mt-ef1817301_GM 09-21_Helena Lewis and Clark Geotechnical Recommendations.pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: April 14, 2021

TO: David Arena Project Manager

FROM: Sean A. Hayter Geotechnical Engineer

SUBJECT: Geotechnical Memorandum 09-21 Geotechnical Site Observations and Recommendations – FSR 109 MP 6.91 Helena – Lewis & Clark 2018 Repairs, Phase 1

MT FS ERFO 2018(1)-73(1)

Teton County, MT

INTRODUCTION

This memorandum provides the observations of our site assessment and geotechnical recommendations in support of the Helena – Lewis & Clark 2018 Repairs, Phase 1 project located within Lewis & Clark National Forest, Montana. The overall project consists of nine separate sites; however, this memorandum is specifically for the Forest Service Road (FSR) 109 MP 6.91, which is located at 47°52'5.05" North and 112°43'59.02" West. For the purpose of this memorandum, FSR 109 MP 6.91 is referred to as “project area.”

From April 2018 to June 2018, the area experienced flooding resulting from rapid snowmelt and heavy rains, which damaged roads within the Lewis & Clark National Forest. The roads provide recreational and administrative access to the National Forest and will be repaired using the Emergency Relief of Federally Owned Roads (ERFO) program.

This geotechnical memorandum should be made available to prospective bidders as part of the physical data.

PROJECT DESCRIPTION

The project area is located along the eastern edge of the Rocky Mountains approximately 27 miles west of Chateau, Montana, as shown in Figure 1 and Figure 2. FSR 109 is primarily a single lane gravel surfaced roadway. The existing minimum roadway width along the project area is 12-feet. FSR 109 is bound by an approximately 40 degree upward slope to the north and an approximately 45 degree downward slope to the south, which terminates at the South

Fork Teton River. FSR 109 decreases 70-feet in elevation from west to east, STA 0+00 to STA 9+16, respectively.

Figure 1. Helena-Lewis & Clark Repairs, Phase 1 project location.

Figure 2. FSR 109 MP 6.91 vicinity map.

The heavy rains and rapid snowmelt in 2018 increased the water level of the South Fork Teton River. The increase in water level likely eroded the toe of the slope, which in turn retrogressed up to the roadway. In addition, the rain and snowmelt runoff traversing the roadway caused additional erosion and saturation of the southern shoulder. The loss of eroded soil and a decrease in soil strength resulted in shallow slope failures and sloughing of the roadway shoulder. Jersey barriers were placed along the shoulder from STA 0+00 to STA 3+90, as shown in Figure 3.

Figure 3. FSR 109 MP 6.91, generally east facing. Photo provided by USFS.

Since the 2018 event, there have not been reports of additional slope instability or sloughing of the roadway shoulder; however, during a site visit completed in August 2020 by Western Federal Lands Highway Division (WFLHD) project team members, the over-steepened slope near the roadway shoulder appeared to be eroding and at risk for additional sloughing and shallow landslides that could retrogress further into the roadway. The proposed repair work includes constructing a reinforced soil slope (RSS) along the southern shoulder from approximately STA 2+50 to STA 4+00. The roadway will likely need to be closed during construction of the RSS. Additional details are provided in the Geotechnical Recommendations section of this memorandum.

GEOLOGY

According to the Geologic Map of the Chateau 30’ x 60’ Quadrangle North Central Montana (Berg, 2008), the project area is mapped as Qal and nearby areas are mapped as Mm and MDtm. As shown in Figure 4, tectonic compression of the rock units from west to east is predominant in this area, resulting in significant thrust faulting with alternating anticlinal and synclinal folds trending north and south at Crab Butte. The beds of the sedimentary rock units, when exposed, dip to the west between 35 and 90 degrees.

The Qal unit is defined as Alluvium of modern channels and flood plans (Holocene in age). It is described as fluvial deposits of locally derived silt, sand, pebbles, and cobbles. Holocene alluvial deposits are typically loose to medium dense and contain rounded to subrounded sand with varying silt content. The age range is reported as less than 12,000 years old.

The Mm unit is defined as Madison Group, undivided (Mississippian) and is mapped to the north and south of the project area. It is described as primarily limestone and dolomite ranging from calcitic dolomite and dolomitic limestone with chert with minor calcareous shale. The thickness of the Madison Group unit is between 900 and 1,800 feet. The age range is reported as 318 and 359 million years old (Ma).

The MDtm unit is defined as Three Forks Formation, Jefferson Dolomite, and Maywood Formation (Devonian and Mississippian) and is mapped approximately 10 miles to the west of the project area. The Three Forks Formation consists of angular fragments less than 3 feet wide of pale, yellowish-brown dolomite and dolomitic limestone, and ranges in thickness from 50 to 200 feet. The Jefferson Dolomite consists of primarily limestone and dolomite, which are mapped between 620 and 800 feet thick. The Maywood Formation consists of thickly bedded limestone and dolomitic limestone with greenish gray mudstone, mapped between 50 and 280 feet thick. The age range is reported as 340 to 383 Ma.

Figure 4. Mapped geology for the project area (Berg, 2008). North is up.

SITE CONDITIONS

The gravel surfaced roadway is generally oriented west-east and varies in travel-way width from approximately 12 to 16 feet. Approximately 400 linear feet of Jersey barriers parallel the southern shoulder of the roadway. From STA 0+00 to STA 9+16 (west to east) the roadway decreases in elevation from approximately 5,438 feet to 5,387 feet, with an average slope grade of -8%. Excluding the roadway shoulders, indications of roadway distress and/or subsidence were not observed. Utilities were not observed within the project vicinity.

According the geologic map provided in Figure 4, the project site is situated between two thrust faults and is mapped as alluvium (Qal). Both the cut slope, north of the roadway, and downslope, south of the roadway, we observed loose to medium dense, light brown, fine to coarse sand with gravel and occasional cobbles, interpreted as alluvial deposits (Qal) and consistent with the mapped geology. Rock outcrops were not observed within the project vicinity. A subsurface investigation was not completed as part of this project.

From STA 2+00 to STA 4+50, surficial sloughing of the exposed alluvial deposits was observed in the cut slope. The slope angle was approximately 40 degrees, and little to no vegetation or top soil was observed. Exposed alluvial deposits at this angle are typically considered moderately to highly erodible; therefore, the surficial sloughing is expected. A 3-foot-wide catchment ditch parallels the cut slope. At the time of the site visit, groundwater/seepage was not observed within the face of the cut slope.

Project Location

From approximately STA 1+00 to STA 4+50, erosion, surficial sloughing, and shallow landslides were observed along the downslope side of the road. The angle of the slope averaged approximately 45 degrees and was moderately vegetated with brush and deciduous trees up to 6 inches in diameter. At the time of our observations, the South Fork Teton River water level was approximately 15 feet below the roadway, which was estimated at STA 3+00. Seepage within the downslope face was not observed at the time of the site visit.

Although shoulder erosion and shallow landslides were observed from STA 1+00 to STA 4+50, the most severely impacted areas observed were between STA 2+50 and STA 4+00, As shown in Figure A1. Within this area, the erosional headscarp/crest of the slope had regressed far enough that it directly impacted the roadway. The remaining slope face is oversteepened, approximately 60 degrees, to an elevation of 4 feet below the crest of the slope and continuing to experience surficial sloughing and erosion.

As previously discussed, it appears the primary mechanisms of failure were likely the combination of higher than normal South Fork Teton River water levels eroding the toe of the slope and rain and snowmelt runoff traversing the roadway over the shoulder. This not only caused erosion of the shoulder material, but also decreased the strength of the soil at the toe of the slope resulting in shallow landslides.

GEOTECHNICAL RECOMMENDATIONS

Three possible options were considered. Option 1 consisted of leaving the south shoulder as-is and shifting the alignment north into the cut slope and away from the erosional headscarp/slope crest of the downslope area. Shifting the roadway would require additional cuts into the already sloughing cut slope to the north, and it was unknown whether rock may be within the proposed excavation limits for shifting the roadway inboard. Remediation of the sloughing cut slope would be cost prohibitive; therefore, Option 1 was dismissed.

Option 2 consisted of addressing the global stability of the downslope. This option would likely include armoring the toe of the slope with Class 5 or 6 riprap and building the slope up with a gabion faced mechanically stabilized earth (MSE) wall. This option was not preferred for environmental reasons and was also considered cost prohibitive and was dismissed.

Option 3 is ultimately recommended and consists of constructing an RSS along the southern shoulder of the existing roadway. This will provide reinforcement along the edge of the roadway, which was lost during the 2018 event, and it will increase resistance against continuing slope erosion that will likely cause future sloughing and shallow landslides retrogressing into the roadway section. The RSS does not fully address global stability of the slope, but is considered to be an appropriate risk reduction solution within the confines of the project scope under ERFO funding authority. To fully address global stability and provide adequate resistance to an event similar to the 2018 event, Option 2 should be considered in the future as a more resistant, river side mitigation solution.

It is recommended to construct the RSS between STA 2+50 and STA 4+00, which was most affected by the 2018 event. The RSS should be 8-feet wide and 3.75-feet thick consisting of 6 layers of reinforcement and constructed in accordance with Section 261 of FP-14. The bottom elevation of the RSS should be determined as the thickness measured below subgrade elevation. The RSS should be constructed with stabilization (biaxial) geogrid meeting Section

714.03 of FP-14 and select borrow meeting Section 704.07 of FP-15. The recommended location of the RSS is displayed in Figure A1, and a typical RSS section is shown in Figure A2 as attachments to this memorandum.

The facing should consist of black or brown, permanent facing geogrid overlying the stabilization geogrid. The facing will allow natural growth of grasses over time and protect the underlying stabilization geogrid from degradation. The permanent facing geogrid should conform to the following:

• Minimum LTDS of 400 lbs/ft in both directions

• Minimum life expectancy of 75 years

• 99 percent UV at 500 hours in accordance with ASTM 4355

• Maximum aperture opening size of 0.08 inches

• Black or brown in color

The RSS reinforcement consists of 6 layers of stabilization geogrid with vertical spacing of 9-inches. The reinforcement should be lapped back into the reinforced section 3-feet, and a thin layer of select borrow should be placed between the lap and the overlying layer to prevent direct contact between the adjacent layers.

An underdrain system should be installed at the back side of the RSS excavation. The underdrain system should consist of a 6-inch diameter perforated collector pipe meeting FP-14 Section 708.05 in a 12-inch by 12-inch deep trench backfilled with select granular backfill (704.08) and incased with separation and stabilization geotextile Class 1, Type A (non-woven) running the length of the RSS. The stabilization and separation geotextile should conform to Section 714.01(a) and be installed according to Section 207.06 of the FP-14. A non-perforated outlet pipe meeting FP-14 Section 708.05 should be installed at the low elevation end of the RSS, STA 2+50, and outlet on the south slope and be tight-lined to the base of the slope to avoid further erosion of the slope below the new RSS.

Based on a minimum overall roadway width of 12 feet, it is anticipated that construction of the RSS will require full roadway closure. In addition, construction traffic should observe a minimum 3-foot setback from the edge of the OSHA mandated RSS temporary excavation.

STABILITY ANALYSES

Global and internal stability analyses were completed for the recommended RSS using Slide2 (RocScience, Inc.) and ReSSA+ (ADAMA Engineering, Inc.), respectively. The global stability analysis was constrained by the north edge of FSR 109 and the South Fork Teton River. Global stability analysis did not include the slope north of the roadway. Soil strength parameters used in the analyses were based on observed soil conditions, moderate slope vegetation, and typical values for alluvium. Soil parameters were then refined by completing a back analysis of the slope failure and matching the model to the existing site conditions and observations of the project site. Because groundwater/seepage was not observed in either the cut slope or downslope, the water level was assumed to be consistent with the observed water level for the South Fork Teton River.

Three force/moment (limit) equilibrium methods were used within Slide2 to estimate global stability: Bishop simplified, Spencer, and GLE/Morgenstern-Price. Table 1 presents a summary of Factors of Safety, and Figures A3 and A4 present the critical cross section results.

Table 1. Summary of global stability results for recommended RSS.

Global Stability Analysis Method Factor of Safety Bishop simplified 1.30 Spencer 1.30 GLE / Morgenstern-Price 1.30

REFERENCES

ADAMA Engineering, Inc. (2021), ReSSA+, Clackamas, Oregon.

Berg, R. B., 2008, Geologic Map of the Choteau 30’ x 60’ Quadrangle North Central Montana, Montana Bureau of Mines and Geology, Open File 571.

Rocscience, Inc. (2021), Slide2, Toronto, Ontario. M5T 1V1, Canada.

CLOSING

Please contact Sean Hayter at (202) 934-0377 or Sean.Hayter@dot.gov with any questions regarding this memorandum.

CC: Steven Davis, Highway Design

Matthew Dillin, Hydraulic Engineer Denise Steele, Environmental Protection Specialist Sean Kilmartin, Highway Safety Engineer Megan Chatfield, Materials Engineer Functional Manager

Geotechnical File mailto:Sean.Hayter@dot.gov

Attachments: Figure A1 – RSS Location Figure A2 – RSS Typical Section Figure A3 – Global Stability Analysis Results Figure A4 – Internal Stability Analysis Results

610 East Fifth Street Vancouver, WA 98661

Project No. RSS Location FigureMT FS ERFO 2018(1)-73(1) Scale

Helena – Lewis & Clark 2018 Repairs, Phase 1 A1

N/A File Name:

Date:

April 2021

Approximate location of recommended RSS.

Approximate location of slope affected by 2018 event.

Approximate location of most severely affected area of slope.

NOT FOR CONSTRUCTION

Vancouver, WA 98661

Project No. RSS Typical Section FigureMT FS ERFO 2018(1)-73(1) Scale

Helena – Lewis & Clark 2018 Repairs, Phase 1 A2

N/A File Name:

Date:

April 2021

NOT FOR CONSTRUCTION

Vancouver, WA 98661

Project No. Global Stability Analysis Results FigureMT FS ERFO 2018(1)-73(1) Scale

Helena – Lewis & Clark 2018 Repairs, Phase 1 A3

N/A File Name:

Date:

Vancouver, WA 98661

Project No. Internal Stability Analysis Results FigureMT FS ERFO 2018(1)-73(1) Scale

Helena – Lewis & Clark 2018 Repairs, Phase 1 A4

N/A File Name:

Date: