THRO 204(1) Buck Hill Road FINAL Geotechnical Report nov 2024 Full Signed.pdf

BUCK HILL ROAD

NP THRO 204(1)

BILLINGS COUNTY, ND

FINAL Geotechnical Report

Prepared by Federal Highway Administration

Central Federal Lands Highway Division

Geotechnical Services Branch

November 2024 ii

SIGNATURE SHEET

Report prepared by: ___________________________________________________________

Dominic J.T. Monarco, Geotechnical Engineer

Report reviewed by: ____________________________________________________________

For James M. Arthurs, P.E., P.G., Ph.D., Senior Geotechnical Engineer

Approved for distribution by: ____________________________________________________

Devin T. Dixon, P.E., Geotechnical Team Leader

Distribution

Electronic:

N:\ND\204(01)\Geotech\7_FinalDocs K:\TechServices\Geotech\1. Geotech Project Reports Project Management Project Development, Lead Designer

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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 SITE GEOLOGY AND SOILS

2.3 REGIONAL SEISMICITY

2.4 SEISMIC DESIGN PARAMETERS

2.5 GEOLOGIC HAZARDS

SECTION THREE - SUBSURFACE INVESTIGATIONS

3.1 SUBSURFACE EXPLORATION PROGRAM

3.2 LABORATORY TESTING PROGRAM

3.3 SUMMARY OF SITE CONDITIONS

3.3.1 General Topography

3.3.2 Surface Reconnaissance

3.3.3 Subsurface Conditions

3.3.4 Subsurface Landslide Movement

3.3.5 Groundwater

SECTION FOUR - ANALYSIS & RECOMMENDATIONS

4.1 BUCK HILL LANDSLIDE MITIGATION

4.1.1 Foundation Type Options

4.2 FOUNDATIONS

4.2.1 Geosynthetic Reinforced Deep Patch

4.2.2 Deep Patch Slope Stability

4.2.3 Deep Patch Internal Stability

4.2.4 Sheet Drains

4.2.5 Ditch Recommendations

4.2.6 Shrink/Swell Recommendations

4.2.7 Drainage Improvements

4.2.8 Roadway Maintenance

4.2.9 Geosynthetics

SECTION FIVE - CONSTRUCTION CONSIDERATIONS

5.1 Roadway Excavation

5.2 Dewatering

5.3 Slope Instability

5.4 SPECIFICATIONS

5.5 DISCLAIMER/LIMITATIONS CLAUSE

SECTION SIX - REFERENCES

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TABLES

TABLE 2.1 Summary of Nearby Seismic Source Faults TABLE 2.2 Summary of Seismic Parameters Corrected for Site Class C TABLE 3.1 Summary of the Field Exploration Program TABLE 3.2 Summaries of Laboratory Index Test Results TABLE 4.1 Generalized Buck Hill Landslide Mitigation Options TABLE 4.2 Estimated Material Properties of Native and Backfill Soils for Deep Patch

Design

FIGURES

FIGURE 1: Typical Roadway Conditions FIGURE 2: Past and Current Buck Hill Road Alignments FIGURE 3: Cross Section Deep Patch

PLATES

PLATE 1 Project Site Map PLATE 2 Geology Map PLATE 3 Landslide Map PLATE 4 Boring Location Map

APPENDICES

APPENDIX A Field Exploration Program APPENDIX B Laboratory Test Results APPENDIX C External and Internal Stability Analysis APPENDIX D-1 Photographs March 9th – 10th APPENDIX D-2 Photographs May 29th – June 3rd APPENDIX E Inclinometers SI-101 and SI-102

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Buck Hill Road, NP THRO 204(1) November 2024

SECTION ONE - Introduction

1.1 BACKGROUND AND LOCATION

Buck Hill Road, also known as Route 204, is located in the southeast portion of the South Unit of Theodore Roosevelt National Park, (Park). The route begins at the intersection of Buck Hill Road and the South Unit Loop Road. The route then proceeds southeasterly for approximately ¾ of a mile and ascends approximately 300 feet vertically to the Buck Hill Overlook parking area. The Buck Hill Overlook is a very popular location in the Park with approximately 32% of Park visitors visiting this overlook. This project proposes to rehabilitate the existing pavement and address subsurface issues observed along the route, including a previously identified slide area. The project further proposes to reconstruct and expand the Buck Hill Overlook parking area at the end of the route. No alternative alignments will be proposed or analyzed.

Typical roadway conditions are presented in Figure 1 and a project vicinity map is presented on Plate 1 of this report.

Figure 1 : Typical Roadway Conditions

Roadway, pavement, and drainage conditions of Buck Hill Road currently exist as poor to fair.

The pavement shows a range of distress, exhibiting longitudinal, transverse, and fatigue cracking along the entire route. The worst area of distress is located from station 113+00 to 122+50 where a persistent landslide has forced maintenance operations to maintain a gravel surface through the aptly named “Buck Hill Landslide.” This stationing is based on the current 95% milestone. Poor subsurface soils and drainage conditions along the roadway have led to the distress caused by the landslide movement. See Appendix D for typical site photographs.

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The proposed project involves rebuilding the existing roadway to a two-lane, two-way roadway with 11-foot lanes and 1-foot shoulders. Drainage improvements including updates to the existing culverts and the addition of new culverts are proposed by the Central Federal Lands Highway Division (CFLHD) Hydraulics Engineering Group as discussed in the Final Hydraulics Report dated October 2024. Geotechnical-related improvements will include subexcavation with stabilization geogrid, an underdrain system, and a deep patch embankment repair. These improvements have the potential to reduce roadway maintenance costs, improve surface drainage, and mitigate pavement cracking.

The Buck Hill Landslide has been known to be a maintenance issue since the 1970’s when it was first formally described during the Buck Hill Road realignment planning and design. Prior to the 1970’s the entrance to Buck Hill Road from the main loop road was further to the west. After years of heavy maintenance and roadway failures the alignment was moved to its current location with the goal of reducing maintenance costs associated with the old alignment. See Figure 2 below where the red line denotes the current alignment, and the blue represents the alignment prior to the 1970’s.

Figure 2: Past and Current Buck Hill Road Alignments. Blue line is approximate location of past Buck Hill Road alignment; red line represents current alignment.

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1.2 SCOPE AND PURPOSE

The scope of work included a geotechnical investigation performed by the CFL Geotechnical Engineering Group, analysis, and recommendations for the mitigation of the Buck Hill Landslide and roadway maintenance. This involved several tasks including field reconnaissance, subsurface sampling, laboratory testing, interpretation and correlation of field measurements, and geotechnical engineering analysis. Specifically, this investigation was conducted to explore the subsurface conditions in the areas where significant ground deformations have damaged the traveled way and to develop recommendations concerning surface/subsurface drainage, geological hazards, material shrink/swell, and construction considerations for the design and construction of drainage features and embankment repairs within the alignment.

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SECTION TWO - Geology and Seismicity

2.1 REGIONAL GEOLOGY

Theodore Roosevelt National Park (THRO) is situated in the Great Plains physiographic province along the boundary separating the glaciated and unglaciated regions of the Missouri Plateau. The bedrock underlying Buck Hill Road consists of Paleocene rocks. The Paleocene Bullion Creek Formation and Sentinel Butte Formation are exposed in the scenic badlands. The Bullion Creek Formation is comprised of yellowish-brown sedimentary deposits of sandstone, siltstone, and mudstone that occur in the Elkhorn and South Units. The overlying Sentinel Butte Formation is widely distributed across the North and South Units of THRO and consists of grayish-brown sedimentary deposits, tuffs, and bentonite beds. The Park also contains Quaternary surficial deposits including upland gravel deposits, mantled pediment deposits, landslide debris, alluvial sediments, and alluvium. Plates 2 and 3 display the geological and landslide maps, respectively.

2.2 SITE GEOLOGY AND SOILS

The project starts at the intersection of Loop Road and Buck Hill Trail Road and spans the full length of Buck Hill Road. The entire stretch of the project sits atop the Sentinel Butte Formation.

This formation consists of grayish-brown silt, sand, clay, sandstone, and lignite. Additionally, it also contains river, lake, and swamp sediment deposits. This formation can have a thickness of up to 600 feet (Sentinel Butte Formation, n.d.). During geotechnical sampling, thick sections of lignite were encountered as described in Section 3.

According to the Natural Resources Conservation Service Web Soil Survey, there are four predominant soil groups in and around the project area – the Rhame-Fleak complex; the Badland-Cabbart complex; the Lonna-Arikara-Cabbart complex; and the Lonna-Kirby-Cabbart complex. A complex consists of two or more soils or miscellaneous areas in such an intricate pattern or in such small areas that they cannot be shown separately on the maps (Billings County, North Dakota (ND007) Map, n.d.). While there are other soil groups present in the area, the primary focus on these four is because they cover the most land surface in and around the project area. The USDA mapping includes a hydraulic soil rating. This rating scale goes from “A” to “D”, with “A” indicating a low soil runoff potential and “D” indicating a high soil runoff potential. A “D” rating has the highest runoff potential with very low infiltration rates.

The Rhame-Fleak complex consists of residuum weathered from calcareous sandstone. It has a Unified Soil Classification rating of sandy clay (SC) due to its percent sand and clay composition. It has a US Department of Agriculture (USDA) hydraulic soil designation of D.

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The Badland-Cabbart complex consists of residuum weathered from shale and siltstone. It has a Unified Soil Classification rating of Lean Clay (CL). It also has a US Department of Agriculture (USDA) hydraulic soil designation of D.

The Lonna-Arikara-Cabbart complex consists of slope alluvium primarily derived from siltstone.

It has a Unified Soil Classification rating of Silt (ML). It has a USDA hydraulic soil designation of B indicative of a moderate infiltration rate and runoff potential.

The Lonna-Kirby-Cabbart complex consists of slope alluvium derived from shale and siltstone and porcellanite. It also has a Unified Soil Classification rating of Silt (ML). It also has a USDA hydraulic soil designation of B.

2.3 REGIONAL SEISMICITY

No seismic source or Quaternary faults are mapped within 50 miles of the project. The closest hazardous fault is the Emigrant Fault, located 353 miles west of the project alignment (USGS, 2020).

2.4 SEISMIC DESIGN PARAMETERS

Recommended seismic response parameters for the Buck Hill Road site design are based on the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications, 9th Edition, 2020 and represents horizontal peak ground acceleration (PGA) with 7 percent probability of exceedance in 75 years (approximate 1,000-year return period). The 1,000-year return period uniform hazard spectrum for the Buck Hill Road Project site, centrally located at 46°55’36.38”N latitude & 103°23’22.57”W longitude, was obtained in accordance with the AASHTO ground motion maps.

The topographic USGS Vs30 online map view (USGS, 2024a) of the site gave an average value of around 1,700 ft/s. Therefore, the site is classified as Class C according to the 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 Table 2.1.

Table 2.1: Summary of Seismic Parameters Corrected for Site Class C Factored Horizontal Peak Ground Acceleration (As) 0.022g

Factored Horizontal Response Spectral Acceleration at Period of 0.2 sec (SDS) 0.054g

Factored Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.028g

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

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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 0.028g, the project is assigned to seismic hazard Zone 1 in accordance with Table 3.10.6-1 of AASHTO. Seismic hazard zones reflect the variation in seismic risk in different regions needing different requirements for design as depicted in Table 4.7.4.3.1-1 in AASHTO. Due to the project location being classified as Zone 1, a seismic analysis is not be required per AASHTO 4.7.4.3.1.

2.5 GEOLOGIC HAZARDS

Geologic hazards exist from both the natural environment of the project site and from existing and proposed construction of the roadway. The main geologic hazards that may exist within the vicinity of the project limits include landslides, frost heave of clays and highly erodible, and fine-grained soils. As stated in the Section 2.2, soils within the project area have a high runoff off potential that could negatively affect the area and cause mass wasting events. A previous Geologic Resource Evaluation Report stated that hazards associated with mass wasting in the Park consist of landslides and soil creep. Soils in the park are also known to be dispersive leading to the occurrence of “pipes” or openings in the surface allowing water infiltration into the subsurface. Landslides have also been exacerbated by road construction in the Park. All existing landslide deposits are capable of renewed activity, and other slopes, especially those in the North Unit containing the Sentinel Butte Formation, may be vulnerable to future movement (Keller Lynn, 2007, p. 7). Refer to Plate 3 to view the USGS Landslide map.

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SECTION THREE - Subsurface Investigations

3.1 SUBSURFACE EXPLORATION PROGRAM

A subsurface investigation was performed by a CFLHD geotechnical engineer on March 9 and 10, 2024, and May 29 and 30, 2024. The geotechnical subsurface exploration program consisted of drilling a total of four (4) borings; consisting of two (2) inclinometer holes and two (2) paired piezometer holes. Five (5) pavement borings were also advanced to a maximum depth of 5 feet.

Please refer to the Final Pavements and Materials report for further information regarding the pavement investigation and design. The purpose of the geotechnical borings was to investigate the subsurface along the project alignment specifically to investigate subsurface movements at the Buck Hill Road landslide. Standard penetration testing (SPT) and sample collection were performed at 5-foot intervals in the geotechnical borings. Additionally, bulk grab samples were collected from the top 5 feet in the pavement borings. Boring locations are presented on Plate 4.

Subsurface conditions were logged and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing. Logs of the explorations and laboratory test results are presented in Appendix A and B, respectively. A summary of the field exploration is provided below in Table 3.1.

Table 3.1: Summary of Field Exploration Program

EXPLORATION

DESIGNATION LOCATION PURPOSE

APPROXIMATE

GROUND ELEVATION

(ft.)

TERMINATION

DEPTH (ft.)

SI-101

STA 117+90

18 ft. LT of centerline

Geotechnical Subsurface Movement

2612 111.5

PZ-101

STA 117+90

18 ft. LT of centerline

Geotechnical Subsurface

Groundwater 2612 60

SI-102

STA 119+00,

13 ft. LT of centerline

Geotechnical Subsurface Movement

2616 126.5

PZ-102

STA 119+00

13 ft. LT of centerline

Geotechnical Subsurface

Groundwater 2616 60

Note: The exploration locations were estimated relative to existing features. Ground elevations were estimated from Google Earth.

3.2 LABORATORY TESTING PROGRAM

Soil samples recovered from the borings by SPT were tested in the laboratory to support the field classifications and to provide an estimate of the engineering characteristics and mechanical properties of the soil. Laboratory tests included moisture content (AASHTO T255), sieve analysis (AASHTO T 11 and T27), classification (AASHTO M145), and Atterberg limits (AASHTO T89 and T90). When the necessary tests were completed, samples were classified using the Unified

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Soil Classification System (USCS) and AASHTO soil classification system. Results of the testing are summarized in Table B-1 in Appendix B.

For SI-101, soil test results indicated a range of material types including silty sands, lean to fat clays, and organic silts and clays, classifying as A-2-7(0) to A-7-5(90) by AASHTO and SM, CL, CH, ML, and MH by USCS.

For SI-102, soil tests indicated a range of fat clays classifying as A-7-6(32) to A-7-6(107) by AASHTO and CH by USCS.

3.3 SUMMARY OF SITE CONDITIONS

A field review of the site conditions was performed in conjunction with the subsurface exploration by a Central Federal Lands Highway Division (CFLHD) geotechnical engineer. The existing roadway consists of oxidized asphalt pavement ranging in width from 16 to 28 feet with high severity fatigue cracking throughout the project. The condition of existing drainage infrastructure is poor with substantial areas of standing water specifically centered around the Buck Hill landslide. This section presents the results of the surface reconnaissance and subsurface exploration.

3.3.1 General Topography

The project is located within the Great Plains physiographic province. The general topography of the project site is characterized by features typical of this geographic region that include hummocky, irregular topography. The landscape of Theodore Rosevelt National Park can be described as rolling. The project area is vegetated in grasses and brush with juniper trees where the topography is less steep. Sparse vegetation in the form of brush and grasses is present on moderately steep slopes.

3.3.2 Surface Reconnaissance

The project begins at the intersection of Scenic Loop Drive and Buck Hill Road. The road proceeds uphill at approximately a 5 to 6 percent grade and then terminates at the Buck Hill Trail parking area approximately 3/4 mile from the intersection. Fatigue cracking was observed on the asphalt pavement throughout the length of the project. Hydraulic concerns including standing water and head cutting near existing culverts was also observed.

From Station 113+00 to 122+50 the roadway is significantly impacted by the Buck Hill Landslide.

This is most evident from Station 117+50 to 122+00 where the Park has been forced to maintain a gravel-surfaced roadway due to large ground deformations destroying the asphalt pavement in the past. There are several lignite seams present in the cutslopes to the east. These lignite seams are approximately 6 inches to 2 feet thick and are interbedded between much thicker layers of siltstones and claystones of the Sentinel Butte Formation. It should be noted that although the

Page | 9 project is underlain by formational units, these materials are not classified as rock for engineering purposes. These units are relatively young and have been subject to variable consolidation and cementation. These materials are typically considered soft to hard soils in terms of strength and engineering behavior.

3.3.3 Subsurface Conditions

In general, silty and clayey material with minor sands and organic (lignite) material were encountered in the borings labeled SI-101 and SI-102. Drilling was noted to be relatively easy, reaching 111.5 feet and 126.5 feet below ground surface in SI-101 and SI-102 respectively.

The geotechnical borings labeled SI-101 and SI-102 were drilled to investigate the subsurface soils and to install slope inclinometer instrumentation in the worst-damaged areas of the Buck Hill landslide. Boring SI-101 was drilled through the existing gravel shoulder at Station 117+90 roughly 18 feet left of the proposed centerline. Boring SI-102 was also drilled through the existing gravel shoulder at Station 119+00 and approximately 13 feet left of the proposed centerline. SI- 101 and SI-102 both had sister piezometer holes labeled PZ-101 and PZ-102, respectively. PZ-101 and PZ-102 were not sampled or logged as these borings were placed within 10 feet of the SI borings with the sole purpose to monitor groundwater conditions to correlate with recorded ground movements.

SI-101: This boring, advanced to 111.5 feet below the ground surface, encountered tan, brown, and dark brown, soft to hard, fine-grained soils, including silt, sandy silt, silty sand, and lean to fat clay. The samples were logged in the field with moisture conditions ranging from slightly moist to wet. Groundwater was encountered around 37 feet below the ground surface during drilling.

Water-carrying lenses of lignite were observed in the boring ranging in thickness from less than 2 inches to greater than 1 foot.

SI-102: This boring, advanced to 126.5 feet below the ground surface, and encountered similar tan, brown, and dark brown, soft to hard, fine-grained soils as boring SI-101. The samples were logged in the field with moisture conditions ranging from slightly moist to wet. Groundwater was encountered around 42 feet below the ground surface during drilling. Water-carrying lenses of lignite were observed in the boring ranging in thickness from less than 2 inches to greater than 1 foot.

In both SI-101 and SI-102 bedrock classifying as claystone or siltstone was encountered. When encountered during drilling it was classified as bedrock. However, after running laboratory index test for engineering classification and design, these samples were described following soil classifications. The reader should interpret these materials as both soil and bedrock due to the minimal consolidation or induration of these soils.

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3.3.4 Subsurface Landslide Movement

The inclinometers were read at the time of installation and then again roughly 2 months and 5 months after install for SI-101 and 2 months after install for SI-102. Typically, inclinometers are read for a minimum of 1 year to determine seasonal precipitation effects on ground movement.

However, due to the project schedule, the monitoring period was abbreviated to maintain delivery of the construction work by Summer 2025. See Appendix E for the inclinometer plots and further explanation.

Movement in SI-101: SI-101 was initially read on March 21, 2024, then again on May 31 and August 19, 2024. Movement was recorded starting at the bottom of the casing with multiple possible slip planes existing approximately at 22 feet, 64 feet, 80 feet, 86 feet, 96 feet and to the bottom of the casing at 106 feet below ground surface. Maximum displacement recorded in SI- 101 was approximately 0.25 inches in 5 months since initial installation.

Movement in SI-102: SI-102 was initially read on June 6, 2024, then again on August 19, 2024.

Movement was recorded starting near the bottom of the casing with multiple possible slip planes existing approximately at 36 feet, 40 feet, 46 feet, 60 feet, 76 feet, 84 feet, 94 feet, and 112 feet below ground surface. Maximum displacement recorded in SI-102 was approximately 0.8 inches in just under 2 months since initial installation.

3.3.5 Groundwater

Groundwater was encountered during drilling in borings SI-101 and SI-102 at depths of 37 and 42 feet below the existing ground surface, respectively. These groundwater levels are noted on the boring logs included in Appendix A. Water levels recorded in August 2024 were 2 feet below the ground surface for PZ-101 and 24 feet below the ground surface for PZ-102. Fluctuations in the groundwater level due to seasonal and climatic effects are expected.

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SECTION FOUR - Analysis & Recommendations

This section presents analysis and recommendations for the Buck Hill landslide mitigation alternatives, management of subsurface water, slope stability, road embankment repair, and construction considerations for the design and construction of the Buck Hill Road project. Based on discussions with the project team, the critical components of work are to address the pavement condition, improve the drainage along the existing road and provide feasible mitigation methods against ground movement due to the Buck Hill Landslide. Generalized subsurface profiles were developed based on field reconnaissance, surficial visual evaluation, and subsurface investigations.

4.1 BUCK HILL LANDSLIDE MITIGATION

Typically, multiple technically feasible options can be considered to protect built infrastructure from landslides. The specific types of mitigation options considered depend on both the types of items to be protected and the nature of the landslide hazard. However, several general categories of mitigation options are considered:

• Monitor and maintain: Under this approach, the infrastructure and landslide are monitored. Maintenance activities are employed to ensure serviceability of the infrastructure. This approach commonly has the lowest upfront costs but can have higher long-term costs.

• Avoid: Under this approach, the infrastructure is relocated to avoid the landslide hazard.

This approach commonly has intermediate to high upfront costs, and the lowest long-term maintenance costs.

• Stabilize: Under this approach, one or a combination of various methods are employed to reduce driving forces or increase resisting forces. Methods of stabilization include toe berms, ground anchors, subsurface drainage, and grading. This approach commonly has high upfront costs and low to moderate long-term costs.

4.1.1 Foundation Type Options

For the Buck Hill Road project, relocation of the road is impractical due to environmental impacts.

Additionally, as previously mentioned the Buck Hill Road has already been realigned to its current location. Other alternative alignments are impracticable and would have significant environmental impacts. The cost of structural stabilization of the slide was considered uneconomical relative to the typical traffic on the Buck Hill Road and the depth to the multiple failure planes. Other stabilization methods, such as constructing a toe berm or removing material from the head of the

Page | 12 slide were deemed impractical due to the high amount of ground disturbance.

A “Deep Patch” type foundation system is proposed for the roadway and embankment between Station 113+00 to 122+50. Other options that were considered included a shallower patch- type foundation, solider pile walls with tiebacks, and coupled drilled shafts. Table 4.1 provides the “PROS” and “CONS” for the Available Mitigation options of the Buck Hill Landslide.

Table 4.1: Generalized Buck Hill Landslide Mitigation Options

MITIGATION OPTION PROS CONS

Shallow Patch-Subexcavation to up to 6 feet in depth with reinforcements every 1 foot vertically

Least expensive alternative.

Smaller and more conventional construction equipment.

Least environmental disturbance.

Relatively quick installation.

Mitigation elements will be completely buried.

Non-rigid elements easily replaceable.

Most susceptible to deformation from ground movements.

Will not arrest landslide movements.

Will generate waste material.

Open excavation during construction during rainy season could lead to longer construction delays.

Potential loss of support from frost depth.

Potential deformation from swelling soils.

Deep Patch-Subexcavation up to 10 feet in depth with reinforcements every 1 foot vertical

Most cost effective.

Smaller and more conventional construction equipment.

Can accommodate large deformations due to close reinforcement spacing.

Minimal environmental disturbance.

Mitigation elements will be completely buried.

Will not arrest ground movements.

Will generate waste material.

Open excavation during construction during rainy season could lead to longer construction delays.

High water table, dewatering methods likely needed.

Potential loss of support from frost depth.

Potential deformation from swelling soils.

Solider Pile Wall with Tiebacks

Higher chance to arrest movement.

Longer design life.

Aesthetics would match current

THRO 11(10) Solider Pile walls;

other facing options are available as desired.

More expensive Not guaranteed to arrest movement due to depth to slide plane.

Require large equipment for installation, more site disturbance, noisy construction operations.

Site topography would require extensive disturbance to install.

Requires specialized contractors.

Corrosion considerations.

Transportation and installation considerations.

More oversite from construction staff

QA-QC.

More time to construct.

High water table, dewatering methods needed.

Culverts passing through walls require time in construction.

Coupled Drilled Shafts Best chance to arrest movement.

High capacity to resist ground movements.

Longer design life.

Most expensive.

Not guaranteed to arrest movement due to depth to slide planes.

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MITIGATION OPTION PROS CONS

Mitigation elements will be completely buried.

Larger equipment, more space to maneuver and install, more site disturbance, noisy construction operations.

Requires specialized contractors.

Concrete considerations (i.e., sulfate attack).

More oversite from construction staff

QA-QC.

More time to construct.

High water table, dewatering methods needed.

4.2 FOUNDATIONS

The most significant deformation observed from the landslide movement is from Station 113+00 to 122+50. The proposed deep patch foundation will consist of the replacement of at least 10 vertical feet of the existing poor subgrade soils and placement of granular backfill with geotextile reinforcement every foot vertically. The reinforced mass should be well-drained and contain at least two levels of sheet drains.

A deep patch foundation system is the most feasible option from a geotechnical perspective. As noted in Section 3 of this report, there are multiple possible failure planes to depths exceeding 100 feet below the ground surface. Attempting to completely arrest the ground movements associated with failure planes to these depths is considered unfeasible and would well exceed the funding available for this project.

Utilizing a deep patch foundation gives the roadway the best chance to remain stable given the project budget and the depth to the failure planes. The deep patch foundation type will allow for the maximum amount of ground movement while maintaining a traversable roadway surface.

Routine maintenance for this alternative is discussed later in this report.

4.2.1 Geosynthetic Reinforced Deep Patch

The deep patch is the recommended repair for the worst extents of the Buck Hill Landslide, between Stations 113+00 to 122+50. The recommended deep patch is a 10-foot-deep road-fill slope repair that is constructed by excavating and replacing the top 10-feet of on-site soils with coarse-grained, granular material and reinforcing the fill material with layers of geosynthetic material, usually geogrid. A cross section of the deep patch developed for this project is shown in Figure 3.

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Figure 3: Cross Section of Deep Patch

4.2.2 Deep Patch Slope Stability

The geosynthetic reinforced patch was analyzed as a reinforced soil block with high shear strength parameters, extending the full width of the roadway subgrade. The deep patch was added to the slope stability model in Slide2.

Back calculation analysis for slope conditions at failure was first conducted using the Slide2 computer program developed by RocScience. Slide2 uses limit equilibrium methods to determine the factors of safety for slope stability. The back-analyzed soil parameters were then used for the deep patch design calculations.

Presumptive soil properties were estimated based on laboratory testing, visual classification, and experience with similar soil materials for initial input in the back analysis. Back analysis of the assumed pre-existing slope included a water table elevation estimated from the water levels observed during drilling, as discussed in Section 3. An incipient static factor of safety of 1.0 was used to refine the estimated soil parameters. Conservative, presumptive properties for deep patch materials were assigned based on experience with similar geologic units and projects with the Park.

Table 4.2 provides a summary of the estimated overburden soil properties used in the stability analysis models. The results of the slope stability analysis are presented in Appendix C.

Table 4.2: Estimate Material Properties of Native and Backfill Soils for Deep Patch Design

MATERIAL

DESCRIPTION

UNIT WEIGHT

ɣ, pcf

COHESION

c, psf

FRICTION ANGLE

φ, deg Lean CLAY (CL) 110 120 20 Silty SAND (SM) 115 75 30 Fat CLAY (CH) 100 100 15

Deep Patch Backfill Material(1) 130 500 35

(1) Cohesion and friction angle were artificially inflated to simulate a reinforced mass.

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4.2.3 Deep Patch Internal Stability

Internal strength of the deep patch was analyzed using the ReSSA+ software published by ADAMA Engineering, Inc. The deep patch was modeled as a select fill material with a unit weight of 130 pounds per cubic foot, friction angle of 30 degrees, and no cohesion. The geosynthetic layers were spaced at 12 inches vertically. The geosynthetic layers were modeled with a long-term allowable strength of 1,000 pounds per foot. The geosynthetic will be laid flat with no wrap or connection; therefore, no pullout capacity was assumed. The ReSSA+ analysis included circular and three-part wedge surfaces. Factors of safety were calculated using the conservative Bishop method. Minimum factors of safety were greater than the acceptable minimums.

The depth of the proposed deep patch is recommended to be 10 feet and the full roadway width.

The reinforcement spacing is recommended to be not more than 1 foot vertically. A Type 1 reinforcement geogrid conforming to Section 714.04 in FP-14 should be used for the deep patch repair. The deep patch should also include a geocomposite sheet drain as shown in the plans.

Backfill material should conform to Section 704.03(b) in the FP-14. Excavated material will not be suitable for backfill. Due to the depth of the excavation, two benches on either side are recommended. Sheet drains should be placed on both sides and on both benches as shown in Figure 3 above.

4.2.4 Sheet Drains

A two-tiered sheet drain system is recommended along the right and left sides of the excavation within the deep patch. Additional areas of poor drainage could be identified during construction.

Sheet drains should be installed in this area to reduce the potential for water to saturate the foundation of the deep patch. Drain outlet pipes should be located at a maximum of 200-feet on-centers spacing.

4.2.5 Ditch Recommendations

The existing roadway has insufficient ditches in locations. Debris filling appears to be the main source of this problem and is impacting the roadway. As stated in the plans, the gradient and width of the roadway ditches and the excavation and embankment ratios may be adjusted by the CO to assure adequate drainage and stability. Refer to the Final Hydraulics Report dated October 2024.

4.2.6 Shrink/Swell Recommendations

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

Page | 16 materials.

4.2.7 Drainage Improvements

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

4.2.8 Roadway Maintenance

A regular slope maintenance program consisting of maintenance personnel that possess the necessary specialized skills and equipment would help to alleviate the concern related to these unstable slopes. In general, routine slope maintenance activities should prevent conditions that promote erosion, deteriorate soil shear strength, or induce adverse loading. Examples of maintenance activities would include, but are not limited to the following:

• Routine observation and monitoring of roadway performance.

o The Buck Hill Landslide is not anticipated to fail catastrophically, but rather slowly and steadily. As this failure progresses the core of the deep patch will remain intact and accommodate deformations well. However, the terminations of the deep patch roadway may experience differential movement relative to the unreinforced roadway. Maintenance should always include observations at each end of the deep patch.

• Sheet drain clean outs.

o The shallow groundwater and the ease of migration and sedimentation by fine-grained soils in locations of the proposed deep patch requires the sheet drains be routinely inspected and regularly cleaned out.

• Ditch clearing and repair.

o The deep patch location is in a relatively flat area of the roadway alignment. The proposed design provided in the plan set has maximized the drainage capabilities for the site. However, the ditches should be regularly inspected and cleared of any blockages, slumps, or dense vegetation disturbing the ditch line. This will reduce the amount of surface water infiltrating the deep patch.

• Culvert cleaning and repair.

o Blocked or partially blocked culverts should be cleaned immediately to maximize the water conveyance. Inspect culverts for damage or separation. Significant ground movement can potentially damage or separate a culvert allowing water to exit prematurely into the landslide.

• Pavement crack filling to prevent water infiltration.

Page | 17 o Regular scheduled crack sealing operations and impromptu crack sealing should be expected along the entire alignment. As mentioned above, the transition zones between the deep patch to native subgrade areas is expected to experience minor differential movement. Expect the need to crack seal at these locations regularly.

4.2.9 Geosynthetics

The geogrid for the deep patch foundation should meet the requirements of stabilization and reinforcement biaxial geogrid, respectively (FP-14 and SCR sections 714.03 & 714.04). Separation geotextile for the underdrains should be Class 2 Type C non-woven geotextile (FP-14 section 714.01).

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SECTION FIVE - CONSTRUCTION

CONSIDERATIONS

5.1 ROADWAY EXCAVATION

Excavate using equipment capable of removing the material while preventing material from escaping outside the construction limits. The Contractor should be prepared with appropriate equipment based on the local geology as discussed in Section 2.1 and 2.2.

5.2 DEWATERING

Dewatering of the deep patch excavation may be necessary due to the shallow groundwater conditions noted in this report and water from seasonal precipitation events. Any free water should be removed prior to placement of the backfill and reinforcement material.

5.3 SLOPE INSTABILITY

Slopes could become unstable during construction operations due to precipitation, flooding events, or heavy loading on soft subgrade soils. Precautions should be taken if movement or cracking is observed. Seepage, as discussed above, is a contributor to slope instability and similar dewatering and stabilization methods could be used to suspend slope movement.

5.4 SPECIFICATIONS

Special provisions were developed to be consistent with geotechnical recommendations stated above and were incorporated into the special contract requirements (SCR) to amend the FHWA Standard Specification for Construction of Roads and Bridges on Federal Highway Projects;

known as FP-14. Final versions of these SCR’s will be provided to the CFLHD Design Engineer to incorporate into the Plans, Specifications, and Estimate package for the 95% review milestone.

5.5 DISCLAIMER/LIMITATIONS CLAUSE

The recommendations in this report are based on the data obtained from exploratory borings, field review, monitoring instrumentation, and laboratory test results. The results of these explorations and tests represent conditions at the specific locations indicated. Subsurface variations across the site are likely and may not become evident until excavation is performed. The Analysis and Recommendations sections in this report include interpretations and recommendations developed by the Government in the process of preparing the design. These interpretations are not intended

Page | 19 as a substitute for the personal investigation, independent interpretation, and judgment of the Contractor.

Page | 20

SECTION SIX - References

ADAMA Engineering Inc, Computer Program, ReSSA+, Version 3.0, Copyright 2024.

American Association of State Highway and Transportation Officials (AASHTO), 2020, LFRD

Bridge Design Specifications, 9th Edition.

Billings County, North Dakota (ND007) map. (n.d.). Web Soil Survey. Retrieved September 13, 2024, from https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx Federal Highway Administration (FHWA), 2007, Geotechnical Technical Guidance Manual, dated May.

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

Federal Highway Administration (FHWA), 2014, Standard Specifications for Construction of

Roads and Bridges on Federal Highway Projects FP-14.

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

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

December.

Federal Highway Administration (FHWA), 2024, “ND NP THRO 204(1) Final Hydraulic

Report,” dated October.

Federal Highway Administration (FHWA), 2024, “ND NP THRO 204(1) Final Pavement and

Materials Report,” dated October.

Keller Lynn, K. (2007). Theodore Roosevelt National Park: Geologic Resource Evaluation

Report. In https://www.nps.gov/subjects/geology/geologic-resources-inventory-products.htm. National Parks Service - Geologic Recourses Inventory. Retrieved September 13, 2024, from https://irma.nps.gov/DataStore/DownloadFile/426420

NPS Geodiversity Atlas—Theodore Roosevelt National Park, North Dakota (U.S. National Park Service). (n.d.). Geologic Resources Inventory. Retrieved September 13, 2024, from https://www.nps.gov/articles/nps-geodiversity-atlas-theodore-roosevelt-national-park-north-dakota.htm#gri

Sentinel Butte Formation. (n.d.). USGS. Retrieved September 13, 2024, from https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=NDTs%3B0

RocScience Inc, Computer Program, Slide2.

U.S. Department of Agriculture (USDA), 2012, “National Engineering Handbook (NEH), Part

631 Geology,” Chapter 4: Engineering Classification of Rock Materials, dated January.

U.S. Geological Survey (USGS), 2024, “Quaternary Fault and Fold Database of the United

States.”

USGS. 2024c. “Vs30 Map Viewer.”, 2024a Earthquake Hazards Program, https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx https://irma.nps.gov/DataStore/DownloadFile/426420 https://www.nps.gov/articles/nps-geodiversity-atlas-theodore-roosevelt-national-park-north-dakota.htm#gri https://www.nps.gov/articles/nps-geodiversity-atlas-theodore-roosevelt-national-park-north-dakota.htm#gri

Page | 21

<https://usgs.maps.arcgis.com/apps/webappviewer/index.html> U.S. Geological Survey (USGS), 2024b, “National Seismic Hazard Map”, <http://earthquake.usgs.gov/research/hazmap> USGS. 2024c. “Quaternary Fault and Fold Database of the United States.” USGS Earthquake

Hazards Program, <http://earthquake.usgs.gov/hazards/qfaults/> http://earthquake.usgs.gov/research/hazmap

Plates PLATE 1 Project Site Map PLATE 2 Geology Map PLATE 3 Landslide Map PLATE 4 Boring Location Map

APPENDIX A

FIELD EXPLORATION PROGRAM

APPENDIX A

FIELD EXPLORATION PROGRAM

A.1 INTRODUCTION

A subsurface investigation was performed by a CFLHD Geotechnical engineer on March 9 and 10, 2024, and May 29 and 30, 2024. Subsurface conditions were logged and representative samples were collected and transported to the CFLHD Materials Laboratory in Lakewood, CO, for physical property testing. Laboratory test results are presented in Appendix B. A summary of the field exploration is provided below in Table A-1. The methods used to conduct the field exploration program are described below. Photos of drilling equipment and field exploration activities are included in Appendix D. Representative soil and rock samples collected during the field exploration program were transported to the CFLHD Materials Laboratory in Lakewood, Colorado for testing. A summary of the laboratory testing program is provided in Appendix B.

A.2 EXPLORATIONS

Borings

Vine Laboratories, Inc of Commerce City, CO provided the drilling services for the soil borings.

Borings were completed using a CME 75 and a CME 55 Buggy drill rig.

The initial phase of borings was drilled between March 9th and 10th 2024 followed by a second phase on May 29th through June 1st, 2024. The geotechnical subsurface exploration program consisted of drilling a total of four (4) borings; consisting of two (2) inclinometer holes and two

(2) paired piezometer holes. Five (5) pavement borings were also advanced to a maximum depth of 5 feet. The purpose of the geotechnical borings was to investigate the subsurface along the project alignment specifically to investigate subsurface movements at the Buck Hill Road landslide. Standard penetration testing (SPT) and sample collection were performed at 5-foot intervals in the geotechnical borings. Additionally, bulk grab samples were collected from the top 5 feet in the pavement borings. Borings were drilled and sampled using Hollow Stem Auger (H.S.A).

If water was encountered at the time of drilling, field personnel measured water levels in the borings. Fluctuations in the ground water level due to seasonal and climatic effects are expected.

The location and elevation of individual borings was estimated using Google Earth Pro. Boring locations are listed on individual boring logs and are show below in Table A-1. Following drilling activities, field personnel installed instrumentation consisting of two inclinometers and two piezometers.

TABLE A-1

FIELD EXPLORATION LOCATIONS

EXPLORATION

DESIGNATION LOCATION PURPOSE

APPROXIMATE

GROUND ELEVATION

(ft.)

TERMINATION

DEPTH (ft.)

SI-101

STA 117+90

18 ft. LT of centerline

Geotechnical Subsurface Movement

2612 111.5

PZ-101

STA 117+90

18 ft. LT of centerline

Geotechnical Subsurface

Groundwater 2612 60

SI-102

STA 119+00,

13 ft. LT of centerline

Geotechnical Subsurface Movement

2616 126.5

PZ-102

STA 119+00

13 ft. LT of centerline

Geotechnical Subsurface

Groundwater 2616 60

A.3 SOIL AND ROCK SAMPLING

Disturbed samples were obtained from the borings in accordance with the Standard Penetration Test (SPT), the procedures of which are detailed in AASHTO T-206. The SPT involves driving a 2-inch outside diameter, 1.375-inch inside diameter split spoon sampler a depth of 18 inches with a 140-pound hammer falling 30 inches. The number of blows required to advance the split-spoon sampler through each of the 6-inch increments was recorded. The SPT resistance, or N-value, is defined as the number of blows required to drive the sampler over the second and third 6-inch increments. The N-value provides a means for evaluating the relative density or compactness of cohesionless (granular) soils and consistency or stiffness of cohesive (fine-grained) soils. An energy corrected N-value, N60, is used to standardize the energy levels of the hammer system in the SPT to 60% efficiency. Because high penetration resistance prevented driving the total length of the sampler at times, the penetration resistance for the partial penetration was recorded where applicable. Representative portions of the split-spoon sample obtained in conjunction with the SPT were placed in plastic baggies and transported to the CFLHD Materials Laboratory for testing.

A.4 SOIL AND ROCK CLASSIFICATION SYSTEM

During the completion of borings, CFLHD geotechnical personnel collected soil/rock samples and prepared field logs of the borings. Soil identification and descriptions, as shown on the field logs, are based on ASTM D2488, a systematic process for identifying and describing individual soil samples by visual and manual means. When sufficient laboratory testing was completed, select samples from borings were classified using the Unified Soil Classification System (USCS) and American Association of State Highway and Transportation Officials (AASHTO) soil classification system. Both the visual soil identification system and the referenced soil classification systems are summarized in the attached Soil Classification Field Reference. Rock samples were classified based on the stratigraphic structure, rock strength, degree of weathering, and other properties. The rock classification system is summarized in the attached Rock Classification Field Reference.

A.5 INSTRUMENTATION

Upon completion, selected borings were instrumented for purposes of measuring groundwater levels and ground movement. Piezometers were installed in Borings PZ-101 and PZ-102.

Inclinometers were installed in Borings SI-101 and SI-102. More details on the employed inclinometer system and the results of inclinometer readings are presented in Appendix E.

A piezometer is used to measure piezometric head levels to determine the stability of slopes and embankments. Piezometers are also used to monitor ground improvement techniques, dewatering schemes, and seepage or groundwater movement. The standpipe piezometer, installed in a borehole, consists of a filter tip joined with a riser pipe, both of which are generally constructed of PVC pipe. The filter tip is slotted and the annular space around the filter tip is backfilled with clean sand. The area above the sand is sealed with bentonite to isolate the pore water pressure at the tip, and the remaining annulus is filled with grout, concrete, or soil cuttings.

The riser pipe is terminated above ground level. Groundwater levels are read to the nearest one-tenth of a foot with an instrumented water level indicator lowered down the riser pipe or a more permanent vibrating wire piezometer which converts water pressure to a…