BB90 Geotechnical Data Report STEP 1 Pg 1-268.pdf

Big Bend Dam (SD01092) Missouri River, Fort Thompson, South Dakota

Geotechnical Data Report

Northwestern Division Omaha District

Date: October 2022

Status: 95% STEP 1

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Big Bend Relief Well Collector Pipe Replacement Omaha District i

Table of Contents

1. Disclaimer

2. Limitations

3. GDR Report Function

4. Overview of Report Contents

GEOTECHNICAL DATA REPORT

1. Disclaimer This Geotechnical Data Report (GDR) contains actual subsurface and other information that was collected at or adjacent to the Site by the Government, or for the Government, for use in preparing the

Contract Documents. This GDR attempts to contain only non-interpretative information. The GDR is a summary, the full-text non-interpretive documents, included in the non-interpretive appendices, are provided for completeness and for the information of the bidder. The Technical Data contained in this report upon whose accuracy the Contractor may rely are logs of borings, logs of trenches/test pits, geophysical information, contractor-reported lab results, SPT hammer results, original construction-era documents, recorded measurements of subsurface water levels, and contractor-reported optical camera images. If opinions, or interpretive or speculative non-factual comments or statements appear in the

GDR, such opinions, comments, or statements are not operative parts of the GDR and do not have contractual standing. Subject to that exception, the GDR is a Contract Document.

2. Limitations The Government does not represent that the boring and test pit/trench logs, field and laboratory test results and other available geologic information show the conditions that will be encountered in performing the work. The Government represents only that such information shows conditions at the locations of the borings, test pits/trenches, or other field data collection locations at the time that the data were collected. Each bidder must form their own opinion of the character of the work and of the materials to be excavated, make their own interpretations, and satisfy themselves by their own interpretations and research regarding all conditions affecting the work to be performed. The

Contractor must assume all responsibility for deductions and conclusions as to the nature or condition of all earth, rock and other materials to be excavated and the difficulties of making and maintaining the required excavations, and of performing other work affected by the geology and subsurface conditions at the Site. The Contractor must assume that any kind of rock and soil may be encountered in excavations, including mud or other materials that are difficult to handle.

Limited Reliance by the Contractor on Technical Data Authorized: The Contractor may rely upon the general accuracy of the Technical Data contained in the GDR. Except for such reliance on this Technical

Data, the Contractor may not rely upon or make any Claim against the Government with respect to:

a) the completeness of this report for the Contractor’s purposes, including but not limited to, any aspects of the means, methods, techniques, sequences, and procedures of construction to be employed by the Contractor, and safety precautions and programs incident thereto, or

b) interpretations and opinions contained in this report, or

c) Any Contractor interpretation of or conclusion drawn from any Technical Data or any such other data, interpretations, opinions, or information.

3. GDR Report Function This report, the Geotechnical Data Report (GDR), functions as a summary of non-interpretative data that have been collected for this project in separate reports at different times. Some of the information within and attached was created by USACE, some by contractors working for USACE.

For more information, please see the full-text appendices that contain more detailed information in each report. They are referenced within the report.

4. Overview of Report Contents The information in the Geotechnical Data Report includes available borehole, pump capacity tests, piezometer logs, Piezometer installation diagrams, relief well boring logs, relief well installation diagrams, relief well pump test data, soil laboratory testing, and other information from or relating to the relief well collector pipe system and nearby instrumentation.

The following information is included in this report:

• Subsurface Explorations o FY20 Geotechnical Investigation Report o FY21 Relief Well Pilot Hole Boring and Borrow Area Investigation Report

• Select Piezometer Data

• Select Relief Well Data

• Pump test capacity Reports o FY20 Dam Safety Instrumentation Rehabilitation Engineering Geology Report

• Geotechnical Lab Results

• Special Inspections and Reports o 1998 Braun Intech Piezometer Rehab and Response Test Report o 2010 Vibrating Wire Piezometer Installation Geologic Field Report o 2015 Camera Inspection Report for Relief Well Collector Pipe

Omaha District Big Bend Replacement of Left Abutment Collector Pipe System Replacement Geotechnical Data Report

SUBSURFACE INVESTIGATION

TOE DRAIN AND RELIEF WELL COLLECTOR PIPE REPLACEMENT GEOTECHNICAL REPORT

(AUG 2021)

FOR OFFICIAL USE ONLY (FOUO)

This document is FOR OFFICIAL USE ONLY (FOUO). It contains information that may be exempt from public release under the Freedom of Information Act (5 USC 552). It is to be controlled, stored, handled, transmitted, distributed, and disposed of in accordance with USACE policy relating to FOUO information and it is not to be released to the public or other personnel who do not have a valid “need to know” without prior written approval of an authorized USACE official.

Big Bend Dam (SD01092)

Missouri River, Fort Thompson, South Dakota Embankment, Powerhouse, and Spillway

TOE DRAIN AND RELIEF WELL COLLECTOR PIPE

REPLACEMENT

GEOTECHNICAL DATA REPORT

Northwestern Division Omaha District

Status: FINAL Report Date: JULY 2020

Revised August 2021

Geotechnical Data Report i July 2020 Big Bend Dam

TABLE OF CONTENTS

1. INTRODUCTION ........................................................................................................................ 1-1

1.1 Project Background .......................................................................................................... 1-1

1.2 Project Description .......................................................................................................... 1-1

1.2.1 Embankment ....................................................................................................... 1-3

1.2.2 Seepage Control Measures .................................................................................. 1-4

1.3 Risk Assessment Status ................................................................................................... 1-7

2. HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE FIELD INVESTIGATION .............. 2-1

2.1 HTW Field Investigation ................................................................................................. 2-2

2.1.1 HTRW Sample Collection .................................................................................. 2-2

2.2 Results .............................................................................................................................. 2-3

3. GEOTECHNICAL FIELD INVESTIGATION ............................................................................ 3-1

3.1 Scope and Purpose ........................................................................................................... 3-1

3.2 Geotechnical Soil Borings ............................................................................................... 3-1

3.2.1 Drilling Methodology ......................................................................................... 3-2

3.2.2 Sampling Methodology ....................................................................................... 3-2

3.2.3 Boring Abandonment .......................................................................................... 3-2

3.2.4 Cutting Disposal ................................................................................................. 3-3

3.3 Laboratory Testing ........................................................................................................... 3-3

3.4 Pump Testing ................................................................................................................... 3-3

3.4.1 Observation Well Installation ............................................................................. 3-4

4. GEOLOGY ................................................................................................................................... 4-1

4.1 Site Geology .................................................................................................................... 4-1

4.1.1 Overburden Deposits .......................................................................................... 4-1

4.1.2 Bedrock ............................................................................................................... 4-1

4.2 Project-Specific Geology ................................................................................................. 4-1

4.2.1 N-values .............................................................................................................. 4-3

4.2.2 Laboratory Data .................................................................................................. 4-3

5. GROUNDWATER ....................................................................................................................... 5-1

5.1 Existing Piezometers ........................................................................................................ 5-1

5.2 2020 Field Investigation .................................................................................................. 5-1

5.3 Pump Testing ................................................................................................................... 5-3

5.4 Constant-Rate Pump Test in RW-67B and Associated Testing ....................................... 5-3

5.4.1 Six-Hour Pump Test on RW-67B (Observation Wells OW20-01 and OW20-02) ........................................................................................................... 5-3

5.4.2 Single-Well Pump Test (RW-67B) ..................................................................... 5-4

5.4.3 Observation Well OW20-01 and OW20-02 Pump Test ..................................... 5-4

5.4.4 Four-Hour Pump Test Relief Well RW-68B (Observation Well PZ-25R) ......... 5-4

5.4.5 Single-Well Pump Test (RW-68B) ..................................................................... 5-5

5.5 Single-well pump tests ..................................................................................................... 5-5

5.6 Two-hour test in relief well RW-66A .............................................................................. 5-5

5.7 Two-hour test in relief well RW-68A .............................................................................. 5-5

5.8 Summary .......................................................................................................................... 5-5

Geotechnical Data Report ii July 2020

LIST OF TABLES

Table 2-1: Soil and groundwater samples tested for TPH-DRO (SW846 8015C) .................................... 2-2 Table 3-1: Soil Boring Designations, Locations, and Depths .................................................................... 3-1 Table 3-2: Existing Relief Well Information (Pump Tests) ....................................................................... 3-4 Table 3-3: Observation well construction details ...................................................................................... 3-4 Table 4-1: Toe drain invert depth, materials encountered, interpreted zones, and clay blanket thickness 4-2 Table 5-1: Groundwater elevation data from the May 2020 sampling event ............................................. 5-1 Table 5-2: Depth groundwater encountered during drilling ...................................................................... 5-2 Table 5-3: Summary of available pump tests ............................................................................................. 5-5

LIST OF FIGURES

Figure 1-1: Big Bend Dam location map ................................................................................................... 1-1 Figure 1-2: Project feature map ................................................................................................................. 1-2 Figure 1-3: Aerial photo showing project features .................................................................................... 1-2 Figure 1-4: General embankment section .................................................................................................. 1-3 Figure 1-5: Left abutment embankment typical section (station 125+00) ................................................. 1-3 Figure 1-6: Project feature map showing location of relief well line and drains ....................................... 1-4 Figure 1-7: Plan view of toe drain and left abutment relief well collector subdrain system...................... 1-5 Figure 1-8: Left abutment relief well collector system RW-68A to T-outfall .......................................... 1-6 Figure 1-9: Left abutment relief well collector system pressurized area by RW-68A T-outfall ............... 1-6 Figure 1-10: Left abutment relief well collector system pressurized area by RW-68B T-outfall .............. 1-7 Figure 2-1: Log from 1974 showing amount of kerosene added to site piezometers ................................ 2-1 Figure 4-1: Geologic profile through centerline of dam (looking upstream) ............................................. 4-1

LIST OF APPENDICES

Appendix A: Location Map and Geologic Profiles Appendix B: HTRW Sampling Report Appendix C: Drilling Logs and Construction Diagrams Appendix D: Laboratory Data Appendix E: Pump Test Data

Geotechnical Data Report 1-1 July 2020

1. INTRODUCTION

The U.S. Army Corps of Engineers (USACE), Omaha District has prepared this engineering geology report to document the results of a drilling program plan (DPP) conducted in support of a planned toe drain and relief well collector pipe replacement project at Big Bend Dam. The objective of the work outlined in the DPP and compiled in this engineering geology report was to provide additional geotechnical, hydrogeological, and hazardous, toxic, and radioactive waste (HTRW) data in the left abutment area of the dam in support of the drain replacement project. The findings from the field investigation as documented in this report will be utilized in conjunction with an analysis of existing site data to refine the team’s understanding of the foundation and groundwater conditions which will aid in the design and construction of the replacement drains, including potential dewatering requirements.

1.1 Project Background

Big Bend Dam is a high hazard potential dam located on the Missouri River in Buffalo and Lyman Counties, South Dakota, approximately 20 miles upstream of the city of Chamberlain. Big Bend is one of six main stem dams on the Missouri River, and was authorized by the Flood Control Act approved 22 December 1944 (Public Law 534, 78th Congress 2nd Session) as part of the general comprehensive plan for flood control, irrigation, navigation, and hydropower in the Missouri River basin (Figure 1-1). The project was constructed from 1963 to 1966.

Figure 1-1: Big Bend Dam location map

1.2 Project Description

Big Bend Dam is a multiple purpose project consisting of a rolled earthfill embankment, hydroelectric generating power plant, spillway, and reservoir (Figure 1-2 and Figure 1-3). The reservoir has a capacity of 1,859,000 acre-feet for flood control, irrigation, conservation, navigation, power development, and other uses. Conventional outlet works structures were not constructed at Big Bend; releases are made through either the spillway or the power plant.

Geotechnical Data Report 1-2 July 2020

Figure 1-2: Project feature map

Figure 1-3: Aerial photo showing project features

Geotechnical Data Report 1-3 July 2020

1.2.1 Embankment

The rolled, zoned (Figure 1-4), earth-filled embankment is 10,570 feet long with a maximum height of 95 feet above the river channel and a crest width of 50 feet. The maximum width at the base of the embankment is 1,200 feet. The embankment makes a gentle S curve across the valley. The embankment was built upon dredged or dumped underwater pervious fill. A central impervious core extends from the pervious fill to five feet below the top of the dam. This core extends through the length of the embankment and transitions into a massive impervious section at the spillway and powerhouse tie-ins.

The core is flanked on the downstream side by a pervious zone that ties into either the pervious downstream embankment fill or a horizontal downstream pervious blanket to provide drainage for seepage through the core. The top five feet of the embankment crest is composed of pervious fill connected to the downstream pervious drain to provide a frost-free base for the highway surface across the embankment.

Figure 1-4: General embankment section

A minimum 10-foot-thick impervious blanket ties into the central impervious core and extends upstream 425 to 540 feet throughout the major portion of the embankment. Near the powerhouse area the blanket is thickened and excavated into bedrock and forms an impervious toe trench that prevents excessive seepage in the powerhouse area.

In the left abutment area near the spillway (Figure 1-5), a minimum 20-foot-thick upstream blanket is present over a portion of the right slope of the spillway approach channel. Above the upstream impervious blanket, the embankment is composed of a massive compacted shale section that is protected by 20 feet of dumped chalk. The downstream fill section consists primarily of random fill with a berm section of dumped chalk. The right abutment section, located between the right side of the powerhouse and the natural abutment ground surface, has 1V on 3H side slopes and is composed of random fill materials obtained from the right bank excavations.

Figure 1-5: Left abutment embankment typical section (station 125+00)

Geotechnical Data Report 1-4 July 2020

1.2.2 Seepage Control Measures

Seepage through the embankment is controlled primarily by the impervious core, impervious upstream blanket, and the pervious drain section on the downstream side of the impervious core. Underseepage control is provided by the upstream impervious blanket, the chalk berm sections, the pervious blanket, and by pressure relief wells along the downstream toe of the embankment. Due to the relatively low head at Big Bend Dam and to the other underseepage control methods provided, a positive cutoff through the foundation sand was not determined to be necessary.

Pressure Relief Wells The original system of pressure relief wells installed in 1963 extends a distance of 7,125 feet along the toe of the dam (Figure 1-6). The system was comprised of 68 relief wells spaced at intervals varying from 75 to 225 feet. The wells were installed to provide relief from any excess hydrostatic uplift pressures that may develop in the valley alluvial sands beneath the downstream impervious natural clay blanket.

Between 2009 and 2012, the relief well system was modified with 35 additional relief wells. The current system has a total of 103 relief wells and extends a distance of 7,425 feet along the toe of the dam. The additional wells were installed between existing wells through the valley and in the left abutment.

Spacing between the relief wells ranges from 35 to 200 feet.

Figure 1-6: Project feature map showing location of relief well line and drains

Toe Drain and Left Abutment Relief Well Collector Subdrain System Seepage was noted on the downstream left abutment shortly after the reservoir began filling during the winter of 1963 (“Downstream Drainage Area” in Figure 1-6). The downstream left abutment area became very marshy which detracted from public use and maintenance. To address the seepage issue, a toe drain and a left abutment relief well collector subdrain system were installed and completed in March 1965 (see Figure 1-7). The work also included lowering the outfalls for relief wells RW-67 and RW-68 and connecting them to the relief well collector subdrain system and grading the storm drainage channels into the relief well collector drainage channel.

The left abutment subdrain system was constructed with 8- to 15-inch-diameter perforated CMP. The toe drain is located between stations 125+00 and 128+00 on the west side of the spillway access road and

Geotechnical Data Report 1-5 July 2020 consists of four risers and approximately 420 feet of 8-inch-diameter and 250 feet of 12-inch-diameter perforated CMP pipe. The toe drain discharges into riser 4 which is part of the left abutment relief well collector subdrain system. The original left abutment relief well collector subdrain system consisted of two manholes, riser 4, and collected discharge from relief wells RW-67 and RW-68. The system discharged into the head of the relief well collector channel. In periodic inspections conducted after installation, the system was noted to be performing well in drying the area immediately adjacent to the embankment toe. During the 2011 flood event, a portion of the relief well collector system became inundated due to high tailwater. In 2010 and 2012, four additional relief wells were installed in the area (RW-67A, RW-67B, RW-68A, and RW-68B) to alleviate pressure observed in the left abutment. The additional relief wells were connected either to existing relief wells or to the relief well collector system.

Figure 1-7: Plan view of toe drain and left abutment relief well collector subdrain system

1.2.2.2.1 2015 Camera Inspection In 2015, a camera inspection was conducted on the relief well collector system. Over eight inches of sediment was reported to be present in the drain approximately 20 feet from the outfall which prohibited further inspection. The contractor subsequently jetted the pipe and was able to inspect additional segments. Significant biofouling was observed in the outfall line of relief wells RW-68B and RW-68A to the T-outfall as shown in Figure 1-8 (note that this section of pipe was not jetted prior to inspection).

Overall, the footage showed the pipe itself to be in good condition. However, from an area approximately 95 feet south of MH-1 to relief well outfall RW-68A, the pipe appears to be pressurized (Figure 1-9 and Figure 1-10). The camera inspection showed multiple locations where water was observed to be shooting through the perforations. Pressurized areas where flow was observed above the flowline along the relief well collector drain are shown on Figure A1 in Appendix A.

The most pressurized area appears to be near relief well RW-68B, where water was observed shooting out of several perforations above the flowline in the pipe (Figure 1-10) indicating either an area of localized high pressure or inadequate collection efficiency of the drain possibly due to plugging. Piezometric levels near relief wells RW-68B and RW-68A typically range from four to five feet above the inverts of the

Geotechnical Data Report 1-6 July 2020 relief well collector pipe (see PZ-25R and PZ-19 on Figure A2 in Appendix A). It is possible that the filter material around the relief well collector pipe has become plugged over the 55 years since the system has been installed. The gradation of the filter material is unknown and it is unclear whether or not the filter material is compatible with embankment and foundation materials.

Based on the recent 2019 PA identifying backward erosion piping (BEP) through the left abutment as a potential risk-driving failure mode, along with the age of the system (55 years), and the current condition of the subdrain system (areas of pressurization), Omaha District Dam Safety has identified the replacement of the toe drain and left abutment relief well collector system as an interim risk reduction measure (IRRM).

Figure 1-8: Left abutment relief well collector system RW-68A to T-outfall

Figure 1-9: Left abutment relief well collector system pressurized area by RW-68A T-outfall

Geotechnical Data Report 1-7 July 2020

Figure 1-10: Left abutment relief well collector system pressurized area by RW-68B T-outfall

1.3 Risk Assessment Status

Previous risk assessments at Big Bend Dam consist of a Screening Portfolio Risk Assessment (SPRA) conducted in 2007, a Potential Failure Mode Analysis (PFMA) performed in 2009, and a Periodic Assessment (PA) conducted in July 2019 (note that the PA is still being drafted). Both the SPRA (2007) and the PFMA (2009) identified seepage and piping in the left abutment as probably inadequate (SPRA) or as a potential failure mode (PFMA).

The PA included a risk assessment in which 54 potential failure modes (PFMs) were identified. Of these, two of the five PFMs listed as risk drivers were also in the area of the left abutment seepage as described below:

• PFM 31: Plugged left abutment relief well subdrain system between stations 120+00 and 130+00 causes blowout resulting in backward erosion piping (BEP) through the foundation alluvium.

• PFM 32: Backward erosion piping (BEP) initiating at an unfiltered exit in the relief well channel in area of historic boils between stations 119+00 and 120+00.

For both PFMs, a large inflow event causes the reservoir to reach top of active storage (TAS) at elevation 1,423 feet local project datum (LPD). High pressures (which have historically been observed during normal pool conditions) exist in the left valley bank alluvial sand. There is a continuous alluvial sand layer between the upstream reservoir and the downstream left abutment bank. Vertical gradients are sufficient to initiate BEP at the left abutment.

The two failure modes (PFM 31 and PFM 32) are differentiated by the exit locations. For PFM 31, the relief well collector subdrain system between stations 120+00 and 130+00 is either plugged or ineffective at relieving the uplift pressures. Vertical exit gradients are sufficient to blow out the relatively thin clay blanket and initiate BEP of the foundation alluvial sands. For PFM 32, the uplift pressures are sufficient to blow out the natural clay blanket in the relief well channel exposing the valley alluvium in the vicinity of stations 119+00 and 120+00. This area was identified as a potential exit location due to historic pinboils at the bottom of the relief well channel. Global gradients would be sufficient for BEP of the foundation sands.

Geotechnical Data Report 2-1 July 2020

2. HAZARDOUS, TOXIC, AND RADIOACTIVE WASTE FIELD INVESTIGATION

In the 1970s and 1980s, kerosene was often added to piezometers at the dam that had water near the surface to prevent the water from freezing and bursting the pipes in the winter. Because several of these piezometers are in the location of the proposed toe drain replacement, specialized samples were also collected from the geotechnical borings to determine any impacts to the soil and groundwater from the kerosene in order to determine if any hazardous or toxic waste would be generated during construction and dewatering. According to project personnel, as the fuel would evaporate off more would be added, typically less than a gallon ever year or two (Figure 2-1). This procedure was phased out in the late- 1980s as vibrating wire transducers were being installed in the open tube pipes and was discontinued completely sometime in 1992 or 1993.

Figure 2-1: Log from 1974 showing amount of kerosene added to site piezometers

During recent routine maintenance operations, it was noted that diesel fuel was still present in some of the piezometers. Project staff later inspected the piezometers and identified two piezometers (J320Ra and J320Rb) with free product. Shallow piezometer J320Ra contained less than two inches of diesel fuel and deep piezometer J320Rb contained approximately five feet of diesel. The combined amount of diesel fuel equated to less than one gallon in the two-inch pipes.

After the diesel was identified, project staff began to remove the diesel by first dipping off the surface fluid. This was completed multiple times to allow for the remaining diesel fuel to rise to the surface.

After the majority of the diesel fuel was removed in this manner, project staff used fuel-only absorbent pigs to collect the residual diesel. Project staff provided all information to the Department of Environment and Natural Resources (DENR) and proposed to continue to use the absorbent pigs to clean any residual fuel out of the piezometers until no further presence of fuel was observed. Based on the location of the piezometers, and fact that no adverse impacts have been identified, the DENR placed this case into the "No Further Action" category with the caveat that US Army Corps of Engineers may be responsible for conducting additional assessment and remediation any future problems arising from the remaining contamination.

Geotechnical Data Report 2-2 July 2020

2.1 HTW Field Investigation

To assess the impact of the diesel fuels on the soil and groundwater in the area of proposed construction, soil samples were collected at depths immediately above the groundwater table from all geotechnical borings drilled for this investigation. Groundwater grab samples were also collected from each open boring to determine any impacts to groundwater which will help determine any mitigation required during dewatering. Field investigation methods are provided in the following sections. An analysis of the results is provided as Appendix B.

2.1.1 HTRW Sample Collection

Environmental soil and groundwater samples were collected from all geotechnical borings drilled for this investigation. Soil and groundwater samples were analyzed for total petroleum hydrocarbons (TPH) of diesel range organics (DRO) from soil borings BH20-01 through BH20-12 as shown in Table 2-1. Soil samples were collected from the two-inch split spoon just above the water table (typically between 3 and 8 feet bgs) in borings BH20-01 through BH20-12. Fuel odor was only noted in one boring during drilling, BH20-12. Groundwater samples were collected as a grab sample from the top of the water column in the augers at the completion of drilling. Free product was not observed either while drilling or during water sampling. Samples were kept on ice at all times after the collection process and were shipped overnight under chain-of-custody to CT Laboratories in Baraboo, Wisconsin. Disposable nitrile gloves were worn when handling soil and groundwater samples. Air monitoring was not required.

Table 2-1: Soil and groundwater samples tested for TPH-DRO (SW846 8015C)

Boring Designation

Soil Sample ID Soil Sample Container

Groundwater Sample

ID

Groundwater Sample Container

BH20-01 BBSD-01A 4 oz. glass jar BBSD-01B 1 liter amber glass jar

BH20-02

BBSD-02A 4 oz. glass jar BBSD-02B 1 liter amber glass jar

BBSD-00A1 4 oz. glass jar BBSD-00B2 1 liter amber glass jar

BH20-03 BBSD-03A 4 oz. glass jar BBSD-03B 1 liter amber glass jar

BH20-04 BBSD-04A 4 oz. glass jar BBSD-04B 1 liter amber glass jar

BH20-05

BBSD-05A 4 oz. glass jar BBSD-05B 1 liter amber glass jar

BBSD-05A-MS 4 oz. glass jar BBSD-05B-MS 1 liter amber glass jar

BBSD-05A-MSD 4 oz. glass jar BBSD-05B-MSD 1 liter amber glass jar

BH20-06 BBSD-06A 4 oz. glass jar BBSD-06B 1 liter amber glass jar

BH20-07 BBSD-07A 4 oz. glass jar BBSD-07B 1 liter amber glass jar

BH20-08

BBSD-08A 4 oz. glass jar BBSD-08B 1 liter amber glass jar

BBSD-XXA3 4 oz. glass jar BBSD-XXB4 1 liter amber glass jar

BH20-09 BBSD-09A 4 oz. glass jar BBSD-09B 1 liter amber glass jar

BH20-10 BBSD-10A 4 oz. glass jar BBSD-10B 1 liter amber glass jar

BH20-11 BBSD-11A 4 oz. glass jar BBSD-11B 1 liter amber glass jar

BH20-12 BBSD-12A 4 oz. glass jar BBSD-12B 1 liter amber glass jar 1Duplicate sample from BH20-02A, 2Duplicate sample from BH20-02B, 3Duplicate sample from BH20-08A 4Duplicate sample from BH20-08B

Geotechnical Data Report 2-3 July 2020

Decontamination Procedures Hollow-stem augers were steam cleaned at the boring location at the completion of drilling prior to moving to the next location. Disposable bailers used for groundwater sampling were not reused and were disposed of in the trash after use along with the disposable nitrile gloves. Split spoon samplers were decontaminated with distilled water and Liquinox between sampling intervals. Wastewater was discharged to the ground in the area of drilling.

Soil Cuttings Soil cuttings from approximately two feet above to two feet below the water table were placed in a 55-gallon drum pending soil analysis. One 55-gallon drum of composite soil cuttings was collected and returned to the Omaha District Boatyard facility for storage. A drum inventory sheet was filled out with the drum numbered and identified. All other soils were disposed of by spreading them at the boring location as no visible impacts were noted nor was it suspected of being impacted due to the nature of the free product occurrence.

2.2 Results

The results of the HTRW sampling indicate the presence of TPH-DRO in soil and groundwater in the location of borings BH20-11 and BH20-12. The soil sample taken from BH20-12 (BBSD-12A) was analyzed at 2830 mg/kg and the groundwater grab sample was analyzed at 27,000 ug/L. The soil sample taken at BH20-11 (BBSD-11A) was analyzed at 96.9 mg/kg. Excavated soils with TPH concentrations greater than 10 mg/kg must be disposed of in accordance with the permitting requirements of the SDDNR Waste Management Program. The groundwater sample taken from BH20-12 (BBSD-12B) was analyzed at 27,000 ug/L. If dewatering is required during construction, groundwater in this area will be containerized and sampled for disposal procedures. The full report is included as Appendix C.

Geotechnical Data Report 3-1 July 2020

3. GEOTECHNICAL FIELD INVESTIGATION

The following section presents a summary of the geotechnical field investigation drilling scope and methodology. Intrusive activities into, in close proximity to, or through embankment dams and their foundations may pose significant risk to the structures if not implemented properly. To mitigate these risks, all drilling and sampling shall be conducted in accordance with EM 1110-1-1804, Geotechnical Investigations (1 January 2001); ER 1110-1-1807, Drilling in Earth Embankment Dams and Levees (31 July 2014); and specific guidance as referenced in this section.

3.1 Scope and Purpose

The geotechnical field investigation was designed to better define the site hydrogeology through geologic logging of the stratigraphy, the collection of blow count (N value) data, the collection of disturbed soil samples for laboratory analysis, and pump testing to obtain estimates of the hydraulic characteristics of the alluvial foundation. To obtain this information, 12 soil borings were advanced to approximately five to ten feet below the construction depth along the length of the toe drain and relief well collector pipe at approximate 100-foot intervals. Representative samples were collected from the soil borings for laboratory testing along with Standard Penetration Tests (SPTs) to obtain density information. Pump tests were also performed to determine estimates of hydraulic characteristics of the alluvial aquifer in the area of construction.

The geotechnical field investigation was conducted from June 1, 2020 through June 18, 2020 by the USACE, Omaha District drill crew. Boring locations were selected by the Project Dam Safety Engineer and staked by Big Bend Project Office personnel. Borings BH20-09, BH20-11, and BH20-12 had to be relocated from the original staked location 40 feet upstream, 50 feet upstream, and 10 feet upstream, respectively, due to overhead power lines located in the area of construction. According to the project office, the overhead lines are 69 kV which requires a minimum 15-foot offset according to EM 385-1-1 (2014). All borings were also cleared through 811 for digging prior to intrusive activities.

3.2 Geotechnical Soil Borings

A total of 12 soil borings (BH20-01 through BH20-12) were installed to log materials and obtain geotechnical samples (and HTRW samples as discussed in Section 2) to better characterize the foundation soils in the area of the proposed toe drain and relief well collector pipe replacement (Table 3-1). Boring locations are shown on Figure A1 in Appendix A.

Table 3-1: Soil Boring Designations, Locations, and Depths

Boring Designation Northing Easting Ground Surface Elevation

Bottom Depth (ft)

BH20-01 629695.581 2199392.547 1384.278 10

BH20-02 629564.363 2199406.379 1381.688 10

BH20-03 629454.701 2199414.736 1379.711 10

BH20-04 629846.579 2199487.894 1386.423 20

BH20-05 629746.368 2199486.945 1382.581 20

BH20-06 629640.983 2199486.485 1380.097 20

BH20-07 629503.822 2199487.639 1379.089 20

BH20-08 629415.776 2199503.251 1376.959 15

Geotechnical Data Report 3-2 July 2020

Boring Designation Northing Easting Ground Surface Elevation

Bottom Depth (ft)

BH20-09 629305.020 2199573.045 1373.840 15

BH20-10 629215.513 2199585.558 1373.094 15

BH20-11 629200.9471 2199634.2021 13691 10

BH20-12 629059.7101 2199722.4521 13681 10

1Estimated using hand-held GPS. Borings were relocated from surveyed location due to overhead power lines.

3.2.1 Drilling Methodology

Drilling and sampling of the foundation materials was accomplished with a Gus Pech 750C (Mite-e-Mite) drill rig equipped with 4 1/4-inch-ID HSA. Slight heave was noted in the augers during drilling through the sands; the drill crew was able to lift the augers with chains and reinsert the center bit to drill out the hole to depth without having to add water to the hole (since HTRW groundwater samples were to be collected at the completion of drilling). Drilling for sample collection and logging purposes was conducted in all borings using a 2-inch-diameter split spoon sampler.

Standard penetration tests were performed by dropping a 140-pound hammer a distance of 30 inches to advance a two-inch-diameter split spoon continuously in the upper 10 feet, then every 2.5 feet to the depth of the boring (if deeper than 10 feet). These penetration tests were performed in accordance with ASTM D 1586-11, “Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils.” Uncorrected blow counts (N-values) were recorded on the drill log included in Appendix C.

SPT energy measurements SPT energy measurements were obtained on the Gus Pech 750C (Mite-e-Mite), equipped with a Boart Longyear automatic hammer, on May 29th, 2019. SPT energy measurements on the Mite-e-Mite automatic hammer resulted in average energy transfer rate (ETR) of 82.6%, 84.1%, and 84.0% during standard operating procedures with the engine running at approximately 750 revolutions per minutes (rpm) for each testing interval. Standard deviations for each test interval were calculated to be 1.2%, 1.0%, and 0.7%, respectively. The overall average ETR of all valid measurements was 83.7% with a standard deviation of 1.1%. For SPT analysis and N-value corrections, it is recommended to use an ETR of 83.7% for the Mite-e-Mite automatic hammer.

3.2.2 Sampling Methodology

Sampling required for this field effort consisted of disturbed samples collected from all borings from each sampling interval and from all changes in material. Representative samples of the subsurface material were placed in pint sample jars and sealed airtight with at least three wraps of electrical tape. The jars were then placed in shipping boxes and hand delivered to the project engineer at the completion of work (along with applicable sample transmittal forms).

3.2.3 Boring Abandonment

Because the borings were located adjacent to both the toe drain and perforated relief well collector pipe, the use of grout was not permitted. All borings were backfilled with medium (3/8-inch) bentonite chips placed in the boring as the augers were removed. The number of bags required for each boring was calculated prior to backfilling (note that one 50-lb bag of chips should fill approximately 2 feet of an 8-inch hole). All borings took approximately the calculated amount of chips.

Geotechnical Data Report 3-3 July 2020

3.2.4 Cutting Disposal

Soil cuttings from approximately two feet above to two feet below the water table were placed in a 55-gallon drum pending soil TPH-DRO analysis. One 55-gallon drum of composite soil cuttings was collected and returned to the Omaha District Boatyard facility for storage. A drum inventory sheet was filled out with the drum numbered and identified. All other soils were disposed of by spreading them at the boring location. The site was restored to pre-investigation conditions prior to demobilization.

3.3 Laboratory Testing

Geotechnical samples collected during this field effort were sorted by the Dam Safety Engineer and select samples were sent to Terracon’s Omaha laboratory for the following analysis:

• ASTM D6913 Sieve Analysis (washed)

• ASTM D422 Hydrometer Analysis

• ASTM D422 Combine Sieve/Hydrometer

• ASTM D4318 Atterberg Limits

Results are provided in Appendix D. Testing results are also provided on the drilling logs included as Appendix C.

3.4 Pump Testing

Pump testing was performed on several existing project relief wells (RW-66A, RW-67B, RW-68A, and RW-68B) to obtain an estimate of the hydraulic characteristics of the alluvium for dewatering efforts that may be required during future construction of the toe drain and relief well collector pipe replacement.

Testing consisted of one six-hour pump test performed on existing relief well RW-67B (which included the installation of two observation wells [OW20-01 and OW20-02]), one four-hour pump test performed on existing relief well RW-68B (with drawdown measured in one existing piezometer [PZ-25R]), and single-well pump tests performed on existing relief wells RW-66A, RW-67B, RW-68A, and RW-68B. A pump test was also conducted using observation wells OW20-01 and OW20-02 to identify any variances in hydraulic parameters vertically in the formation. All pump tests were performed at constant pumping rates. The results of the testing are discussed in Section 5.3.

A summary of construction details and initial specific capacities for the relief wells is provided in Table 3-2. All of the relief wells tested were installed in 2010 or 2012 and were constructed with 40- or 50-slot Muni-Pak™ stainless steel well screens with a five-inch-diameter inner screen and a seven-inch-diameter outer screen. The blank casings consist of five-inch-diameter, Schedule-40, stainless steel flush-threaded pipe. The upper ten-foot-section of riser is five-inch-diameter, Schedule-40, PVC flush-treaded pipe.

Locations are shown on Figure A1 in Appendix A. Results are discussed in Section 4.

It should be noted that all of the relief wells were rehabilitated and capacity tested prior to pump testing along with several other relief wells in the area of the drain replacement as documented in the FY20 Dam Safety Instrumentation Rehabilitation Engineering Geology Report (USACE, 2020). The post-rehabilitation specific capacities are also included in Table 3-2.

Geotechnical Data Report 3-4 July 2020

Table 3-2: Existing Relief Well Information (Pump Tests)

Relief Well Designation

Bottom Depth (ft bgs)

Screen Interval (ft bgs)

Initial Pumping Rate (gpm)/

Drawdown (ft)

Initial Average Specific Capacity (gpm/ft drawdown)

2020 Post-Rehab Pumping Rate

(gpm)/ Drawdown (ft)

2020 Post- Rehab Specific

Capacity (gpm/ft drawdown)

RW-66A 81.4 31.4-81.1 61/10.3 5.9 59.5/15.7 3.8

RW-67B 83.0 37.0-82.0

23/12.1

1.8 20.7/10.8 1.9 35/18.7

49/28.2

RW-68A 89.0 47.9-87.9

23.5/1.7 (13.8)

9.5 33/26.5 1.2 33.6/4.0 (8.4)

45.7/7.4 (6.2)

RW-68B 90.0 48.6-88.6

20.5/14.6

1.4 16.2/20.0 0.8 31.2/19.5

41.4/30.8

3.4.1 Observation Well Installation

Two observations wells (OW20-01 and OW20-02) were installed a distance of five and ten feet, respectively, from existing relief well RW-67B as shown on Figure A4 in Appendix A for the six-hour pump test. Drilling for the observation wells was accomplished with a Gus Pech 7500C (Mite-e-Mite) drill rig equipped with 4 1/4-inch-ID HSA. The borings were advanced using a center bit and logged from drill cuttings. The observation wells were constructed of two-inch nominal-diameter, Schedule 40, PVC casing with a ten-foot-long, 0.010-inch-slotted screen and 20-40 gradation clean silica sand filter pack (Table 3-3). The filter pack was tremied into place and extends approximately three feet above the top of the well screen. The remaining borehole annulus was backfilled with medium (3/8-inch) bentonite chips due to the proximity of the borings to the perforated drains. The observation wells were completed with 4-inch-square (observation wells) by 5-foot long locking steel protective covers. Construction diagrams are included in Appendix C. Both observation wells were developed and response tested at the completion of installation.

Table 3-3: Observation well construction details

Observation Well

Designation

Northing Easting Distance from

RW-67B

Bottom Depth (ft bgs)

Filter Pack Interval (ft bgs)

Screen Interval (ft bgs)

OW20-01 629356.653 2199535.284 5.0 20 7-20 9.4-19.4

OW20-02 629349.363 2199535.361 10.0 19.7 7-20 9.4-19.4

Geotechnical Data Report 4-1 July 2020

4. GEOLOGY

4.1 Site Geology

4.1.1 Overburden Deposits

The overburden soils at the dam site consist of residual clays, alluvial clays, silts and sands, and glacial outwash sand and gravel (Figure 4-1). The overburden materials are described below:

• Left abutment glacial deposits: In the upper reaches of the left abutment above the Pierre shale.

Consist of silts, sands, and gravels, mantled locally with loess.

• Left bank flood plain deposits: Form part of the embankment foundation. Mantle of fat clay (CH) up to 25 feet thick overlying pervious valley alluvium.

• Pervious valley alluvium: Pervious alluvial deposits of the Missouri River. Consist primarily of silty fine sands that become coarser with depth, some gravel strata.

• Right bank overburden: In the upper reaches of the right abutment above the Pierre shale.

Consists of fat clays (CH) derived from the underlying Pierre shale and silts, sands, and gravels (Good Soldier Creek).

Figure 4-1: Geologic profile through centerline of dam (looking upstream)

4.1.2 Bedrock

Bedrock at the dam site consists of (in descending order): Pierre Formation shale, Niobrara Formation chalk, and Carlile Formation shale, all of Cretaceous age.

• Pierre Shale: Occurs in the spillway and powerhouse intake and discharge channels and beneath a short section of embankment on the right abutment. Gray to dark gray compaction-type shale composed chiefly of clay and silt constituents. Has well-defined bedding planes and numerous bentonite seams.

• Niobrara Chalk: Supports all of the appurtenant structures at the project. Lead gray, medium hard, impervious, argillaceous chalk or chalky shale derived chiefly from marine microorganisms and precipitates of calcium carbonate materials. Bentonite, bentonite clay, and shale seams occur throughout the formation.

• Carlile Shale: Unrelated to any structure foundation at the project (72 feet below the deepest part of the powerhouse foundation).

4.2 Project-Specific Geology

In the area of the drain replacement, the foundation geology consists an alluvial clay blanket overlying pervious sands with interbedded clay layers as shown in Table 4-1 and on the geologic profiles in Appendix A. The thickness of the clay blanket varies across the area of the investigation ranging from 0 to >10 feet. The blanket clays are predominantly lean (CL) or sandy lean clays (s[CL]) and occasionally contain sand and gravel. The alluvial sands beneath the clay blanket are predominantly silty and poorly graded fine sands (SM, SP-SM, and SP). Within the alluvial sands, clay layers composed of both lean

Geotechnical Data Report 4-2 July 2020

(CL) and fat (CH) clays are present. Available boring data is limited in this area (note that the relief well borings shown on the geologic profiles were logged from cuttings) but most of the available data indicates the presence of an intermediate clay layer approximately 20 feet thick separating the sand unit. Several thinner clay layers also appear to be present, though none appear to be laterally continuous. Fill material was also noted in several of the borings as discussed in the following section.

Table 4-1: Toe drain invert depth, materials encountered, interpreted zones, and clay blanket thickness

Boring Designation

Approximate Toe Drain Invert Depth (ft bgs)

Materials Encountered/ Depth (ft bgs)

Interpreted Zone Approximate Clay Blanket Thickness (ft)

Toe Drain Borings

BH20-01 3.5

CL (0-2)

SP (2-3)

(CL)s (3-5)

SC (5-9)

SP (9-10)

Random Embankment Fill Horizontal Drain

Alluvial Clay Alluvial Clay/Sand

Alluvial Sand

6.0

BH20-02 5.0 CL (0-4.5)

SM (4.5-10)

Random Embankment Fill Alluvial Sand 0.0

BH20-03 2.5

CL (0-1)

SM (1-3)

SW-SM (3-4.5)

CL (4.5-8)

SM (8-10)

Random Embankment Fill Horizontal Drain Horizontal Drain

Alluvial Clay Alluvial Sand

3.5

Relief Well Collector Pipe Borings

BH20-04 10.0

CL-ML (0-2)

CL (2-6)

SC (6-6.5)

SP-SM (6.5-10)

SP (10-20)

Reworked clay /trench backfill

Alluvial Clay Alluvial Clay/Sand

Alluvial Sand Alluvial Sand

4.5

BH20-05 8.5

CL-ML (0-2)

CL (2-5)

CL-ML (5-6)

SP (6-20)

Reworked clay /trench backfill

Alluvial Clay Alluvial Clay Alluvial Sand

4.0

BH20-06 8.0

CL-ML (0-2.5)

CL (2-6)

(ML)s (6-8)

SP (8-10)

SP-SM (10-15)

SP (15-20)

Reworked clay /trench backfill

Alluvial Clay Alluvial Silt

Alluvial Sand Alluvial Sand Alluvial Sand

4.0

BH20-07 9.5

CL-ML (0-3)

(CL-ML)s (3-4)

SP-SM (4-6)

SP (6-18.5)

SM (18.5-20)

Alluvial Clay Alluvial Clay Alluvial Sand Alluvial Sand Alluvial Sand

4.0

BH20-08 10.0

CL (0-4)

SP-SM (4-6)

SM (6-15)

Alluvial Clay Alluvial Sand Alluvial Sand

4.0

Geotechnical Data Report 4-3 July 2020

Boring Designation

Approximate Toe Drain Invert Depth (ft bgs)

Materials Encountered/ Depth (ft bgs)

Interpreted Zone Approximate Clay Blanket Thickness (ft)

BH20-09 7.0

CL (0-3.5)

S(CL) (3.5-4.2)

CL (4.7-9)

SM (9-10)

SP (10-15)

Alluvia…