B08_Att_5_Geotechnical_Report_2020_12_08.pdf

DETO 228872 0.2.3.2 Geotechnical Investigation

National Park Service

DEVILS TOWER NATIONAL MONUMENT

Replace Obsolete Housing Unit with Multiplex Unit

Devil Tower, Wyoming

Geotechnical Investigation

December 8, 2020

Prepared by:

Wood Environment & Infrastructure Solutions, Inc.

2000 South Colorado Boulevard, Suite 2-1000 Denver, Colorado, 8022

0.2.3.2 Geotechnical Investigation DETO 228872

Replace Obsolete Housing Unit with Multiplex Unit at DETO December 8, 2020

12/3/2020

David Esterline, RA, AIA, NCARB Project Manager Croft & Associates, PC 3380 Blue Springs Road Kennesaw, Georgia 30144

Dear Mr. Esterline, Wood Environment & Infrastructure Solutions, Inc. (Wood) has completed the Geotechnical Engineering Report for the referenced project located at Devils Tower National Monument, Wyoming. This study was performed in accordance with the Scope of Services (SOS) document dated August 19, 2020. Results of field exploration, laboratory testing, engineering analyses, and recommendations for foundations and pavements are included in this report.

We appreciate the opportunity to be of assistance on this project. Please contact us if you have any questions, require additional information, or if we can be of further service.

Sincerely, Wood Environment & Infrastructure

Solutions, Inc.

Reviewed by:

Alexander Vega Technical Professional II, Geotechnical

Daniel Servigna, PE Associate Engineer, Geotechnical

Reviewed by:

<Signature> Aaron Murray, PE, CFM Associate Engineer, Civil

EXECUTIVE SUMMARY

This project involves the design and construction of a two-story 9-plex housing building and associated utility pavement improvements. An existing septic leach field system located approximately 400 feet southeast of the existing housing building will also be expanded. An existing 6-plex housing building will be demolished to facilitate the new housing and pavement improvements. The proposed housing improvements will be located adjacent to the existing 6-plex housing building situated at 149 State Highway 110, Devils Tower, Wyoming, 82714, near the southern end of Devils Tower National Monument.

Wood has completed a geotechnical study to evaluate the subsurface conditions across the project site to support the design of the proposed housing development. A summary of our findings, opinions, and recommendations are listed below. This Executive Summary provides a brief overview of the geotechnical observations and recommendations for the project and is not intended to replace more detailed information contained elsewhere in this Geotechnical

Engineering Report.

• This report addresses and provides geotechnical design and construction recommendations relative to subsurface conditions, earthwork, foundations and design parameters, flatwork, slab-on-grade floors, pavement sections, and percolation testing. The study was performed in accordance with the Croft & Associates, PC Scope of Services document dated August 19, 2020.

• The proposed building footprint will measure approximately 90 by 40 feet (3,600 square feet).

We understand that the final location of the proposed housing structure has not been finalized but will generally be located either north or west of the existing housing building.

The elevation difference across the site is +/- 2 feet, therefore minimal cut and fill grading is anticipated.

• We understand the preferred building foundation system consists of a thickened edge concrete slab (also known as a turned-down slab). The outer wall structure will support 1 to 2 kips per lineal foot and the maximum anticipated slab load is 125 pounds per square foot.

• We completed a field exploration at the project site between October 28 and 30, 2020 to characterize the subsurface conditions. A total of five (5) borehole locations were drilled and sampled across proposed building and pavement areas to planned depths ranging between approximately 20 and 25 feet below the existing grade. Additionally, one (1) test pit was excavated to a depth of 8 feet near the center of the proposed septic leach field expansion area to observe the soil conditions. Four (4) percolation tests were also performed within the proposed septic leach field expansion area.

• Samples recovered from the field exploration were transported to a laboratory for further evaluation and testing to determine soil classifications, material properties, swell/consolidation characteristics, and chemical analyses.

• The site soils are relatively uniform across the project site and match the published geological description for the alluvial soils. A surficial layer of topsoil with loose silty to clayey sands and organic materials was encountered within the upper 1.5 to 3 feet of the soil profile. The topsoil layer is underlain by up to 5 feet of stiff fat clay soils processing a high expansive potential, an appreciable sulfate content, susceptibility to frost heave. The clay soils are underlain by up to 3 feet of soft to stiff silt, followed by a layer of loose to medium dense silty sands with varying amounts of gravel, until reaching auger refusal conditions in Spearfish

Formation sandstone bedrock at depths of 23 to 25.5 feet below ground surface.

• Groundwater was encountered during the field exploration at depths ranging from approximately 10 to 12.5 feet below ground surface. Redoximorphic features (e.g., soil mottling formed by oxidation/reduction of soil iron) indicate that the seasonal high groundwater table may be as shallow as 8 feet below ground surface. The frost depth line is

3.5 feet below ground surface.

• It is our opinion the project site appears suitable for the proposed housing and pavement improvements provided the recommendations included in this report are implemented in the design and construction of the project. However, the expansive soils encountered near the surface at foundation, slab-on-grade, flatwork, and pavement bearing elevations are of most concern and require complete or partial removal and replacement with compacted granular materials to mitigate the effects of swell and frost heave. Recommendations for excavation removal, subgrade preparation, and fill compaction are provided herein.

• Close monitoring of the construction operations and implementation of earthwork recommendations discussed herein will be critical in achieving the intended foundation, slab-on-grade floor, flatwork, and pavement performance. We therefore recommend a

Geotechnical Engineer, or their representative be on-site to monitor this portion of the work.

The Owner/Designer/Contractor should not rely solely upon the summary above. This memorandum should be read in its entirety prior to implementing the recommendations in the preparation of design and construction documents. Wood’s Geotechnical Engineer should be retained to perform sufficient services to determine plan/specification compliance with the recommendations in this memorandum.

TABLE OF CONTENTS

Executive Summary 3

1.0 Site and Project Description 7

2.0 Scope of Work 7

3.0 Field Exploration 8

• 3.1 Drilling and Soil Sampling Procedures 9

• 3.2 Percolation Testing 9 o 3.2.1 Test Pit 10 o 3.2.2 Percolation Test Holes 11

4.0 Site Conditions 11

• 4.1 Local Geology 11

• 4.2 Surface Conditions 12

• 4.3 Subsurface Conditions 12

• 4.4 Groundwater 13

• 4.5 Geological Hazards 14 o 4.5.1 Earthquakes 14 o 4.5.2 Landslides 14 o 4.5.3 Expansive Soils 15

5.0 Laboratory Testing 15

• 5.1 Soil Classification and Indexing 16

• 5.2 Atterberg Limits and Grain Size Analysis 16

• 5.3 Density and Moisture Content 16

• 5.4 Swell 16

• 5.5 Consolidation 17

• 5.6 CBR 17

• 5.7 UU Triaxial Shear 18

6.0 Conclusions and Recommendations 18

• 6.1 Earthwork 18 o 6.1.1 Site Preparation 18 o 6.1.2 Subgrade Preparation 21 o 6.1.3 Fill Placement 22 o 6.1.4 Utility Trenches 25 o 6.1.5 Construction Dewatering 25 o 6.1.6 Grading and Drainage 25 o 6.1.7 Landscaping 26 o 6.1.8 Construction Observation and 26

Testing

• 6.2 Foundation 26

• 6.3 Slab-On-Grade 28

• 6.4 Pedestrian Flatwork 29 o 6.4.1 Concrete Sidewalks and Pavements 29 o 6.4.2 Concrete and Stone Pavers 29

• 6.5 Seismic Site Classification 30

• 6.6 Corrosivity 32

• 6.7 Pavement 33 o 6.7.1 Pavements Subgrade Preparation 33 o 6.7.2 Pavements Section Design 33 o 6.7.3 Pavements Drainage 35 o 6.7.4 Pavements Maintenance 35

7.0 Closing 35

8.0 References 36

List of Tables

1 Summary of Borehole and Test Pit Locations 2 Summary of Percolation Test Hole Locations 3 Test Pit Subsurface Conditions 4 Borehole Subsurface Conditions 5 Swell Test Results 6 Consolidation Test Results 7 Fill Requirements 8 Structural Fill Specification 9 WYDOT Grading W Aggregate Base Specification 10 Fill Compaction Requirements 11 Foundation Design Parameters 12 Lateral Earth Pressure Design Parameters 13 Slab-On-Grade Design Recommendations 14 Seismic Design Parameters 15 Corrosivity Test Results 16 Minimum Pavement Sections

Appendices 38

A Conceptual Site Plans B Field Exploration Plans C Borehole and Test Pit Logs D Laboratory Test Results E Field Exploration Photography Log F Supporting Documents

1.0 SITE AND PROJECT DESCRIPTION

The project site is located at the existing 6-plex housing building situated at 149 State Highway

110, Devils Tower, Wyoming, 82714 near the southern end of Devils Tower National Monument.

We understand the project will consist of the design and construction of a two-story 9-plex housing building consisting of three (3) two-bedroom units and six (6) single-bedroom units, a laundry and shower facility attached to the building, with storage and community gathering areas.

The project will also include utility upgrades and pavement improvements, including two (2) RV parking spaces. The existing septic leach field system located approximately 400 feet southeast of the existing housing building will be expanded. The existing 6-plex housing building will be demolished to facilitate the new housing and pavement improvements.

The proposed building footprint will measure approximately 90 by 40 feet (3,600 square feet).

We understand that the final location of the proposed housing structure has not been finalized but will generally be located either north or west of the existing housing building. The conceptual site plans provided by Croft & Associates, PC (Croft) are presented in Appendix A, Conceptual

Site Plans.

Based on our discussion with Croft’s subconsultant, PES Structural Engineers, we understand the preferred building foundation system consists of a thickened edge concrete slab (also known as a turned-down slab). The outer thickened edge of the slab-on-grade floor will support wall loads between 1 to 2 kips per lineal foot (kips/ft.). The maximum anticipated slab load is 125 pounds per square foot (psf).

2.0 SCOPE OF WORK

The geotechnical study was completed at the project site in accordance with the Croft Scope of

Services (SOS) document dated August 19, 2020. The purpose of this study is to evaluate the subsurface conditions at selected locations and to develop geotechnical design and construction recommendations relative to:

1. Subsurface conditions;

2. Earthwork;

3. Foundations and design parameters;

4. Slab-on-grade floors;

5. Flatwork;

6. Pavement sections; and

7. Percolation testing

3.0 FIELD EXPLORATION

Wood conducted a field exploration at the project site between October 28 and 30, 2020 to characterize the subsurface conditions. The field exploration was accomplished by drilling and sampling five (5) borehole locations (B-1 to B-5) to planned depths ranging between approximately 20 and 25 feet below the existing grade within the proposed building footprint and pavement areas.

The borehole locations were surveyed in the field using GPS survey equipment. Table 1 below summarizes the borehole locations, surface elevations, and exploration depths. The approximate locations of the boreholes and topographic information from the survey are provided in

Appendix B, Field Exploration Plans.

Table 1: Summary of Borehole and Test Pit Locations

Borehole/ Test Pit ID

Latitude (deg.) 1

Northing (ft.) 1

Longitude (deg.) 1

Easting (ft.) 1

Surface Elevation

(ft.) 2

Exploration Depth (ft.)

B-1 44.5822178° 1487980.891 -104.7192539° 772735.607 3868.2 19

B-2 44.5820867° 1487932.892 -104.7194137° 772694.225 3868.3 24

B-3 44.5821171° 1487944.457 -104.7190614° 772785.968 3868.0 23.2

B-4 44.5819921° 1487898.200 -104.7195545° 772657.744 3868.6 25.3

B-5 44.5818091° 1487831.759 -104.7193754° 772704.778 3867.7 25.8

Notes

1. GPS coordinates are based on Wyoming State Plane Coordinate System East Zone 4901

2. The elevations are based on North American Vertical Datum (NAVD) 88, 2001 adjustment, derived from Wood’s GPS observations.

A field representative of the Wood Geotechnical Engineer was present during the field exploration to coordinate, supervise, and record geotechnical data during the field investigation.

Vista Geoscience, LLC of Golden, Colorado performed drilling with a Geoprobe 7822DT rubber-tracked drill rig. The boreholes were advanced using conventional hollow-stem augur (HSA) drilling methods with an 8-inch outer diameter (3¼-inch inner diameter) continuous flight auger.

To minimize damage to the existing landscaping during drilling, rubber mats were used to traverse or work over lawn surfaces. The boreholes were backfilled upon completion with hydrated bentonite pellets to within a foot of ground surface and then capped using auger cuttings to form a level ground surface.

3.1 Drilling and Soil Sampling Procedures

Disturbed soil samples were recovered in accordance with Standard Penetration Test (SPT) drive sampling procedures (ASTM D1586). A 2-inch outer diameter split barrel sampler was driven into the ground using the Geoprobe’s DH103 Automatic Drop Hammer system, which consists of a

140-pound automatic hammer that falls from a vertical height of 30 inches. The number of blows required to advance a sampler the last 12 inches of an 18-inch penetration is recorded as the SPT

N-value. Similarly, less disturbed samples were obtained using a 2.5-inch outer diameter split-barrel sampler (also known as a Modified California Barrel), lined with 6-inch long brass rings, in accordance with ring-lined split-barrel sampling procedures (ASTM D3550).

Relatively undisturbed soil samples of cohesive (clay) materials were obtained using thin-walled tube sampling procedures (ASTM D 1587) The thin-walled tube consisted of a 3-inch diameter, 30-inch long, seamless steel tube (also known as a Shelby tube) with a sharp cutting edge. To obtain relatively undisturbed samples, the Shelby tubes carefully pushed hydraulically into the subsurface using the drill rig.

The soil samples recovered were visually classified during the field exploration and immediately sealed in plastic bags to prevent the loss of moisture or disturbance in preparation for their transfer to the laboratory. Grab samples were obtained from auger cuttings within the upper 5 feet of select boreholes to obtain representative samples of subsurface materials. These grab samples were sealed in plastic 5-gallon size containers.

A field log was prepared for each borehole location during drilling, including sampling depths, SPT N-values, groundwater levels, visual soil classifications, and other relevant sampling information. The Wood Geotechnical Engineer's interpretation of subsurface conditions, including modifications based on visual observations and laboratory tests are provided in

Appendix C, Borehole and Test Pit Logs. A general summary of the borehole subsurface soils is presented below in Section 4.2, Subsurface Conditions.

3.2 Percolation Testing

The test pit and percolation test locations were surveyed in the field using GPS survey equipment.

The approximate locations of the test pit and percolation tests are also shown in Appendix B, Field Exploration Plans. The percolation test hole locations and surface elevations are summarized in Table 2. The percolation test field study costing of a test pit excavation and test holes is discussed in subsequent sections below.

Table 2: Summary of Percolation Test Hole Locations

Borehole/ Test Pit ID

Latitude (deg.) 1

Northing (ft.)

Longitude (deg.) 1

Easting (ft.)

Surface Elevation

(ft.) 2

TP-1 44.5809239° 1487510.865 -104.7180887° 773041.794 3863.5

P-1 44.5810033° 1487539.779 -104.7181290° 773031.12 3862.2

P-2 44.5809346° 1487514.831 -104.7180526° 773051.162 3862.3

P-3 44.5808891° 1487498.134 -104.7181375° 773029.125 3862.4

P-4 44.5808356° 1487478.726 -104.7180576° 773050.068 3862.1

Notes

1. GPS coordinates are based on Wyoming State Plane Coordinate System East Zone 4901

2. The elevations are based on North American Vertical Datum (NAVD) 88, 2001 adjustment, derived from Wood’s GPS observations.

3.2.1 Test Pit

Prior to performing the percolation tests, one (1) test pit (TP-1) measuring 4 by 6 feet was excavated to a depth of 8 feet near the center of the proposed septic leach field expansion area to observe for evidence of a seasonal high water table, the presence of rock or impermeable soil that may block or limit absorption of water, and to classify the soils based on visual observations.

The test pit excavation was performed by Oedekoven Excavating & Construction, LLC of Gillette, Wyoming. The test pit was excavated using a Bobcat E45 rubber-tracked mini excavator.

A field representative of the Wood Geotechnical Engineer was present during the test pit excavation and subsequent percolation tests to coordinate and record geotechnical data. Based on our visual observations, there appears to be no evidence of a limiting layer (e.g., rock or impermeable soil), groundwater, or seasonal high groundwater table (i.e., redoximorphic features) below the proposed leach field area (approximately 3 feet below existing grade). The test pit excavation was backfilled upon completion by placing and tamping removed spoils in 1-foot thick lifts using the excavator bucket.

Table 3 summarizes the subsurface conditions encountered within the test pit excavation and is provided to highlight the major soil strata encountered. Additional descriptions and the interpreted distribution (approximate depths) of the subsurface materials are presented in

Appendix C, Borehole and Test Pit Logs. Stratification lines shown on the test pit log represent the approximate boundaries between material types and the actual transitions may be gradual and vary with location.

Table 3: Test Pit Subsurface Conditions

Unit Material Description (USCS Classification) Depth to Bottom of

Stratum (ft.)

Topsoil silty to clayey SAND (SM and SC) with organics 1 1.5

Clay fat CLAY (CH) 2

Sand silty SAND with varying amounts of gravel (SM) 7

Sand and

Cobble silty SAND and COBBLE with varying amounts of gravel (SM) undetermined 2

Note

1. Organics include varying amounts of roots, stems, leaves and mulch.

2. The test pits excavation was terminated at a planned depth of approximately 8 feet below ground surface.

3.2.2 Percolation Test Holes

A total of four (4) percolation tests were performed adjacent to the east of the existing septic leach field at nearly equal spaced distances within the proposed leach field expansion area. The percolation tests were performed in accordance with Crook County Small Wastewater

Regulations, Appendix A (Crook County, 2018) and Wyoming Water Quality Division, Wyoming

Water Quality Rules and Regulations, Chapter 25 (WY Water Quality, 2017).

The percolation test results are provided in Appendix F, Supporting Documents and interpreted in the wastewater section of the Integrated Design Narrative.

4.0 SITE CONDITIONS

4.1 Local Geology

Based on our review of United States Geological Survey (USGS) maps, the project site is mapped as Quaternary alluvium, Unit Qal. These alluvial deposits are found in the valleys of streams encompassing Devils Tower National Monument, particularly along the valley of the Belle Fourche

River, which flows approximately 800 feet southeast of the project site. The alluvium deposits consist of unconsolidated sand, silt, clay, gravels, and cobbles in and along streams, which vary in thickness from a thin veneer to several tens of feet (Sutherland, 2007).

The alluvium deposits are underlain by the Spearfish Formation sandstone, which is visible exposed (outcrops) in the southern parts of Devils Tower National Monument along the valley of the Belle Fourche River and forms prominent brownish-red to maroon cliffs that border the river valley. The upper Spearfish Formation is described as thinly and poorly bedded, fine-grained sandstone ,siltstone, and sandy shale, with a thickness that varies between approximately 450 to

600 feet (Robison, 1956).

4.2 Surface Conditions

The site is developed with an existing single-story housing building the occupies an area measuring approximately 3,500 square feet, associated concrete sidewalks, asphalt concrete paved parking and access roads, and utility infrastructure. Additional surface features at the project site include several large ash and oak trees measuring approximately 20 to 50 feet tall with trunk bases up to 3-feet in diameter. The existing housing building also features a septic leach field system located 500 feet to the southeast, which occupies an approximately 40 by 90-foot area.

At the time of our field exploration, the ground surface at the location of Boreholes B-3 and B-5 were covered with lawn. Elsewhere, the ground surface at borehole and test pit locations were densely covered with native grasses and weeds. The topography across the location of the proposed building is relatively flat with an elevation difference that varies +/- 2 feet. The ground surface remains relatively flat until dropping at a 10H:1V (horizontal to vertical ratio) slope to a flat lower terrace approximately 300 to 500 feet southwest of the proposed building location, where the existing septic leach field system is located.

4.3 Subsurface Conditions

The subsurface conditions at the project site have been interpreted from geotechnical data obtained from the completed field exploration and laboratory testing. The site soils are relatively uniform across the project site and match published geological description for the alluvial soils. A surficial layer of topsoil with loose silty to clayey sands (SM and SC) and organic materials was encountered within the upper 1.5 to 3 feet of the soil profile. The topsoil layer is underlain by up to 5 feet of stiff fat clay (CH) soils with a high expansive potential, an appreciable sulfate content, and susceptible to frost heave. The clay soils are underlain by up to 3 feet of stiff silt followed by loose to medium dense silty sands (SM, SM-SP, SP) with varying amounts of gravel until reaching refusal in Spearfish Formation sandstone bedrock at depths of 23 to 25.5 feet below ground surface.

Table 4 summarizes the subsurface conditions at the project site and is provided to highlight the major soil strata encountered. Additional descriptions and the interpreted distribution

(approximate depths) of the subsurface materials are presented in Appendix C, Borehole and

Test Pit Logs. Stratification lines shown on the borehole logs represent the approximate boundaries between material types and the actual transitions may be gradual and vary with location.

Table 4: Borehole Subsurface Conditions

Unit Material Description

(USCS Classification)

Depth to

Bottom of

Stratum (ft.)

Soil

Density/Consistency

SPT N-

Values

Topsoil silty to clayey SAND (SM and SC) with organics 1

1.5 to 3 very soft to stiff 2 to 9 bpf

Expansive

Clay fat CLAY (CH) 3.5 to 6 stiff 14 bpf

Silt SILT (ML) 5.5 to 8.5 soft to stiff 3 to 11 bpf

Sand silty to poorly graded

SAND with varying amounts of silt and gravel (SM, SP-SM, SP)

23 to 25.5 very loose to medium dense 3 to 13 bpf

Bedrock SANDSTONE undetermined n/a

50 blows per 1 to 3 inches

Note

1. Organics include varying amounts of roots, stems, leaves and mulch.

2. Borehole were terminated at planned depths of approximately 20 to 25 feet below ground surface. Auger refusal conditions were encountered in bedrock.

Abbreviation

Bpf: blows per foot

4.4 Groundwater

Depths to groundwater were recorded where wet soils were encountered during drilling and sampling. Groundwater was encountered in each borehole at depths ranging from approximately 10 to 12.5 feet below ground surface. Based on redoximorphic features, which are color patterns in the soil formed by the oxidation and reduction of iron, the seasonal high groundwater table may be as shallow as 8 feet below ground surface.

The boreholes were not intended as groundwater monitoring wells and time was not allowed for water to reach equilibrium in them before abandonment. Groundwater and soil moisture conditions within the area will vary depending on rainfall, water level changes along the Belle

Fourche River and other factors that may not have been apparent during the field exploration.

The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.

4.5 Geological Hazards

4.5.1 Earthquakes

Data published by the Wyoming State Geological Survey (WSGS) indicates that earthquakes with at least a magnitude 2.5 have occurred in every county in Wyoming over the past 120 years; with more frequency and intensity near the northwest corner of the state near Yellowstone National

Park (Case and Green, 2000; WSGS, 2015). The largest earthquake recorded to date in Wyoming was registered as a magnitude 6.5 in Yellowstone National Park on August 18, 1959; which was an aftershock of the Hebgen Lake-Red Canyon seismic event that registered as a magnitude 7.3 west of Yellowstone National Park near Hegben Lake, Montana on August 17, 1959. The project site is located approximately 300 miles east of this historical seismic event.

Only two magnitude 3.0 and greater earthquakes have been recorded in or around Crook

County, Wyoming; however, no significant damage was reported (Case et al., 2002). One of which occurred near Sundance, Wyoming on February 3, 1897; the other being a magnitude 4.3 earthquake occurring approximately 18 miles east of Gillette, Wyoming near the Crook County-

Campbell County border. Furthermore, Crook County is in a region of Wyoming considered to be least seismic active zone in that state “Seismic Zone 0” where effective peak ground accelerations (PGA) with a 90% chance of non-exceedance in 50 years can range from 0 to 0.05g

(WSGS, 2002). As of November 2020, Crook County has not adopted any standard building codes (Crook County, 2020). However, the State of Wyoming has since replaced UBC with the

International Building Code (IBC) 2018 where seismic motions are based upon probabilistic analyses.

Quaternary faults, with evidence of movement in the last 1.6 million years, are considered to have the greatest potential to be the sources of future earthquakes with magnitudes ranging between

6.75 and 7.5; however, there are no known active or suspected faults in Crook County, Wyoming

(WSGS, 2000). The Quaternary faults nearest to the project site is the South Granite Mountain

Fault System located approximately 220 miles southwest of the project site (Larsen and Wittke, 2013a).

Therefore, it is our opinion that the project site is situated in an area that is not likely to be subject to strong ground motions due to earthquakes. The performance of a site-specific seismic hazards analysis was beyond the scope of this investigation.

4.5.2 Landslides

The sedimentary rock outcrops and colluvial deposits that encompass the immediate area base of

Devils Tower National Monument are mapped as susceptible to landslides (Larsen and Wittke, 2013b). However, these areas are generally located north of the Devils Tower National

Monument Road which is about 300 feet north of the project site. Therefore, it is our opinion the project site is not located in an area susceptible to landslide. Additionally, the project site is located within an area of relatively level topography. Since little-to-no earthwork grading is anticipated for the project, it is our opinion that earthwork grading (if any) should not result in a potential for slope instability within or in the immediate vicinity of the site.

4.5.3 Expansive Soils

Expansive soils are characterized by their ability to undergo significant volume change (shrink or swell) due to variations in moisture content. Changes in soil moisture content can result from rainfall infiltration, landscape irrigation, utility leakage, roof drainage, elevated groundwater levels, drought, or other factors and may cause unacceptable settlement or heave of structures, concrete slabs supported-on-grade, or pavements supported over these expansive materials.

Based on the results of laboratory testing performed on selected samples obtained during our field exploration, the subsurface soils encountered across the project site, beneath the topsoil, consists of fat clay (CH) to depths ranging between approximately 3.5 to 6 feet below ground surface. The swell test results and Plasticity Index (PI) value of 52 (highly expansive) obtained from a representative soil sample indicate that these clay materials have a high expansion potential.

Unless mitigated, heave movements are typically not uniform and can result in damage to foundation, slab, and pavement structures. Recommendations for mitigating expansive clay soils are discussed in Section 6.1, Earthwork.

5.0 LABORATORY TESTING

Samples recovered from the field investigation were transported to Advanced Terra Testing, Inc.

of Lakewood, Colorado to conduct the laboratory testing to classify and evaluate the material properties of the soils. Soil geochemical testing, including corrosivity and soil nutrient tests were performed by Colorado Analytical Laboratories, Inc. of Lakewood, Colorado. The laboratory tests were performed in accordance with applicable ASTM and AASHTO test procedures and included the following tests:

• Moisture content and density (ASTM D7263 and D2216)

• Atterberg limits (ASTM D4318)

• Grain size analysis (ASTM D6913)

• Swell (ASTM D4546)

• Consolidation - ASTM D2435

• Moisture-density relationship (ASTM D1557)

• California Bearing Ratio (CBR) (ASTM D1833)

• Undrained Unconsolidated (UU) Triaxial Shear (ASTM D2850)

• Corrosivity, including pH (AASHTO T289-91), electrical resistivity (AASHTO T289-91), soil soluble sulfates (ASTM D4327), soil soluble chlorides (ASTM D4327)

• Soil nutrient (n/a)

The laboratory testing procedures are discussed below. Results are provided in a summary table and individual results in Appendix D, Laboratory Test Results.

5.1 Soil Classification and Indexing

All recovered soil samples were classified using the Unified Soil Classification System (USCS), in accordance with ASTM D2488. Field identifications were then supplemented with additional visual classifications and/or by laboratory testing. The USCS classifications are shown in

Appendix C, Borehole Logs, the general soil profile in Section 4.3, Subsurface Conditions, and are periodically referenced throughout this report.

5.2 Atterberg Limits and Grain Size Analysis

Atterberg limits and grain size analyses have been performed on several selected samples, in accordance with ASTM D4318 and D6913. Atterberg limits are a basic measure of the critical water contents of a fine-grained soil, such as its shrinkage limit, plastic limit, and liquid limit. The sieve analyses are used to assess the particle size distribution also called gradation of granular materials. The weight of the portion of the sample retained on each sieve screen is recorded and the percentage finer or coarser of the total weight is calculated.

5.3 Density and Moisture Content

The density has been determined for selected relatively undisturbed soil samples. These densities were determined in general accordance with the method presented in ASTM D7263. The results are recorded as dry unit weight in pounds per cubic foot (pcf). The moisture contents are determined in accordance with ASTM D2216, and are expressed as a percentage.

5.4 Swell

The swell potential for selected relatively undisturbed clay soils was determined in general accordance with ASTM D4546. The testing apparatus is designed to accept cut portions of relatively undisturbed Shelby tube samples. The sample were initially loaded to specified surcharge loads equivalent to either 150 or 2,000 psf. The sample were then inundated with water and allowed to swell against the surcharge. The resultant swell or consolidation is recorded after a 24-hour period and expressed as a percent of the sample’s initial thickness. After the inundation period, additional incremental loads are applied to evaluate the swell pressure and consolidation response of the tested sample.

The percent swell values for the tested clay soils samples indicate a high swell potential under a load of 150 psf with a swell of 7.74%, whereas a higher load of 2,000 psf resulted in a lower swell of 0.74%. The results indicate that the relatively light slab-on-grade floor and vehicle surcharge loads (on the order of 150 psf) provide insufficient restraint to the expansive clay activity, whereas a more heavily loaded foundation (on the order of 2,000 psf) tends to restrain swelling movements. The swell test results are summarized in Table 5. Mitigation measures for expansive soils are discussed in Section 6.1, Earthwork.

Table 5: Swell Test Results

Borehole

ID

Sample

Depth

(ft.)

Material

Description (USCS

Classification)

Inundation

Stress (psf) Swell 1 (%)

Swell

Pressure 1

(psf)

B-1 3 to 5 fat CLAY (CH) 150 7.7 3,296

B-5 5 to 7 fat CLAY (CH) 2,000 0.7 4,457

Note

1. The swell pressure required to restore the sample to its original height prior to inundation.

5.5 Consolidation

Consolidation tests, also referred to as one-dimensional compression tests, were performed on select clay soil samples to determine their rate and magnitude of settlement in accordance with

ASTM D2435. The testing apparatus is designed to accept cut portions of relatively undisturbed

Shelby tube samples. Each sample is then loaded incrementally, and the resulting deflection is recorded at selected time intervals. Porous stones are in contact with the top and bottom of the sample to permit the addition or release of pore water. Considering the expansive potential of the clay soils, we anticipate these materials will be removed during earthwork. Therefore, these results were not considered in our settlement analysis.

Table 6: Consolidation Test Results

Borehole

ID

Sample

Depth

(ft.)

Material

Description

(USCS

Classification)

Preconsolidation

Stress, ’p (psf)

Compression

Index, CC

Recompression

Index, Cr

B-1 3 to 5 fat CLAY (CL) 5,660 0.284 0.076

B-5 5 to 7 fat CLAY (CL) 12,580 0.210 0.078

5.6 CBR

CBR testing was performed in accordance with ASTM D1833 on a representative composite sample of pavement subgrade soils collected from Borehole B-1 and B-5 from 0 to 5 feet. The representative composite sample was tested for its maximum dry density and optimum moisture content in accordance with the modified Proctor compaction (ASTM D1557) and remolded accordingly for CBR testing. The CBR tests were performed at varying moisture contents (at optimum moisture and +/- 2% of optimum). The test results indicate that clay soils have a an average CBR value of 13.5.

However, we believe the CBR results are artificially high as the auger cutting obtained within the upper 5 feet are mixed with the overlying topsoil materials containing sand. Based on typical published values, we have conservatively estimated that a lower CBR design value of 3 should be used to design the pavement sections. The CBR design value was used to evaluate the recommended pavement sections presented in Section 6.7, Pavement.

5.7 UU Triaxial Shear

A selected soil sample was tested to determine the undrained strength shear strength of subsurface clays in accordance with ASTM D2850. A UU triaxial tests was performed on a cut portion of a relatively undisturbed Shelby tube sample obtained from Borehole B-1 at a depth of

3 to 5 feet. During the tests, the sample was placed on a triaxial cell using an at depth confining pressure before loading the sample to failure. The results indicate the undrained shear strength of the tested stiff fat clay (CH) is approximately 3,500 psf. (half the UU shear strength at failure).

6.0 CONCLUSIONS AND RECOMMENDATIONS

Based on data collected from the geotechnical engineering analyses, field exploration, and laboratory test results, it is our opinion the project site appears suitable for the proposed development provided the recommendations included in this report are implemented during the design and construction of the project.

6.1 Earthwork

The project site is relatively level with an approximate elevation difference of 2 feet within the proposed building footprint. Therefore, we anticipate minimal cut and fill to achieve site grades.

Based on the encountered site conditions and project understanding, we anticipate earthwork will involve demolition, clearing and grubbing, and the removal and replacement of expansive fat clay (CH) soils with low volume changing structural fill materials. The following sections present recommendations for site preparation, excavation, subgrade preparation, and fill placement.

6.1.1 Site Preparation

6.1.1.1 Demolition and Removal of Existing Structures

We understand that the existing, on-site structures and associated improvements will be demolished, and the resulting debris will be removed prior to construction of the proposed housing improvements. During demolition, we recommend existing foundations, below-grade utilities, pavements, and other buried improvements which may be present but not currently visible be removed and disposed of at approved locations off-site or outside the construction limits.

Although no evidence of existing fill was encountered within the completed boreholes, we anticipate some fill may be encountered within the vicinity and below the existing building structure and possibly other areas at the project site. Any debris and existing fill materials encountered within planned building, flatwork, or pavement areas should be removed in their entirety and replaced with or as structural fill, if suitable. The extent of removal should extend down from the edge of foundations at a minimum slope of 2 vertical to 1 horizontal (2V:1H) and extend horizontally beyond the edge of the foundation a distance equal to the depth of fill. For flatwork and pavement areas, the extent of removal should be at least 1 to 3 feet beyond their perimeters, respectively.

6.1.1.2 Stripping and Grubbing

Prior to general site grading and/or construction of the proposed building foundations, slab-on-grade, flatwork, and pavements, any vegetation (including the entire root ball), organic topsoil, or debris should be removed and disposed of at approved locations off-site or outside the construction limits. Deep stripping or grubbing will be required where concentrations of organic soils, tree roots, or debris are encountered. Stripped topsoil free of debris and large tree roots may be stockpiled and reused for landscape purposes; however, this material should not be incorporated into any structural fill. Based on the subsurface conditions encountered, we anticipate the topsoil thickness will vary from 1.5 to 3 feet across the site. Based on the size of the observed ash and oak trees, they will likely have root systems extending at least 4 feet that may also spread laterally a distance equivalent to or greater than the tree canopy.

6.1.1.3 Abandoned Utilities

Below-grade utility lines may exist within areas of the site other than those currently occupied by existing building. Existing utilities that will be abandoned should be removed and disposed of at approved locations off-site or outside the construction limits. Existing below-grade utility conduits that extend beyond the limits of the proposed construction that will be abandoned in-place should be plugged with cement grout to prevent migration of soil and/or water. All excavations resulting from utility removal activities should be cleaned of loose or disturbed material, scarified, and compacted (see Section 6.1.2 Subgrade Preparation), prior to placing any structural fill or backfill.

6.1.1.4 Excavations

The anticipated excavations for the proposed construction will encounter soils that can be managed using conventional earthmoving equipment. Unsuitable and excess materials resulting from earthwork operation should be hauled to an available waste site approved by the owner or should be disposed of in such a manner as to meet all requirements of State, county and municipal regulations regarding health, safety, and public welfare.

The subgrade soil conditions should be evaluated during the excavation process and the stability of the soils determined at that time by the earthwork contractors’ Competent Person. The contractor should be made responsible for designing and constructing stable, temporary excavations as required to maintain stability of both the excavation sides and bottom. At a minimum, all excavations should be performed in accordance with Occupational Safety and

Health Administration (OSHA) 29 CFR, Part 1926, Subpart P, Excavations and in accordance with any applicable local, and/or state regulations. Construction site safety is the sole responsibility of the contractor who controls the means, methods, and sequencing of construction operations.

6.1.1.5 Expansive Clay Soils

Expansive fat clay (CH) soils were observed in all boreholes at depths ranging from approximately

3.5 to 6 feet below ground surface. These clay soils are susceptible to volume change resulting in excessive and differential ground movement with variations in moisture level. Based on the swell test results, anticipated loads, and heave calculations adapted from Fredlund and Rahardjo

(1993), we have estimated potential ground heave up to 0.8 inches below foundations, and up to

3.5 inches below slab-on-grade floors and pavements. The PI for test clay was also 52 (very high expansive potential). Therefore, the clay soils encountered on-site are not suitable for use as structure fill or an ideally suited subgrade support for foundations, slab-on-grade floors, flatwork, or pavements. In addition, these clay soils are susceptible to frost heave during freezing temperatures.

Where fat clay (CH) soils are encountered beneath foundation and slab-on-grade floors, the expansive clay soils should be removed to a minimum depth of 5 feet and replaced with structural fill to mitigate excessive foundation movements. The extent of removal should extend down from the edge of foundations at a minimum slope of 2 vertical to 1 horizontal (2V:1H), and at least 5 feet, or the depth of fill placed beyond the edge of the foundation (the greater of the two).

Elsewhere on-site, the pavement subgrade soils are likely to consist of expansive fat clay (CH) soils. Based on sieve analysis and Atterberg limits for this material, these clay soils also classified as AASHTO A-7-6 soils, which are rated as poor pavement subgrade soils susceptible to swell and frost heave. The presence of the expansive subgrade soils implies risk that pavements may heave and be damaged unless mitigated. Therefore, we recommend expansive clay soils should be removed to a minimum depth of 2 feet and replaced with structural fill if encountered during pavement subgrade preparation to create a low-volume change layer beneath pavements. The horizontal extent of removal should be at least 3 feet beyond the pavement perimeter.

To minimize the effects of frost heave or swelling and enhance drainage beneath flatwork, we recommend that a minimum of 12 inches of the expansive clay be removed. The flatwork should then be supported over a minimum 6-inch thick layer of granular draining materials consisting of

WYDOT Grading W aggregate base. The horizontal extent of removal should be at least 1 foot beyond the perimeter of flatwork.

Methods such as lime stabilization are an effective means of reducing the plasticity of clays, but they have been found to aggravate frost susceptibility in sulfate-bearing clay soils. In particular, the lime stabilization may result in added sulfate-induced heaving due to the formation of ettringite formation (i.e., mineral crystals) from the reactions between lime and sulfates in soils. As presented in Section 6.6, Corrosivity, the fat clay (CH) soils encountered on-site have a sulfate content of 0.6 %. Based on National Cooperative Highway Research Program (NCHRP, 2009), section titled “Threshold for Sulfate Levels in Soils” and recommended practices for stabilization of sulfate-rich subgrade soils, the sulfate-bearing clays on-site have a moderate to high risk associated with lime stabilization (between 0.5 and 0.8%). Therefore, removal and replacement of the clay soils is preferred to lime stabilization methods.

6.1.2 Subgrade Preparation

6.1.2.1 Scarification and Compaction

Prior to construction, the subgrades of foundations, slab-on-grade floors, flatwork, and pavement should be scarified to a depth of 8 inches, moisture-conditioned to within +/-3 % of the optimum moisture content, and compacted to at least 95% of the maximum dry density as determined by standard Proctor compaction (ASTM D698). The compaction is required to densify any soft/loose or disturbed native subgrade soils.

The scarification and compaction of the site soils should extend horizontally beyond the edge of the foundation a distance equivalent to the depth of fill. For flatwork and pavement areas the extent of removal should be at least 1 to 3 feet beyond their perimeters, respectively.

Compaction should not be performed on excessively wet or frozen ground and should not be allowed to freeze during moisture conditioning or prior to placing fill and foundation construction. Unstable soil conditions as described in the section below should be addressed prior to subgrade scarification and compaction.

6.1.2.2 Unstable Soil Conditions

This subgrade preparation should be performed in the presence of a Geotechnical Engineer to ensure that soft/loose zones of soils are identified and improved. The Geotechnical Engineer can observe proof rolling with a heavy-loaded wheel vehicle (minimum 20-ton load) to identify potentially unstable soil conditions. Alternatively, areas that are inaccessible to proof rolling equipment can be evaluated using hand equipment such as a steel T-probe. Where soft/loose zones are encountered that do not improve with repeated compaction, they should be undercut and replaced with compacted structural fill.

Construction traffic over the completed subgrades should be minimized to the extent possible.

The site should also be graded to prevent ponding of surface water on the prepared subgrades or in excavations. Water collecting over or adjacent to construction areas should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the affected material should be removed, or the materials should be scarified, moisture conditioned, and recompacted, prior to placing fill and foundation construction.

If site preparation or grading is performed during the winter or spring season, or shortly after significant precipitation, near-surface site soils may be significantly over the optimum moisture content. This condition could impede construction as well as efforts to compact soils to specified levels of compaction. Excessive moisture conditions can be improved by disking to aerate soils, replacement with imported fill material, chemical treatment, stabilization with a geotextile materials or coarse aggregates (such as cobbles), and/or other suitable methods. The applicable method will depend on the Contractor's capabilities as well as other project-related factors beyond the scope of this study. Therefore, if excessive soil moisture conditions are encountered during construction, the Geotechnical Engineer should review these conditions (as well as the contractor's capabilities) and, if appropriate, provide recommendations for their treatment.

6.1.3 Fill Placement

6.1.3.1 Fill Types

Topsoil and fat clay (CH) soils are not suited for use as structural fill but can be incorporated into landscaping outside structural building and pavement areas. If encountered during earthwork operations, native soils and/or existing fill free of organics, debris, or other unsuitable materials may be considered for use as structural fill if the requirements specified later in Table 8 are met.

Frozen fill materials should not be used or placed on a frozen subgrade. Material testing to determine conformance to structural fill requirements should be submitted to the Geotechnical

Engineer for evaluation prior to use on this site. Recommended backfill locations and types are summarized in Table 7.

Table 7: Fill Requirements

Location and Use Backfill Type

Foundation and Slab-On-Grade

Foundation subgrades

(up to 5 feet below the bottom of foundation) Structural Fill

Slab-on-grade floor subgrades

(up to 5 feet below the bottom of slab) Structural Fill

Slab-on-grade floor subgrades

(6 inches below) WYDOT Grading W Aggregate Base

Pavement and Flat Work

Pavement subgrades

(up to 2 feet below the bottom of pavement) Structural Fill

Pavement aggregate base WYDOT Grading W Aggregate Base

Flatwork (6 inches below) WYDOT Grading W Aggregate Base

Flatwork subgrade (12 inches below aggregate base) Structural Fill

Utility Trench and Landscaping

Utility trench bedding (4 inches below and 6 inches above utility trench pipes or conduits WYDOT Grading W Aggregate Base

Utility trench backfill Native SILT (ML) and silty/clayey SAND (SM, SP-

SM, SM, SC) or Structural Fill

Landscaped and other non-structural areas

Topsoil (SC/SM), native fat CLAY (CH), SILT

(ML), and SAND (SM, SP-SM, SM) or Structural

Fill

Abbreviation

WYDOT: Wyoming Department of Transportation

Structural fill shall be a select, homogeneous, granular material free from organic matter with a PI of 7% or less and a maximum Liquid Limit (LL) of 30%. Structural fill shall meet the required material gradation specified in Table 8.

Table 8: Structural Fill Specification

Sieve Size Percent Finer by Weight 1

#4 50 to 100

#40 10 to 70

#200 0 to 20

Note

1. In accordance with ASTM C136.

Wyoming Department of…