NPS HQ and Swiftcurrent Geotech Rpt.pdf

SUBMITTED TO:

HDR Engineering, Inc.

700 SW Higgins Ave., Suite 200 Missoula, MT 59803

BY:

Shannon & Wilson 1321 Bannock Street, Ste 200 Denver, CO 80204

(303) 825-3800 www.shannonwilson.com

PRELIMINARY GEOTECHNICAL REPORT

NPS Glacier National Park Headquarters and Swiftcurrent Utility Improvements

FLATHEAD & GLACIER COUNITES, MONTANA

December 2022

Shannon & Wilson No: 107857-001

Headquarters and Swiftcurrent

Preliminary Geotechnical Report

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CONTENTS

1 Introduction

2 Site Description

2.1 Headquarters Site

2.2 Swiftcurrent Site

3 Project Description

3.1 GLAC 307440 Replace Headquarters Wastewater & Backup Power Systems

3.2 GLAC 307606 Swiftcurrent Water Distribution System Improvements

4 Field Explorations and Laboratory Testing

5 Geologic Setting and Subsurface Conditions

5.1 Regional Geology

5.1.1 Headquarters Site

5.1.2 Swiftcurrent Site

5.2 Subsurface Conditions

5.2.1 Headquarters Site

5.2.2 Swiftcurrent Site

5.3 Groundwater

5.4 Subsurface Variation

6 Geologic Hazard Evaluation

6.1 Seismic Hazards

6.2 Corrosive Soil and Bedrock

7 Preliminary Geotechnical Engineering Recommendations

7.1 Seismic Design Considerations

7.2 Below Grade Water Tank

7.2.1 Foundations

7.2.2 Lateral Earth Pressures

7.2.3 Lateral Resistance

7.2.4 Drainage

7.3 Pipelines and Manholes

7.3.1 Loads on Pipes

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7.3.2 Thrust Restraint

7.3.3 Manhole Foundations

8 Additional Explorations

9 Construction Considerations

9.1 Dewatering

9.2 Earthwork

9.2.1 Site Preparation

9.2.2 Excavation and Rippability

9.2.3 Temporary Slopes and Shoring

9.2.3.1 Temporary Slopes

9.2.3.2 Temporary Shoring

9.2.4 Foundation Subgrade Preparation

9.2.5 Fill Placement and Compaction

9.3 Pipeline and Manhole Considerations

9.3.1 Trench Excavation

9.3.2 Subgrade Preparation

9.3.3 Pipe and Manhole Bedding and Backfill

9.3.4 Pipe Trench and Manhole Backfill

9.4 Horizontal Directional Drilling (HDD)

9.4.1 Trenchless Construction

9.4.2 Anticipated Ground Conditions

9.4.3 Potential Risks

9.4.3.1 Inadvertent Drilling Fluid Release

9.4.3.2 Obstructions

10 Closure

11 References iv

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Exhibits Exhibit 4-1: Site Conditions in June 2022 during drilling at boring SW-15 Exhibit 5-1: Geologic Map of the Headquarters Site Exhibit 5-2: Geologic Map of the Swiftcurrent Site Exhibit 5-3: Summary of Groundwater Depths During Drilling Exhibit 6-1: Corrosivity Ratings Based on Soil Resistivity Exhibit 6-2: Corrosivity Ratings Based on Water Soluble Sulfate Exposure Exhibit 7-1: Seismic Design Parameters Exhibit 7-2: Recommended Lateral Earth Pressures Exhibit 9-1: Bedding and Embedment Material Gradation

Figures Figure 1: Vicinity Map Figure 2: Headquarters Site and Exploration Plan Figure 3: Swiftcurrent Site and Exploration Plan Figure 4: Recommended Surcharge Load for Temporary and Permanent Walls

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Important Information

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1 INTRODUCTION

This preliminary geotechnical report presents the results of our geotechnical explorations and provides our preliminary geotechnical engineering recommendations for the Glacier National Park Headquarters Wastewater System Improvements (GLAC 307440), and the Swiftcurrent Water Distribution System Replacement Project (GLAC 307606) (the Project) in Flathead and Glacier Counties, Montana. Our work was completed in accordance with our signed Geotechnical Subconsultant Agreement Number 1000100068490 with HDR Engineering, Inc. (HDR).

Our activities under this scope of services included the following:

Completing geotechnical explorations within the two Project areas, Conducting laboratory testing on select soil/rock samples, and

Preparing this preliminary report, which summarizes the results of our explorations and laboratory testing and provides our preliminary geotechnical engineering recommendations for the proposed improvements.

If a service is not specifically indicated in this report, do not assume it was performed.

2 SITE DESCRIPTION

The Project is located at two sites: at the West Glacier Headquarters site in Flathead County and the Swiftcurrent area in Glacier County, Montana, inside Glacier National Park (the Park), (Figure 1).

2.1 Headquarters Site

The Headquarters site is located at 64 Grinnel Drive, West Glacier, in Flathead County, near the West Entrance to the Park. The Headquarters site is generally bordered to the north and west by Going-to-the-Sun Road, to the east by Belton Hills, and to the south by the Middle Fork Flathead River. The site is occupied by the Headquarters Building, residential housing and apartment buildings, maintenance facilities, and asphalt paved roadways (Figure 2).

The terrain with the Headquarters Site is relatively flat and sloped gently to the west.

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2.2 Swiftcurrent Site

The Swiftcurrent site is generally located approximately 25 miles northeast of the Headquarters site and ½ mile west of Swiftcurrent Ridge Lake, near the east side of the Park. The Swiftcurrent site is bordered to the south by Swiftcurrent Creek, to the west by Wilbur Creek, and to the north by Altyn Peak. The site is occupied by the Swiftcurrent Motor Inn and lodging area, the Many Glacier Campground, and a National Park Service (NPS) administrative area, with asphalt paved roadways throughout (Figure 3). A service road extends northwest from the loop road north of the Swiftcurrent Motor Inn to the location where the existing water storage tank is buried. Large boulders and rock outcrops are exposed at the surface near the tank site and along the service road. This location lies approximately 100 feet above the remainder of the site, which slopes gently to the south and east.

3 PROJECT DESCRIPTION

We based our understanding of the Project improvements on the HDR plans for the GLAC 307440 Headquarters Improvements and plans for the GLAC 307606 Swiftcurrent Improvements, dated March 2022. HDR is currently preparing design-build procurement documents for the Project and limited information was available for the planned final design and construction at the time we prepared our report.

3.1 GLAC 307440 Replace Headquarters Wastewater & Backup

Power Systems

The Project improvements will include a new sanitary sewer system to service the existing facilities at the Headquarters site. More than 8,700 feet of sanitary sewer pipeline will be installed via open trench construction methods, generally following the existing roadway alignments. The minimum depth of cover of the pipeline will be 7 feet. The sewer pipeline is expected to consist of 8-inch inside diameter (I.D.) Polyvinyl chloride (PVC) pipe. The sanitary sewer system will also include 57 manholes.

Trench cuts in asphalt paved roadways are expected to be patched with 6 inches of crushed base course and 3 inches of asphalt pavement.

Other improvements are expected to include mechanical improvements at the lift station building and a new backup generator at the Headquarters Building.

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3.2 GLAC 307606 Swiftcurrent Water Distribution System Improvements

The Project improvements at the Swiftcurrent site will include a new precast or cast-in-place concrete (CIP), below-grade water storage tank, with dimensions of approximately 17 feet by 17 feet (20,000 gallons). The proposed water storage tank will be located near the existing 40 by 40-foot (125,000 gallons) below-grade water storage tank at the north end of the site (see Figure 3).

More than 11,000 feet of water pipeline will be installed primarily via open trench construction methods. The new water pipeline will begin at the storage tank and flow south at grades ranging from < 1% to > 13%. The pipeline alignments will generally parallel the existing roadway alignments. The water pipeline is expected to consist of 6- to 10-inch I.D.

high-density polyethylene (HDPE) pipe. The water lines will be installed in trench excavations with a minimum embedment depth of 3 feet for seasonal (winterized) operations.

Trench cuts in asphalt paved roadways are expected to be patched with 6 inches of crushed base course and 3 inches of asphalt pavement.

While the majority of the water pipelines will be installed via open trench constructions, a relatively short section (~475 feet) of pipeline is expected to be installed by horizontal directional drilling (HDD) at the east end of the site in the picnic area to reduce surface disturbance and potential impacts to cultural resources. In addition, a new water well is planned in the administrative area at the east end of the site.

Other improvements are expected to include a new generator at the ranger station and mechanical improvements at the Chlorination Building.

4 FIELD EXPLORATIONS AND LABORATORY TESTING

Shannon & Wilson conducted a subsurface exploration program between June 6 and 9, 2022 to evaluate the subsurface conditions at the Headquarters and Swiftcurrent sites. The exploration program consisted of the following:

8 borings (SW-01 through SW-08) were completed at the Headquarters site, along the proposed sanitary sewer line alignments to depths of approximately 21.5 feet below ground surface (bgs), 8 borings (SW-09 through SW-16) were completed at the Swiftcurrent site. Borings SW- 10 through SW-15 were drilled along the proposed water line alignments to depths of

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107857-001 December 2022 approximately 11.5 feet bgs. Boring SW-09 was drilled to 77 feet bgs at the proposed water well location and boring SW-16 was drilled to a depth of 40 feet bgs for the proposed water storage tank.

The locations of the borings are shown on Figure 2. Exhibit 4-1 shows site conditions at boring SW-15 at the time of our field exploration program.

Exhibit 4-1: Site Conditions in June 2022 during drilling at boring SW-15 (view approximately southeast).

Appendix A describes the procedures used to complete the drilling and sampling of the geotechnical borings and presents the individual exploration logs along with an explanation of the symbols and terminology used on the logs.

We completed geotechnical laboratory testing to determine index and engineering properties of selected soil and rock samples retrieved from the borings. Laboratory tests included natural water content, moist unit weight, Atterberg limits, grain-size distribution, corrosion testing, Hveem Stabilometer (R-value), and unconfined compressive strength

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(UCS) on rock cores. Laboratory test methods and results are provided in Appendix B. The natural water contents, Atterberg limits, and fines content are also indicated on the individual boring logs in Appendix A.

5 GEOLOGIC SETTING AND SUBSURFACE CONDITIONS

We based our understanding of the geology and subsurface conditions at the site on regional geologic maps, our field reconnaissance, and the results of our field exploration program.

5.1 Regional Geology

Based on geologic mapping by Whipple (1992), the Project traverses multiple geologic units as described below and presented Exhibit 5-1 and Exhibit 5-2.

5.1.1 Headquarters Site

Alluvium (Qal) – Holocene-age deposits of gravel, sand, and silt in canyon bottoms.

Helena Formation (Yh) – middle Proterozoic-age, interbedded limestone, dolomite, and quartz arenite.

Empire Formation (Ye) – middle Proterozoic-age argillite and siltite with lesser amounts of arenite and dolomite.

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Exhibit 5-1: Geologic Map of the Headquarters Site (adapted from Whipple, 1992) (not to scale)

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5.1.2 Swiftcurrent Site

Glacial and fluvioglacial deposits (Qg) – Holocene and upper Pleistocene age deposits of unsorted boulder rubble with lesser amounts of sand, silt, and clay (till).

Appekunny Formation (Yap) – middle Proterozoic-age, interlaminated siltite and argillite.

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Exhibit 5-2: Geologic Map of the Swiftcurrent Site (adapted from Whipple, 1992) (not to scale)

5.2 Subsurface Conditions

The subsections below present summaries of the subsurface conditions encountered at each project location. Soil density and consistency and bedrock strength terminology are defined on Figure A-1 and A-2, respectively.

5.2.1 Headquarters Site

The overburden soils generally consisted of: (1) loose to medium dense, clayey/silty sand to poorly graded sand with clay/silt and gravel, and (2) medium dense to very dense, poorly graded gravel with clay/silt and sand (alluvium). Cobbles were inferred in the subsurface from drilling action in the borings. Bedrock was not encountered to the depths explored.

5.2.2 Swiftcurrent Site

The overburden soils generally consisted of: (1) poorly graded gravel with clay/silt and sand, and (2) clayey/silty sand with gravel (till). The soils were predominantly medium

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107857-001 December 2022 dense to very dense; however, very loose to loose deposits were encountered in four borings (SW-10, SW-11, SW-13, and SW-14).

Claystone and siltstone boulders were inferred in boring SW-09 from depths of approximately 29 to 38 feet bgs and 44 to 59 feet bgs, respectively. Siltite bedrock was encountered in boring SW-16 at a depth of 5.5 feet bgs; the upper 10 feet of bedrock was drilled by ODEX methods and described as weak to medium strong (see Figure A-2 for a definition of strength descriptions). NQ2 double tube wireline coring methods were used to advance the boring to a total depth of 40 feet. Laboratory test results indicate the core samples tested were very strong. Completely weathered siltstone bedrock was encountered in boring SW-15 at a depth of 9.5 feet.

5.3 Groundwater

As indicated in Exhibit 5-3, groundwater was measured in five borings during drilling at depths between 5 feet and 19 feet. Groundwater measurements were recorded using an electronic water level indicator.

Exhibit 5-3: Summary of Groundwater Depths During Drilling

Boring No.

Groundwater

Depth1 Measurement

Date

SW-01 19.0 6/9/2022

SW-09 16.9 6/7/2022

SW-10 8.2 6/8/2022

SW-13 5.3 6/8/2022

SW-15 6.1 6/8/2022

NOTES:

Depth values are in feet.

Groundwater fluctuations are possible and will depend on many factors, including seasonal variations, local precipitation, runoff, snow melt, and stage of the Middle Fork Flathead River, Swiftcurrent Creek, and Wilbur Creek.

5.4 Subsurface Variation

The explorations were performed to evaluate geotechnical soil and bedrock conditions at the Project sites. Our observations are specific to the locations, depths, and times noted on our boring logs and may not be applicable to all areas of the Project. No amount of exploration or testing can precisely predict the characteristics, quality, or distribution of subsurface and site conditions. Potential variation includes, but is not limited to:

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The conditions between explorations may be different.

The passage of time or intervening causes (natural and manmade) may result in changes to site and subsurface conditions.

Penetration test results in gravelly soils may be unrealistic. Actual soil relative density may be lower than estimated if the test was performed on gravel or cobbles.

6 GEOLOGIC HAZARD EVALUATION

6.1 Seismic Hazards

Based on geologic mapping, the nearest fault to the Project site is the South Fork Flathead Fault. The northern terminus of the mapped fault trace is shown approximately 2 miles south of the Headquarters site and approximately 27 miles southwest of the Swiftcurrent site (USGS, 2021 and Haller, 2006). Haller (2006) notes that the fault has not been studied in detail, but that no scarps (e.g., ground ruptures) have been identified along this fault, suggesting that there have not been movements on the fault in the Holocene (more recently than about 11,000 years before present). Other workers (Bryant and others, 1984 and Erdmann, 1944) concluded that there is no evidence of fault movements causing ground ruptures since the Tertiary Period (more than about 2.5 million years before present). In our opinion, the above sources suggest that the Project site is unlikely to be affected by ground ruptures related to fault movements.

Liquefaction can occur in loose, saturated, cohesionless soils when subjected to earthquake ground shaking. Seismic compression may occur when loose, granular soils above the groundwater table are rearranged into a tighter packing configuration during seismic shaking, which can cause settlement. Based on subsurface soil and groundwater conditions encountered in the borings, it is our opinion there is a risk of liquefaction and seismic compression along the pipeline alignment. The risk cannot be fully understood because most of the pipeline borings were only drilled to depths of 10 or 20 feet. However, the risk of liquefaction and seismic compression at the proposed water tank is nil because of the presence of shallow bedrock. NPS will need to determine if seismic hazards beneath the pipeline will need to be addressed during final design and, if so, provide direction as to what degree.

6.2 Corrosive Soil and Bedrock

The soil/rock encountered at the Project site can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, selected samples were tested for pH, Utility Improvements Preliminary Geotechnical Report

107857-001 December 2022 resistivity, water soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B.

The resistivity measured in the samples was 2,200 to 3,800 ohm-centimeters. Based on correlations developed by Roberge (2012) as shown in Exhibit 6-1, these values suggest corrosive to highly corrosive subsurface conditions for metallic pipe in contact with subsurface materials across the site.

Exhibit 6-1: Corrosivity Ratings Based on Soil Resistivity

Soil Resistivity (ohm-cm)

Corrosivity Rating

>20,000 Essentially Noncorrosive

10,000-20,000 Mildly Corrosive

5,000-10,000 Moderately Corrosive

3,000-5,000 Corrosive

1,000-3,000 Highly Corrosive

<1,000 Extremely Corrosive

The concentration of water-soluble sulfates measured in the samples was 0.02% by weight.

Based on classifications as defined by ACI-318-19 (ACI, 2019) as shown in Exhibit 6-2, these test results suggest an exposure class S0 on concrete exposed to site soils.

Exhibit 6-2: Corrosivity Ratings Based on Water Soluble Sulfate Exposure

Water Soluble Sulfate in Soil (Percent by Weight)

Sulfate Severity (Class)

<0.10 Negligible (S0)

0.10-<0.20 Moderate (S1)

0.20-2.00 Severe (S2)

>2.00 Very Severe (S3)

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion. As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

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7 PRELIMINARY GEOTECHNICAL ENGINEERING

RECOMMENDATIONS

The preliminary recommendations presented in the following sections are based on our understanding of the Project based on information provided by HDR and our subsurface exploration and laboratory testing program.

7.1 Seismic Design Considerations

We understand that design ground motion parameters will be based on the requirements of the 2021 International Building Code (International Code Council, 2020), which has adopted the ASCE 7-16 design code for minimum design loads on buildings (ASCE, 2017). These design loads are in turn based on the USGS National Seismic Hazard Maps that were updated in 2008 (Petersen and others, 2008). Based on the soil and rock conditions encountered at the water storage tank location, the site can best be described as Site Class A (hard rock). The following exhibit provides seismic design parameters for the Project.

Exhibit 7-1: Seismic Design Parameters

Design Parameter Value

Peak Ground Acceleration (PGAB)¹ 0.183 g

Short-period Spectral Acceleration, SS 0.4 g

Long-period Spectral Acceleration, S1 0.127 g

Site Class A

Site Factor, FPGA 0.8

Site Factor, Fa 0.8

Site Factor, Fv 0.8

Site Modified Peak Ground Acceleration, PGAm 0.146 g

Site Modified Spectral Acceleration (0.2 s), SMS 0.320g

Site Modified Spectral Acceleration (1.0 s), SM1 0.102 g

Seismic Design Value (0.2 s), SDS 0.214 g

Seismic Design Value (1.0 s), SD1 0.068 g

NOTES:

PGAB refers to peak ground acceleration for a site underlain by Site Class B soil (rock).

g = gravity; sec. = seconds

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7.2 Below Grade Water Tank

7.2.1 Foundations

The proposed 20,000-gallon water storage tank is expected to be a buried structure bearing at depth of approximately 15 to 20 feet bgs. A mat foundation is suitable for the water storage tank. We anticipate that the tank will be bearing on siltite bedrock and will essentially fully compensated (i.e., the weight of the structure will be less than the excavated soil/rock). Therefore, we anticipate only nominal settlement to occur (less than approximately ½ inch).

Allowable bearing capacity for the mat foundation will be high because of the presence of rock and should not control the design. For preliminary design of the mat foundation, we recommend using an allowable service bearing pressure of 10,000 pounds per square foot (psf), which corresponds to estimated total settlement of 1 inch, or a service bearing pressure of 5,000 psf, with an estimated total settlement of 0.5 inches. These values assume a 17-foot square water tank. Settlement should occur essentially as load is applied.

7.2.2 Lateral Earth Pressures

The lateral earth pressures on structure walls depend on many factors, including surcharge loads, the soil type, excavation extents and backfill type, and whether the top of the wall can yield or deflect laterally during and after excavation. If the top of the wall is free to yield a minimum of 0.1% of the wall height (i.e., 0.001 times the height [H]), the soil pressures will generally be lower (i.e., active condition), than if the wall is not able to yield this amount due to stiffness or resistance of the wall (i.e., at-rest condition). We anticipate that at-rest earth pressures will be appropriate for the buried tank.

For preliminary design purposes, we assumed that the tank will be excavated such that a 1H:1V (horizontal to vertical) zone extending upward from the base of the structure from a point 1.5 feet behind the wall zone will need to be backfilled. We further assumed that the overburden soils encountered in boring SW-16 (silty gravel with sand with any cobbles and boulders removed) would be used as backfill and that water does not accumulate in the backfill. Excavated rock should not be used for backfill. Exhibit 7-2 provides recommended lateral earth pressures values for these conditions. These pressures also assume a vertical wall face and a flat backslope.

Seismic active lateral earth pressure coefficients were determined based on the Mononabe- Okabe method (AASHTO, 2020), and seismic at-rest earth pressure coefficients were determined based on methods developed by Zhang and others (1998). Per AASHTO (2020), Utility Improvements Preliminary Geotechnical Report

107857-001 December 2022 the pressure distribution shape for dynamic active earth pressures is triangular, with the resultant force (PAE) acting at the lower third of the wall height (h/3). The parameters are provided in Exhibit 7-2.

Exhibit 7-2: Recommended Lateral Earth Pressures

Design Parameter Coefficient Equivalent Fluid Density (pcf)

Static At-Rest Earth Pressure Coefficient, ko 0.44 60

Dynamic At-Rest Earth Pressure Coefficient, koe 0.51 68

NOTES:

Based on granular soil with an assumed unit weight of 135 pcf and a friction angle of 34 degrees for the retained soil.

Values assume a vertical face and flat backslope.

pcf = pounds per cubic foot

Surcharge loads from motor vehicles/traffic and construction equipment will induce additional lateral loads on structure walls. The pressures provided above do not account for surcharge loads. Lateral loads due to various types of surcharges may be calculated by multiplying the surcharge by the at-rest earth pressure coefficient (k0). Surcharge loads should be applied as recommended in Figure 4.

The values in Exhibit 7-2 could be significantly reduced if the excavation for the tank is less than the assumed 1H:1V zone assumed in developing the above values.

7.2.3 Lateral Resistance

Lateral loads can be resisted by friction along the base of the foundation using an allowable coefficient of sliding resistance of 0.3 for concrete sliding on siltite bedrock. This allowable value for coefficient of sliding resistance includes a factor of safety of 1.5.

7.2.4 Drainage

As indicated in Section 7.2.2, we assume that groundwater will not accumulate in the backfill around the tank. Groundwater was not encountered while drilling in boring SW-16, near the proposed water storage tank. As such, groundwater is not anticipated to impact design of the structure. However, surface water from rainfall and snowmelt can make its way into the backfill materials and result in increased pressure on walls and potential uplift conditions. As such, we recommend that the design-build contractor consider this potential in the design and either account for the potential for saturation of backfill to result in higher lateral pressures/uplift, provide for subsurface drainage, or take measures to prevent the infiltration of surface water into backfill materials.

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7.3 Pipelines and Manholes

We anticipate that foundation soils at pipeline invert elevations will consist of primarily granular overburden soils with varying amounts of silt and clay. Based on the Project descriptions presented in Section 3 of this report, bedrock is not expected to be encountered in the trench excavations at both Project areas (except near the proposed water tank).

Groundwater is not expected to be encountered at the Headquarters site, but it may be within a foot or two below the bottom of trench in portions of the Swiftcurrent site, which could result in unstable trench subgrade.

7.3.1 Loads on Pipes

The structural design of the pipeline should consider the loads acting on the pipe. The earth load that will be imposed on a buried pipe depends on the subsurface soil and groundwater conditions, the type of backfill, the height of backfill above the pipe, the method of installation, the relative rigidity of the pipe, and the pipe dimensions. Because the proposed pipeline across the alignment is flexible pipe (PVC or HDPE) we anticipate that the loading will be transferred through the pipe and carried by the bedding and subgrade soils beneath the pipe.

Loads on flexible pipes are supported by the passive resistance of the soil as the pipe deflects and the sides move outward and downward into the soil. To estimate the potential settlement of flexible pipes under loading, the Modified Iowa Formula (ASCE and Water Pollution Control Federation, 1982) is typically used. The soil parameter required in this formula is the modulus of subgrade reaction (E’). Based on the recommendations made in the American Water Works Association (AWWA) PVC Pipe Design and Installation Manual (AWWA, 2002) and the AWWA’s PE Pipe – Design and Installation Manual (AWWA, 2006), we recommend a modulus of soil reaction (E') of 3,000 psi for slightly compact granular soils and 5,000 psi for compact granular soil.

7.3.2 Thrust Restraint

Thrust restraint for the pipelines may be provided through the use of restrained joints or thrust blocks. Thrust blocks bearing against the native soils and used to encase tee connections and bends may be designed using an allowable passive earth pressure of 375 pcf, with an average pressure over the face of the thrust block not to exceed 2,000 psf. For restrained joints, a coefficient of friction of 0.25 may be used between the pipe and pipe bedding material in combination with a soil unit weight of 125 pcf. These values include a factor of safety of 1.5 on ultimate soil strengths.

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7.3.3 Manhole Foundations

We recommend the use of a structural mat (raft) foundation for manholes. We recommend a gross allowable bearing capacity of 1,500 psf for soil or overburden and 2,000 psf for bedrock. Settlement of the manholes is anticipated to be about ½ inch. The at-rest earth pressures provided in Section 7.2.2 can be used for manhole design. Uplift pressure on manhole foundations should be considered at the Swiftcurrent site based on the relatively shallower groundwater conditions encountered in the borings during drilling. Because a long-term groundwater monitoring system was not completed as part of the Project, we recommend assuming a groundwater level at the ground surface for this evaluation.

Sufficient mass or uplift resistance will be required to counteract uplift loads created by hydrostatic forces acting on the bottom of the foundation. For the manholes, the uplift resistance will be provided by the weight of the manhole and the weight of soil above the manhole base. We recommend a total unit weight of soil of 125 pcf be used for soil above any extended manhole base in the uplift analysis, along with an appropriate factor of safety.

8 ADDITIONAL EXPLORATIONS

Our geotechnical exploration program was conducted to support preliminary design for the Project. The design-build contractor should evaluate the adequacy of the subsurface explorations provided in this preliminary report, and if necessary, complete additional explorations to support final design.

9 CONSTRUCTION CONSIDERATIONS

The applicability of the design recommendations provided in this report is contingent on good construction practice. Poor construction techniques may alter conditions from those on which our recommendations are based, resulting in reduced foundation capacity and increased settlement. The following sections present construction considerations for this project.

9.1 Dewatering

As discussed in Section 5.3, groundwater was encountered during our subsurface exploration program in five borings. At the Headquarters site, groundwater was encountered at 16.9 and 19 feet bgs, and as such, groundwater is not generally anticipated to be encountered in excavations for relatively shallow utility lines that are anticipated to be

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107857-001 December 2022 less than 7 feet in depth. At the Swiftcurrent site, groundwater was encountered in three borings from 5.3 to 8.2 feet bgs, therefore, the Contractor should be prepared to dewater the excavations for shallow utility lines. We recommend that the groundwater be drawn down at least 3 feet below the subgrade elevation to maintain a suitable subgrade condition during construction. Based on the depth of excavation, dewatering can likely be accomplished by the use of sumps and pumps. However, if the excavation extends below the water level in the adjacent Swiftcurrent Creek (borings SW-10 and SW-13), relatively high pumping rates may be required, and the Contractor may prefer to implement a seepage cutoff (e.g. drive sheet pile) to decrease seepage into the excavation.

9.2 Earthwork

9.2.1 Site Preparation

We recommend that grass and other vegetation, including roots, be cleared from all areas to be improved. Any existing surficial organic topsoil and any soil containing organics also should be stripped and removed. The depth of this removal is anticipated to vary over the site. Topsoil and organic-rich soils are not considered suitable for reuse as fill and should be removed from the site or stockpiled for reuse in landscaping areas.

9.2.2 Excavation and Rippability

We understand that the depth to the proposed utility lines will be approximately 3 to 7 feet bgs. Based on our subsurface explorations, trench excavation will generally be performed within granular overburden consisting of glacial till and alluvium with cobbles and boulders. We anticipate that the overburden materials can be excavated with conventional construction equipment, such as a rubber-tired backhoe or a tracked hydraulic excavator with ripper attachments.

However, the excavation for the proposed water storage tank is expected to extend 15 to 20 feet bgs, within the siltite bedrock, which will be more challenging. The two laboratory samples of siltite bedrock tested exhibited high unconfined compressive strength. The rock quality designation (RQD) may be used to provide an indication of the degree of fractures or jointing in a rock mass. The RQD of 20 % from approximately 15 to 20 feet bgs indicates that the upper portion of the bedrock encountered in the boring is highly fractured. Where highly fractured bedrock is exposed in the excavation, techniques and equipment such as ripping, hydraulic rock breakers, and the use of percussive drilling to break up rock in the excavation for the underground storage tank may be considered. However, the rock quality in the boring generally increased with depth, corresponding to an RQD of approximately 60 %; therefore, less fractured zones of bedrock may also be encountered in the excavation for

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107857-001 December 2022 the storage tank. Due to the high strength of the intact bedrock, rock excavation using drilling and blasting techniques may be needed to excavate the more intact portions of the bedrock. The Contractor should use controlled blasting techniques to provide safety to onsite personnel and prevent damaging the existing underground water tank.

9.2.3 Temporary Slopes and Shoring

A temporary excavation will be required for the proposed utility lines and water storage tank. The Contractor will be responsible for excavation stability during construction. As such, the Contractor should evaluate the stability of temporary slope and excavation configurations that may be required during construction.

9.2.3.1 Temporary Slopes

The safe slope for the excavation of subsurface materials depends on many factors, including: (1) the presence of groundwater; (2) the type, density, and shear strength of the subsurface materials; (3) the depth of excavation; (4) the presence of adjacent structures; (5) surcharge loading adjacent to the excavation (including excavated material, existing dead or live loads, and construction equipment); and (6) time of construction.

To the extent that temporary excavation slopes are feasible, the following slope conditions are for cost estimating and planning purposes only and limited to slopes up to 10 feet in height. We anticipate that temporary excavations in cohesionless soils can be made at slopes of 1.5H:1V and in bedrock at 0.75H:1V. Flatter slopes should be anticipated if groundwater is encountered during excavation. Temporary slopes should be consistent with the Occupational Safety and Health Administration (OSHA) guidelines contained in 29 CFR 1926, Subpart P (OSHA, 1999).

Consistent with conventional practice, the Contractor should be responsible for the actual temporary excavation slopes and shoring, including methods, sequence, and schedule of construction. The Contractor can observe the nature and conditions of the subsurface materials encountered and should evaluate the factors discussed above. If instability is observed, slopes should be flattened or shored. All excavations should be accomplished in accordance with local, state, and federal safety regulations.

9.2.3.2 Temporary Shoring

Due to geometric considerations, temporary slopes may not be feasible at all locations of the site. Temporary shoring may be required adjacent to the existing underground storage tank. In our opinion, potential shoring systems that are typically considered include soil

Utility Improvements Preliminary Geotechnical Report

107857-001 December 2022 nail, sheet piles and soldier pile walls. However, depending on the depth of shoring, it may not be feasible to achieve adequate embedment of sheet piles into the rocky ground or bedrock. Additionally, soil nail length may be limited due to the close proximity to the adjacent tank and construction of soil nail shoring may be challenging due to limited stand-up time of granular soils. Nail installation through rocky soils may also be difficult in some areas. Therefore, a soldier pile wall may be preferable.

The final selection of shoring type, temporary shoring design, and method of construction should be the responsibility of the Contractor. Further, it should be the Contractor’s responsibility to monitor the stability of shored excavations and adjacent structures and take corrective action if any potentially unstable conditions are encountered. We recommend the Project specifications stipulate that any shoring design be performed by a Montana-licensed Professional Engineer.

Trench boxes are commonly used for pipe installation applications. It should be understood that trench boxes generally do not provide adequate support of the trench, but only provide for safety of workers within the trench. Because the trench box is placed after excavation and is not typically snug against the walls, a significant amount of movement can occur, especially in granular soils.

9.2.4 Foundation Subgrade Preparation

Excavations for the pipeline and mat subgrade should be observed by a geotechnical engineering representative familiar with the subsurface conditions to evaluate the adequacy of the subgrade and confirm that the subsurface conditions are consistent with design assumptions.

For the water tank mat foundation on bedrock, the bottom of the excavation should be relatively smooth. Any sharp bedrock projections should be removed and any significant overbreaks should be filled with concrete. A layer of gravel such as ASTM C33 (gradation 67), graded gravel or uniformly graded pea gravel may be used as a leveling course and to help provide uniform support for the buried tank if the drainage is accounted for in the design (see Section 7.2.4). Additional guidelines on suitable gravel materials are provided in Section 9.3 of this report.

Subgrade excavations should be cleaned of all undocumented fill, debris, or loose/soft material prior to concrete placement and should be kept free of water at all times. If any structures require dewatering to reach the base of excavation, it will be important to maintain the soil or rock at the base of the excavation in a firm and undisturbed condition.

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If groundwater is flowing into the base of the excavation or is near the subgrade elevation it can loosen/soften the subgrade material.

9.2.5 Fill Placement and Compaction

Granular Fill should consist of native or imported well-graded sand and gravel with less than 20 % fines (material passing the U.S. Standard No. 200 sieve). Based on the grain size analyses in Appendix B, the majority of the on-site soils should meet this criteria. Imported material classified as GW, GW-GM, SW, or SW-SM according to the Unified Soil Classification System is suitable for Granular Fill, with the additional requirement that the maximum particle size should be 3 inches.

Fill materials should be placed in horizontal lifts, with the loose lift thickness not to exceed 12 inches for heavy equipment compactors and 6 inches for hand-operated compactors.

Compaction of backfill adjacent to buried structures can result in higher lateral earth pressures. As such, only small equipment that is hand-operated should be used within a line extending upward from the base of the structure at 0.5H:1V, or at least 3 feet from the wall, whichever is greater. Unless the structural engineer indicates differently, we recommend that backfill around structures be brought up in uniform horizontal layers and not be unbalanced around the structure.

All fill materials should be compacted to a dense/firm and unyielding condition. Granular Fill should be compacted to a minimum of 95 % of its Modified Proctor maximum dry density (ASTM D1557) with moisture content within ± 3 % of the optimum moisture content.

9.3 Pipeline and Manhole Considerations

9.3.1 Trench Excavation

The Contractor should be made responsible for trench safety, excavation, and shoring methods, as the Contractor is responsible for planning and conducting the work. Provisions should be incorporated into the contract documents that require the Contractor to minimize adverse ground movements, including lateral deformations and ground surface settlements, particularly within existing roadways where numerous underground utilities are present.

The Contractor should be made responsible for the repair of adjacent improvements if the Contractor’s activities disturb them.

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9.3.2 Subgrade Preparation

Granular soils are expected at the pipe invert based on the anticipated depths and observations from our borings. Based on our subsurface explorations, these materials will provide adequate foundation support for the pipe if maintained in an undisturbed condition free of water during construction. If loose soils are encountered at the invert elevations these soils should be compacted in place to a dense and unyielding condition using machine or hand operated compaction equipment. If sufficient compaction is not able to be achieved, the soils should be over-excavated and replaced with crushed rock. While not anticipated, wet or saturated subgrade soils are not considered suitable as subgrade material for the pipe or manholes. Wet soils should be over-excavated and replaced with crushed rock. We recommend that unsuitable materials be over-excavated a minimum of 6 inches.

9.3.3 Pipe and Manhole Bedding and Backfill

We recommend that a minimum of 6 inches of pipe bedding material be placed beneath the pipe and manholes. We further recommend that the bedding extend a minimum of 12 inches and a maximum of 24 inches on either side of the pipe.

We recommend that the bedding and pipe embedment material consist of one of the following materials: ASTM C33 (gradation 67), graded gravel, or uniformly graded pea gravel. The allowable gradation for each material is provided in Exhibit 9-1 below.

Alternately, flowfill could be utilized for bedding and embedment.

Exhibit 9-1: Bedding and Embedment Material Gradation

Material

Sieve Size (Percent Passing by Weight)1

1-Inch ¾ - Inch ½ - Inch ⅜-Inch No. 4 No. 8 No.

No.

No.

No.

ASTM C33

(Gradation 67)

100 90-100 - 20-55 0-10 - - 0 - -

Graded Gravel - 100 90-100 40-100 0-30 0-10 0-5 - - -

Uniformly Graded Pea Gravel

- - - 100 70-100 25-90 5-80 - 1-15 0-5

NOTES:

Fraction passing the No. 4 sieve should have a liquid limit less than 25 and a plasticity index less than 5.

The bedding and embedment materials should be placed in lift thicknesses of 6 inches or less and carefully worked under and around the pipe by means of slicing with a shovel, vibration, tamping, or other approved procedures.

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Bedding and pipe zone material should be compacted to 95% of the laboratory maximum dry density per ASTM D698. Ideally, compaction testing would be performed with a nuclear density gauge to evaluate the effectiveness of material placement and compaction.

However, in our experience, testing clean, granular material can be difficult to test due to the inherent variabilities in the testing method combined with the small range of allowable test results. Alternative methods, such as following a method-based specification may be more appropriate than traditional compaction testing.

9.3.4 Pipe Trench and Manhole Backfill

Native soils are considered suitable for reuse as trench and manhole backfill. Backfill should be placed in horizontal lifts and compacted to at least 95% of its Modified Proctor maximum dry density (ASTM D1557) and be deemed to be in a dense and unyielding condition. The moisture content should be within + 2% /- 2% of optimum.

Backfill material should be placed in uncompacted lift thicknesses of 8 inches or less and compacted by vibrating, tamping, or a combination. Backfill shall not be placed when the temperature is less than 40 degrees Fahrenheit and subgrade is frozen, wet, loose, or soft.

Compaction testing should be performed at regular intervals of one set of tests per 250 linear feet.

9.4 Horizontal Directional Drilling (HDD)

9.4.1 Trenchless Construction

HDD methods of pipe installation are proposed near the east end of the Swiftcurrent site in the picnic area. In this method of construction, a smooth alignment arc (or drill path) is established by drilling a small pilot hole beneath the crossing with a continuous string of steel drill rod; sharp bends or turns cannot be made. After the pilot hole reaches the exit point, the drill bit is replaced with a larger-diameter reaming tool and the borehole is enlarged by back-reaming. Drilling fluid slurry is circulated through the drill rods, maintaining the stability of the bore, and lubricating/cooling the tooling. The drilling fluid flushes drill cuttings to the entry or exit point.

Trenchless methods are proposed to limit ground surface disturbance in this area, which is densely vegetated with trees. In addition, trenchless methods are proposed to reduce the potential impact to cultural resources in the area. Five 20-foot by 20-foot drilling pits have been identified in this area; these are the only zones where ground disturbances are permitted. The proposed 6-inch I.D. water pipeline will be installed with 3 feet of cover over the pipe.

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9.4.2 Anticipated Ground Conditions

While no borings were advanced at this location, borings SW-09 and SW-10 were drilled approximately 800 feet west of the proposed HDD alignment. These borings encountered very loose to dense gravel with varying amounts of sand and clay/silt (GP-GC and GM) along with dense sand with gravel within the upper 7 feet. Cobbles were inferred from drill action in boring SW-10. Groundwater was encountered at a depth of about 17 feet in boring SW-09, and no groundwater was encountered in boring SW-10. Bedrock was not encountered in either boring.

9.4.3 Potential Risks

Assuming the ground conditions encountered during trenchless construction are similar to those observed in borings SW-09 and SW-10, we anticipate that pipeline installation using HDD is feasible; however, there are potential risks associated with trenchless construction that should be considered as described below.

9.4.3.1 Inadvertent Drilling Fluid Release

Inadequate soil cover between the drill path and the ground surface could lead to hydrofracture (“frac-out”) and inadvertent drilling fluid release during drilling. Frac-out can occur when the drilling fluid pressure exceeds the confining pressure available from the existing overburden. To mitigate this concern, the Contractor should take appropriate measures to evaluate hydrofracture risks (e.g., cavity expansion model) and maximum allowable drilling fluid pressures in overburden soils, then manage drilling pressures accordingly. A contingency plan for addressing potential hydrofracture should be developed.

9.4.3.2 Obstructions

Cobbles, boulders, or existing utilities could stop the advance of the drill rods. Cobbles were inferred from drill action in boring SW-09, and cobbles and boulders were encountered in other borings throughout the Project area. In addition, it is our understanding that the proposed HDD alignment will parallel the existing waterline in this area.

Obstructions such as cobbles, boulders, and existing utilities can hinder steering, require significant deviation of the proposed drill path, and cause the drill steel to get stuck in the ground. If an obstruction is encountered, the drill path is typically modified by retracting the drill steel about 15 feet and trying to steer around the obstruction. The Contractor

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107857-001 December 2022 should utilize appropriate equipment that will allow for pull-back and steering as may be required. To mitigate concerns of a utility strike, the existing water line should be located and surveyed before HDD operations commence.

10 CLOSURE

This preliminary report was prepared for the exclusive use of HDR and the National Park Service for specific application to the design of the Project at the sites as it relates to the geotechnical aspects discussed herein. The purpose of this report was to provide information on factual data only, and it should not be construed as a warranty of subsurface conditions, such as those interpreted from the logs and discussions of subsurface conditions in this report. This report is preliminary. It will be necessary for the design-build contractor to develop final geotechnical recommendations for the Project, including completing additional explorations as necessary.

The preliminary conclusions and recommendations presented in this report are based on the site conditions as they presently exist. These opinions further assume that explorations are representative of the subsurface conditions beneath the site; that is, the subsurface conditions everywhere are not significantly different than those disclosed by the explorations.

Within the limitations of the scope and budget, the analyses, conclusions, and recommendations presented in this report were prepared in accordance with generally accepted professional geotechnical engineering principles and practice in this area at the time this report was prepared. We make no other warranty, either express or implied.

Shannon & Wilson has prepared the attached “Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our reports.

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