Brush Creek Site 14 Final Geotech Invest Report_rev1 10-28-21.pdf

FINAL GEOTECHNICAL INVESTIGATION

REPORT, REVISION 1

Brush Creek Site 14 Dam Rehabilitation Mercer County, West Virginia

Prepared For:

NRCS – West Virginia

Aterra-Schnabel Reference 18proj007 Schnabel Reference 18C35013.03 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 14 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 October 28, 2021, 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 14 Dam Rehabilitation Design

October 28, 2021 Page i Aterra-Schnabel JV Project 18C35013.03 ©2021 All Rights Reserved

FINAL GEOTECHNICAL INVESTIGATION REPORT

BRUSH CREEK SITE 14 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 and Training Dike D. Borrow Area E. Toe Drain F. Water Supply

III. LOGS

IV. LIMITATIONS

V. REFERENCES

A. Site-Specific Studies B. General

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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 2 Figure 10: Geologic Profiles and Sections – Sheet 2 of 2

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 14 Dam is located in Mercer County, West Virginia, almost 9-miles northeast of the City of Bluefield and about 4-miles southwest of the City of Princeton on the North Fork of Brush Creek. The site can be accessed via Glenwood Park Road. The latitude and longitude of the approximate midpoint of the embankment centerline are 37.3350° N and 81.1557° W, respectively. A Site Location Map is provided on Figure 1, and a Site Vicinity Map is provided on Figure 2.

The Site 14 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 2011, NRCS, with support from Gannett Fleming, conducted an initial assessment of the dam’s condition and hydraulic performance.

Using this study as a baseline, NRCS completed the Final Supplemental Watershed Plan #8 – Environmental Assessment for the Rehabilitation of Brush Creek Site 14 Watershed (Plan-EA) in 2018.

The Plan-EA process included an additional 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 demonstrated that the dam did not meet current West Virginia Dam Safety and/or NRCS criteria for auxiliary spillway integrity and stability, and filter compatibility between the existing drain materials and adjacent embankment fill materials.

NRCS also indicated the following dam safety deficiencies and concerns:

Above ground utilities are present on the embankment, and a gravel access road is present in the auxiliary spillway.

The auxiliary spillway training dike length is insufficient to divert auxiliary spillway flows away from the toe of the embankment.

The seepage collection drain pipes are susceptible to corrosion.

The principal spillway riser configuration is susceptible to accumulation of debris and potentially does not meet current seismic design standards.

The principal spillway outlet configuration does not meet current design standards.

The embankment does not have adequate protection from wave erosion.

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

Replacing the existing principal spillway riser.

Installing a concrete cutoff wall in the auxiliary spillway.

Replacing the existing seepage collection drain system.

Leveling the dam crest.

Constructing a principal spillway outlet plunge pool.

Installing embankment wave protection.

Constructing/reconfiguring auxiliary spillway training dikes.

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Relocating the access road.

Removing existing utility infrastructure from the embankment.

Aterra-Schnabel JV was selected by NRCS in September 2018 to develop the rehabilitation design for Site 14.

2. Purpose

This report presents the results of the geotechnical investigation performed by Aterra-Schnabel JV at Brush Creek Watershed Site 14. 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.

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 14 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 3 test borings in the auxiliary spillway and foundation o Drilling 1 test boring through the right auxiliary spillway training dike and 1 test boring at the downstream toe 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

October 28, 2021 Page 3 Aterra-Schnabel JV 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 in November 2013 and January 2014. 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, 2014). 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. Ground surface elevations at the test boring locations were derived from the Gannett Fleming survey because all of the control points shown in Gannett Fleming’s survey were either buried or in the woods. 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 14 Dam is a multiple-purpose dam designed by the West Virginia SCS (now NRCS) used for flood control, recreation, and municipal water supply. The dam was constructed in 1966 and impounds an approximately 55-acre reservoir at its normal pool of EL 2420.2. The storage capacity at the auxiliary spillway crest is approximately 1,550 acre-feet, and the top of dam storage capacity is approximately 3,750 acre-feet (Gannett Fleming, 2011).

The dam is a 423-ft long zoned earthen embankment with a top width of about 14 ft, and a maximum height of about 40 ft measured from the invert of the principal spillway to the low point on the crest of the dam. The embankment fill volume is approximately 49,900 cubic yards (USDA-NRCS, 2018). According to the 2014 survey by Gannett Fleming, the minimum crest of the dam is EL 2444.8. The upstream slope of the embankment is approximately 3H:1V, both above and below an 8-ft wide wave berm located at about EL 2420. The downstream slope is approximately 2.3H:1V above a 25-ft wide toe berm located at

October 28, 2021 Page 4 Aterra-Schnabel JV about EL 2423. The slope below the toe berm averages about 3H:1V. The As-Built Drawings provided by NRCS (USDA-SCS, 1966, Appendix E) show that the embankment consists of upstream and downstream shells (Section No. 1), a low-permeability core (Section No. 2), and a rockfill berm at the downstream toe of the dam (Section No. 3).

According to the As-Built Drawings, a 3-ft wide seepage collection drain system is located approximately 77-ft downstream of the embankment centerline. The filter material is a blend of sand and gravel that is keyed into rock on the left side of the principal spillway conduit, and is keyed a minimum of 2 ft into natural sand on the right side of the principal spillway conduit. The seepage collection drain system includes two 8-inch diameter perforated CMPs that outlet downstream of the principal spillway outlet.

The principal spillway consists of an open-top reinforced concrete riser that is about 21-ft tall and contains multiple bays. The inside dimensions of the main bay are about 4 ft by 12 ft. The invert of the riser crest is at the normal pool elevation of the reservoir: EL 2420.2. The riser conveys flows from the reservoir through the embankment via an approximately 228-ft long, 48-inch diameter RCP that outlets into an excavated riprap-lined plunge pool. The principal spillway conduit was constructed with a concrete cradle founded on rock and includes reinforced concrete anti-seep collars along its length. To the right of the principal spillway conduit is a 16-inch diameter cast-iron water supply pipe that is controlled by an 18-inch sluice gate located about 4.5 ft below the riser crest. A second 18-inch water supply sluice gate is located 2 ft above the base of the riser. Reservoir drain releases are controlled using a 24-inch slide gate located at the base of the riser.

The auxiliary spillway is a 280-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 2432.6, approximately 14.1 ft below the minimum top of dam elevation. The approach channel of the auxiliary spillway slopes from the control section back toward the reservoir at about a 2 percent grade, and the auxiliary spillway exit channel slopes approximately 5 percent downstream away from the control section.

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 14:

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 the geology report dated August 27, 1963, and the soil mechanics memorandum dated October 21, 1963. Test pit and test boring logs are also included on the 1966 As-Built Drawings.

2013 Investigation by NRCS: Three test borings were completed in the dam to support the evaluation of the existing embankment. Four additional test borings were located within the auxiliary spillway channel to support an evaluation of the existing auxiliary spillway integrity.

Borings were advanced with hollow-stem augers and NX rock coring. The boring drilled from the embankment crest included soil sampling at 5-ft intervals, and the borings drilled from the embankment slopes and in the auxiliary spillway included continuous soil sampling. Rock coring was performed in the auxiliary spillway borings only and included continuous sampling. The

October 28, 2021 Page 5 Aterra-Schnabel JV locations of these borings are shown on the Boring Location Plan on Figure 3, and the Boring Logs are included in Appendix F.

8. Geotechnical Investigation Program

We performed geotechnical investigation and field testing programs between March 26 and April 2, 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.

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. 1 and 2 and foundation materials (Borings B-41 and B-42).

One auger probe drilled from the crest of the dam for piezometer installation (B-42A).

Two borings drilled in the downstream slope (B-641 and B-642) and one boring drilled in the upstream slope (B-643) of the dam to target Embankment Section No. 1 and the foundation materials.

Two auger probes drilled near the principal spillway outlet to evaluate the depth to rock (AP-341 and AP-342).

Three borings drilled in the auxiliary spillway (B-241, B-242, and B-244).

One boring drilled through the right auxiliary spillway training dike (B-243) and one boring drilled at the downstream toe of the auxiliary spillway training dike (B-245).

Test borings were numbered as follows:

Centerline of embankment: B-4x Auxiliary spillway: B-24x Principal spillway outlet: AP-34x

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Embankment slopes and toes: B-64x

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.

(2) Test Borings

Soil drilling in the B-series testing borings 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.

Auger Probes AP-341 and AP-342 were 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 Borings B-42, B- 642, and B-643 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 the 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.

Upon completion of the investigation, areas disturbed by the drilling equipment were strawed and seeded in accordance with NRCS requirements.

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.

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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

H&A installed two permanent open-standpipe piezometers at Site 14. One piezometer was installed in Boring B-42A in the embankment core trench fill material (Section No. 2), and one piezometer was installed in Boring B-641 in alluvial soils. 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 to 2.5 ft below ground surface (bgs). A concrete pad encasing a flush-mount steel monitoring well cover was installed at the ground surface.

The piezometers were developed by bailing until the piezometers ran dry. Development was terminated after the observed water was clear (B-42A) or after bailing 12 well volumes (B-641).

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

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 permanent piezometers were measured multiple times after the completion of piezometer installation. 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.

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:

87 SPTs in soil 2 hydraulic conductivity tests using piezometers in soil (slug tests) 6 hydraulic conductivity pressure tests in rock (packer tests) 2 hydraulic conductivity tests in rock (falling head tests)

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

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(2) Standard Penetration Tests

H&A performed the SPTs in soil using either 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 testing) was completed in the piezometers in Borings B-42A and B-641. Slug tests included both falling head and rising head tests.

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).

2. Topography

According to the 2014 Gannett Fleming survey, the left abutment slope above the auxiliary spillway ranges from approximately 3H:1V to 2H:1V, and the slope of the right abutment ranges from approximately 2H:1V to 1.5H:1V. The floodplain is about 160-ft wide immediately downstream of the embankment and widens to almost 400-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:

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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 7.5ꞌ 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.

b) Site Geology

The shales and sandstones of the Bluestone Formation are present at Site 14. The 1963 SCS geology report states that the primary surface units present at the site are the Lower Mud Shale, the Glady Fork Sandstone, and the Pipestem Shale. (These rock unit formation names mentioned in the 1963 SCS geology report are taken from Reger and Prince, 1925.) The underlying bedrock is weathered, moderately soft, brown sandy shale, and below the shale is a coarse grained, blue, moderately hard sandstone with limey cement. Prior to construction in 1966, outcrops of the Glady Fork Sandstone were observed on both abutments (USDA-SCS, 1963a). The general orientation of the bedding is striking northeast-southwest, with a dip of about 5 degrees toward the southeast (downstream) (USDA-NRCS, 2015a).

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c) Soil Survey Mapping

A portion of the NRCS Soil Survey Map for Mercer County, West Virginia, within the vicinity of the site (USDA-NRCS, 2017), is shown on the Soils Map (Figure 6). According to this map, the primary series of soils at the site are from the Orrville-Lobdell Complex (map unit OI) and the Dekalb-Gilpin-Jefferson Complex (map unit DgF). The Orrville-Lobdell Complex consists of well drained silty alluvium derived from sandstone, shale, limestone, and siltstone, and is typically present in the floodplain. The Dekalb- Gilpin-Jefferson Complex consists of well drained residual and colluvial soils derived from sandstone, shale, and siltstone, and is typically found along mountain slopes on 35 to 80 percent slopes. Lily Loam (map unit LlD) is also present at the site, which typically consists of residual soils derived from sandstone and is found along hillslopes on 15 to 25 percent slopes.

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 14 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 that the project site structural geology bedding orientation trends northeast-southwest and dips at 5 degrees toward the southeast. This orientation correlates to the structural measurements shown on the geology map by Beuthin et al. (2000), who mapped nearby Bluestone Formation bedrock with strikes trending northeast-southwest, and dips ranging between 4 and 8 degrees toward 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, and according to NRCS (2018), no seepage has been recorded in previous dam inspection reports.

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 principal spillway outlet structure is expected to have a slightly larger footprint than the existing principal spillway 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.

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8. Seismicity

The seismicity at Brush Creek Site 14 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:

Body wave magnitude (Mb) 4.9 – 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 14 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 14 is in the vicinity of the Eastern Tennessee Seismic Zone, Central Virginia Seismic Zone, and the Giles 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.

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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 14 (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 (Sections 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 will be included in the Final Soil and Rock Laboratory Testing Report. Subsurface profiles and sections of the embankment and auxiliary spillway are included as Figures 9 and 10.

A. Embankment and Foundation

1. Test Borings

Borings B-41 and B-42 were drilled vertically from the embankment crest through the shell (Section No. 1) and core (Section No. 2). The borings were drilled up to 48 ft through embankment fill and up to 2 ft through foundation soils before encountering bedrock. The borings were then advanced up to 30 ft into rock. The total depth of these borings ranged from approximately 44 ft (B-41) to 81 ft (B-42). In addition, one vertical auger probe, B-42A, was drilled approximately 5 ft to the left of Boring B-42 for the installation of an open standpipe piezometer in the core trench.

Borings B-641 and B-642 were drilled vertically through the downstream shell of the embankment (Section No. 1). The borings were drilled up to approximately 17 ft through fill material and up to approximately 10 ft through foundation soils before encountering bedrock. The borings were then advanced up to 15 ft into rock. The total depths of these borings were approximately 41 ft (B-641) and 30 ft (B-642).

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Boring B-643 was drilled vertically through the upstream shell of the embankment (Section No. 1). This boring was drilled through approximately 14 ft of embankment fill material and approximately 14 ft of foundation soils before encountering bedrock. The boring was then advanced 10 ft into rock. The total depth of this boring was approximately 38 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 borings.

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 fragments. These materials were divided into three sub-strata corresponding to their section number shown on the As-Built Drawings (1966). The materials encountered in each embankment section are described in more detail in the following paragraphs.

S1 Section No. 1, Shell – Described as “fine-grained silty sand (SM)” on the As-Built Drawings.

These materials were sampled as both fine grained (material with >50% passing No. 200 sieve) and coarse grained (material with <50% passing No. 200 sieve) soils. Fine grained soils were sampled as 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 sampled as Clayey SAND (SC) and Silty SAND (SM), each with varying amounts of fines and gravel. The samples were generally moist with varying shades of brown and gray and included rock fragments. Section No. 2 materials were encountered in Borings B-41 and B-42 to depths up to approximately 7 ft below the crest of the dam. Section No. 1 materials were also encountered in Borings B-641 through B-643 from the ground surface up to approximately 17 ft bgs. SPT N-values ranged from 3 to 16 blows per foot, with an average value of 9 blows per foot.

S2 Section No. 2, Core – Described as “fine-grained clayey material (CL)” on the As-Built Drawings.

These materials were sampled as fine grained soils classifying as LEAN CLAY (CL) and Sandy LEAN CLAY (CL), each with varying amounts of sand and gravel, and as coarse grained soils classifying as Clayey SAND (SC) with varying amounts of fines and gravel. Samples were moist with varying shades of brown and gray and included rock fragments. The top of Section No. 2 materials was observed in Borings B-41 and B-42 between approximately 7 and 6 ft bgs, respectively, and the bottom was observed at approximately 48 and 26 ft, respectively. SPT N-values ranged from 6 to 30 blows per foot, with an average value of 14 blows per foot. An approximately 2.5-ft thick layer of Poorly Graded SAND with Silt (SP-SM) with gravel was observed in Boring B-42 between approximately 14 and 16.5 ft below the crest of the dam.

S3 Section No. 3, Rockfill Toe – Described as “coarse-grained (rock) material (max size 12 inches)” on the As-Built Drawings. These materials were not observed in the downstream bench boring (B-642) or the boring drilled on the slope of the downstream toe berm (B-642).

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b) Alluvium (Stratum A)

Alluvial soils were encountered below the embankment fill soils in Borings B-641 through B-643 at depths ranging from approximately 9 to 17 ft bgs, with thicknesses ranging from approximately 6 to 10 ft. These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as LEAN CLAY (CL) with gravel. Coarse grained soils were sampled as Clayey SAND (SC), Poorly Graded SAND with Silt (SP-SM), and Well Graded SAND with Clay (SW-SC), with varying amounts of fines and gravel. The samples were moist to wet with varying shades of gray, and included rock and wood fragments. SPT N-values ranged from 2 to 14 blows per foot, with an average value of 7 blows per foot.

c) Residuum (Stratum R)

Residual soils were encountered in all embankment borings, with the exception of Boring B-42A, beneath the fill and/or alluvial soils. The residual soils were encountered between approximately 19 ft (B-642) and 48 ft (B-42) bgs, with thicknesses up to approximately 3 ft. These materials were sampled as both fine grained and coarse grained soils. Fine grained soils were sampled as Gravely LEAN CLAY (CL) and Sandy LEAN CLAY (CL), with varying amounts of sand and gravel. Coarse grained soils were sampled as Clayey GRAVEL (GC), Clayey SAND (SC), and Silty SAND (SM). The samples were typically moist with varying shades of brown and gray, and included rock fragments. SPT N-values ranged from 19 to 56 blows per foot, with an average value of 28 blows per foot.

d) 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 an 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 Borings B-42, B-641, and B-642 beneath the alluvial and/or residual soils with thicknesses up to approximately 1 ft.

e) Bedrock (Stratum B)

SPT refusal (50 blows for 6 inches or less penetration) was used to define top of rock. Sandstone and shale were encountered below the embankment fill and foundation soils in the embankment borings.

Sandstone was observed in Borings B-41, B-641, and B-642 below the foundation soils with top depths ranging approximately from 20 ft (B-642) to 29 ft (B-41) bgs. The sandstone was sampled as varying shades of brown and gray, fresh to severely weathered, very strong to very weak, and slightly to intensely fractured. Some of the sampled sandstone exhibited a strong reaction to hydrochloric acid. REC ranged

October 28, 2021 Page 16 Aterra-Schnabel JV from 37 to 100 percent, and RQD ranged from 13 to 93 percent. The bedding was dipping approximately 0 to 20 degrees.

Shale was observed below the sandstone in Borings B-41, B-641, and B-643 with top depths ranging approximately from 28 ft (B-643) to 41 ft (B-41) bgs. Shale was also observed in Boring B-42 at approximately 51 ft bgs. The shale was sampled as dark gray, fresh to severely weathered, strong to very weak, and moderately to intensely fractured. REC ranged from 50 to 100 percent, and RQD ranged from 63 to 95 percent. The bedding was generally very thin (i.e., less than 2.5 inches) and dipping approximately 0 to 10 degrees.

Two primary discontinuity sets were identified in the shale and sandstone: one near-horizontal bedding joint set, and one joint set dipping between approximately 30 and 50 degrees. Additional joints dipping between approximately 60 and 85 degrees were also identified. The joints were generally fresh to slightly weathered, planar, and smooth to slightly rough.

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 in the embankment borings were generally as expected.

Artesian conditions were not observed in these borings. We will continue to monitor the water levels in the piezometers to obtain longer-term water level readings to support analyses and design.

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) Hydraulic Conductivity Testing Using Piezometers (Slug Tests)

Slug tests were performed in the Boring B-42A piezometer, where the screened interval was installed in the embankment core trench material (Section No. 2) between approximately 43 and 48 ft bgs. The estimated hydraulic conductivity from the falling head and rising head test methods was 1.8x10-5 cm/s (5.9x10-7 ft/s) and 1.6x10-5 cm/s (5.2x10-7 ft/s), respectively.

Slug tests were performed in the Boring B-641 piezometer, where the screened interval was installed in alluvium between approximately 17 and 22 ft bgs. The estimated hydraulic conductivity from the falling head and rising head test methods was 1.8x10-5 cm/s (5.9x10-7 ft/s) and 2.2x10-5 cm/s (7.1x10-7 ft/s), respectively.

The slug test method and detailed results are presented in Appendix C.

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c) Hydraulic Conductivity Testing in Rock

Falling head and packer tests were performed in rock in the embankment borings. Both methods are presented in the following subsections. Additional details and test results are presented in Appendix D, and a summary of the results is listed in Table 3.

(1) Packer Testing

H&A performed packer testing in rock in Borings B-41, B-42, B-641, and B-642. Testing was performed at depths ranging from 21.8 to 80.8 ft bgs, using either single or double packer assemblies. The minimum recorded transmissivity value of 1 Lugeon was observed in Boring B-641 from 28.4 to 41.4 ft bgs, and the maximum recorded transmissivity value of 40 Lugeons was observed in Boring B-41 from

37.0 to 44.4 ft bgs.

(2) Falling Head Testing

Due to concerns of hydrofracturing the embankment/foundation interface, one falling head test was performed immediately beneath the foundation soils in Borings B-41 and B-42 in lieu of packer testing.

These tests were completed between 33.0 and 63.5 ft bgs. The hydraulic conductivity interpreted from the falling head tests in Borings B-41 and B-42 was approximately 1.1x10-5 cm/s (3.5x10-7 ft/s) and 2.0x10-5 cm/s (6.4x10-7 ft/s), respectively.

B. Centerline of Outlet Structure

1. Auger Probes

Two auger probes (AP-341 and AP-342) were performed near the existing principal spillway outlet structure to evaluate the depth to top of rock. Auger refusal was observed at approximately 4 ft bgs in AP-341 and 15 ft bgs in AP-342.

2. Generalized Subsurface Stratigraphy

The following subsections provide a brief description of the subsurface soils encountered near the outlet structure. Soil descriptions and strata breaks are based on auger cuttings observed during drilling.

a) General Fill (Stratum F)

This stratum includes non-embankment fill material. General Fill was encountered in both auger probes.

These materials were classified as moist Clayey SAND (SC) and Silty SAND (SM), with varying shades of brown and gray. The fill materials were encountered from the ground surface up to approximately 4 ft bgs, the depth of auger refusal in AP-341.

b) Residuum (Stratum R)

Residuum was encountered in Auger Probe AP-342 beneath the General Fill. These soils were classified as moist, brown Silty SAND (SM). The residual soils were approximately 12-ft thick.

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3. Hydrogeologic Conditions

Auger Probes AP-341 and AP-342 were dry upon completion. We did not obtain long-term water level readings because the auger probes were backfilled upon completion for safety.

Hydraulic conductivity testing in the auger probes was excluded from this program.

C. Auxiliary Spillway…