CRFQ AGR26-16 - Exhibit B 5-BC15 Geotechnical Investigation Report.pdf

FINAL GEOTECHNICAL INVESTIGATION

REPORT, REVISION 1

Brush Creek Site 15 Dam Rehabilitation Mercer County, West Virginia

Prepared For:

NRCS – West Virginia

Aterra-Schnabel Reference 18proj007 Schnabel Reference 18C35013.04 June 19, 2019 Revised October 28, 2021

300 Brookside Avenue www.aterra-schnabel.com Building 18, Suite 160 T/ 610-405-7708 Ambler, PA 19002

October 28, 2021

Mr. Derrick Crane, PE Contracting Officer’s Representative NRCS – West Virginia 1550 Earl L. Core Road, Suite 200 Morgantown, WV 26505

Subject: Final Geotechnical Investigation Report, Revision 1 Brush Creek Site 15 Dam Rehabilitation Design, Mercer County, West Virginia Contract #12SPEC18D0023 / Task Order #12SPEC18F0053

Dear Mr. Crane:

Aterra-Schnabel JV is pleased to submit our revised Final Geotechnical Investigation Report for this project. This report includes tables, figures, and appendices with relevant data collected for this study.

This study was performed in accordance with the approved Plan of Work dated December 28, 2018. The organization of this report follows the Geologic Investigation Report outline presented in NEH Part 631.0205(e) to the extent practical. The previous final version of this report, which was dated June 19, 2019, was revised to remove our interpretations and conclusions based on your email request on September 15, 2021.

We appreciate the opportunity to be of service for this project. Please call us if you have any questions regarding this report.

Sincerely, Aterra-Schnabel JV

Jonathan M. Pittman, PE Principal

Evan Binder, PE

CDS:SMB:EBB:ARB:JMP:PIW:vm

Distribution:

NRCS – West Virginia Attn: Mr. Derrick Crane and Mr. Andy Deichert http://www.aterra-schnabel.com/

NRCS – West Virginia Final Geotechnical Investigation Report, Revision 1 – Brush Creek Site 15 Dam Rehabilitation Design

October 28, 2021 Page i Aterra-Schnabel JV Project 18C35013.04 ©2021 All Rights Reserved

FINAL GEOTECHNICAL INVESTIGATION REPORT, REVISION 1

BRUSH CREEK SITE 15 DAM REHABILITATION DESIGN

NRCS – WEST VIRGINIA

MERCER COUNTY, WEST VIRGINIA

TABLE OF CONTENTS

I. INTRODUCTION

A. General B. Surface Geology and Physiography

II. SUBSURFACE GEOLOGY

A. Embankment and Foundation B. Centerline of Outlet Structure C. Auxiliary Spillway D. Borrow Area E. Toe Drain F. Water Supply

III. LOGS

IV. LIMITATIONS

V. REFERENCES

A. Site-Specific Studies B. General

October 28, 2021 Page ii Aterra-Schnabel JV

LIST OF FIGURES

Figure 1: Site Location Map Figure 2: Site Vicinity Map Figure 3: Boring Location Plan Figure 4: Geologic Map Figure 5: Structural Geology Map Figure 6: Soils Map Figure 7: Earthquakes Within 100 km Figure 8: Local Seismic Sources Figure 9: Geologic Profiles and Sections – Sheet 1 of 3 Figure 10: Geologic Profiles and Sections – Sheet 2 of 3 Figure 11: Geologic Profiles and Sections – Sheet 3 of 3

LIST OF TABLES

Table 1: Summary of Boring Locations, Depths, and Termination Information Table 2: Summary of Water Levels in Observations Table 3: Summary of Hydraulic Conductivity Testing in Rock

APPENDICES

Appendix A: Subsurface Drilling and Sampling Data Appendix B: Piezometer Installation Sketches and Development Records Appendix C: Results of Hydraulic Conductivity Testing in Soil Appendix D: Results of Hydraulic Conductivity Testing in Rock Appendix E: As-Built Drawings Appendix F: Historical Subsurface Drilling and Sampling Data

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I. INTRODUCTION

A. General

1. Background

The Brush Creek Watershed Site 15 Dam is located in Mercer County, West Virginia, about 9.5-miles northeast of the City of Bluefield and about 6-miles southwest of the City of Princeton, on the North Fork of Brush Creek. The site can be accessed via Dan Hale Reservoir Lake Road and Gobbler Run Road.

The latitude and longitude of the approximate midpoint of the embankment centerline are 37.3246° N and 81.1848° W, respectively. A Site Location Map is provided on Figure 1, and a Site Vicinity Map is provided on Figure 2.

The Site 15 Dam is categorized as a High Hazard dam by both West Virginia Dam Safety and NRCS and is subject to both West Virginia Dam Safety and NRCS requirements. In 2017, the West Virginia Conservation Agency (WVCA), with support from Gannett Fleming, completed the Draft Supplemental Watershed Plan – Environmental Assessment for the Rehabilitation of Brush Creek Site 15 Watershed Dam (Plan-EA) (Gannett Fleming, 2017b). The Plan-EA process included an assessment of the condition of the dam and its compliance with current West Virginia Dam Safety and NRCS standards. The results of engineering analyses performed by NRCS (2013) and Gannett Fleming (2017a) demonstrated that the dam did not meet current West Virginia Dam Safety and/or NRCS criteria for auxiliary spillway integrity and stability, filter compatibility between the existing drain materials and adjacent embankment fill materials, embankment rapid drawdown and seismic slope stability, and principal spillway riser seismic stability.

The Plan-EA includes the following rehabilitation design concepts to bring the dam into compliance with NRCS requirements and West Virginia Dam Safety requirements:

Installing a roller-compacted concrete cutoff wall in the auxiliary spillway.

Constructing stability berms on the upstream and downstream slopes of the embankment.

Constructing a new embankment toe drain system.

Extending the principal spillway conduit and reconstructing the impact basin.

Seismic retrofitting of the principal spillway riser.

Repairing the existing sluice gates.

Aterra-Schnabel JV was selected by NRCS in September 2018 to develop the rehabilitation design for Site 15.

2. Purpose

This report presents the results of the geotechnical investigation performed by Aterra-Schnabel JV at Brush Creek Watershed Site 15. The purpose of the geotechnical investigation was to:

Perform field testing and sampling of soil and rock materials to provide geologic, hydrogeologic, and geotechnical information to serve as a basis for the rehabilitation design.

Characterize the embankment and foundation materials to analyze embankment stability and evaluate seepage.

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Characterize the auxiliary spillway soils and rock to evaluate auxiliary spillway integrity and stability.

Evaluate the potential for dispersive soils at the site.

3. Scope of Services

The Specifications for Engineering Services – Dam Rehabilitation Design, Mercer County, West Virginia (Design Specifications), dated September 2018, and the Plan of Work – Dam Rehabilitation Design, Brush Creek Sites 9, 14, and 15, Mercer County, West Virginia, dated December 28, 2018, outline 12 tasks (items) for engineering services associated with the Brush Creek Site 15 dam rehabilitation. Item 4 includes the geotechnical investigation required to support the evaluation of the dam and subsequent rehabilitation design. This report is limited to the presentation of data gathered as a part of Item 4. The scope of services for Item 4 generally included:

Development of Preliminary and Final Geotechnical Investigation Plans Geotechnical investigation to include:

o Drilling 5 test borings through the embankment and foundation o Drilling 5 test borings in the auxiliary spillway and foundation o Drilling 1 test boring through the right auxiliary spillway training dike and 1 test boring on the slope to the right of the auxiliary spillway training dike o Drilling up to 2 auger probes near the principal spillway outlet o Installing up to 3 permanent piezometers in the embankment o Standard penetration testing (SPTs) and undisturbed sampling of soil o In situ hydraulic conductivity testing in soil and rock o Backfilling the test borings and auger probes, and site restoration o Drilling oversight

Preparation of Preliminary and Final Geotechnical Investigation Reports to, at a minimum, include:

o The investigation findings, geologic interpretations, and the geotechnical engineer’s professional conclusions and opinions pertaining to design, construction, and performance of works o Test boring logs, field test data, and photographs o Maps, profiles, and cross-sections

Item 5 of the Plan of Work includes laboratory testing of select soil and rock samples collected during the investigation. The laboratory test results are not included in this report but will be included in the Final Soil and Rock Laboratory Testing Report.

4. Land Surveying and Control

Gannett Fleming performed a topographic survey of the site between March 7 and 9, 2016. NRCS provided the results of that survey to us for use in this study. Survey control points were established by a static GPS session processed by the NGS-OPUS website (Gannett Fleming, 2017a). Based on these reference points from the Gannett Fleming survey, we used an unmanned aerial vehicle (UAV or drone) and 3DR’s aerial surveying software, Site Scan, to obtain horizontal coordinates of the as-drilled test boring locations. 3DR’s online Site Scan processing engine developed the orthophotograph and DEM

October 28, 2021 Page 3 Aterra-Schnabel JV used to obtain the as-drilled test boring elevations. Additional details on the results of the drone flight will be provided in the Aerial Photography and Mapping Report.

Horizontal coordinates in the Gannett Fleming survey reference NAD83, GEOID 12A, West Virginia State Plane Coordinate System, South Zone. Elevations in the Gannett Fleming survey and in this report are in feet and reference the North American Vertical Datum of 1988 (NAVD88). Elevations on the As-Built Drawings reference the National Geodetic Vertical Datum of 1929 (NGVD29). Locations and elevations should be considered no more accurate than the methods used to determine them.

5. Terminology

Descriptive nomenclature for dams is based upon one looking downstream in the direction of flow.

“Right” and “left” are referenced in this manner. The reservoir side is known as the upstream side with the opposite slope referred to as the downstream slope.

The abbreviation “EL” in this report represents elevation in feet.

6. Site Description

The Brush Creek Watershed Site 15 Dam is a multiple-purpose dam designed by the West Virginia SCS (now NRCS) and constructed in 1967, and is used for flood control, recreation, and municipal water supply. The contributing watershed is 1,719 acres (2.7 sq mi). The dam impounds an approximately 75-acre reservoir with a capacity of 1,516 acre-feet at its normal pool of EL 2489.5. At normal pool, the depth of water at the upstream toe of the embankment is about 53 ft. The top of dam storage capacity is approximately 3,080 acre-feet.

The dam is a 692-ft long zoned earthen embankment with a top width of about 22 ft, and a maximum height of about 76 ft measured from the invert of the principal spillway conduit to the low point on the crest of the dam. The embankment fill volume is approximately 236,100 cubic yards (Gannett Fleming, 2017b). According to the 2016 survey by Gannett Fleming, the minimum crest elevation of the dam is EL 2506.3. The upstream slope of the embankment ranges from about 2.5H:1V to 2H:1V. The downstream slope is approximately 2.5H:1V. The embankment consists of a low-permeability core (Section No. 1) located upstream of the embankment centerline, an upstream shell and transition zone downstream of the core (both designated as Section No. 2), and a downstream shell (Section No. 3).

A 4-ft wide seepage collection drain system is located 100-ft downstream of the embankment centerline at the downstream edge of Section No. 2. The As-Built Drawings provided by NRCS (USDA-SCS, 1967, Appendix E) indicate the filter material is a blend of sand and gravel that extends up to 4 ft into rock. The seepage collection drain system includes an 8-inch diameter perforated CMP that outlets downstream of the principal spillway outlet.

According to the As-Built Drawings, the principal spillway consists of a covered-top reinforced concrete riser that is about 72-ft tall. The inside dimensions of the riser are about 2 ft by 6 ft. The invert of the riser crest is at the normal pool elevation of the reservoir: EL 2489.5. The riser conveys flows from the reservoir through the embankment via an approximately 368-ft long, 24-inch diameter RCP that outlets into a reinforced concrete impact basin. The invert of the principal spillway outlet is EL 2430.4. The principal spillway conduit was constructed with a concrete cradle founded on rock and includes reinforced

October 28, 2021 Page 4 Aterra-Schnabel JV concrete anti-seep collars along its length. The reservoirs drain release are controlled by using an 18-inch slide gate located at the base of the riser.

The auxiliary spillway is a 125-ft wide vegetated earth auxiliary spillway constructed in the left abutment.

The auxiliary spillway has a 20-ft long (upstream to downstream) control section at EL 2496.3, approximately 10 ft below the minimum top of the dam elevation. Both the approach channel and exit channel of the auxiliary spillway slope away from the control section at about a 2 percent grade. Auxiliary spillway flows drain into the North Fork of Brush Creek.

7. Previous Subsurface Explorations by Others

The following subsurface explorations have been performed at Site 15:

1963 Investigation by SCS: Numerous test pits and test borings were performed across the site in 1963 to support the original design of the dam. Results and findings are summarized in a memorandum dated June 27, 1963, and the soil mechanics memorandum dated August 23, 1963. Test pit and test boring logs are included on the 1967 As-Built Drawings.

2013–2014 Investigation by NRCS: This investigation was performed to support a planning level evaluation of the existing structure. Four test borings were performed in the dam along a section near the principal spillway conduit, and 3 test borings were performed within the auxiliary spillway. Borings were advanced with hollow-stem augers and NX rock coring. Soil sampling intervals in the embankment ranged from 5 ft to continuous. The borings drilled in the auxiliary spillway included continuous soil sampling. Rock coring was performed only in the auxiliary spillway borings and included continuous sampling. The locations of these borings are shown on the Boring Location Plan on Figure 3, and the Boring Logs are included in Appendix F.

2016 Investigation by Gannett Fleming: Three test borings were performed along a section of the embankment near the left abutment to obtain additional geotechnical data to support the planning level study. Borings were advanced with hollow-stem augers and NX rock coring.

Continuous sampling was performed in both soil and rock. Piezometers were installed in each boring upon completion of sampling and testing. The locations of these borings/piezometers are shown on Figure 3, and the Boring Logs are included in Appendix F.

8. Geotechnical Investigation Program

We performed a geotechnical investigation and field-testing programs between February 26 and March 20, 2019, to further characterize the subsurface conditions at the site in support of the rehabilitation design. This program included subsurface drilling, sampling, and in situ testing. The subsurface investigation methods are presented in the subsections below, and the investigation results are discussed in Section II.

Prior to beginning the subsurface exploration, we presented the proposed exploration and field-testing approach and procedures in the Geotechnical Investigation Plan, Brush Creek Sites 9, 14, and 15, Mercer County West Virginia. This plan was reviewed and approved by the NRCS and finalized on March 28, 2019.

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a) Subcontractors

Our drilling subcontractor, Horn and Associates, Inc. (H&A) of Winchester, Kentucky, performed subsurface drilling. Drilling was completed using track-mounted CME-55 and Diedrich D-50 drill rigs. To access the embankment slope borings, the drill rig was anchored to a John Deere 650K XLT bulldozer positioned at the embankment crest, and the drill rig mast was mechanically locked at the complementary angle to produce a vertical boring. The drilling, sampling, and in-situ testing were performed in general accordance with current, applicable ASTM procedures, as described in the following subsections.

b) Subsurface Drilling and Sampling

(1) General

The geotechnical investigation program included the following test borings:

Two borings drilled from the crest of the dam to target embankment Section Nos. 2 and 3 (Borings B-51 and B-52).

One auger probe drilled from the crest of the dam for piezometer installation (B-51A).

Two borings drilled in the downstream toe of the embankment (B-651 and B-652).

One boring drilled in the upstream slope of the dam to target embankment Section No. 1 and the interface between embankment Section Nos. 1 and 2 (B-653).

One auger probe drilled on the left side of the principal spillway outlet (AP-351).

Five borings drilled in the auxiliary spillway (B-251, B-252, B-254, B-255, and B-257).

Two borings drilled in the right auxiliary spillway training dike (B-253 and B-256).

Test borings were numbered as follows:

Centerline of embankment: B-5x Auxiliary spillway: B-25x Principal spillway outlet: AP-35x Embankment slopes and toes: B-65x

The locations, depths, and termination information for the borings are summarized in Table 1. Boring locations are shown on Figure 3.

A licensed professional geologist from Aterra-Schnabel JV either observed or supervised the drilling, sampling, and borehole logging. Additional support personnel included experienced geotechnical engineers. A geologist from NRCS was also present during portions of the investigation.

(2) Test Borings

Soil drilling was performed using 4¼-inch inside diameter (ID) hollow-stem augers. Soil samples were obtained with either a 2-inch outside diameter (OD) split-barrel sampler, 3-inch OD split-barrel sampler, or 3-inch ID Shelby tube. At select locations, where low recoveries were due to the presence of gravel particles, the 2-inch and 3-inch OD samplers were used in an alternating sequence. In general, each Shelby tube sampling interval consisted of pushing two Shelby tubes to increase the likelihood of obtaining sufficient relatively undisturbed samples for laboratory testing.

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Auger Probe AP-351 was performed by drilling without sampling until auger refusal using 2¼-inch OD solid-stem augers.

Due to concerns of heaving sands in the embankment foundation, water was added to Boring B-51 prior to advancing beyond 10 ft above the estimated top of foundation soils. The water level in the test boring was generally maintained above the estimated pre-drilling piezometric surface to develop a slight downward gradient.

Rock coring and sampling were performed using an HQ triple-tube wireline system, diamond rotary coring bit, and tube core barrel.

Test boring backfill material consisted of cement/bentonite grout mixed with about 30 gallons of water to one bag (94 pounds) of Type I/II Portland cement and about 25 pounds of bentonite powder. This material was selected in order to reduce the potential for vertical movement of water along the borehole sidewalls and to reduce the potential for settlement at the top of the test boring. The grout was placed by tremie pipe to the bottom of test borings. After allowing time for the backfill materials to settle, the test borings were topped off as necessary with additional grout or bentonite chips. Backfill methods for each test boring are shown on the test boring logs and borehole backfill records included in Appendix A.

Areas disturbed by the drilling equipment were strawed and seeded in accordance with NRCS requirements upon completion of the investigation.

Aterra-Schnabel JV personnel logged the borings and described the materials using the Unified Soil Classification System and Schnabel’s Descriptive Criteria for Rock (Appendix A), in general accordance with ASTM D2488. Rock Quality Designation (RQD) and Recovery (REC) for each rock core run were recorded prior to sample transport.

Appendix A includes additional details on subsurface exploration procedures, sampling protocols, soil identification, rock core descriptions, test boring logs, undisturbed sampling records, and rock core photographs.

c) Piezometer Installation and Development

(1) 2019 Geotechnical Investigation Piezometers

H&A installed two permanent piezometers at Site 15. One open standpipe piezometer was installed in Boring B-51A in the embankment fill material (Section No. 2), and one capped open standpipe piezometer was installed in Boring B-651 in bedrock. Each piezometer was constructed with 2-inch diameter Schedule 40 PVC pipe and one 5-ft, 0.01-inch well screen. Global Drilling Suppliers, Inc. #5 sand was placed around the riser pipe to about 1 to 2 ft above the well screen. Then, a 2-ft seal of ¼-inch coated bentonite pellets was placed above the sand. Cement/bentonite grout, with a mixture of about 30 gallons of water to one bag (94 pounds) of Portland Type I/II cement and about 25 pounds of bentonite powder, was placed by tremie pipe from the top of the bentonite seal to about 1.3 to 2 ft bgs. A concrete pad encasing a flush-mount steel monitoring well cover was installed at the ground surface. A glycerin-filled pressure gage was installed on the Boring B-651 piezometer due to artesian conditions observed after drilling.

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The Boring B-51A piezometer was developed by bailing until the piezometer ran dry. Development of Piezometer B-51A was terminated after the observed water was clear to light brown after bailing 20 well volumes. One well volume of clear water with silt was bailed from the Boring B-651 piezometer; however, the Boring B-651 piezometer was not completely developed by bailing due to artesian flow from the top of the piezometer.

Details of piezometer installation and development are included in Appendix B.

(2) 2016 Gannett Fleming Subsurface Investigation Piezometers

Gannett Fleming (2017a) installed three sets of piezometers in the embankment during the 2016 subsurface investigation. Two nested piezometers were installed in the embankment crest and downstream slope borings (GF-1 and GF-2, respectively), and a single piezometer was installed at the embankment toe (GF-3). The upper piezometer screen in Boring GF-1 (GF-1S) was installed in embankment fill from 51 to 53 ft below ground surface (bgs), and the lower piezometer screen in Boring GF-1 (GF-1D) was installed in bedrock from 91.5 to 93.5 ft bgs. The upper piezometer screen in Boring GF-2 (GF-2S) was installed in embankment fill from 32 to 34 ft bgs, and the lower piezometer screen in Boring GF-2 (GF-2D) was installed in bedrock from 53.5 to 55.5 ft bgs. The piezometer screen in Boring GF-3 was installed in alluvial soils from 8 to 10 ft bgs.

The piezometers were constructed with 1-inch diameter PVC pipe and 2-ft long well screen. A sand filter pack was placed around the well screen. Then, a bentonite seal was placed above the sand.

Cement/bentonite grout was placed from the top of the bentonite seal to the surface. A steel riser encasing the piezometer(s) was also installed. Piezometer construction logs are provided in Appendix F, and additional details are included in the Gannett Fleming report (2017a). We were not present during the installation of these piezometers and cannot confirm that the piezometers were constructed as recorded. It is not clear whether these piezometers were developed during the 2016 investigation.

d) Water Level Observations

Aterra-Schnabel JV field staff measured water levels in the test borings using an electronic water level meter. Water levels were typically measured several times in each boring, including during drilling, after completion of drilling, at the beginning and end of each day, and immediately prior to grouting. Water levels in the Aterra-Schnabel JV and Gannett Fleming permanent piezometers were measured multiple times after the completion of drilling activities. Select water level observations are included on the test boring logs in Appendix A, and all water level observations are included in Table 2. Additional discussion on the water level observations is included in Section II.

Aterra-Schnabel JV field staff also recorded water levels in the existing 2016 Gannett Fleming piezometers several times during the geotechnical investigation. The recorded water levels are given in Table 2.

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e) Field and In Situ Testing

(1) Summary

The testing performed in the field on in situ materials is presented in the following subsections and includes:

139 SPTs in soil 1 hydraulic conductivity test using piezometers in soil (slug and well recovery tests) 6 hydraulic conductivity pressure tests in rock (packer tests) 1 hydraulic conductivity test in rock (falling head test)

An Aterra-Schnabel JV representative observed the sampling and testing, and recorded the data presented herein.

(2) Standard Penetration Tests

H&A performed the SPTs in soil using a 24-inch long, 2-inch OD split-spoon sampler or a 3-inch OD split-spoon sampler to collect soil samples and to measure the relative penetration resistance of soils.

Samples taken from the embankment were collected continuously during drilling, and samples taken from the auxiliary spillway were collected at 5-ft intervals. The number of blows required to drive the split-spoon sampler four consecutive 6-inch increments using a 140-pound hammer falling 30 inches was recorded. These SPT “N-values,” which are shown on the test boring logs in Appendix A, are defined as the sum of the second and third 6-inch blow count intervals. Additional details on the SPTs are provided in Appendix A.

(3) Hydraulic Conductivity Testing Using Piezometers (Slug Tests)

Hydraulic conductivity testing of soil (slug and well recovery testing) was completed in the Boring B-51A piezometer. Due to slow static water level recovery in this piezometer after well development, one falling head test with a water slug was performed to calculate the hydraulic conductivity of the soil. A summary of the results is included in Section II. Additional details and test results are included in Appendix C.

(4) Hydraulic Conductivity Testing in Rock

Hydraulic conductivity testing in rock included packer testing and falling head testing in select borings.

Both methods are presented and summarized in Section II. Additional details and test results are included in Appendix D.

B. Surface Geology and Physiography

1. Physiographic Area

The project site is located within the Appalachian Plateau Physiographic Province of West Virginia. The Appalachian Plateau spans the majority of the western part of the state and is bounded by the Valley and Ridge Province to the east. The Appalachian Plateau is generally characterized by flat-lying sedimentary rocks and rugged topography carved by streams (WVGES, 2017).

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2. Topography

According to the 2016 Gannett Fleming survey, the left abutment slope above the auxiliary spillway ranges from approximately 3H:1V to 2.5H:1V, and the slope of the right abutment ranges from approximately 3.5H:1V to 5H:1V. The floodplain is about 310-ft wide immediately downstream of the embankment and widens to roughly 450-ft downstream of the auxiliary spillway exit channel.

3. Geologic Formations and Surficial Deposits

a) Regional Geology

Several regional geology maps, at different scales, have been completed for the Bluefield, Mercer County, West Virginia area:

1) Reger, D.B. and Price P.H. (1925). Map II Mercer County showing General and Economic Geology, West Virginia Geological Survey, scale 1:62,500.

2) Cardwell, D.H., Erwin, R.B., and Woodward, H.P. (1968). Geologic Map of West Virginia. West Virginia Geological and Economic Survey, Map 1, scale 1:250,000 (Figure 4).

3) Beuthin, J.D., Blake Jr., B.M., and Rader, E. (2000). Bedrock Geologic Map of the Bluefield 7.5’ Quadrangle, West Virginia – Virginia. West Virginia Geological and Economic Survey Open File Publication OF-0003, scale 1:24,000 (Figure 5).

The regional underlying geology in the vicinity of the project is described below using geologic formation names from Cardwell et al. (1968), and Beuthin et al. (2000). The regional geology consists of upper Mississippian-aged sedimentary rock of the Mauch Chunk Group, detailed below from youngest to oldest with approximate unit thickness.

The Mauch Chunk Group is subdivided into four formations:

Bluestone Formation (400–800 ft) is a variable sequence of mudstone, shale, siltstone, and sandstone, with discontinuous beds of limestone and siderite (iron) nodules.

o Unnamed upper layer consists of red and green mudstone, siltstone, shale, and very fine-grained sandstone capped by a claystone.

o The Glady Fork Sandstone is a light gray to light brown, weathering light brown to brown and greenish brown, fine to coarse grained silty, massive, and may contain carbonaceous films and flakes.

o The Pride Shale is dark gray to tan shale, siltstone, and sandstone with some lenses of siderite nodules.

Princeton Formation (20–50 ft) is a sequence of sandstone, conglomerate, silty shales, and silty mudstone, and may contain a massive basal sandstone.

Hinton Formation (800–1,350 ft) is red, green, and medium-gray mudstone, sandstone, and limestone, including the Avis Formation, with limited occurrences of coal.

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b) Site Geology

The shales and sandstones of the Bluestone Formation are present at Site 15. The 1963 SCS geology report states that the primary rock type is shale, which was characterized as dark, soft, iron stained, and clayey. The general orientation of the shale bedding (strike and dip) is about N35°E, 9°SE on the right abutment to dipping at 14°SE on the left abutment. Outcrops of the overlying Glady Fork Sandstone are present on the right abutment, upslope of the dam (USDA-SCS, 1963a).

c) Soil Survey Mapping

A portion of the NRCS Soil Survey Map for Mercer County, West Virginia, within the vicinity of the site (NRCS 2017), is shown on the Soils Map (Figure 6). According to this map, the primary series of soils at the site are the Cateache-Berks soils, which consist of well drained soils formed in residual shale, sandstone, and siltstone. At the site, there are three Cateache-Berks sub-units:

Cateache-Berks channery silt loam (map unit CeD3) – 15 to 35 percent slopes, found along slopes, ridges, and structural benches

Cateache-Berks channery silt loam (map unit CeF) – 35 to 70 percent slopes, found along slopes and ridges

Cateache-Berks channery silt loam (map unit CeF3) – 35 to 70 percent slopes, found along slopes and ridges

Orrville-Lobdell soils (map unit OI) are found within the floodplain downstream of the dam. This unit consists of silty alluvium derived from sandstone, shale, limestone, and siltstone.

4. Structure

According to Reger and White (1926), severe folding of the Mississippian-aged strata has occurred in the project area. The Hurricane Ridge Syncline is a long, deep basin that spans from northeast to southwest across the southern part of Mercer County (Figure 4). The basin is generally asymmetrical where the northwest limb slopes gently compared to the steep and often overturned southeastern limb. Site 15 is located on the northwest limb roughly 1.5-miles northwest of the Hurricane Ridge Syncline fold axis (Figure 5).

The 1963 SCS geology report noted the project site structural geology using shale bedrock orientations.

The general orientation of the shale bedrock was measured striking N35°E, and dipping between 9°SE and 14°SE. This orientation correlates to the structural measurements shown on the Beuthin et al. (2000) geology map who mapped nearby Bluestone Formation bedrock with strikes trending northeast-southwest, with dips ranging between 8 and 10 degrees to the southeast (Figure 5).

5. Evidence of Landslides, Seepage, or Springs

Based on our review of the USGS (2016) Bluefield Quadrangle at a scale of 1:24,000, as well as our observations, there is no evidence of landslides at the site.

We did not observe any seepage or springs during the geotechnical investigation. Slight artesian conditions were observed in one of the borings drilled at the downstream toe of the embankment (B-651), which are discussed in Section II. Gannett Fleming observed wet areas along the downstream toe of the

October 28, 2021 Page 11 Aterra-Schnabel JV embankment during their March 2016 inspection (Gannett Fleming, 2017a), but these wet areas were not observed during our investigation.

6. Sediment and Erosion

An evaluation of reservoir sedimentation and erosion was not included in this geotechnical investigation.

7. Downstream Channel Stability

Evaluation of the downstream channel was not included in this investigation. Although the proposed replacement outlet structure is expected to have a slightly larger footprint than the existing outlet structure, it is not expected to negatively impact the downstream channel. Additionally, the principal spillway conduit diameter is not expected to be modified at this time; therefore, discharge characteristics will essentially remain the same as the existing conditions.

8. Seismicity

The seismicity at Brush Creek Site 15 is typical of the Central and Eastern United States (CEUS). The CEUS is a stable, intraplate region that is not nearly as active as the Western United States (WUS) that borders a plate boundary; however, much less attenuation occurs with CEUS earthquakes compared to those in the WUS because of the thick, denser crust in the CEUS. Therefore, earthquakes of similar magnitude will impact a much larger region in the CEUS (USGS, 2018).

A literature review of regional seismicity was performed to identify seismogenic sources in the area and locations of significant past earthquakes. Figure 7 shows the locations of magnitude 4 (M4.0) or greater earthquakes within a 100-km radius of the project site that are part of the CEUS Seismic Source Characterization Model earthquake catalog (CEUS-SSCn, 2012). These events include:

Mb4.9 – 4/29/1852 – Wytheville, Virginia Mb4.6 – 5/3/1897 – Pulaski County, Virginia Mb5.8 – 5/31/1897 – Pearisburg, Virginia Mb4.3 – 11/5/1898 – Pulaski County, Virginia Mb4.3 – 11/25/1898 – Pulaski County, Virginia Mb4.4 – 2/13/1899 – Pulaski County, Virginia Mb4.3 – 11/20/1969 – Mercer County, West Virginia Mb4.7 – 6/19/1976 – Welch, West Virginia Mb 4.1 to 4.3 – 4/14/1988 to 11/23/2006 – Mining Collapse/Blasting Events

Most of these earthquakes occurred before the creation of the modern-day seismic network; therefore, there is a great degree of uncertainty associated with their location and magnitude. Additionally, the body wave magnitude for events prior to 1976 was estimated from the intensity of shaking that was reported in the area.

Brush Creek Site 15 is located in an area of moderate seismicity for the CEUS and is near several seismic source zones as shown in Figure 8. In areas where active faults cannot be identified such as the CEUS, seismic source zones are developed to capture the seismicity of the region. Brush Creek Site 15 is in the vicinity of the Eastern Tennessee Seismic Zone, Central Virginia Seismic Zone, and the Giles

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County Seismic Zone. The Giles County Seismic Zone is within 100 km of the site and is believed to be the source of the 1897, 1898, 1899, and 1969 events. Following the May 3, 1897, foreshock, the May 31, 1897, event was one of the largest observed in the CEUS. The earthquake was felt from Georgia to Pennsylvania, and from the coast to Indiana. Chimneys fell and the flow in springs changed during the event (VDEM, 2018).

The USGS Quaternary Fault and Fold Database identifies the Giles County Seismic Zone as the Pembroke Faults; however, the Pembroke Faults are defined as Class B faults. Class B faults have geologic evidence that either demonstrates the existence of a fault or suggests Quaternary deformation, but either: (1) the fault might not extend deeply enough to be a potential source of significant earthquakes, or (2) the evidence is not strong enough to assign it to Class A. Class A faults are either exposed for mapping or can be inferred from liquefaction or other deformational features.

Bollinger and Wheeler (1998) first theorized that the seismicity in the Giles County Seismic Zone was potentially caused by the reactivation of faults that developed in the Late Proterozoic or early Paleozoic era. Faults were then discovered during excavations for landfill material in 1992, but it is unclear whether the faults are tectonic or the result of solution collapse. Regardless of the specific tectonic mechanism, the Giles County Seismic Zone contributes to the seismic hazard at Brush Creek Site 15 (Wheeler, 1998).

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II. SUBSURFACE GEOLOGY

Several distinct geologic material units were identified at the site during the geotechnical investigation program. Project-specific “stratum” names have been assigned to these materials based on visual descriptions and the results of field testing in order to characterize the materials in terms of their general geologic and engineering properties.

The geologic materials observed at the site include:

Fill material placed during construction of the embankment dam (Section Nos. S1 through S3 and Stratum F)

Alluvial material transported by the stream and deposited in and near the former stream channel (Stratum A)

Residual soil formed from chemical weathering of in-place rock (Stratum R) Disintegrated Rock formed by the near-complete weathering of in-place rock, but still maintaining a relatively high density and relict structure of the parent rock (Stratum D) Bedrock (Stratum B).

The characteristics of the geologic materials encountered in the test borings are discussed in the following sections. Detailed descriptions of these materials and strata designations are also included on the test boring logs in Appendix A. The Standard Penetration Test results (N-values) listed below and on the test borings logs are the values recorded in the field.

The geologic material descriptions given below and elsewhere in this report are based on field classifications. The test boring logs included in this report are considered draft logs. The field classifications will be revised based on laboratory test results, and updated test boring logs will be submitted with the final geotechnical engineering report. Detailed laboratory test results are included in the Final Soil and Rock Laboratory Testing Report dated June 19, 2019.

Subsurface profiles and sections of the embankment and auxiliary spillway are included on Figures 9 through 11.

A. Embankment and Foundation

1. Test Borings

Borings B-51 and B-52 were drilled vertically from the embankment crest through the transition zone and shell (Section Nos. 2 and 3, respectively). The borings were drilled up to 77 ft through embankment fill and up to 8 ft through foundation soils before encountering bedrock. The borings were then advanced up to 57 ft into rock. In addition, one vertical auger probe, B-51A, was drilled approximately 5 ft to the right of Boring B-51 for the installation of an open standpipe piezometer. The total depth of these borings ranged from 76 ft (B-51A) to approximately 142 ft (B-51).

Borings B-651 and B-652 were drilled near the downstream toe of the embankment. The borings were drilled up to about 8 ft through fill material and up to about 9 ft through foundation soils before encountering bedrock. The two borings were then advanced 10 ft into rock. The total depths of these borings were 26 ft (B-651) and approximately 18 ft (B-652).

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Boring B-653 was drilled vertically through the embankment core and upstream shell (Section Nos. 1 and 2, respectively). This boring was drilled through 69 ft of embankment fill material and 7 ft of foundation soils before encountering bedrock. The boring was then advanced 10 ft into rock. The total depth of this boring was 86 ft.

2. Generalized Subsurface Stratigraphy

The following subsections provide a brief description of the subsurface soil and rock strata encountered in the embankment and foundation.

a) Embankment Fill (Sections S1 through S3)

These soils generally include variable amounts of clay, silt, sand, and gravel, and may contain rock fragments, root fragments, organics, and coal. These materials were divided into three sub-strata corresponding to their section number shown on the As-Built Drawings (1967). The materials encountered in each embankment section are described in more detail in the following paragraphs.

S1 Section No. 1, Core – Described as “clayey material (CL)” on the As-Built Drawings. This material was sampled primarily as fine grained soils (material with >50% passing No. 200 sieve) classifying as LEAN CLAY (CL) with varying amounts of sand and gravel. Samples were moist with varying shades of brown, and included rock, root, and coal fragments. Section No. 1 materials were encountered in Boring B-653 between approximately 22 and 69 ft bgs. These depths correspond to a top elevation of EL 2486 and a bottom elevation of EL 2439. SPT N-values ranged from 11 blows per foot to 28 blows per foot, with an average value of 19 blows per foot.

S2 Section No. 2, Transition and Upstream Shell – Described as “clayey sand and silty sand (SC, ML-CL)” on the As-Built Drawings. These materials were sampled as both fine grained and coarse grained (material with <50% passing No. 200 sieve) soils. Fine grained soils were sampled as LEAN CLAY (CL), Gravelly LEAN CLAY (CL), Sandy LEAN CLAY (CL), and FAT CLAY (CH), each with varying amounts of sand and gravel. The samples were moist with varying shades of brown and gray and included rock fragments. Coarse grained soils were predominantly sampled as Clayey GRAVEL (GC) and Clayey SAND (SC), each with varying amounts of fines, sand, and gravel. The samples were moist to wet with varying shades of brown and gray and included rock fragments. SPT N-values ranged from 9 blows per foot to 32 blows per foot, with an average value of 17 blows per foot. Thin layers of Well Graded SAND (SW) and Well Graded SAND with Clay (SW-SC) were observed in Boring B-51.

Section No. 2 materials were encountered in Borings B-51 and B-52 to depths of approximately 65 to 77 ft below the crest of the dam. These depths correspond to bottom elevations of EL 2443 and EL 2431, respectively. Based on our observations during drilling, the Section No. 2 and Section No. 3 materials are very similar. We estimate the top of Section No. 2 is just below the crest of the dam (EL 2508) in Boring B-51 and 4 ft below the crest of the dam (EL 2504) in Boring B-52. According to the As-Built Drawings, the top of Section No. 2 is about 8 ft below the crest of the dam (EL 2500).

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Section No. 2 materials were also encountered in Boring B-653 from the ground surface to approximately 22 ft bgs (EL 2478). The observed depth to the bottom of the Section No. 2 materials in Boring B-653 generally agrees with the anticipated depth to the bottom of the Section No. 2 materials based on the As-Built Drawings.

S3 Section No. 3, Downstream Shell – Described as “shale and siltstone” on the As-Built Drawings.

These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as LEAN CLAY (CL) with sand and containing gravel. Coarse grained soils were sampled as Poorly Graded GRAVEL with Silt and Sand (GP-GM). The samples were moist with varying shades of brown and gray and included rock fragments. SPT N-values ranged from 9 blows per foot to 17 blows per foot, with an average value of 13 blows per foot. Section No. 3 materials were encountered in Boring B-52 to a depth of approximately 4 ft below the crest of the dam (EL 2504). Materials matching the Section No. 3 description on the As-Built Drawings were not observed in Boring B-51.

b) General Fill (Stratum F)

This stratum includes fill material observed downstream of the embankment in Borings B-651 and B-652.

These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as Sandy SILT (ML) containing gravel. Coarse grained soils were sampled as Clayey SAND (SC) and Silty SAND (SM) with varying amounts of fines and gravel. The samples were moist with varying shades of brown and gray and included rock fragments. These materials were encountered from the ground surface to depths ranging from approximately 6 to 8 ft bgs. SPT N-values ranged from 3 to 33 blows per foot, with an average value of 10 blows per foot.

c) Alluvium (Stratum A)

Alluvial soils were encountered below the embankment fill soils in Borings B-51 and B-653 at a depth of approximately 77 ft below the crest of the dam, with thicknesses ranging from approximately 4 to 6 ft.

Alluvium was also observed below the general fill soils in Boring B-651 approximately 8 ft bgs, with a thickness of approximately 3 ft. These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as Sandy LEAN CLAY (CL). Coarse grained soils were sampled as Clayey SAND (SC), Poorly Graded SAND (SP), and Poorly Graded SAND with Silt (SP-SM), with varying amounts of fines. The samples were wet with varying shades of gray and included rock fragments and organics. SPT N-values ranged from 6 to 21 blows per foot, with an average value of 14 blows per foot.

d) Residuum (Stratum R)

Residual soils were encountered in all embankment borings, with the exception of Boring B-51A, beneath the fill and/or alluvial soils. The residual soils were encountered between approximately 65 to 83 ft below the crest of the dam (EL 2425 to EL 2443), with thicknesses ranging from approximately 2 to 6 ft. These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as LEAN CLAY (CL), Sandy LEAN CLAY (CL), and FAT CLAY (CH), with varying amounts of sand and gravel. Coarse grained soils were sampled as Clayey GRAVEL (GC). The samples were typically moist with varying shades of brown and gray and included rock fragments. SPT N-values ranged from 8 to 100 blows per foot, with an average value of 39 blows per foot.

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e) Disintegrated Rock (Stratum D)

The boundary between soil and rock is typically not sharply defined. A transitional zone of disintegrated rock material is normally found overlying bedrock and is a product of in-place chemical weathering of the parent material. The material in this zone typically retains the texture and structure of the parent rock.

Disintegrated Rock is defined as residual material with a SPT N-value between 60 blows per foot and SPT refusal. The contact between the disintegrated rock and rock can be quite irregular and erratic, even over short horizontal distances. Lenses and boulders of hard rock and zones of disintegrated rock are commonly present within the residual soil above the general bedrock level. Disintegrated rock or soil seams may be present beneath relatively unweathered bedrock, which is commonly developed by weathering along bedrock fractures.

Disintegrated rock was observed in all embankment borings, with the exception of Boring B-51A, beneath the residual soils, with thicknesses ranging from less than 1 ft to almost 3 ft.

f) Bedrock (Stratum B)

SPT refusal (50 blows for 6 inches or less penetration) was used to define top of rock. Shale was the primary rock type encountered below the embankment fill and foundation soils in all borings. Claystone was also observed in Boring B-652 at a depth of approximately 10 ft (EL 2429) and was approximately 4-ft thick. The shale and claystone are believed to be part of the Bluestone Group.

Shale was sampled as gray to dark gray, fresh to severely weathered, strong to very weak, and slightly to intensely fractured. Rock core recoveries (REC) ranged from 38 to 100 percent. Rock Quality Designation (RQD) ranged from 0 to 93 percent and generally increased with depth, although zones of relatively low RQD were observed between zones of relatively high RQD. The bedding was generally very thin and dipping approximately 0 to 10 degrees. Two primary discontinuity sets were identified: one near-horizontal bedding joint set and one near-vertical joint set. Additional joints dipping between approximately 20 and 60 degrees were also identified. Severely to highly weathered zones were observed near bedding joints. The shale was also highly susceptible to slaking along bedding planes.

Claystone was sampled as gray, fresh to slightly weathered, weak to very weak, and moderately to intensely fractured. REC ranged from 73 to 100 percent, and RQD ranged from 42 to 50 percent.

3. Hydrogeologic Conditions

a) Water Level Observations

Several water level readings were recorded in the open boreholes during the field investigation.

Recorded water levels are presented in Table 2, and select water level measurements are shown on the test boring logs in Appendix A. The water level readings were generally within the expected range.

However, the water levels observed in the B-51A piezometer were deeper than expected, but this may be the result of the slow recharge rate from the low-permeability embankment fill material adjacent to the well screen. We will continue to monitor the water levels in the piezometers following the investigation to obtain longer-term water level readings to support analyses and design.

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Reported water levels should not be considered globally representative. It is anticipated that the observed water levels could have been affected by drilling procedures, and would be expected to fluctuate due to variations in precipitation, surface runoff, reservoir levels, and other factors.

b) Artesian Conditions

Artesian conditions were observed in Boring B-651 after piezometer installation, but were not encountered during soil drilling and…