BB90 Geotechnical Data Report STEP 1 Pg 563-684.pdf

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

PUMP TEST CAPACITY REPORTS

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

FY20 DAM SAFETY INSTRUMENTATION REHABILITATION

ENGINEERING GEOLOGY REPORT

Northwestern Division Omaha District

Status: FINAL Report Date: JUNE 2020

Engineering Geology Report i June 2020 Big Bend Dam Instrumentation

TABLE OF CONTENTS

1. INTRODUCTION

1.1 Big Bend Dam Risk Assessment Status

1.2 Objectives and Justification

2. PROJECT BACKGROUND

2.1 Project Description

2.1.1 Embankment

2.1.2 Seepage Control Measures

3. FY20 DAM SAFETY INSTRUMENTATION FIELD WORK

3.1 Relief Well Construction Details

3.1.1 Original Project Relief Wells (1963)

3.1.2 2010 Relief Wells

3.1.3 2012 Relief Wells

3.2 Relief Well Inspection

3.2.1 Pre-capacity procedures

3.3 Capacity Testing

3.4 Mechanical Rehabilitation

3.4.2 Post-Rehabilitation Capacity Testing

3.5 Summary of Results and Recommendations

LIST OF TABLES

Table 1: Construction details for relief wells to be capacity tested in FY20 Table 2: Relief well inspection and sediment removal summary Table 3: Manhole and outfall information Table 4: Initial Capacity Test Results Table 5: Final Capacity Test Results Table 6: Summary and Recommendations

LIST OF FIGURES

Figure 1: Project feature map Figure 2: Aerial photo showing project features Figure 3: General embankment section Figure 4: Left abutment embankment typical section (station 125+00) Figure 5: Typical construction details for original project relief wells Figure 6: Field sketch showing the configuration of the outfalls, collector pipes, and manholes (relief wells RW-64B to RW-67). Not to scale Figure 7: Photograph of biological materials in manhole for relief well RW-67A Figure 8: Groundwater flowing around well riser pipe at base of manhole vault during capacity testing in relief well RW-67B Figure 9: Airlifted material (post-surge) from relief well RW-67A Figure 10: Airlifted material (post-surge) from relief well-68A Figure 11: Initial discharge water at the start of post-surge airlifting (RW-68) Figure 12: Final discharge water at the end of post-surge airlifting (RW-68)

LIST OF APPENDICES

Appendix A Location Maps Appendix B Field Forms

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1. INTRODUCTION

The purpose of this report is to document the results of an FY20 drilling program plan to capacity test and rehabilitate (as required) 11 relief wells at Big Bend Dam. The 11 relief wells are located near the left abutment of the dam in areas with identified risk-driving failure modes. The purpose of this work was to measure and mitigate any losses in well efficiency that may have occurred since installation of the relief wells; thereby ensuring that they continue to relieve pressures in the left abutment foundation.

1.1 Big Bend Dam 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.

1.2 Objectives and Justification

The objectives of the work outlined in this report were to capacity test and rehabilitate (if required) relief wells RW-65A through RW-68B. These 11 relief wells are located in the general area of two PA-identified risk-driving failure modes (PFM 31 and PFM 32). The capacity testing and rehabilitation of relief wells is part of the routine operation and maintenance procedures at the dam in accordance with EM 1110-2-1914, Design, Construction, and Maintenance of Relief Wells which states that all relief wells should be pump tested every five years to measure and mitigate any appreciable losses in well efficiency.

Based on the funding received in FY20, relief wells RW-65A through RW-68B were identified as the highest priority due to their proximity to PFM31 and PFM32. None of the 11 relief wells had been tested within the last five years.

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2. 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. The project was constructed from 1963 to 1966.

2.1 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 and Figure 2). 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.

Figure 1: Project feature map

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Figure 2: Aerial photo showing project features

2.1.1 Embankment

The rolled, zoned (Figure 3), 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 3: General embankment section

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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 4), 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 4: Left abutment embankment typical section (station 125+00)

2.1.2 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.

2.1.2.1 Pressure Relief Wells

The original system of pressure relief wells was installed in 1963 and extends a distance of 7,125 feet along the toe of the dam (Figure 2). 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.

3. FY20 DAM SAFETY INSTRUMENTATION FIELD WORK

The FY20 dam safety instrumentation field effort at Big Bend Dam was conducted in two phases from May 4 through May 21, 2020 and from June 15 through June 20, 2020 by the U.S. Army Corps of Engineers (USACE), Omaha District drill crew. The primary goal of this investigation was to rehabilitate and capacity test instrumentation to relieve uplift pressures at Big Bend Dam. Specific tasks included the inspection, capacity testing, and rehabilitation of relief wells RW-65A through RW-68B as discussed in

Engineering Geology Report 6 June 2020 the following sections. Maps with the instrument locations are included as Appendix A. Field forms are included as Appendix B.

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 was 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); EM 1110-2- 1914, Design, Construction, and Maintenance of Relief Wells (29 May 1992), the drilling program plan, and specific guidance as referenced in these sections.

3.1 Relief Well Construction Details

A total of 11 relief wells in the left abutment area were capacity tested and rehabilitated for this effort.

Construction details for the relief wells are provided in the following sections. Three of the relief wells (RW-66, RW-67, and RW-68) are original wood-stave project relief wells constructed in 1963. Eight of the relief wells are stainless steel Muni-Pak™ wells installed in 2010 and 2012. Relief well construction details are summarized in Table 1. Locations are shown on Figures A-1 and A-2 in Appendix A.

3.1.1 Original Project Relief Wells (1963)

The original project relief wells (RW-66, RW-67, and RW-68) consist of a gravel-packed, eight-inch-diameter, slotted, wood stave well screen attached to an eight-inch-diameter plastic riser pipe. Access to the well is through a corrugated metal well pit covered with a steel cover. Discharge of the well is either into the relief well channel or into the relief well collector pipe. The screen slots are 3/16-inch wide with 30 square inches of slotting per lineal foot of screen. Construction details are provided on Figure 5.

Locations are shown on Figures A-1 and A-2 in Appendix A.

Figure 5: Typical construction details for original project relief wells

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3.1.2 2010 Relief Wells The relief wells installed in 2010 (RW-65A, RW-66A, RW-66B, and RW-67A) consist of 50-slot Muni- Pak™ stainless steel well screens with a five-inch-diameter inner screen and a seven-inch-diameter outer screen. The riser consists of five-inch-diameter, Schedule-80, PVC flush-treaded pipe.

3.1.3 2012 Relief Wells The relief wells installed in 2012 (RW-66C, RW-67B, RW-68A, and RW-68B) consist of 40-slot Muni- Pak™ stainless steel well screens with a five-inch-diameter inner screen and a seven-inch-diameter outer screen. The riser consists of five-inch-diameter, stainless steel flush-treaded pipe.

Table 1: Construction details for relief wells to be capacity tested in FY20

Relief Well Designation

Year Installed

Bottom Depth (ft bgs)1

Screen Interval (ft bgs)1

Construction Material

RW-65A 2010 80.1 30.1-79.8 5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 80 PVC riser

RW-66 1963 80.0 25.0-80.0 8-inch ID wire-wrapped wood-stave screen, 8-inch plastic riser pipe

RW-66A 2010 81.4 31.4-81.1 5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 80 PVC riser

RW-66B 2010 82.4 32.4-82.1 5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 80 PVC riser

RW-66C 2012 82.0 36.5-81.5

5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 40 stainless steel riser

RW-67 1963 84.7 22.4-84.7 8-inch ID wire-wrapped wood-stave screen, 8-inch plastic riser pipe

RW-67A 2010 82.3 32.3-82.0 5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 80 PVC riser

RW-67B 2012 83.0 37.0-82.0

5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 40 stainless steel riser

RW-68 1963 87.1 26.5-87.1 8-inch ID wire-wrapped wood-stave screen, 8-inch plastic riser pipe

RW-68A 2012 89.0 47.9-87.9

5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 40 stainless steel riser

RW-68B 2012 90.0 48.6-88.6

5x7-inch ID Muni-Pak pre-packed stainless steel screen, 5-inch Schedule 40 stainless steel riser

1All depths are below ground surface.

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3.2 Relief Well Inspection

Prior to capacity testing and/or rehabilitative efforts, each of the relief wells was sounded for depth and the results compared to the original as-built design depths as described in the following sections and summarized in Table 2. All of the wells were noted to be in good condition with sediment accumulations less than 2.6 feet.

Table 2: Relief well inspection and sediment removal summary

Relief Well Designation

As-Built Bottom Depth (ft bgs)

Sounded Depth May 2020 (ft btoc)

Top of outer casing stick-up

(ft ags)

Sounded Depth May

(ft bgs)

Depth of Sediment

(ft)

Condition/Comments

RW-65A 80.1 82.3 1.3 81.0 -0.9

Top of well riser pipe is 5.6 feet below top of outer casing or 4.3 feet below ground surface. Good condition.

RW-66 80.0 81.3 2.4 78.9 1.1

Top of well riser pipe is 5.2 feet below top of outer casing or 2.8 feet below ground surface. Good condition.

RW-66A 81.4 83.2 1.5 81.7 -0.3

Top of well riser pipe is 5.2 feet below top of outer casing or 3.7 feet below ground surface. Good condition.

RW-66B 82.4 83.0 0.7 82.3 0.1

Top of well riser pipe is 4.8 feet below top of outer casing or 4.1 feet below ground surface. Good condition.

RW-66C 82.0 83.3 1.5 81.8 0.2

Top of well riser pipe is 7.8 feet below top of outer casing or 6.3 feet below ground surface. Good condition.

RW-67 84.7 85.4 1.8 83.6 1.1

Top of well riser pipe is 8.3 feet below top of outer casing or 6.5 feet below ground surface. Good condition.

RW-67A 82.3 82.4 0.5 81.9 0.4

Top of well riser pipe is 4.9 feet below top of outer casing or 4.4 feet below ground surface. Good condition.

RW-67B 83.0 83.8 1.1 82.7 0.3

Top of well riser pipe is 6.1 feet below top of outer casing or 5.0 feet below ground surface. Good condition.

RW-68 87.1 85.4 0.9 84.5 2.6

Top of well riser pipe is 9.3 feet below top of outer casing or 8.4 feet below ground surface. Good condition.

RW-68A 89.0 90.9 1.8 89.1 -0.1

Top of well riser pipe is 8.9 feet below top of outer casing or 7.1 feet below ground surface. Good condition.

RW-68B 90.0 89.7 1.6 88.1 1.9

Top of well riser pipe is 8.2 feet below top of outer casing or 6.6 feet below ground surface. Good condition.

ft = feet, bgs = below ground surface, btoc = below top of outer casing, ags = above ground surface

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3.2.1 Pre-capacity procedures

Typically, relief well outfall pipes are shut or packered off during capacity testing to allow the static water levels to equilibrate prior to the measurement of drawdown. At Big Bend, none of the eleven relief well outfalls are equipped with shut-off valves to stop flow. Three of the eleven wells (RW-66, RW-66A, and RW-66B) discharge into the relief well channel. The outfalls for the other eight relief wells flow into collector pipes or other relief wells/manholes as noted in Table 3. A field sketch showing the configuration of the outfalls and collector pipes from relief well RW-64B to RW-67 is provided as Figure 6.

Table 3: Manhole and outfall information

Relief Well Designation

Outfall/manhole details Notes

RW-65A RW-65A flows into the manhole for RW-65 which discharges into the relief well channel.

No packer or sump pump needed. Drawdown measured from top of flowing water level.

RW-66 Outfalls directly into the relief well channel -outfall invert at elevation 1362.92 ft.

Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level.

RW-66A Outfalls directly into the relief well channel -outfall invert at elevation 1365.32 ft.

Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level.

RW-66B Outfalls directly into the relief well channel -outfall invert at elevation 1365.94 ft.

Outfall for RW-66C was packered off during capacity testing on RW-66B to keep water from flowing into well during test. Drawdown measured from top of flowing water level.

RW-66C RW-66C flows into the manhole for RW-66B.

Groundwater was observed flowing up from ground around well riser pipe during capacity testing. Sump used to dewater manhole for RW-66C to prevent water from flowing into well during test. Drawdown measured from top of flowing water level.

RW-67 Outfalls into MH-2.

Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level. Water barely flowing from top of casing.

RW-67A Outfalls into MH-2.

Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level. Water barely flowing from top of casing.

RW-67B RW-67B flows into RW-67A which outfalls into MH-2.

Groundwater was observed flowing up from ground around well riser pipe during capacity testing. Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level.

RW-68 In series to the collector drain. No packer or sump pump needed. Drawdown measured from top of flowing water level.

RW-68A Outfalls into the collector drain.

Groundwater was observed flowing up from ground around well riser pipe during capacity testing. Drawdown measured from top of flowing water level.

RW-68B Outfalls into the collector drain.

Manhole filled with algae/gunk. Removed with sump pump prior to testing. Drawdown measured from top of flowing water level.

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Figure 6: Field sketch showing the configuration of the outfalls, collector pipes, and manholes (relief wells RW-64B to RW-67). Not to scale.

Further complicating the testing, all relief well riser pipes were cut off below ground surface near the depths of the outfalls and completed to the surface with 36-inch diameter outer casings placed over the wells (Figure 7). For this reason, the drawdown was measured from the existing water levels in all relief wells (typically near the base of each outfall). Because the drawdown readings were taken from flowing conditions, the measured drawdown is less than what it would be under static conditions. Because of this, the specific capacity measured from the outfall will be higher (less drawdown at the same pumping rate) than what would be recorded under static conditions. It should be noted, however, that the original capacity tests on the newer wells were also performed after the outfalls were installed and under flowing conditions.

As shown in Table 3, sump pumps were required in several of the relief well manholes due to algae/debris that had accumulated in the bottom of the vaults. The growth had to be removed with a sump pump prior to testing to avoid drawing the material into the well during testing and rehabilitation. Groundwater was also observed to be flowing around the outside of the riser pipe at the base of the vault in three of the relief wells, RW-66C, RW-67B, and RW-68A. It appears that the seal between the bottom of the vault and the riser pipe has been compromised in these three wells and groundwater is flowing up through the seal.

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Figure 7: Photograph of biological materials in manhole for relief well RW-67A

Figure 8: Groundwater flowing around well riser pipe at base of manhole vault during capacity testing in relief well RW-67B

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3.3 Capacity Testing

Once the initial inspection was made (note that no sediment was required to be removed), the 11 relief wells were capacity tested. The capacity tests were performed using a five-inch-diameter submersible pump. Each relief well was pumped to achieve a drawdown similar to the original pumping rates/drawdowns for a minimum of two hours unless rapid drawdown was obtained (RW-67 and RW- 67A). Where rapid drawdown was obtained at a very low pumping rate (~1 gpm), the test was stopped and the well was identified for rehabilitation. The pump inlet was placed the minimum required distance below the water level in the relief well to obtain adequate drawdown, yet remain above the top of the well screen. Flow was determined by the use of an in-line flow meter and bucket and stopwatch. Recharge was monitored immediately after pumping until the water level recovered to 95% of the measured pre-pumping static water level. Pre-rehabilitation capacity test data is summarized in Table 4. All data was recorded on the Capacity Test Data Form (Appendix B). The results were compared to the original pump test results obtained after installation (if available) to determine if the wells have experienced diminished capacity and to compare to the post-rehabilitation capacity testing to measure the effectiveness of the rehabilitative efforts as discussed in Section 3.5.

Table 4: Initial Capacity Test Results

Relief Well Designation

2020 Pre-Rehab Pumping Rate

(gpm)

2020 Pre-Rehab Drawdown (ft)

2020 Pre-Rehab Specific Capacity

(gpm/ft of drawdown)

2020 Pre-Rehab Approx. Recovery

Time (>95%)

Original Specific Capacity

(gpm/ft of drawdown)

RW-65A 13.3 15.9 0.8 1 minute 17/13.6 (1.3)

RW-66 23.3 16.2 1.4 2 minutes, 30 seconds 24.4/6.1 (4.0)

RW-66A 32.5 18.8 1.7 40 seconds 61/10.3 (5.9)

RW-66B 12.7 18.0 0.7 40 seconds 24/17.7 (1.4)

RW-66C 20.3 18.0 1.1 1 minute, 10 seconds

23/12.3 (1.9)

33/17.8 (1.9)

49/26.9 (1.8)

RW-67 1 >10 NA Not measured (slow recovery) 46.3/5.9 (7.8)

RW-67A 1 >10 NA 20 minutes NA

RW-67B 12.2 17.0 0.7 2 minutes, 30 seconds

23/12.1 (1.9)

35/18.7 (1.9)

49/28.2 (1.7)

RW-68 10.1 9.8 1.0 3 minutes, 20 seconds 7.2

RW-68A 21.2 11.4 1.9 40 seconds

23.5/1.7 (13.8)

33.6/4.0 (8.4)

45.7/7.4 (6.2)

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Relief Well Designation

2020 Pre-Rehab Pumping Rate

(gpm)

2020 Pre-Rehab Drawdown (ft)

2020 Pre-Rehab Specific Capacity

(gpm/ft of drawdown)

2020 Pre-Rehab Approx. Recovery

Time (>95%)

Original Specific Capacity

(gpm/ft of drawdown)

RW-68B 9.2 18.8 0.5 4 minutes

20.5/14.6 (1.4)

31.2/19.5 (1.6)

41.4/30.8 (1.3)

3.4 Mechanical Rehabilitation

After the initial capacity tests were completed, the wells were mechanically cleaned to remove as much biomass, mineral scale, and sediment from each well as possible. In accordance with EM 1110-2-1914, the wells were only to be mechanically rehabilitated if determined to be below 80 percent of the original capacity (all tested wells met the guideline for rehabilitation). Most relief wells undergo some loss in specific capacity primarily due to the slow movement of foundation fines into the filter pack with a corresponding reduction in permeability. The mechanical rehabilitation was conducted on wells with diminished capacity to remove any debris and break apart any materials that may be obstructing the well screens.

Chemical treatment was not utilized for this field effort. The process of mechanical rehabilitation continued until the water was generally free and clear of fines. All relief wells were sounded before and after rehabilitation and those depths were compared to the as-builts to ensure that all sediment was removed after the rehabilitation process. Significant amounts of sediment (between 2 and 4 feet) were only observed to be entering the well through the screen from the surrounding material/filter pack during development in relief wells RW-68A and RW-68B (mostly fines and fine sand). The steps for the mechanical rehabilitation are described in the following sections.

3.4.1.1 Scrubbing and Airlifting

The initial phase of mechanical rehabilitation consisted of mechanically scrubbing the entire length of each well with a nylon brush to remove bio-foul and mineral scale within the well casing and screen.

Before any actual scrubbing was initiated, a pre-pass test was conducted which consisted of the brush assembly being slowly lowered the entire length of the well to determine if the brush assembly would pass without binding or encountering obstructions. No unknown obstructions were encountered in any of the relief wells. Brushing was performed with stroke lengths of no more than five feet. Pertinent details of the brushing process were recorded on the attached Relief Well Rehabilitation Forms. Once brushing was complete, the water column in the well was pumped utilizing an airlift method until the well was clear of biomass and sediment (note that none of the wells was noted to contain significant bio-mass or mineral scaling).

3.4.1.2 Surging and Airlifting (Stainless Steel Wells)

At the completion of brushing and airlifting, the stainless steel relief wells were surged with a Q-water development tool (with built-in relief valve) to attempt to break up any bridged fines around the filter pack and increase the capacity of the well. Each well screen was surged in sections with the stroke length not exceeding10 feet with a stroke rate of no more than 4 feet/second. Each 10-foot length of rehabilitation zone proceeded for 15 minutes. Most of the material that entered the wells was noted to be fines and fine sand, as shown in Figure 9. Relief wells RW-68A and RW-68B had significant amounts of sand in the discharge, as shown in Figure 10. Once surging was complete, the water column in the well was pumped utilizing an airlift method until the well was clear of biomass and sediment. After airlifting was complete, the bottom depth was measured to confirm that all material at the bottom of the well from surging has been removed.

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Figure 9: Airlifted material (post-surge) from relief well RW-67A

Figure 10: Airlifted material (post-surge) from relief well-68A

3.4.1.3 Airlift Surging and Airlifting (Wood-Stave Wells)

Because the wood-stave wells are more than 50 years old and more fragile/susceptible to collapse, surge blocks were not utilized for these wells (RW-66, RW-67, and RW-68). Instead, the wells were rehabilitated by creating a surging action with the airlift mechanism by alternately drawing the water column down to a depth just above the top of the screen and shutting the air off allowing the well to recharge. This process continued until the recharge improved enough to allow continuous pumping at which time continuous airlifting was conducted until the water was free and clear of fines.

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Figure 11: Initial discharge water at the start of post-surge airlifting (RW-68)

Figure 12: Final discharge water at the end of post-surge airlifting (RW-68)

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3.4.2 Post-Rehabilitation Capacity Testing

Once the mechanical rehabilitation was complete, the relief wells were again tested for capacity utilizing the steps identified in Section 3.3. All data was recorded on the Capacity Test Data Form (attached).

Table 5: Final Capacity Test Results

Relief Well Designation

2020 Post-Rehab Pumping Rate

(gpm)

2020 Post-Rehab Drawdown (ft)

2020 Post-Rehab Specific Capacity

(gpm/ft of drawdown)

2020 Recovery Time (>95%)

Original Specific Capacity

(gpm/ft of drawdown)

RW-65A 26.3 9.3 2.8 15 seconds 17/13.6 (1.3)

RW-66 29.9 13.5 2.2 90 seconds 24.4/6.1 (4.0)

RW-66A 59.5 15.7 3.8 20 seconds 61/10.3 (5.9)

RW-66B 28.7 10.2 2.8 20 seconds 24/17.7 (1.4)

RW-66C 32.7 10.1 3.2 20 seconds

23/12.3 (1.9)

33/17.8 (1.9)

49/26.9 (1.8)

RW-67 14 10.3 1.4 2 minutes, 30 seconds 46.3/5.9 (7.8)

RW-67A 14.5 13.6 1.1 45 seconds NA

RW-67B 20.7 10.8 1.9 40 seconds

23/12.1 (1.9)

35/18.7 (1.9)

49/28.2 (1.7)

RW-68 16.0 8.8 1.8 1 minute, 45 seconds 7.2

RW-68A 33 26.5 1.2 2 minutes

23.5/1.7 (13.8)

33.6/4.0 (8.4)

45.7/7.4 (6.2)

RW-68B 16.2 20.0 0.8 2 minutes, 15 seconds

20.5/14.6 (1.4)

31.2/19.5 (1.6)

41.4/30.8 (1.3)

3.5 Summary of Results and Recommendations

A summary of the results of the relief well rehabilitation and capacity testing are include in Table 6 along with recommendations for each well based on the testing.

Engineering Geology Report 17 June 2020

Table 6: Summary and Recommendations

Relief Well Designation

Baseline Specific Capacity

(gpm/ft of drawdown)

2020 Pre-Rehab Specific Capacity

(gpm/ft of drawdown)

2020 Post-Rehab Specific Capacity

(gpm/ft of drawdown)

Comments Recommendation

RW-65A 17/13.6 (1.3) 13.3/15.9 (0.8) 26.3/9.3 (2.8)

Specific capacity increased from 0.8 to 2.8 gpm/ft drawdown after rehabilitation.

Recovery improved from 60 seconds to 15 seconds.

Relief well is currently at ~215% of original capacity. Relief well should be capacity tested again in five years and rehabilitated as required.

RW-66 24.4/6.1 (4.0) 23.3/16.2 (1.4) 29.9/13.5 (2.2)

Specific capacity increased from 1.4 to 2.2 gpm/ft drawdown after rehabilitation.

Recovery improved from 150 seconds to 90 seconds.

Relief well is currently at ~55% of original capacity. Relief well should be evaluated and considered for abandonment and replacement (if determined to be necessary) due to its age and the decrease in capacity. If not replaced, relief well should be capacity tested again in five years and rehabilitated as required.

RW-66A 61/10.3 (5.9) 32.5/18.8 (1.7) 59.5/15.7 (3.8)

Specific capacity increased from 1.7 to 3.8 gpm/ft drawdown after rehabilitation.

Recovery improved from 40 seconds to 20 seconds.

Relief well is currently at ~64% of original capacity but is still higher than surrounding wells. Relief well should be capacity tested again in five years and rehabilitated as required.

RW-66B 24/17.7 (1.4) 12.7/18.0 (0.7) 28.7/10.2 (2.8)

Specific capacity increased from 0.7 to 2.8 gpm/ft drawdown after rehabilitation.

Recovery improved from 40 seconds to 20 seconds.

Relief well is currently at ~200% of original capacity. Relief well should be capacity tested again in five years and rehabilitated as required.

RW-66C

23/12.3 (1.9)

20.3/18.0 (1.1) 32.7/10.1 (3.2)

Specific capacity increased from 1.1 to 3.2 gpm/ft drawdown after rehabilitation.

Recovery improved from 70 seconds to 20 seconds.

Relief well is currently at ~170% of original capacity. Relief well should be capacity tested again in five years and rehabilitated as required.

Repair leak around outside of pipe (bottom of vault).

33/17.8 (1.9)

49/26.9 (1.8)

RW-67 46.3/5.9 (7.8) 1 gpm/drawdown below screen 14.0/10.3 (1.4)

Specific capacity increased from drawdown into screen at 1 gpm to 1.4 gpm/ft drawdown after rehabilitation. Recovery 150 seconds after rehabilitation (not measured after pre-rehab test).

Relief well responded to rehabilitation; however, it is currently at only 18% of original capacity.

Relief well should be evaluated and considered for abandonment and replacement (if determined to be necessary) due to its age and the decrease in capacity. If not replaced, relief well should be capacity tested again in five years and rehabilitated as required.

Engineering Geology Report 18 June 2020

Relief Well Designation

Baseline Specific Capacity

(gpm/ft of drawdown)

2020 Pre-Rehab Specific Capacity

(gpm/ft of drawdown)

2020 Post-Rehab Specific Capacity

(gpm/ft of drawdown)

Comments Recommendation

RW-67A NA <1 gpm/drawdown below screen 14.5/13.6 (1.1)

Specific capacity increased from drawdown below top of screen at 1 gpm to 1.1 gpm/ft after rehabilitation (note the initial capacity could not be measured due to immediate drawdown). Recovery improved significantly from 20 minutes to 45 seconds.

Original capacity data is not available for this relief well due to immediate drawdown below top of screen occurring during initial test (and also during the pre-rehabilitation test). Relief well responded to rehabilitation and is currently at 1.1 gpm/ft capacity. Relief well should be capacity tested again in five years and rehabilitated as required.

RW-67B

23/12.1 (1.9)

12.2/17.0 (0.7) 20.7/10.8 (1.9)

Specific capacity increased from 0.7 to 1.9 gpm/ft drawdown after rehabilitation.

Recovery improved from 150 seconds to 45 seconds.

Relief well is currently at ~100% of original capacity. Relief well should be capacity tested again in five years and rehabilitated as required.

Repair leak around outside of pipe (bottom of vault).

35/18.7 (1.9)

49/28.2 (1.7)

RW-68 7.2 10.1/9.8 (1.0) 16/8.8 (1.8)

Specific capacity increased from 1.0 to 1.8 gpm/ft drawdown after rehabilitation.

Recovery improved from 200 seconds to 105 seconds.

Relief well is currently at ~25% of original capacity. Relief well should be evaluated and considered for abandonment and replacement (if determined to be necessary) due to its age and the decrease in capacity. If not replaced, relief well should be capacity tested again in five years and rehabilitated as required.

RW-68A

23.5/1.7 (13.8)

21.2/11.4 (1.9) 33/26.5 (1.2)

Specific capacity decreased from 1.9 to 1.2 gpm/ft drawdown after rehabilitation.

Recovery decreased from 40 seconds to 120 seconds. More substantial sediment was drawn into the well during pumping than other relief wells tested.

Relief well is currently at ~10% of original capacity and had significant sediment drawn in during testing. Relief well design should be investigated and the well should be considered for abandonment, redesign, and replacement. If not replaced, relief well should be capacity tested again in five years and rehabilitated as required.

Repair leak around outside of pipe (bottom of vault).

33.6/4.0 (8.4)

45.7/7.4 (6.2)

RW-68B

20.5/14.6 (1.4)

9.2/18.8 (0.5) 16.2/20.0 (0.8)

Specific capacity increased from 0.5 to 0.8 gpm/ft drawdown after rehabilitation.

Four feet of sediment was drawn into the well during post-surge airlifting. Recovery improved slightly from 150 seconds to 135 seconds.

Relief well is currently at ~50% of original capacity and had significant sediment drawn in during testing. Relief well design should be investigated and the well should be considered for abandonment, redesign, and replacement. If not replaced, relief well should be capacity tested again in five years and rehabilitated as required.

31.2/19.5 (1.6)

41.4/30.8 (1.3)

Appendix A: Location Maps

&%RW-68

RW-67

RW-66

RW-67A

RW-66B

RW-65A

RW-68B

RW-68A

RW-67B

RW-66C

RW-66A

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community, Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

LEGEND

Relief Wells Rehabilitated and Capacity Tested (FY20)

Relief Wells not Rehabilitated or Capacity Tested (FY20)

³ U.S. Army Corps of Engineers

Omaha District

Big Bend Dam Buffalo and Lyman Counties, SD

Relief Well Location Map

March 2020 FIGURE A1

800 0 800400 Ft

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RW-66C

RW-66A

PZ-24

PZ-22

PZ-21

PZ-19

PZ 30

PZ 29

PZ 28

PZ 27

PZ 26

J970R J662R

J474R

J320RJ171R

H810R

H560R

D760R

PZ-25R

J662RaJ474Rb J474Ra

J320RbJ171Rb

H810Ra

H560Ra

D760Ra

J970Ra J662Rb

J320Ra

H1060R H1060Rb

H1060Ra

D760Rba

D560R-12

PZ-18R-12

J970Rba-12

H810Rba-12

Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community

LEGEND

Relief Wells to be Rehabilitated and Capacity Tested (FY20)

@A Existing Piezometer

³

_̂ South Dakota

Nebraska

Iowa

MinnesotaPierrePierre

U.S. Army Corps of Engineers Omaha District

Big Bend Dam Buffalo and Lyman Counties, SD

Relief Well Rehabilitation Location Map

March 2020 FIGURE A2

100 0 10050 Ft

Appendix B: Field Forms

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

GEOTECHNICAL LAB REPORTS

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No. Size (mm) Coarser Coarser

MRD Lab. No.: 3 inch 76.2

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

SPECIAL INSPECTIONS

1998 BRAUN INTECH PIEZOMETER REHAB AND RESPONSE TEST REPORT

BRAUN

I INTERTEC

Eng,,eers and Scientists

Serv , --,9 the Bu ,it and

NotorD; Er v , rorvnents

Piezometer Rehabilitation and Response Tests Big Bend Dam, South Dakota Contract No. DACW45-96-D-0004 Delivery Order No. 7

Prepared for

Department of the Army Corps of Engineers, Omaha District

Project No. DACW-45-9607 May 18, 1998

Braun Intertec Corporation

A. u , P.E.

ice President

BRAUN'

INTERTEC

Braun Intertec Corporation 6875 Washington Avenue South P 0 Box 39108

Minneapolis, Minnesota 55439-0108 612-941-5600 Fax. 942-4844

Engineers and Scientists Serving the Built and Natural Enoronments'

May 18, 1998 Project No. DACW-45-9607

Department of the Army Corps of Engineers, Omaha District Attn: CENWO-ED-GH (Skeen) 215 North 17th Street Omaha, NE 68102-4978

Dear Mr. Skeen:

Re: Contract No. DACA45-96-D-0004, Delivery Order No. 7

Braun Intertec Corporation has completed piezometer rehabilitation and response tests at the Big Bend Dam in South Dakota. Our observations, results and conclusions are provided in the attached report.

We appreciate the opportunity to provide our services to you. Should you have any questions regarding this report or the project, please call Pat Terhaar at (612) 683-8756 or Ray Huber at

(612) 942-4831.

Sincerely, Patricia M. Terhaar, PG Senior eologist

Attachment:

Report r\dacw\45\9607\9607-R01.wpd

A. Introduction

As a result of extremely high Lake Francis Case tailwaters, 55 piezometers on the down stream side of Big Bend Dam, South Dakota were submerged. The piezometers required rehabilitation and response testing to assure and confirm that they were working properly for their intended purpose of determining the groundwater piezometric elevation under areas of the dam.

On December 9, 1997 Braun Intertec Corporation (Braun Intertec) was authorized by the U.S.

Army Corps of Engineers (USACE) Omaha District Office to complete piezometer rehabilitation and response tests at the Big Bend Dam. This work was carried out under Contract No.

DACA45-96-D-0004, Delivery Order No. 7.

B. Scope of Work

The scope of work for this project was provided to Braun Intertec by the USACE in a Scope of Services document dated November 14, 1997. The following tasks were carried out by Braun Intertec.

• Elevation data for Lake Sharp and the Lake Francis Case tailwater for the period of piezometer rehabilitation was obtained from the Big Bend Project Office.

• Prior to rehabilitating each piezometer, the depth to water and depth of the piezometer were measured and recorded to the nearest hundredth of a foot. The condition of the piezometer was also recorded. For piezometers having a protective outer casing, depth measurements were made from the top of the well riser pipe (inner casing or TOR), except in some instances where the riser pipe was considerably lower than the outer casing. In those instances, the measurements were made from the top of the protective casing and are noted as TOC on the field data forms.

• Sediment was removed from each piezometer using compressed air. Plastic tubing was lowered into the piezometer 1 to 2 inches above the base. Compressed air was blown through the tubing, gradually increasing the pressure until it was sufficient to eject water and sediment from the top of the pipe. An up and down surging motion was used on the tube until the water appeared clear and free of sediment. Potable water was added as necessary to remove all of the sediment.

Department of the Army Corps of Engineers Project No. DACW-45-9607 May 18, 1998

• Following rehabilitation, all water (to the degree possible) was removed from the piezometer with compressed air. A recovery rate (response) test was then performed by measuring and recording water levels in the piezometer at specified intervals as indicated on the Piezometer Test Data Forms in Appendix A.

• After completion of the response testing, the final water level and depth of the piezometer were measured and recorded.

• The recovery rate test data were analyzed using the aquifer test analysis computer program AQTESOLV, developed by Geraghty and Miller, Inc. An estimate of hydraulic conductivity was obtained for the aquifer in the vicinity of each piezometer at the horizon of the well screen.

C. Deviations

The following deviations to the proposed scope of work occurred.

• A post-response testing piezometer depth was not measured on the following wells:

D-760-RB, PZ-2, PZ-2A, PZ-4, PZ-6, PZ-8, PZ-8A, 11810-RB-A, and H1060-RA.

• Piezometer F560-RB was damaged prior to our arrival at the site. We were able to repair this piezometer by reattaching the riser pipe; however, the lock ring remained broken and a new cap is needed.

• Ten of the piezometers contained diesel fuel. According to the Big Bend Project Office, the diesel fuel was added several years ago to keep the water in the piezometers from freezing. Based on information provided to us, at one time the thickness of diesel fuel in the piezometers ranged from 1 to 7 feet. At the direction of the Project Office, wearing protective clothing, Braun Intertec personnel used compressed air to eject the diesel fuel along with the water onto the ground surface.

Due to malfunction of one of the water level tapes, we used a water level tape borrowed from the Big Bend Project Office for some of the response test measurements in three of the piezometers. Since the borrowed water level tape was marked only in 1-foot increments, we were only able to take measurements to the nearest tenth of a foot using a ruler held against the water level tape.

Project No. DACW-45-9607 May 18, 1998

A piece of PVC pipe was inadvertently dropped down PZ-5, but was later retrieved.

The depth of a piezometer outside of the flooded area, E130-RB, was measured and assessed for the presence of sediment as a comparison to the wells in the flooded area. A noticeable thickness of sediment was observed. We were not able to determine the thickness of the sediment without knowing the constructed depth of the piezometer, but the feel of the water level probe at the base of the well was similar to many of the rehabilitated wells.

Two piezometers (PZ-2A and PZ-8A) were dry when initially checked and remained dry after rehabilitation. Therefore, response tests could not be performed. Piezometers

B480-R and PZ-22 initially had water but remained dry after rehabilitation, and piezometers PZ-11A and PZ-14A had very little recovery. D760-RB-A,, J970-RB-A and

PZ-25R had a very fast recovery, with most of the recovery occurring before measurements could be made. Therefore, hydraulic conductivity values were not calculated for any of these wells.

• Six piezometers (see Table 1) had ice which prevented us from obtaining an accurate initial water level measurement. Likewise, ten piezometers contained diesel fuel, which did not allow an accurate initial water level measurement.

None of the deviations described above prevented us from achieving the goal of removing the sediment in the piezometers that resulted from flooding. With the exception of the dry piezometers and the ones showing very little recovery, none of the deviations are believed to have affected the recovery rate test results.

D. Results and Conclusions

Table 1 provides a summary of water level measurements, well depth measurements and well conditions. General field notes and completed Piezometer Response Test Data Forms for each of the piezometers are contained in Appendix A. Well depths measured after the response testing are indicated in the comments column. On most of the forms a notation of H (hard) or S (soft) was used to indicate whether sediment was felt (i.e., H indicates that no sediment was felt).

Project No. DACW-45-9607 May 18, 1998.

As shown in Table 1, the initial depths measured by Braun Intertec agreed fairly well with the depth measurements made in Augus C xeept for in piezometers F560-RB, PZ-2 PZ-2Aand 199 PZ-21. The likely cause of this discrepancy for PZ-2 and PZ-21 is that the August measurements were made relative to the top of protective casing while our December 1997 measurements were made from the top of riser. The August depth of F560-RB was nearly 1 foot deeper than in

December indicating additional sediment entered the well after August, or an inaccurate measurement was made. The August depth for PZ-2A was approximately 0.5 feet greater than the December measurement; however, the well was dry in December and no sediment was felt.

Sediment was generally felt prior to rehabilitation in the piezometers unless otherwise noted in the comments section of Table 1. Fourteen of the piezometers exhibited an increase in measured depth greater than 0.1 feet after rehabilitation. Of these, five had an increase greater than 1 foot;

these are listed below.

Piezometer Feet

F560-RB 1.33

H1060-RB 0.95

H1310-RB 2.20

J474-RB 0.64

J662-RB 0.63

With the exception of F560-RB, clear, sediment-free water was obtained from all of the wells at the end of rehabilitation and prior to the response testing. Piezometer F560-RB was flushed for one hour without achieving clear water. An additional well volume of water was removed prior to response testing; however, sediment was still present. This was the piezometer that required repair prior to rehabilitation. A white precipitate believed to be scaling was observed in the water removed from some of the piezometers.

At the end of the response testing, a small amount of sediment could be felt using the water level probe at the base of many of the piezometers. This sediment is believed to have come into the piezometer through the screen during water level recovery.