WY.FLAP.30400(1).Gros Ventre Road.Geotechnical Memo.FINAL.PDF

To: Micah Leadford, Project Manager, CFLHD, Lakewood CO

From: James Arthurs, Geotechnical Engineer, CFLHD, Lakewood CO

Through: Marilyn Dodson, Lead Geotechnical Engineer, CFLHD, Lakewood CO

Subject: WY FLAP 30400(1) Gros Ventre Road Geotechnical Memorandum

1.0 Introduction and Project Description

This memo presents the review of available geotechnical information and geotechnical recommendations for the WY FLAP 30400(1) Gros Ventre Road project. The project consists of resurfacing, restoration, and rehabilitation (3R) of 3.75 miles of Gros Ventre Road, starting at the boundary of Grand Teton National Park (approximate milepost 2.2) and continues east to the Atherton Campground entrance road. The project is in Teton County approximately 15 miles northeast of the Town of Jackson, Wyoming.

Project Description

The project consists of the re-construction of the asphalt pavement from approximately MP 2.2 to

5.95. The existing road roadway consists of a 19- to 24-foot paved width (including paved shoulders) two-way road. Horizontal and vertical profile changes are not anticipated. Additional bench width may be required at limited locations and is anticipated to be provided by widening to the outboard shoulder, rather than expansion of the existing cut slopes. Additional construction such as signage and drainage improvements is also included in the project plans.

Technical Memorandum

Date: April 30, 2020

Figure 1:- Project Location Map

Geologic Setting

Regional Geology

The Gros Ventre Road is located on the eastern side of the Jackson Hole basin, formed by a downdropped fault block bounded to the west by the Teton Mountain Range and to the east by the Gros Ventre and Washakie Ranges. While the source rocks in the Tetons are Precambrian age (2.8 to 2.7 Ba) metamorphic rocks intruded by igneous dikes, the uplift which created the mountain range began with the Teton Fault vertically offsetting rocks (13 Ma) followed with the formation of the Tetons through erosion and basin deposition (10 Ma). Rocks found within the Gros Ventre Range are representative of sedimentary deposits typical of the Western Interior Seaway and earlier inundation periods. Along the eastern boundary of the Rocky Mountains, fine grained marine and sedimentary rock sequences over 15,000 feet thick are representative deposits typical to the period of origin. Regional bedrock types found in the area include limestone, sandstone, and shale with localized deposits of bentonite, gypsum, and coal beds. Formation of the Rockies during the Laramide Orogeny and subsequent tectonic faulting resulted in deformational bedrock structures within the Gros Ventre Range typically dipping westwards, away from the uplifting mountains.

Recent geologic activity in the area includes the glaciation which carved the Tetons and provided the source material for the Jackson Hole basin, as well as the massive volcanic eruptions from the Yellowstone Volcano. The Buffalo Glaciation (200,000 years ago) is estimated to have completely covered the Tetons, Gros Ventre Range and surrounding area in over 2,000 feet of ice. This event is credited as the most severe glaciation and led to the formation of subsequent drainage paths and valleys. Later glacial periods led to lacustrine, fluvatile, and glacial deposits of outwash moraine alluvium, wind-blown loess, and laminated silts and sands.

Local Geology

The site is mapped as underlain by Quaternary glacial, alluvial, and landslide deposits and Mesozoic sedimentary rocks. The Quaternary deposits generally range from fine-grained lacustrine silts and clays to colluvial silt, sand, gravel, and boulders deposits. Mesozoic rock units include sandstone, siltstone, and shale with interbeds and lenses of limestone and dolostone.

Numerous ancient and modern landslides are mapped in the project area, including the 1925 Gros Ventre landslide, which dammed the Gros Ventre River. The geologic map in Figure 2 shows the road alignment (red line) through glacial and landslide deposits typical of the region.

The Gros Ventre Landslide is located near the project, on the south side of the Gros Ventre River.

In the landslide source area, the Tensleep Formation overlies the Amsden Formation and dips north-northeast toward the Gros Ventre River. Tensleep Formation is white to gray sandstone containing thin limestone and dolomite beds, while Amsden Formation is red and green shale and dolomite. Both formations range in age between Late Pennsylvanian to Early Permian (299 Ma).

Numerous landslide deposits and jumbled bedrock outcrops north and south of the river indicate that the area has experienced landslides regularly.

The June 23, 1925 landslide originated in the Tensleep Sandstone south of the Gros Ventre River and carried 50-million cubic yards of debris into the valley, which blocked the river creating Lower Slide Lake. While the historic Gros Ventre and prehistoric Devil’s Elbow Landslides are notable large-scale failures, there is a high density of mapped landslide deposits (Qls) north of the road.

Local geology, geologic structure, hydrology, and precipitation are the primary reasons that landslides occur in specific areas. Changes in the loading regime through the construction of cuts and fills can increase the risk of future instability of inactive and active slides in the vicinity of the road.

Figure 2:- Project Area Geologic Map

2.0 Available Geotechnical Information

Geotechnical information was obtained from sources that include available published geologic maps as well as boring logs and laboratory test results presented in the Pavements Report and geotechnical reports for nearby features. The following references have been reviewed:

• Wyoming Office of Homeland Security, “Wyoming State Mitigation Plan 2016-2021”.

• Geotechnical report for Gros Ventre Road Landslides prepared by Kumar and Associates.

• “A Brief History of the Gros Ventre Slide Geologic Site” published by the U.S. Forest Service.

• “Geologic Map of the Shadow Mountain Quadrangle, Teton County, Wyoming” published by the Wyoming State Geologic Survey.

Subsurface Information

The geologic mapping indicates that the current alignment is underlain predominantly by various Quaternary materials that are described as consisting of unconsolidated deposits of silt, sand, gravel, and cobbles. Portions of the alignment are underlain by areas mapped as landslide debris.

These general descriptions appear to be consistent with the soil descriptions and laboratory test results presented in the Pavements investigation. Bedrock in the area consists of sandstone, siltstone, and shale with interbeds and lenses of limestone.

A pavements investigation was performed by CFLHD in November, 2017. This investigation indicates that the subgrade soils along Gros Ventre Road consist of silty to clayey sand and gravel and silty clay with sand and gravel (A-2-4, A-2-6, A-4, and A-6 in the AASHTO classification).

Seismic Design Parameters

Three seismic hazard faults are located within 40 miles of the project. The Teton Fault is a north-south trending fault that is located west of the project, the East Mount Sheridan faults are north-south trending and located north of the project, and the Hoback fault is a northwest-southeast trending fault located south of the project.

No bridges or other major structures are proposed for this project. If minor structures such as retaining walls or reinforced soil slopes are required to provide sufficient bench width, they should be designed using the following seismic 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, and represents horizontal peak ground acceleration (PGA) with a 7 percent probability of exceedance in 75 years (approximate 1000-year return period). The 1000-year return period uniform hazard spectrum for the center of the Gros Ventre Road project (43.633397° N latitude and 110.557191° W longitude) was obtained in accordance with the AASHTO ground motion maps. The site was classified as “Class D” according to site class definitions specified in Table 3.10.3.1-1 of AASHTO and soil profile assumptions based on the available geotechnical information, including boring completed for nearby projects. The recommended spectral acceleration coefficient values for probabilistic design are summarized on Table 1.

Table 1:- Summary of Seismic Parameters Corrected for Site Class D

Peak Seismic Ground Acceleration Coefficient (As) 0.353

Horizontal Response Spectral Acceleration at Period of 0.2 sec (SDS) 0.856

Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.432

Site Factor at Zero-Period of Acceleration Spectrum (FPGA) 1.224

Site Factor at Short-Period of Acceleration Spectrum (Fa) 1.254

Site Factor at Long-Period Range of Acceleration Spectrum (Fv) 1.959

Based on the long acceleration coefficient SD1 value of 0.432, the site is assigned to Seismic Hazard Zone 3, in accordance with Table 3.10.6-1 of AASHTO.

3.0 Field Exploration

A geotechnical subsurface investigation was not performed for this scope of work. Limited surficial evaluation and reconnaissance was performed based on areas identified during office review of the preliminary 30% plans and the associated site visit. Two samples of cut slope materials were collected for laboratory testing as part of the reconnaissance. Laboratory tests included gradation (AASHTO T88), classification (AASHTO M145), and Atterberg limits (AASHTO T89 and T90). When the necessary tests were completed, samples were classified using the Unified Soil Classification System (USCS) and AASHTO soil classification system. Results of the classification testing are summarized on Table 2 below.

Table 2 – Summary of Laboratory Index Test Results

STATION

SAMPLE

DEPTH

PERCENT

GRAVEL

PERCENT

SAND

PERCENT

PASSING

#200

LIQUID

LIMIT

PLASTICITY

INDEX

USCS

CLASS.

AASHTO

CLASS.

(inches)

299+00 Surface 0 22 78 36 19 CL A-6(13)

308+25 Surface 0 49 51 25 11 CL A-6(2)

Resistivity (AASHTO T288), pH (AASHTO T289), sulfate (AASHTO T290), and chloride (AASHTO T291) tests were also performed on the samples. Results of the corrosivity testing are presented on Table 3 below.

Table 3 – Summary of Laboratory Corrosivity Results

STATION

SAMPLE

DEPTH

RESISTIVITY PH

SULFATE ION

CONTENT

CHLORIDE

ION CONTENT

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

299+00 Surface 1,390 7.2 50 3

308+25 Surface 2,060 6.8 40 6

Detailed laboratory test results are attached to this memorandum.

Cut slopes from Sta. 119+00 to approximately Sta, 190+00 appeared to be stable, with well-developed vegetation. Slopes between Sta. 190+00 and 319+00 were generally marginally stable, with little vegetation. Some surficial sliding and raveling was noted during the site visit. Isolated seepage locations were noted between Sta. 300+00 and 310+00 (Figure 3).

Figure 3:- Seepage area near Sta. 305+00.

4.0 Preliminary Geotechnical Recommendations

The following analysis is based on the preliminary 30% design plans, the available geotechnical information presented in Section 2 of this memo, and limited site reconnaissance performed during the 30% design field review. No site specific geotechnical subsurface investigation has been performed.

Based on the 30% field review, the scope of geotechnical work on this project is expected to be limited to minor cuts and fills (less than 5-foot high) and subgrade stabilization. Recommendations for these items are presented in this section.

Cut Slopes

Numerous small scale landslide and creep features have been identified in the project area through review of the available geotechnical data and limited site reconnaissance performed during project scoping. Removing load from the toe of an unstable or quasi-stable slope has a high potential to reactivate ground movement. Due to this potential, expansion of the existing cut slopes or construction of new cut slopes is not recommended without thorough geotechnical investigations and analysis. This work is beyond the current geotechnical scope of this project.

Embankments and Earthwork

Construct permanent long-term embankments with a maximum slope ratio of 1V:2H to maintain slope stability and promote vegetation of the slope.

On-site soils expected to be encountered within the project limits generally consist of alluvial and colluvial clayey sands and gravels with varying amounts of silt and cobbles/boulders. It is estimated that these soils will have a 10% shrink percentage, corresponding to a shrink/swell factor of 0.90 when placed and compacted according to Section 204 of the FP-14. 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.

Subgrade Stabilization

During the 30% review site visit, one location of potential subgrade failure was noted. Significant cracking was noted across the existing pavement between Sta. 303+00 and Sta. 312+00 (Figure 4). Springs and seeps were also observed in the cut slope in this area. Based on the pattern of distress observed, saturation and yielding of the subgrade is interpreted to be the primary failure mechanism. A reinforced subexcavation section is recommended to stabilize the subgrade. The subexcavation should be 2 feet deep and extend the full width of the paved roadway. The subexcavation should be lined with non-woven separation geotextile, Class 1, Type C to prevent migration of fines. The subexcavation should be reinforced with two layers of stabilization geogrid, with one layer placed at the bottom of the subexcavation, and the other placed 12 inches above the bottom. The subexcavation should extend at least 25 feet beyond the identified failure areas into stable ground. The subgrade and pavement section should also be protected with an underdrain constructed on the cut-slope side of the roadway. The underdrain should be at least 5 feet deep with regularly spaced discharge pipes. The bottom of the subexcavation should be sloped to direct drainage into the underdrain.

Based on the soils encountered during the Pavements investigation, use of import material will be preferable for the backfill. Material from the USFS borrow pit will be suitable for use as fill for subexcavation with limited processing to remove cobbles and boulders greater than 4-inch diameter.

Additional areas of unstable subgrade may be identified during construction. These areas should be stabilized using a subgrade detail similar to the one described above, except that the stabilization geogrid may be eliminated. Approximately 500 cubic yards of this subexcavation should be included in the contract for use at the discretion of the CO and CFL field staff. We understand that the Pavements Engineer may also provide recommendations for subexcavation locations.

Figure 4:- Damaged pavement at proposed subexcavation location.

Drainage

Collection and removal of groundwater using interceptor trench drains or cutoff systems will prevent hydrostatic pressures from developing within engineered fills. Maintaining internal stability and localized global stability with a robust drainage system could prolong service life of embankments. Subsurface groundwater sources and surface runoff from ditches should be evaluated for the capacity of any drain, with the expectation that seasonal runoff from springtime snowmelt would likely be the maximum design flow rate. An underdrain should be installed in the inboard shoulder at locations where springs and seeps are evident. Surface water should be appropriately channelized and conveyed away from the roadway embankment to prevent infiltration into the embankment backfill.

Forest Service Borrow Area

We understand that the US Forest Service maintains a borrow source off of Forest Road 30380, approximately 8 miles east of the project site. Materials in this borrow source generally consist of alluvial (stream terrace) deposits, primarily silty sand and gravel with cobbles and boulders.

Boulders up to approximately 3 feet were observed on the surface. This material is anticipated to be suitable for Subexcavation Type 1 Backfill with processing to remove particles larger than 4 inches in diameter. Photos of the borrow area are presented in Figures 5 and 6.

Figure 5:- Proposed Borrow Area

Figure 6:- Proposed Borrow Area

5.0 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.

Attachments

1. Laboratory Test Results

2. Reinforced Subexcavation Detail

3. Subexcavation Backfill SCR

Prepared by:

James Arthurs, Ph.D., P.E.

Geotechnical Engineer, CFLHD

Reviewed by:

Todd Hansen, P.E.

Geotechnical Engineer, CFLHD

Approved by:

Marilyn D. Dodson, P.E.

Lead Geotechnical Engineer, CFLHD

References American Association of State Highway and Transportation Officials (AASHTO). “AASHTO LRFD Bridge Design Specifications.” Seventh Edition, 2014.

Love, J.D. (2001). “Geologic Map of the Shadow Mountain Quadrangle, Teton County, Wyoming.” 1:24,000. J. David Love Historical Geologic Map Series, Geology of the Teton- Jackson Hole Region, Map LMS-7.

Monley, Greg J., (1999). “Geotechnical Engineering Study Gros Ventre Road Landslides near Jackson, Wyoming.” Kumar and Associates, Inc Project No. 98-1-468.

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2014). “Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.”

Publication No. FHWA-FLH-14-001.

U.S. Department of Transportation, Federal Highway Administration (FHWA). (2009). “Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes.”

Publication No. FHWA-NHI-10-024.

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

U.S. Geologic Survey (2019). “U.S. Seismic Design Maps.” USGS Earthquake Hazards Program, <http://earthquake.usgs.gov/designmaps/us/application.php>

Wyoming Office of Homeland Security. (2016). Wyoming State Mitigation Plan. Cheyenne, WY.

324 pgs.

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GRAIN SIZE DISTRIBUTION

COBBLES

GRAVEL

78.0

51.0

4.75

4.75

SAND

GRAIN SIZE IN MILLIMETERS

coarse fine

LEAN CLAY with SAND(CL)

SANDY LEAN CLAY(CL)

Classification

D100 D60 D30 D10 %Gravel

0.1

Sta. 299+00

Sta. 308+25 coarse

SILT OR CLAY

finemedium

0.0

0.0

%Sand %Silt %Clay

0.0

0.0

22.0

49.0

BOREHOLE DEPTH

BOREHOLE DEPTH

3 100

Sta. 299+00

Sta. 308+25

24 16 301 2006 10 501/2

HYDROMETERU.S. SIEVE OPENING IN INCHES U.S. SIEVE NUMBERS

1403 4 20 406 601.5 8 143/4 3/8

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PROJECT NUMBER WY FLAP 30400(1)

PROJECT NAME Gros Ventre Road

PROJECT LOCATION Teton County, Wyoming

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P L A S T I C I T Y

I N D E X

LIQUID LIMIT

Fines Classification

LEAN CLAY with SAND(CL)

SANDY LEAN CLAY(CL)

LL PL PI

ATTERBERG LIMITS' RESULTS

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BOREHOLE DEPTH

CLIENT

PROJECT NUMBER WY FLAP 30400(1)

PROJECT NAME Gros Ventre Road

PROJECT LOCATION Teton County, Wyoming

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NO SCALE

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204-B structural section information

See Typical Sections for ground

Existing

F2

10' 10'

Type C (Non-woven)

Geotextile, Class 1, 12" vertical spacing (typ.)

Stabilization Geogrid

Station 302+50 - 312+50 details

See Special 605-A for

Geocomposite underdrain

4.

3.

2.

1.

Notes:

See note 1.

USFS Material Pit

Backfill with material from

(TYPE 1)

SUBEXCAVATION

(TYPE 1)

SUBEXCAVATION

26.8' or As Directed by CO

1:4 1:4

See FP Subsection 714.01(a) for Geotextile requirements.

See SCR Subsection 207 for Stablilization Geogrid requirements.

Do not place backfill material within the structural section.

See SCR Subsection 704.09 for backfill requirements.

towards the geocomposite sheet drain.

Slope bottom of subexcavation at a minimum of 2%

STATE PROJECT

P

M N

W Y w y R o a d w a y C

A D

D S h e e ts F T y p e S u b e x .d g n

CENTRAL FEDERAL LANDS HIGHWAY DIVISION

FEDERAL HIGHWAY ADMINISTRATION

U.S. DEPARTMENT OF TRANSPORTATION

CENTRAL FEDERAL LANDS HIGHWAY DIVISION

FEDERAL HIGHWAY ADMINISTRATION

U.S. DEPARTMENT OF TRANSPORTATION

SPECIAL

U.S. CUSTOMARY SPECIAL

U s e r S ta c y .D ic c ic c o

NUMBER

SHEET

WY

Gros Ventre Road

WY FLAP 30400(1)

Section 704. – SOIL

Add the following Subsection:

704.09 Type 1 Subexcavation Backfill. Furnish sound, durable, granular soil free from organic matter or other deleterious material (such as shale or other soft particles with poor durability).

Conform to the following:

(a) Quality requirements.

(1) Gradation Table 704-3

(2) Liquid limit, AASHTO R 58 and T 89 40 max.

(3) Plastic index, AASHTO R 58 and T 90 20 max.

Table 704-3 Type 1 Subexcavation Gradation

Sieve Size Percent by Mass Passing

Designated Sieve (AASHTO T 27 and T 11)

4 inch 100 No. 4 20–100 No. 40 0 – 60

No. 200 0 – 35