slope (top) & Looking Downstation (bottom)

CUT SLOPE 5 PHOTOS

Approximately STA 775+00 (L)

Figure D-5-1: Direct View of Slope (top) & Looking Upstation (bottom) {*Note daylighting rock joints.}

CUT SLOPE 6 PHOTOS

Approximately STA 780+00 (L)

Figure D-6-1: Looking Upstation (top) & Direct View of Toe of Slope (bottom)

Figure D-6-2: Looking Downstation (top) & Direct View of Slope (bottom)

References