FINAL_Geotechnical_Report_Cottonwood_Pass_East_CO_FLAP_CHA306(1).pdf

COTTONWOOD PASS ROAD

CHAFFEE COUNTY ROAD 306

SAN ISABEL NATIONAL FOREST, CO

CO FLAP 306 (1)

FINAL Geotechnical Report

Report No. CO-CX-306-17-01

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch November 2017

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 – SUBSURFACE INVESTIGATION

3.1 SUMMARY OF SUBSURFACE CONDITIONS

3.2 CUT AND FILL SLOPES

SECTION FOUR – ANALYSIS & RECOMMENDATIONS

4.1 GEO-PROFILE OF SLOPE MATERIALS

4.2 CUT-SLOPE EVALUATION

4.2.1 Assumed Shrink/Swell Factors

4.2.2 Rockfall Catchment Area

4.2.3 Scaling

4.3 EMBANKMENT EVALUATION

4.4 CUTSLOPE STABILIZATION ASSESSMENT

4.4.1 Rock Buttress Design

4.5 ROCK SLOPE ASSESSMENT

4.6 DEBRIS FLOW MITIGATION OPTIONS

4.7 EXISTING RETAINING WALL EVALUATION

4.7 DISCLAIMER/LIMITATIONS CLAUSE

SECTION FIVE - REFERENCES

Page | ii

TABLES

TABLE 1:- Summary of Nearby Faults TABLE 2:- Summary of Seismic Parameters for Site Class C TABLE 3:- Soil Type Categories within the Cutslopes TABLE 4:- Summary of the Cut Slope Recommendations TABLE 5:- Estimated Material Properties for Design TABLE 6:- Summary of Recommended Rock Buttress

FIGURES

FIGURE 1:- View of Cottonwood Pass Road near summit FIGURE 2:- Existing rock buttress (left) and slumping slope (right) located near Sta.

3215+00 FIGURE 3:- - Debris flow channel near Sta. 3783+00 FIGURE 4:- - Existing rock buttress near Sta. 3215+00 FIGURE 5:- Cut-slope near Sta. 3825+00. A concrete traffic barrier has been installed by

Chaffee County to increase catchment area of the roadside ditch. Differential weathering of block-in-matrix material creates a significant rockfall hazard.

FIGURE 6:– Cut-slope near Sta. 3825+00. Concrete traffic barrier providing additional catchment capacity to the roadside ditch. However, at time of site visit, this catchment was filled, increasing the likelihood of rockfall reaching the roadway

FIGURE 7:- Berm constructed from debris flow material at mouth of channel near Sta.

3800+00

FIGURE 8:- Existing MSE located near Sta. 3794+00

APPENDICES

APPENDIX A - Project Location and Geologic Maps APPENDIX B - Buttress and Soil Slope Analyses APPENDIX C - Special Contract Requirements file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721343 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721344 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721344 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721345 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721346 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721347 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721347 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721347 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721347 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721348 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721348 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721348 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721348 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721349 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721349 file://cflcadd3.cflhd.local/projects/CO/cha306(1)/Geotech/6_DraftDocs/Geotechnical%20Report%20Cottonwood%20Pass.docx#_Toc490721350

Page | 3 Cottonwood Pass CO FLAP 306 (1) August 2017

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 18.5-mile long section along Cottonwood Pass Road, CO FLAP 306(1) project, on the east side of the Continental Divide. The project involves improvements to the Cottonwood Pass Road, also known as County Road (CR) 306 and Forest Highway 59. The project corridor is in the Rocky Mountains, within the San Isabel National Forest, Chaffee County, Colorado. The project begins at the summit of Cottonwood Pass and the Gunnison/Chaffee County boundary (Sta. 3077+23) at an approximate elevation of 12,110 and descends in an easterly direction, ending at an approximate elevation of 8,888 at the junction with CR 321 (Sta. 4052+92). The project is located in undeveloped forested terrain between the aforementioned elevations with varying seasonal precipitation. Due to the high elevations along the continental divide, construction on this route will be limited to summer months (May through October) depending on winter precipitation and temperatures. A site map is presented on Figure 1 of this report.

The Cottonwood Pass Road provides access to the town of Crested Butte, Taylor Park Reservoir, Spring Creek Reservoir, Mirror Lake, Kebler Pass, and campground sites. The route is also an access to the forest amenities with recreational activities including fishing, boating, biking/hiking trails, hunting, and camping. The overall goal of the Cottonwood Pass Roadway improvement project is to balance transportation requirements and roadway maintenance within an environmentally sensitive setting. In the early 1980s, Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA) completed the design and reconstruction of the east side of Cottonwood Pass Road between the Continental Divide parking area and Buena Vista. Construction is in progress to improve and pave the west side of Cottonwood Pass (Gunnison County Road 209) under CFL Project CO FLAP 209(1). Based upon concerns related to roadway maintenance, geometrics, safety, current and projected traffic volumes, and other needs, the east segment of the Cottonwood Pass roadway is now proposed for rehabilitation and reconstruction.

The existing route is characterized as a narrow two-lane thoroughfare with various widths and many sharp switchbacks and relatively steep grades. The roadway traverses mountainous terrain with most cuts excavated in colluvium soils and side cast embankments to provide two, 11-foot asphalt paved lanes that pass through forest, shrub land, and alpine tundra habitat. The current average daily traffic is estimated at 500 with a significant number of large vehicles including

Page | 4 recreational vehicles, vehicles pulling campers, and boats creating hazardous traffic conditions.

Within the first 0.5 miles, the proposed roadway alignment will shift the roadway away from an embankment failure. The remainder of the roadway will match the existing alignment with increased shoulder width between MP 15 and MP 18.5 to improve bicycle access and safety.

This segment of the roadway currently has several distress failures, short sight distance, drainage issues, and does not meet AASHTO design standards. The alternatives presently under consideration include improvements to the horizontal and vertical alignment, drainage, roadway width, and roadside cut and fill slopes to accommodate 11-foot paved travel lanes with 0- to 6-foot shoulders For consistency with previous projects within the region and for cost effectiveness, all widening will be accomplished through slope cuts and embankment fills to achieve desired roadway width.

Areas with noted pavement distress will be overexcavated and stabilized using geotextile fabric.

Existing pavement will be pulverized and overlain with new asphaltic concrete pavement.

FIGURE 1:- View of Cottonwood Pass Road near summit.

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1.2 SCOPE AND PURPOSE

The scope of work included geotechnical investigation, analysis, and recommendations of cuts and fills within the project limits for use in design and construction. This involved several tasks including field reconnaissance, interpretation and correlation of field measurements, 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, corrosion potential, geological hazards, material shrink/swell, and construction considerations for the design and construction of slopes within the alignment. The stationing in this report is based on the preliminary 30 percent project plans, dated July 5, 2017.

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SECTION TWO – Geology and Seismicity

2.1 REGIONAL GEOLOGY

The project is located within the Sawatch Range, an extensive northwest-southeast oriented mountain range containing many of Colorado’s highest peaks. The range forms a portion of the Continental Divide and is comprised largely of Precambrian and Paleozoic rocks (approximately

1.8 billion to 260 million years old) at its core and younger sedimentary along its flanks. Various intrusive and volcanic rocks (predominantly 34 to 5 million years old) are scattered throughout the range. The principal mapped surficial deposits include glacial material, stream gravel, and alluvium (Tweto and others, 1976 [1]; Brock and Barker, 1972 [2]; Scott, 1975 [3]).

The majority of the roadway alignment traverses surficial materials mapped as glacial drift formed by weathering and eroding of granitic rock. Scattered outcrops of lower Paleozoic rocks occur as bedrock windows within the surficial deposits. At the western end of the alignment, near the continental divide, the roadway traverses Precambrian rocks.

The scattered bedrock outcrops adjacent to the roadway include a wide range of lithologies deposited approximately 325 to 540 million years ago (Cambrian to Mississippian). Rock units may include sandstone, quartzite, limestone, dolomite, and shale. Near the westernmost end of the alignment, bedrock consists of Precambrian interlayered felsic and hornblendic gneiss.

Glacial drift is the principal geologic unit along most of the roadway. Glacial drift typically consists of unconsolidated clay, sand, and gravel with cobbles and boulders. The thickness of this unit may be on the order of 100 feet or more in some locations.

Several known Quaternary faults are mapped within approximately 40 miles of the roadway alignment. These faults are considered inactive and are summarized on Table 1 below. These structures are on the east flank of the Sawatch Range and the western boundary of the upper Arkansas Valley graben, the northernmost expression of the Rio Grande Rift. Movement on the Sawatch Fault has occurred within the last 15,000 years. The timing of most recent movement on the other faults is less clear, but is estimated to have occurred within the last 1.6 million years.

No known active faults underlie or are closely associated with the project site (U.S. Geological Survey (2017) [4]).

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TABLE 1:- Summary of Nearby Faults

FAULT OR FAULT ZONE

DISTANCE

FROM

CENTER OF

PROJECT

FAULT

PARALLEL

SLIP RATE

FAULT

LENGTH AGE

(miles) (inch/year) (miles) (years)

Unamed Fault West of Buena Vista 3 <0.01 2 <1,600,000

Sawatch Fault, Southern Section 0 <0.01 25 <15,000

Sawatch Fault, Northen Section 16 <0.01 21 <130,000

Unamed Faults near Twin Lakes Reservoir 16 <0.01 8 <1,600,000

Northern Boundary Fault System 15 <0.01 30 <750,000

Unnamed Fault South of Shavano Peak 17 <0.01 4 <1,600,000

Unnamed Fault of Missouri Park 21 <0.01 4 <130,000

Unnamed Faults near Leadville 32 <0.01 12 <1,600,000

Mosquito Fault 24 <0.01 39 <130,000

Western Boundary Fault 34 <0.01 12 <1,600,000

Northern Sangre de Cristo Fault, Crestone Section 35 <0.01 49 <15,000

Villa Grove Fault Zone 39 <0.01 12 <15,000

East-Side Chase Gulch Fault 38 <0.01 19 <130,000

West-Side Chase Gulch Fault 35 <0.01 2 <130,000

Eleven Mile Fault 39 <0.01 3 <130,000

2.2 SEISMIC DESIGN PARAMETERS

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 [5], 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 Cottonwood Pass Road (38.8353º N latitude and -106.4189º W longitudes) was obtained in accordance with the AASHTO ground motion maps. The site was classified as “Class C” according to site class definitions specified in Table 3.10.3.1-1 of AASHTO. The recommended spectral acceleration coefficient values for probabilistic design are summarized in the following table.

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TABLE 2:- Summary of Seismic Parameters for Site Class C

Peak Seismic Ground Acceleration Coefficient, (As) 0.083

Horizontal Response Spectral Acceleration at Period of 0.2 sec, (SDS) 0.174

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.047

Site Factor at Zero-Period of Acceleration Spectrum, (Fpga) 1.2

Site Factor at Short-Period of Acceleration Spectrum, (Fa) 1.2

Site Factor at Long-Period Range of Acceleration Spectrum, (Fv) 1.7

Based on the long acceleration coefficient SD1 value of 0.047, the site is assigned to seismic hazard Zone 1 in accordance with Table 3.10.6-1 of AASHTO.

2.3 GEOLOGIC HAZARDS

The Cottonwood Pass is a high elevation road with rugged topography and adverse climatic conditions. Past glaciation within the area had a considerable influence on the morphology, geology, and groundwater conditions and is expected to impact proposed roadway design and construction. Slope stability conditions and the potential for landslides in the region have previously been identified by others (Bukovansky, 1995 and Rold, 1999). However, these studies do not have a consensus on the extent of actively moving landslides and do not address the presence of landslides within the project area. No evidence of active landslides that directly affect the roadway prism was observed during the investigation. Due to the mountainous terrain and the type of materials present nearby the roadway, it is probable that there are historic or inactive landslides near the roadway. These slides are not anticipated to activate from the proposed roadway construction.

Slumps or shallow transitional landslides were observed in slopes between 1V:1H and 1V:1.5H slope ratio that are generally composed of loose residual materials such as sands and gravels with wet to saturated conditions. Where unconsolidated materials are breached along roadway cuts, water infiltration is increased, saturating underlying deposits, which then fail by slumping. The existing uphill cuts along the upper 3,000-foot section of the roadway from the start of project at Station 3077+25 to approximate Station 3107+00 have higher potential for slumping and sliding.

Additionally, there are sections of cut slopes along the alignment that have a rockfall hazard due to excavations into glacial drift with boulders. Rockfall hazards generally include block-in-matrix type materials that have a long-term potential to produce small to moderate volume rockfalls.

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Existing cut and fill slopes along the project alignment were likely constructed at slope angles near or slightly flatter than the angle of repose of the in-situ material. Therefore, it is not likely that they were designed at a factor of safety meeting AASHTO requirements for engineered cuts and fills.

Several steep, narrow drainages exist in the eastern portion of the project, between Sta. 3758+00 and Sta. 3803+00. Based on geologic maps, the existing landforms, and news reports [6], [7], [8], these drainages have sourced debris flows in the past and are likely to source them again in the future. Debris flow and mudslides have also been recently reported along Chaffee County Road 344 [9]. Debris flows are a type of very rapid landslide movement consisting of soil, rock, and water, along with trees and other materials. Due to their high density and high velocity, debris flows have a large amount of energy, and are therefore very difficult to mitigate using barriers or diversions. Debris flows occur in steep, confined channels. As the gradient and confinement of the channel decline, the energy and velocity of the flow will also be reduced, leading to deposition of the flow materials. The volume of these deposits is significant, and can easily bury houses and other structures, and disrupt transportation infrastructure. The deposits are frequently saturated and can contain significant void spaces, which makes them hazardous to

FIGURE 2:- Existing rock buttress (left) and slumping slope (right) located near Sta.

3215+00.

Page | 10 work on or near immediately following the flow event. Days or weeks may be required to remove deposits from a road and re-establish through access.

In summary, the geologic hazards that exist near the project area are rock fall, avalanches, highly erodible granular soils, debris flows, and slope instability at cut and fill slope locations.

Variability of the geologic profile and groundwater conditions found at the sites increase the risk of failure of cut and fill slopes. Cutslope failures, rock fall, and debris flows are likely to occur during periods of high precipitation or periods of rapid snow melt. The risk is particularly high during the summer monsoon season and in the wet winter and early spring months.

FIGURE 3:- - Debris flow channel near Sta. 3783+00.

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SECTION THREE – Subsurface Investigation

3.1 SUMMARY OF SUBSURFACE CONDITIONS

A subsurface investigation targeting the existing slopes, debris flow areas, and proposed cut slopes was not performed for this scope of work. Surficial evaluation and mapping was performed based on areas identified during project scoping. The Pavement Design Memorandum dated October 24, 2016, presents limited subsurface information based on test pits excavated within or near the existing roadway prism.

3.2 CUT AND FILL SLOPES

Cut- and fill- 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.

Existing cut-slopes extend to a height of approximately 100 feet; there is some evidence of continuous raveling and slumping in select cut slopes. Cut-slopes in soil within the roadway alignment generally vary between 25 to 45 degrees from horizontal and cut slopes in weathered bedrock range from 55 to 65 degrees while naturally occurring slopes are typically found between 14 and 30 degrees. Near vertical slopes exist where resistant granitic bedrock is exposed. Fill slopes are typically less than 30 feet in height and banked at angles of about 25 to 40 degrees.

Visual inspection of slopes, along with geological knowledge provided the means for evaluating slope stability. In general, slopes with smooth contours indicated that the slopes have been formed by surface erosion processes without slip movement. On the other hand, irregular surfaces with stair stepping contours, scarp lines, slopping trees, or changes of vegetation indicated slope slumping. Although the majority of the existing soil cut-slopes along the roadway appeared to meet a stable factor of safety for low volume roads, a few indicated minor slope movements due to water seeps. None of these movements had impacted the roadway surface. For cut-slope design purpose, materials encountered were categorized into six different types as shown in Table 3.

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TABLE 3:- Soil Type Categories within the Cutslopes

GENERALIZED SLOPE MATERIAL DESCRIPTION

Soil Type ① Gray, silty sand, gravel, cobble, and boulder, fractured and weathered gneiss/schist, colluvium, and talus.

Soil Type② Gray, clayey sand and gravel, with cobble and boulder.

Soil Type③ Brown-gray, silty to clayey sand with gravel, rare cobble and boulder, colluvium.

Soil Type④ White and grey, silty sand, gravel, cobble, and boulder, rounded to subrounded, river terrace deposit.

Soil Type⑤ Light brownish gray, silty sand and gravel, with cobble and boulder, weathered granite alluvial fan deposit.

Rock Type⑥ Gray gneiss/schist

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SECTION FOUR – Analysis & Recommendations Based on the investigations previously described, this section presents analysis and recommendations pertaining to cut and fill slope ratios, buttress repair and cut-slope stabilization, and debris flow hazard mitigation. In addition, this section provides general interpretation of visual site reconnaissance and slope stability analysis. Generalized subsurface profiles were developed based on field reconnaissance and on test-pit data collected during the pavement design investigation. Cut- and fill- slopes are anticipated between Stations 3077+25 and 3107+00. The remainder of the project is anticipated to follow the existing road footprint.

However, to ensure long-term slope stability, cuts and cut-slope stabilization methods are necessary in localized segments of the project alignment. Stabilization methods were established for each cut site using the most feasible method that best fit the natural setting, and match existing stabilization methods along Cottonwood Pass Road. Rock buttresses were deemed to meet these criteria.

4.1 GEO-PROFILE OF SLOPE MATERIALS

Soil deposits encountered along the proposed roadway alignment were mostly of colluvial or alluvial origin and are characterized for design purposes as a mixture of silty gravel and silty clayey sand with varying sizes of cobbles and boulders. The slope materials are generally formed by the weathering and eroding process of various crystalline rocks including granite, gneiss, and schist. The materials are considered to have high friction angle and low cohesion which is a good quality material for roadway construction. The overburden slope materials are, however, susceptible to water infiltation and will require underdrains along certain segments.

Recommendations for subexcavation and underdrain installation are presented in the Pavement Design Memorandum, dated October 24, 2016.

The natural soils are highly permeable and are likely to be heterogeneous in nature. The soil profile consists of zones of partially weathered bedrock material containing remnants of the parent rock (boulders) of varying sizes, and zones of fully weathered material (soil).

The extent to which the slope stability within the on-site soils can be evaluated by analytical methods may be limited, due to uncertainties in the soil strength parameters and in the water saturation conditions. This may not, however, entirely rule out the use of analytical methods (as was done in analyzing the effectiveness of mitigation methods), as it may still be possible to determine the strength parameters from back analysis and infinite slope methods applied to existing slopes. Visual inspection, geological appraisal of the slope and surrounding area, Page | 14 topography, soil conditions and maintenance history of the existing slopes along the roadway are all critical for assessing slope stability conditions and predicting future behavior.

4.2 CUT-SLOPE EVALUATION

Expansions of existing cut-slopes are proposed to accommodate roadway design concepts from Station 3077+25 to Station 3107+00. The remainder of the project will stay within the existing road prism. However, based on field reconnaissance, minor modification of limited cut slopes are recommended to improve safety and stability of the slopes. Table 4 summarizes cut-slope materials and geometry and provides recommendations on shrink/swell factors, catchment ditch width, and cut-slope ratios for stable slope. Recommended slope ratios are based on observed conditions at the existing cut face and anticipated slope height. In general, non-saturated course grained granular soil slopes can be constructed at a slope ratio of 1V:1.5H or flatter; bedrock slopes can be constructed at a slope ratio of 2V:1H or flatter unless it is fractured and weathered;

and combination soil and rock slopes can be cut at 1V:1H. Cut-slopes should not exceed slope ratios presented in Table 4. Areas of observed springs and seeps encountered during construction may require flattening or additional support and should be evaluated by the 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.

Modification of the cut slope between Stations 3822+45 and 3827+90 is contingent on acquisition of appropriate right-of-way. If this acquisition is not practicable, the existing roadside ditch should be cleared and the slope scaled to the right-of-way or temporary construction easement limits. Light scaling efforts should be limited to removal of blocks already significantly undercut by differential weathering. Removal of blocks should be avoided where it will likely further destabilized the slope.

Page | 15

TABLE 4:- Summary of the Cut Slope Recommendations

BEGIN

STA.

END

STA.

OFF-

SET

MAX

SLOPE

RATIO

(V:H)

MAX

SLOPE

HEIGHT

(FT)

MINIMUM

DITCH

WIDTH

ASSUMED

SHRINK (-)/

SWELL (+)

SLOPE

MATERIAL

TYPE

(TABLE 3)

NOTES AND

RECOMMEND.

3077+25 3090+00 LT 1:1.15 50 8 -10 ①

3090+00 3091+15 LT 1:1.15 30 8 -10 ②

3091+15 3096+25 LT 1:1.15 40 8 -10 ①

3098+00 3100+25 LT 1:1.15 14 8 -10 ②

3100+25 3101+25 LT 1:1.15 Varies 8 -10 ②

3157+25 3158+50 RT 1:1.5 20 6 -10 ③

Install rock buttress

3166+00 3167+00 RT 1:1.5 20 6 -10 ③

Install rock buttress

3213+60 3215+50 LT 1:1.5 20 6 - ③

Reconstruct existing rock buttress and install rock buttress

3425+00 3430+00 LT 0.5:1 30 8 10 ⑥

Remove existing rockery, except at spring outlet near Sta. 3425+75.

Cut rock face with controlled blasting, scale finished face.

3753+00 3756+00 LT 1:1.5 30 - - ④

Light slope scaling

3756+00 3757+00 LT 1:1.2 65 10 10 ⑤

Brow rounding at new cut slopes

3758+40 3760+00 LT 1:1 15 - - ⑤

Brow rounding at new cut-slopes

3762+50 3765+00 LT 1:1 40 - - ⑤

Light slope scaling and brow rounding

3765+00 3770+00 LT 1:1 30 - - ⑤

Light slope scaling and brow rounding

3822+45 3827+90 RT 1:3 70 - - ④

Light slope scaling and brow rounding if slope not modified

Page | 16

4.2.1 Assumed Shrink/Swell Factors

The assumed shrink/swell factors are 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. Estimated shrink/swell values within a station range were weighted to represent the area of anticipated excavation; therefore, they are intended to be an average for station ranges shown. Materials shrink/swell percentage shown in the table represents the difference in volume between undisturbed bank materials and then compacted embankment volume. A material that occupies a larger volume as fill from the excavation volume (e.g. rock) is considered to swell and represented as a positive. A material that occupies a smaller volume as fill from the bank excavation volume (e.g. soil) is considered to shrink and represented as a negative.

Shrink/Swell values are dependent on subsurface conditions estimated from the observations of materials found in existing cut-slopes. On-site soils encountered along the route generally consist of brown silty sand with cobble to boulder size rock fragments with larger boulders exceeding 5 feet present in existing cut-slopes near the beginning and end of the route. Without the cobble and boulder size rock fragments, the material classifies as a SM to GM material and is anticipated to be used as embankment. However, it is estimated that such soils will have a shrink percentage ranging from -5 to -10 percent. Bedrock encountered along the route exhibited varying degrees of weathering with an estimated swell percentage of +10 to +15 percent.

4.2.2 Rockfall Catchment Area

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 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 effective rockfall protective measures. The Oregon Department of Transportation (ODOT) Rockfall Catchment Area Design (RCAD) design charts were used to evaluate various slope cut ratios and slope heights. For cut-slopes in the upper portion of the project, a standard catchment area using an 8-foot wide area sloping at 1V:4H with a 1-foot shoulder is recommended for an estimated 50 to 75 percent (depending on slope height) rockfall containment. Due to limitations of space in the lower portions of the project, concrete barriers are recommended to improve catchment area. Routine ditch cleaning should be included in the county’s maintenance plan to avoid decreased rockfall retention.

Page | 17

4.2.3 Scaling

Scaling and/or crest rounding of existing cut slopes is recommended in locations not mitigated by proposed permanent cut slopes. Scaling is conducted to reduce the amount and frequency of rockfall debris on the roadway by removal of loose and unstable material from existing slopes.

Scaling is anticipated to be achieved by hand and light tools. Approximately 50 crew hours are recommended to be included in the contact documents. General locations for scaling are identified in Table 4. The exact location for areas that may require scaling will be determined during construction.

Scaling is only considered a temporary or short term rockfall mitigation technique to remove any loose material that could potentially dislodge. Routine scaling should be included in the county’s maintenance plan.

4.3 EMBANKMENT EVALUATION

Embankment construction is required in certain areas to facilitate roadway widening and realignment, side-road approaches, and culvert replacement. Due to the granular nature of the on-site soils and embankment heights anticipated, it is recommended that proposed embankments be constructed at 1V:2H slopes or flatter when possible to maintain slope stability, limit settlement, reduce environmental impacts, and facilitate vegetation growth. Embankment settlement is expected to be negligible with most of the settlement occurring immediately after construction.

Groundwater may be encountered within embankment and culvert foundations. Dewatering and stabilization methods will depend on conditions encountered. Based on on-site observations, it is not likely that a significant depth of top soil will be present on the project sites. For estimation purposes, it should be assumed that limited topsoil of 3-4 feet will be stripped and stockpiled for re-use on the project.

4.4 CUTSLOPE STABILIZATION ASSESSMENT

Observations indicated that slope failures along the route are not likely to be deep seated circular failures and are more likely to be relatively shallow, with generally translational failure surfaces.

In large slopes with a limited depth of weathered material overlying rock, slope failures are likely to be predominantly translational slides. Shallow slips and slumps (≤ 10 feet) in the natural and cut-slopes are common and are typically triggered by freeze-thaw, periods of prolonged precipitation, and rapid snow melt. These shallow landslides are associated with soil saturation and associated pore pressure due to rainfall infiltrating the soil along the gravitational gradient that forms a perched water table in the soil eventually affecting the slope equilibrium. The rate of

Page | 18 rainfall infiltration is influenced by various soil parameters, such as the unit weight, porosity and mineral composition. Increasing water content in a soil slope affects stability in two ways: 1) increase in weight of slope materials, 2) decrease in shear resistance. These processes may result in a sufficient decrease in the effective stress in the near-surface soil and may ultimately lead to shallow slope failure (slumps).

To provide appropriate remedial measures to stabilize cut-slopes along the proposed alignment or to increase the factor of safety of a marginally-stable slope, one or more of the following may be done; 1) increase the shear resistance by improving drainage to lower the pore and seepage pressures, 2) decrease the driving forces by flattening the slope or decreasing the height, or 3) adding a buttress at the toe of the moving soil mass to increase the resisting moment. Typically, increasing the shear resistance is more effective in rotational type of failure. Since most failures along the project alignment are characterized as translational shallow slips, methods 2 or 3 above are more effective. Due to the regional topography and the slope sizes, modifying slope ratios and heights in this setting may have severe environmental impacts; therefore, we recommend that slope stability be achieved by adding a toe weight such as a rock buttress.

FIGURE 4:- - Existing rock buttress near Sta. 3215+00.

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4.4.1 Rock Buttress Design

Several cut-slopes along the roadway alignment were proposed to gain roadway width. Cut-slope geometries were determined from the critical cross-section along each proposed cut-slope segment based on the 30 percent project plans. Slope material properties were estimated for rock buttress design based on observations and geology, and presumptive empirical strength characteristics of similar materials. Back-analysis of slopes with evidence of creeping or slumps was performed to further refine the estimated material properties. This process was also informed by an infinite slope stability analysis performed for the Cottonwood Pass (West Side) project (CO FLAP 209(1)). The infinite slope method is a method of slope stability analysis in which the length of the sliding plane is large relative to its depth and the sliding plane is assumed to follow a straight-line parallel to the ground surface. Along the project alignment, shallow landslides occur when water infiltration reaches a certain depth into the surface soil layer which are mostly classified as silty-clayey gravels and sands and have the tendency to lose shear strength when saturated.

Based on site observations, groundwater was primarily located near the surface of the proposed buttress areas creating surficial erosion and soil riling. For the analysis, it was assumed that the upper three feet of the subsurface profile was saturated and the remaining soil (beneath the surficial three feet) was considered unsaturated. Table 5 provides a summary of the estimated overburden soil and rock buttress properties used in the stability analysis models.

TABLE 5:- Estimated Material Properties for Design

MATERIAL

DESCRIPTION

UNIT WEIGHT

ɣ, pcf

COHESION

c, psf

FRICTION ANGLE

φ, deg

Fill (GM-SM) [Lower Layer & Upper Layer]

120 100 40 120 100 38

Silty GRAVEL (GM) [Lower Layer & Upper Layer]

125 100 40 125 100 38

Silty clayey SAND (SM-SC) [Lower Layer and Upper Layer]

120 50 36 115 30 36

Rock Buttress 140 0 45 Bedrock 130 500 45

The critical cross sections at each cut-slope location, as identified from the 30 percent project plans, were modeled and checked for global stability using the computer program SLIDE 7.0.

The program uses limit equilibrium theory to compute the factor of safety of soil slopes. The factors of safety were computed using the modified Bishop Method. Due to the low probability (seismic hazard zone 1) for a seismic event to occur in this region, seismic effects were considered negligible with regards to buttress design and were not considered in the analysis.

Page | 20

A critical section was modeled based on maximum slope heights and maximum back slope angles within the project. The proposed cut slope was evaluated with a three-foot thick buttress.

The results are summarized in Table 6 and are presented in Appendix B.

A buttress is recommended to stabilize slopes that are currently experiencing partial or full height slumping. A buttress is a thicker section of an inlay or overlay and provides a restoring moment to stabilize slumping or shallow rotational sliding. The thickness and height of the buttress are designed to provide necessary resisting moment. The results indicated that a three-foot thick buttress provides suitable mitigation measures to meet the required factor of safety.

Based on these results it is recommended that a three-foot thick buttress be incorporated in the design with the option that a few might need to be increased to five-foot thick during construction. Table 6 summarizes areas by stations where rock buttresses are recommended to stabilize the cuts.

Other areas may require rock buttress and will be evaluated during construction once cut slopes are constructed. The rock buttress should conform to section 705.05 of the FP-14 [13] and should be mechanically placed in accordance with section 251.05. A Class 2 Type B separation geotextile conforming to Section 714.01 in FP-14 should be placed between the native soil and the buttress. The buttress final slope ratio should be 1:1.5 or flatter which is steep enough to provide sufficient restoring moment and flat enough to maintain stability of the buttress. The buttress should also be keyed into the granular soil a minimum of 4 feet. A minimum 8-inch diameter perforated drain pipe should be included at the bottom of this buttress key.

TABLE 6:- Summary of Recommended Rock Buttress

BEGIN

STATION

END

STATION

LENGTH, ft CUT SIDE

SLOPE

HEIGHT, ft

SLOPE RATIO NOTES

3157+25 3158+50 75 RT 20 1:1.5

3166+00 3167+00 100 RT 20 1:1.5

3213+60 3214+80 120 LT 20

1:1.5 Remove and reconstruct existing buttress

3214+80 3215+50 70 LT 20 1:1.5

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4.5 ROCK SLOPE ASSESSMENT

Most of the cut- and fill- slopes along the alignment are generally performing satisfactorily for a low volume, winding mountainous roadway at very high elevation. Steeper roadway cuts along the alignment are sparsely vegetated and erosion from surface water is evident. For slopes steeper than 1V:1.5 H, it is unlikely that vegetation can be re-established within a short period.

Few rock slopes exist along the roadway alignment and are typically composed of granodiorite and gneiss. In localized areas, the rock is composed of weak, highly fractured, steeply dipping rock resulting in rockfall potential. Highly fractured bedrock with steeply dipping joint sets and large boulders encountered along rock outcrops may require additional treatment and scaling.

Scaling by hand methods and removal of potentially unstable boulders and blocks is required to reduce the rockfall hazard at various locations along the alignment following construction. In areas where differential weathering of the slope surface produces rockfall hazards, scaling is considered as a temporary solution. Weathering along the pass will continue to expose new, unstable boulders and blocks.

FIGURE 5:- Cut-slope near Sta. 3825+00. A concrete traffic barrier has been installed by Chaffee County to increase catchment area of the roadside ditch. Differential weathering of block-in-matrix material creates a significant rockfall hazard.

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A rockfall catchment ditch is a relatively steep-sloped ditch that is designed to catch rockfall and prevent most rocks from reaching the road. The installation of a minimum of eight-foot wide catchment ditch or concrete barrier along areas with rockfalls can reduce the risk to vehicles from rockfall. The eight-foot wide catchment ditch is for preventing approximately 50 to 75 percent of rockfalls of reaching the paved surface. Without scaling or ditch installation, Chaffee County should expect routine maintenance for the removal of rocks from the roadway after construction. Scaling of slope faces to remove loose, unstable rock and debris should be in accordance with SCR Section 623.

FIGURE 6:– Cut-slope near Sta. 3825+00. Concrete traffic barrier providing additional catchment capacity to the roadside ditch. However, at time of site visit, this catchment was filled, increasing the likelihood of rockfall reaching the roadway.

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4.6 DEBRIS FLOW MITIGATION OPTIONS

As previously discussed, several small steep drainages that cross the project have or potentially could source debris flows that would impact the roadway and pose a risk to the public. Due to their rapid rate of movement, high energy, and lack of predictability, debris flows can be challenging to mitigate. Some options that could be considered for the Cottonwood Pass Road are as follows:

• Debris Flow Netting: Debris flow ring net barriers are becoming a more common mitigation method for controlling debris flows in the U.S. The number and size of the debris flow barriers is dependent on the volume of materials to be retained before reaching capacity. Predicting debris flow volumes is not straightforward and is therefore a shortcoming of existing design methodologies. Volumes of the flow are generally estimated based on the drainage-channel geometry and slope steepness. Such estimates typically require ground survey measurements of the channels to be collected. Debris flow nets require regular maintenance to remove accumulated material and maintain system capacity. The debris flow net system must therefore be located based on both maximizing catchment and providing maintenance access. Without detailed analysis, the estimated cost of such a system cannot be determined with a high degree of accuracy.

However, for preliminary planning purposes, a cost between $50,000 and $100,000 can be assumed for each installation.

• Debris Flow Warning Systems: The U.S. Geological Survey [14] and various other government organizations [15] [16] have set up limited debris flow warning systems.

Generally speaking, these are regional scale systems that tie into existing weather observation and forecasting networks. Smaller systems, while technically feasible, have not entered the market at this time. If a warning system is used to reduce the public risk due to debris flow, such a system would require custom design. Based on the limited scope and schedule of this project, such custom design work may not provide an appropriate solution.

• Channel Improvements: Another strategy to mitigate debris flows is to modify the debris flow channel to reduce impacts to infrastructure and hazard to the public. These modifications can include basins to retain debris material and channelization to convey the debris flow safely. A combination of these methods could be employed on Cottonwood Pass Road by constructing basins on the uphill (left) side of the road and providing oversized culverts to convey debris materials under the roadway. Based on the debris flow deposits exposed in cut-slopes, flows with volumes between 1,000 and 2,000 cubic yards have occurred with some frequency. Ideally, the basin and culverts would be

Page | 24 sized to meet or exceed this capacity. A smaller system can potentially fill and overtop, causing damage to infrastructure and endangering the public. Culverts should be equipped with racks to reduce the potential for clogging. Basins and culverts will require regular maintenance to ensure adequate capacity.

These recommendations can be considered individually, or in combination, to reduce the hazard related to debris flows along Cottonwood Pass Road.

4.7 EXISTING RETAINING WALL EVALUATION

Existing block-faced Mechanically Stabilized Earth (MSE) walls are present along the alignment between Sta. 3750+00 and Sta. 3815+00. The walls are located on the outboard side of the roadway, between the paved surface and Cottonwood Creek, and vary between 1 foot and 8 feet in exposed wall height. Based on the original project plans and observations in the field, the walls appear to have incorporated geogrid reinforcement to stabilize the wall backfill. In general, the walls are in fair to good condition, with a few cap units missing and some minor weathering

FIGURE 7:- Berm constructed from debris flow material at mouth of channel near Sta.

3800+00.

Page | 25 and damage to the facing blocks. No significant signs of distress such as settlement, tilting, or bulging were observed. Where metal pipe culverts are proposed to penetrate the existing walls, the walls should be carefully deconstructed to allow installation of the pipe and facing block should be salvaged to the extent possible. After installation of the culvert pipe, the wall should be reconstructed using new geogrid meeting the same long-term design strength as the original material. The project plans indicate that Tensar UX1500 high-density polyethylene (HDPE) geogrid was originally used to construct the wall. During construction, the Contractor should make a reasonable effort to confirm the type of geogrid incorporated into the wall. Tensar UX1500 has a Minimum Average Roll Value (MARV) ultimate tensile strength (Tult) of 7,810 lb/ft. Based on documented test results performed by the manufacturer and minimum strength reduction factors specified in FP-14, the allowable long-term strength (Tal) of UX1500 is 2,482 lb/ft. The Geotechnical Engineer should be contacted to evaluate the proposed replacement material prior to initiation of reconstruction work. A cast-in-place concrete collar or headwall should be constructed around the culvert where it penetrates the wall face. The facing blocks should be replaced to the extent possible. Where blocks abut to the concrete headwall, blocks should be sawcut to fit and a minimum ¼-inch expansion joint material should be placed between the block and the concrete.

FIGURE 8:- Existing MSE wall located near Sta. 3794+00.

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4.7 DISCLAIMER/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 Analysis and Recommendations Section in 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 judgment of the Contractor.

Page | 27

SECTION FIVE - References [1] Tweeto, O., Steven, T.A., Hail, W.J. Jr., Moench, R.H. (1976). “Preliminary Geologic

Map of the Montrose 1° x 2° Quadrangle, Southwestern Colorado”. 1:250,000.

Miscellaneous Field Studies, Map MF-761. Reston, VA: U.S. Geologic Survey

[2] Brock, M.R., and Barker, Fred (1972). “Geologic Map of the Mount Harvard Quandrangle, Chaffee and Gunnison Counties, Colorado.” 1:62,500. Geologic Quadrangle Map, GQ-952. Reston, VA: U.S. Geologic Survey

[3] Scott, G.R. (1975). “Reconnaissance Geologic Map of the Buena Vista Quadrangle, Chaffee and Park Counties, Colorado.” 1:62,500. Miscellaneous Field Studies, Map MF-

657. Reston, VA: U.S. Geological Survey

[4] U.S. Geological Survey (2017). “Quaternary Fault and Fold Database of the United States.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/hazards/qfaults/>

[5] American Association of State Highway and Transportation Officials (AASHTO).

“AASHTO LRFD Bridge Design Specifications.” Seventh Edition, 2014.

[6] National Weather Service (NWS). “Chaffee County Rock and Mudslides – July 22, 2002”. <http://www.crh.noaa.gov/pub/events/mudslides/mudslides.php>

[7] TheDenverChannel.com. “Denver Breaks Rainfall Record”. 19 August 2004.

<http://www.thedenverchannel.com/news/3665466/detail.html> Accessed on July 24, 2017.

[8] Goetz, P. “Alpine Residents Clean Up After July 21 mudslide”. Chaffee County Times.

25 July 2007.

<https://web.archive.org/web/20110708133700/http://www.chaffeecountytimes.com/mai n.asp?SectionID=2&SubSectionID=2&ArticleID=3347&TM=7788.078> Accessed on July 24, 2017.

[9] “Cottonwood Lake remains closed ‘until further notice’ after multiple mudslides”.

Chaffee County Times. 6 July 2015.

<http://www.chaffeecountytimes.com/free_content/cottonwood-lake-remains-closed-until-further-notice-after-multiple-mudslides/article_2bd92cc8-2419-11e5-bcc4-c3e2a700ea40.html> Accessed on July 24, 2017.

Page | 28

[10] U.S. Geologic Survey (2017). “U.S. Seismic Design Maps.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/designmaps/us/application.php>

[11] ReSSA Version 3.0, Adama Engineering, Inc.

[12] Slide Version 6.0, RocScience, Inc.

[13] U.S. Department of Transportation, Federal Highway Administration. “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP- 14.” Publication No. FHWA-FLH-14-001.

[14] U.S. Geological Survey (2005). “NOAA-USGS Debris-Flow Warning System—Final Report.” Circular 1283.

[15] Romag, H., Zappa, M., Hilker, N., Gerber, M., Dufour, F., Frede, V., Berod, D., Oplatka, M., Hegg, C., and Rhyner, J. (2011). “IFKIS-Hydro: an early warning and information system for floods and debris flows.” Natural Hazards. Vol 56, Issue 2, pp 508-527.

[16] Norwegian Water Resources and Energy Directorate “About flood warning and landslide warning” <http://www.varsom.no/flom-og-jordskredvarsling/om-flomvarslingen-og-jordskredvarslingen/> Accessed on August 15, 2017, translated using Google Translate service.

APPENDIX A Project Location and Geologic Maps

APPENDIX B Buttress and Soil Slope Analyses

1.0271.027

W

W

1.0271.027

Material Name Color Unit Weight (lbs/ 3)

Cohesion (psf)

Phi (deg) Water Surface

Silty Gravel (Upper) 125 100 38 Water Surface

Silty Gravel (Lower) 125 100 40 None

Bedrock 130 500 45 None

S…