A22c_Finalized Arkabutla IRRM DIPP.pdf

September 2023

Emergency Installation of Structural Interim Risk Reduction Measures Drilling Program Plan (DIPP)

United States Army Corps of Engineers

Arkabutla Dam (NID MS 01496) Yazoo River Basin, Mississippi Dam Embankment

Drilling Program Plan

Disclaimer: The contract specifications and plans govern in the event of a conflict between the contract plans and specifications and the Drilling Program Plan.

Vicksburg District CUI

Drilling and Invasive Program Plan ii Arkabutla Lake Dam

Table of Contents

1 DAM SAFETY OFFICER / LEVEE SAFETY OFFICER CERTIFICATION

2 PROFESSIONAL ENGINEER/ PROFESSIONAL GEOLOGIST CERTIFICATION

3 OVERVIEW

3.1 Program Objective

3.1.1 Justification

3.1.2 Program Scope

3.2 Program Team

3.2.1 Program Team Requirements

4 GEOLOGY, GEOTECHNICAL, AND HYDROGEOLOGIC SITE CONDITIONS

4.1 Regional Geology

4.2 Site Engineering Geology

5 PROPOSED DRILLING AND INVASIVE ACTIVITIES PROGRAM PLAN (DIPP)

5.1 Site Layout and Access

5.2 Anticipated Subsurface Conditions

5.3 Drilling and Sampling Methodology

5.3.1 Drilling Methodology

5.3.2 Sampling Methodology

5.3.3 Drill Log Requirements

5.4 Relief Well Construction

5.5 Piezometer Construction

5.6 Boring Abandonment and Site Restoration

6 RISK EVALUATION AND MITIGATION

7 REFERENCES

APPENDIX A: Historic Borings and Plates

APPENDIX B. Instrumentation Layout

APPENDIX C. Hydraulic Fracture Calculations

APPENDIX D. Piezometer Specifications

Drilling and Invasive Program Plan iii Arkabutla Lake Dam

Table of Figures

Figure 1. Project Location Map Figure 2. Aerial view showing key project features (red arrow orients north) Figure 3. Site Layout Map Figure 4. Schematic drawing of Stilling Basin and Underslab Drainage exit Figure 5. Plan view of Relief Wells Figure 6. Site layout of new wells and piezometers to be installed Figure 7. Generalized Geologic Profile along Center Line of Arkabutla Dam Figure 8. Typical Section of the Main Embankment Figure 9. North pilot boring particle size distribution report. Graphic indicates depth of the critical geologic unit at 74’ Figure 10. South pilot boring particle size distribution report indicating depth of the critical geologic unit at 74’ Figure 11. Particle size distribution report for the south pilot boring illustrating a typical SP foundation sand Figure 12. Outlet works piezometer locations Figure 13. Boring log view of Pilot Borings drilled in June 2023. Red outline is the proposed screened interval for the relief wells Figure 14. Plan view of Borings 2-72 and OSB 2-78 Figure 15. Relief well schematic indicating screen and riser lengths and lateral tie in elevation Figure 16. Relief well schematic illustrating desired thicknesses of filter pack, bentonite seal, and grout cap Figure 17. Schematic of underslab piezometers from sheet C‐501

Table of Tables Table 1. Relief Well Specifications Table 2. New Automated Piezometers Table 3. MVK Drilling Program Plan Team Roster

Drilling and Invasive Program Plan i Arkabutla Lake Dam

1 DAM SAFETY OFFICER / LEVEE SAFETY OFFICER CERTIFICATION

The Risk Management Center’s (RMC) Drill Plan Review Team has reviewed the DPP for the installation of the automated instrumentation along the profile of the outlet works of Arkabutla Dam. They provided comments back to the District; those comments were evaluated, incorporated, and back checked with no additional review items. The review will be attached to the end of the DPP, and the signed final copy of the DPP will be placed on the CE-DALS ProjectWise datasource.

This Drilling Program Plan has been developed and reviewed by experienced professionals and is in compliance with all the requirements of ER-1110-1-1807. The proposed actions are justified and have been developed to minimize the likelihood of damage to the existing structure.

Lanny B. Barfield, P.E.

Dam Safety Officer Vicksburg District, Mississippi Valley Division

2 PROFESSIONAL ENGINEER/ PROFESSIONAL GEOLOGIST

CERTIFICATION

This Drilling Program Plan has been developed and reviewed by experienced professionals and is in compliance with all the requirements of ER-1110-1-1807. The proposed actions are justified and have been developed to minimize the likelihood of damage to the existing structure.

SIGNATURE

Name

Title

Date

Professional Geologist

01/18/2024

Drilling and Invasive Program Plan 2 Arkabutla Lake Dam

3 OVERVIEW

Arkabutla Dam is located in northwest Mississippi on the Coldwater River, a tributary of the Yazoo River.

The dam is located about 4.25 miles north of Arkabutla, MS, and it is classified as high hazard in accordance with ER 1110-2-1156. The location of the dam is shown in Figure 1 below. A site overview of the Arkabutla Dam Project is shown on Figure 2.

The Arkabutla Dam Project, along with three other flood control reservoirs in north Mississippi and other Yazoo headwater projects, were completed to reduce flooding in the Yazoo basin, and it has a drainage area of 1,000 square miles and has a flood pool surface area of 33,400 acres.

Figure 2-1.

Figure 1. Project Location Map

Drilling and Invasive Program Plan 3 Arkabutla Lake Dam

Figure 2. Aerial view showing key project features (red arrow orients north)

3.1 Program Objective

The objective of this Drilling and Invasive Activities Program Plan (DIPP) is to outline methods and procedures required to conduct a multi-staged drilling program to include the drilling and installation of a passive relief well system and new instrumentation located both upstream and downstream of the dam crest.

The scope of work is further detailed below in Section 3.1.2.

All drilling operations will be conducted within the footprint of the outlet works conduit and channel (see Figure 3 for site layout map). The placement of the new relief wells will be in close proximity to the existing relief wells which are situated nearest to the conduit in the north and south backfill areas of the outlet works.

The implementation of the additional relief wells will be the critical interim risk reduction measure (IRRM) in relieving underslab pressures once the underslab drainage system has been properly abandoned. Moreover, the installation and implementation of additional instrumentation at both the upstream slope of the dam and proximate to the conduit within the stilling basin will provide USACE personnel with more frequent data to assist in assessing pressure conditions that currently exist at the project for the ongoing Dam Modification Study.

Drilling and Invasive Program Plan 4 Arkabutla Lake Dam

Figure 3. Site Layout Map

3.1.1 Justification

The Vicksburg District declared a Dam Safety Emergency for Arkabutla Dam (NID MS01496) based on observed signs of distress associated with a risk-driving potential failure mode on 9 May 2023. Foundation material was found to be actively piping through the outlet works stilling basin underslab drainage system in an unfiltered manner leading to backwards erosion and piping. This failure mode was initially detected as a depression at the toe of the dam near the conduit, and was verified through observations of cloudy, sandy flow from the referenced drainage system’s discharge pipes. Many structural and non-structural interim risk reduction measures (IRRMs) have been implemented and/or will be implemented in the near future due to the potential breach emergency. MVK is developing plans and specifications for structural IRRMs which include the abandonment of the underslab drainage system, relief well installation, and additional instrumentation installation. The following DIPP was developed to outline approved drilling methodologies necessary for the installation of the proposed structural IRRMs as well as assess potential hazards associated with drilling in the proposed IRRM locations.

The 2021 Issue Evaluation Study (IES) of the Arkabutla Lake Dam evaluated a potential failure mode (PFM) of backward erosion piping (BEP) of conduit foundation material through an unfiltered exit (underslab drainage system) into the outlet works stilling basin (PFM 08). Samples obtained during the installation of six piezometers along the conduit indicated a continuous, fine uniform sand that extends from the upstream side of the dam to the downstream. Based on these samples and data from existing borings, the IES evaluation team concluded there was a high likelihood that the foundation materials beneath the conduit and stilling basin consisted of this sand. Laboratory analysis of the fine uniform sands resulted in computed Cu values that were indicative of materials susceptible to BEP. Additionally, the team determined that the most likely exit for this PFM was through the porous concrete underslab drainage system.

Drilling and Invasive Program Plan 5 Arkabutla Lake Dam

Performance issues related to the underslab drainage system at Arkabutla have been noted throughout the life of the project. Sand and organics have been observed in the stilling basin adjacent to the drains during past dewatering events. Moreover, the underslab drainage system at the spillway was experiencing similar issues, and therefore, was abandoned in 1968 and six relief wells were installed to mitigate increased underslab pressures.

Figure 4. Schematic drawing of Stilling Basin and Underslab Drainage exit

The two existing relief wells at the outlet works stilling basin were installed in 1993 to alleviate increased pressures beneath the outlet structure in case the porous concrete underslab drainage system was compromised. Due to recent investigations and IES results, MVK has concluded the underslab drainage system is compromised and will need to be properly abandoned, thus, requiring additional relief wells to replace the underslab pressure relief system of the stilling basin floor. In addition to the installation of the proposed relief wells, MVK plans to install additional instrumentation to monitor foundation pressures on the upstream side of the dam (closer to the tower) as well as additional instrumentation within the stilling basin to capture underslab pressures closer to the conduit.

3.1.2 Program Scope

The program scope includes:

(1) the drilling of two 4” pilot holes to inform relief well screen slot size and filter design, (this work has been completed by MVK Drilling Production – June 2023)

(2) the drilling and installation of six additional pressure relief wells at the outlet works (three wells on both the north and south sides of the Outlet Works),

(3) the drilling of eleven (11) boreholes, for the installation of twenty-four (24) new open standpipe piezometers.

a. Nine (9), 10-inch boreholes for the installation of twenty-two (22) foundation piezometers to monitor foundation pressures and provide denser data for analysis of the critical area of the dam will be drilled through the embankment, and

b. Two (2), 4-inch boreholes for the installation of two (2) piezometers within the stilling basin

Drilling and Invasive Program Plan 6 Arkabutla Lake Dam to capture underslab pressures proximate to the conduit.

Below the drilling campaign follows the listing above to further elaborate on both the approved drilling methods and the scope of work for each of the four items.

3.1.2.1 Drilling two 4 in. Pilot Holes adjacent to the Outlet Works (Work completed June 2023)

Prior to the drilling and installation of the wells, MVK drilling personnel drilled two, 4-inch pilot holes using hollow stem auger (HSA) and direct mud rotary drilling to allow for continuous sampling and logging of subsurface materials to better inform screen slot size selection and filter design. Hollow stem auger drilling was verbally approved by the Risk Management Center (RMC) since there were concerns with contaminating the pervious backfill with the infiltration of drilling fluids. Therefore, the MVK Drilling Production Center was able to successfully case the entire pervious backfill (~ 25-30 ft. depth) using 4.25-inch diameter auger flights and then proceed to drill foundation materials with direct mud rotary using a 4-inch fishtail bit.

The pilot boreholes were drilled approximately 25 ft. off the wingwalls on both the north and south sides of the Outlet Works to ensure that the bottom of the wingwall was not damaged by drilling activities. The boreholes were situated between the desired locations of the proposed wells. The boreholes were sampled continuously once the foundation materials were encountered and all drilling in the pervious backfill was completed. A professional geologist was onsite during drilling operations and assisted the field geologist with sampling and field classifications.

The Vicksburg District Soils Laboratory conducted the testing of samples collected for design analysis of a filter and relief well screen selection. Sieve analysis was completed on all samples collected while drilling in the foundation materials underlying the pervious backfill.

3.1.2.2 Drilling and Installation of a passive relief well system

The drilling and installation of a passive relief well system at the Outlet Works is considered the critical structural interim risk reduction measure (IRRM) to mitigate the risk of dam breach due to backward erosion and piping along the conduit. Installing the passive relief well system will allow the District to effectively abandon the underslab drainage system, which is currently acting as an unfiltered exit, eliminate the potential for further removal of foundation sands under the conduit, and adequately relieve uplift pressures beneath the stilling basin floor.

The passive relief well system will consist of six (6) 12-inch diameter relief wells with 8-inch lateral discharge pipes that will exit through the outlet channel wingwall into the stilling basin. Figure 5 below depicts the well configuration as set in the final plans and the coordinates are provided in Table 1. Three (3) wells are located on either side of Outlet Works within their respective backfill areas. A conical check valve will be added to the exit of the lateral discharge pipe to prevent backflow into the relief well. The relief well lateral inverts will be set at Elevation 172 ft.

All six relief wells consist of 30 ft. of screened interval and 40 ft. of riser pipe as tabulated below in Table 1.

Laboratory analysis results gathered from samples collected at the two pilot borings informed the design of the proposed relief wells. Screens will consist of a Hi-flow stainless steel screen with wire wrapping. The specifications for screen size and length for each well is in Table 1. Details and plans concerning the construction of the relief wells are articulated further in Section 5.4 of this report.

Current approved methods for all drilling to be conducted on site, as discussed with the RMC, include:

Relief Well Installation:

- Reverse Circulation – preferred method.

Relief well drilling and installation will be carried out by an external contractor utilizing the preferred drilling method, reverse circulation drilling. If a comparable/safer method is suggested by the contractor, it must not only have district approval, but it must also be approved by the RMC. A USACE Vicksburg District professional engineer and/or professional geologist will oversee drilling operations.

Drilling and Invasive Program Plan 7 Arkabutla Lake Dam

Table 1. Relief Well Specifications

Well Northing Easting Diameter (in)

Riser Length (ft)

Screen Length (ft)

Lateral Elevation (ft)

Screen Slot Size

N Outlet Well (existing)

1912901.8 2358944.2 8

35 30 No Lateral 0.030 in

North Well No. 1 1912905.8 2358927.3

12 40 30

172 0.030 in

North Well No. 2 1912926.3 2358905.1 12 40 30 172 0.030 in North Well No. 3 1912948.1 2358884.2 12 40 30 172 0.030 in S Outlet Well (existing)

1912831.1 2358906.7 8 27 40 No Lateral 0.030 in

South Well No. 1 1912839.4 2358892.4 12 40 30 172 0.030 in South Well No. 2 1912846.1 2358862.9 12 40 30 172 0.030 in South Well No. 3 1912851.0 2358833.1 12 40 30 172 0.030 in

Section 5 of this report provides further explanation of anticipated subsurface conditions and important information pertaining to the execution of this proposed scope of work.

Drilling and Invasive Program Plan 8 Arkabutla Lake Dam

Figure 5. Plan view of Relief Wells.

Drilling and Invasive Program Plan 9 Arkabutla Lake Dam

Figure 6. Site layout of new wells and piezometers to be installed.

Drilling and Invasive Program Plan 10 Arkabutla Lake Dam

3.1.2.3 Drilling and installation of twenty-four (24) foundation piezometers

In addition to the relief wells, the Vicksburg District has developed additional structural IRRMs that include the drilling of eleven (11) boreholes of varying diameter and depth and the installation of twenty-four (24) piezometers both upstream of the dam centerline and within the stilling basin. Table 2 below provides pertinent details for each of the new piezometers including, location, type of instrumentation, diameter, open standpipe material, quantity of open standpipes, screened elevations, borehole diameter, and total length of piezometer. All of the piezometers will be tipped in the foundation sands found underlying the conduit and stilling basin. The locations of these instruments are depicted on the above figure (Figure 6).

All new piezometers installed will have a diameter of 1.25 in. and be constructed with schedule 80 PVC with a #10 slot screen (0.010 in.), with the exception of 2 piezometers located in the stilling basin.

Piezometers located in the stilling basing shall be 1.25 in. and be constructed with stainless steel with a #10 slot screen. The slotted screens will be 1 and 2 ft. in length. Details concerning the construction and installation of the piezometers is covered in Section 5.5 of this report.

Each embankment borehole consists of "nested" open standpipe piezometers. This means that multiple open standpipe piezometers will be installed in each borehole at varying tip depths. The "nested" piezometers range from 2 to 3 open standpipes in each borehole. Each embankment borehole will be 10 in. in diameter to accommodate for multiple open standpipes, excepting two (2) boreholes in the stilling basin, which will be 4

in. in diameter. Two (2) of the boreholes and piezometers will be cored through the centerline of the outlet works concrete floor and set into the subsurface foundation. This will consist of a 4 in. diameter coring that must penetrate 5 feet of concrete slab, 1 foot of porous concrete, an unknown depth of cement grout that exist from previous repairs, and 3 feet of foundation material in which the piezometer screen will be tipped.

The piezometer riser for each underslab piezometer must be trenched into the concrete floor and wall to protect the riser.

Current approved drilling methods for Piezometer Installation:

- Hollow Stem Auger (HSA) – dry methods are required through the dam embankment and until foundation materials are encountered.

- Direct Mud Rotary – can be utilized in the drilling of the piezometers once the foundation materials are encountered and drilling within embankment materials has been completed.

Table 2. New Automated Piezometers.

Additionally, twenty-one (21) existing piezometers will be outfitted with vibrating wire instrumentation by the contractor; however, this structural IRRM does not require any drilling and/or invasive activities, therefore, it has been excluded from this DIPP.

P‐7‐23 1912821.3 2359299.6 101+53 VWP 1.25 PVC 3 200 183 168.5 10 252 166.5

P‐8‐23 1912856.9 2359228.1 182+85 102+21 VWP 1.25 PVC 2 183 168.5 ‐‐‐ 10 267 166.5

P‐9‐23 1912914 2359114.6 182+85 103+61 VWP 1.25 PVC 2 184 168.5 ‐‐‐ 10 221 166.5

P‐10‐23 1912735.9 2359250.2 ~183+83 ~101+61 VWP 1.25 PVC 2 183 168.5 ‐‐‐ 10 267 166.5 P‐11‐23 1912687.2 2359225.9 ~184+37 ~101+61 VWP 1.25 PVC 3 183 175 168.5 10 267 166.5 P‐12‐23 1912600.5 2359187.4 185+35 101+53 VWP 1.25 PVC 3 200 175 168.5 10 252 166.5 P‐13‐23 1912638.1 2359112.3 185+35 102+21 VWP 1.25 PVC 3 183 175 168.5 10 267 166.5 P‐14‐23 1912692.6 2359003.6 185+35 103+61 VWP 1.25 PVC 2 184 168.5 ‐‐‐ 10 221 166.5 P‐15U‐23 1912848.1 2358966.9 184+10 104+63.5 VWP 1.25 STEEL 1 165 ‐‐‐ ‐‐‐ 4 205 163*

P‐16U‐23 1912871 2358922 184+10 105+13.5 VWP 1.25 STEEL 1 159 ‐‐‐ ‐‐‐ 4 205 157*

P‐17‐23 1911678.2 2358520.4 196+50 103+03 VWP 1.25 PVC 2 165 130 ‐‐‐ 10 255 128

Boring ID NORTHING EASTING DAM

PIEZOMETER SCREEN

ELEVATIONS, FT.

MIN.

HOLE

DIA., IN.

TYPE DIA., IN. CASE QTY

BOTTOM

OF HOLE,

FT.

TOP OF

RISER

EL, FT.

CONDUIT

APPROX.

COORDINATES

STATIONING

Drilling and Invasive Program Plan 11 Arkabutla Lake Dam

3.2 Program Team

The project delivery team for the design, along with the preparation of this DIPP consisted of the following individuals:

Table 3. MVK Drilling Program Plan Team Roster

Name Function Organization

Kolb, Dakota Professional Geologist CEMVK-ECG-G Dees, Jillian Geological Engineer CEMVK-ECG-G Griffin, Wiley Geologist CEMVK-ECG-G Jarabica, Mason DSPM – Geotechnical CEMVK-ECG-I Reves, Ryan Chief, Dam Safety CEMVK-ECG-I Cummins, Brandon Chief, Geology and Lab CEMVK-ECG-G Leopard, Jarsh (Lin) Chief, Geodetic Section CEMVK-ECG-D Penley, William (Rusty) Field Foreman, Drilling Crew CEMVK-ECG-D Woodward, Chris Lead Driller, Drilling Crew CEMVK-ECG-D Barfield, Lanny Dam Safety Officer CEMVK-ECG

The drilling and installation of the proposed pressure relief wells and piezometers will be contracted out by MVK; however, the two, 4” pilot boreholes for sampling purposes were conducted by the MVK Drilling Production Center (DPC) (see the project delivery team listed above).

3.2.1 Program Team Requirements

As per ER 1110-1-1807, the contractor selected to execute this work will have a minimum of 3 years of experience in using reverse circulatory drilling on at least two dam and/or levee projects, as well as the other approved drilling methodologies listed above. Additionally, the contractor must maintain a State of Mississippi Water Well Drillers license. Drilling and onsite oversight of reverse circulation drilling operations will be performed by experienced drilling and technical staff which will be included in appropriate contracting documents.

Revisions to this drilling program plan will be made to include the contractor’s team members and their specific work experience once a selection has been made by MVK. If an alternative drilling methodology is proposed by the contractor and reviewed and approved by district staff and the RMC this document will be amended with the appropriate methods, calculations, and specifications. Only methods determined safer to the structural integrity of the existing foundation conditions and dam structures will be evaluated and must adhere to USACE Engineering Regulation (ER)1110-1-1807.

Vicksburg District will be responsible for providing a professional geologist and/or professional engineer for onsite oversight during drilling operations. For the duration of this document, relief well borings will be discussed as planned reverse circulation (RC) drilling until/unless another submitted and approved method is decided on.

All drilling and sampling will be conducted in accordance with EM 1110-1-1804, Geotechnical Investigations (January 2001); ER 1110-1-1807, Drilling in Earth Embankment Dams and Levees (June 2023); and specific guidance as referenced in this section.

Drilling and Invasive Program Plan 12 Arkabutla Lake Dam

4 GEOLOGY, GEOTECHNICAL, AND HYDROGEOLOGIC SITE

CONDITIONS

The Arkabutla Dam site lies within the drainage basin of the Coldwater River in what is known as the “Loess or Bluff Hills” physiographic region of north Mississippi. The eastern edge of the Mississippi River Floodplain is limited by these bluffs, which are capped with a mantle of loess. The topography of the area is that of a typical loess type, characterized by pronounced hills with steep slopes, narrow flat-topped ridges, and narrow intervening valleys.

4.1 Regional Geology

The geologic formations of the hills around the dam site consist of several terrace deposits (Bentley, Montgomery, and Prairie) that were laid down by the Mississippi River during periods of glacial retreat. The highest hills in the area are 380 feet above mean seal level and the lowest part of the Coldwater River valley is 195 feet above mean sea level. Except for the bluffs just north of the dam site, the change in relief in any direction is seldom more than 40 feet per mile. Immediately underneath the main embankment of the dam are Recent Alluvial deposits composed of generally impervious topstratum, underlain by pervious substratum.

The topstratum varies in thickness from 20 to 40 feet and is composed of relatively impervious clays, silts, and fine sands. The substratum varies in thickness from 20 to 50 feet and is composed of pervious sands with some gravel layers, as well as minor silt and clay lenses. The Recent Alluvial deposits under the dam generally underlain by Tertiary clays of the Cook Mountain Formation (Eocene Age).

4.2 Site Engineering Geology

The Coldwater River valley at the dam site is filled with alluvium that has an average thickness of 76 feet.

The alluvium is divided into a topstratum of fine-grained sediments (clays, silts, and some fine sands) and a substratum of predominately coarser grained sediments (sands and gravels). The topstratum ranges from 20 to 40 feet in thickness with the average being 32 feet. Clay (CL) varying from 3 to 24 feet in thickness caps the surface and is underlain by interbedded silts (ML) that sometimes contain clay strata; silty sands, (SM) that sometimes contain clay strata; fine sands, or traces of gravel; and very soft to very stiff clay (CL-CH) that may contain rootlets, wood lignite, or traces of gravel. The topstratum-substratum contact varies between elevations 174 to 154. The substratum consists of fine to medium sands (SP, Cu<1) that may or may not contain gravel. Generally, these are underlain by sandy or clayey gravel (GP). However, when lenses or strata of silty sand or clay (CL) are present, some may by as thick as 15 feet. The substratum ranges from 20 to 50 feet thick with the average being 36 feet. The alluvium is underlain by a minimum of 20 and possibly as much as 50 feet of medium-stiff to hard Tertiary clay of the Eocene, Claiborne Group, Cook Mountain Formation from Station 145+00 to Station 182+00. However, from Station 135+00 to 145+00 and 182+00 to 190+00, the alluvium is underlain by a thick section of Tertiary sands and silty sands. General foundation condition is shown below on Figure 6.

Drilling and Invasive Program Plan 13 Arkabutla Lake Dam

Figure 7. Generalized Geologic Profile along Center Line of Arkabutla Dam

Construction of Arkabutla Dam began in August 1940 and it was completed in June 1943 with the conduit being placed into operation sometime after August 1941 and the project being into operation sometime after June 1943. The dam is founded on topstratum alluvial deposits of clays and silts underlain by a substratum of dense sands.

The main embankment is of rolled earth fill construction approximately 7,300 feet in length. The entire dam including the abutment dikes is 10,700 feet in length. The crest elevation is at 264.3 with a maximum height above the valley floor of 85 feet. The embankment is constructed primarily of a uniform lean clay (CL) material. The embankment was generally constructed with 1V on 2.5H slopes from the crest down to elevation 239, and 1V on 3H below elevation 239. The embankment has a 40 feet wide crest. The upstream face of the embankment is protected by a 2.5 feet thick layer of riprap placed on a 8 inch thick layer of bedding stone. The downstream face of the embankment is protected by sod cover. A 10 feet wide berm is location at elevation 239 on the downstream face of the dam. Concrete abutment and toe ditches are provided on the downstream face to intercept surface runoff. Upstream and downstream stability berms are provided at the south end in the closure section. Both these berms are approximately 140 feet in width, 20 feet in thickness, and extend from approximately Station 187+00 to the south abutment.

Figure 8. Typical Section of the Main Embankment

The outlet works consists of gated intake structure, conduit, and stilling basin. The conduit’s location was selected such that the conduit could be founded on sands to reduce the amount of settlement. The conduit’s foundation consists of course to medium sand (Cu>1) extending to approximately Elevation 130. The foundation is underlain by a dense layer of fine to medium sands with an average blow count of 55.

For the construction of the conduit, the alluvial deposits were excavated so that the conduit is founded on substratum sands. Where the conduit crosses the old river channel the channel was infilled with waste material. The placement of the conduit on substratum sands rather than on the alluvium clays and silts was intended to reduce the issue of differential settlement (because of the continuous pour design). In doing so, Drilling and Invasive Program Plan 14 Arkabutla Lake Dam the monolithically constructed conduit is now directly in contact with the pervious foundation sands.

Drawings indicate that two feet of impervious backfill was compacted around the conduit in 2-inch loose lifts (compacted by a pneumatic hand tamper), but the bottom portion of the conduit still sits directly on the foundation sands.

5 PROPOSED DRILLING AND INVASIVE ACTIVITIES PROGRAM PLAN

(DIPP)

The following sections detail key components of the proposed drilling campaign including site access, anticipated subsurface conditions, drilling and sampling methodology, relief well construction, piezometer construction, and lastly, boring abandonment and site restoration.

5.1 Site Layout and Access

All drilling and installation of the proposed structural IRRMs will be localized to an affected area adjacent to the upstream and downstream ends of the conduit and the Outlet Works as seen in Figure 6 in Section 3.1 above. Access roads will consist of the Arkabutla Dam Road that is situated along the dam crest and HWY 301 which is situated along the toe of the dam.

5.2 Anticipated Subsurface Conditions

Several foundation borings exist in the area of the outlet works and dam that will be used to anticipate subsurface conditions. This includes several borings located along the center line of the dam completed in 1940 pre-construction, three borings completed in the 1970’s located downstream of the dam and adjacent to both the north and south wing wall, and two borings completed in 2020 located upstream of the dam.

Borings located along the axis of the dam and centerline of the outlet works used to interpret subsurface conditions include boring C-40, K-86, C-45, K-87, C-50, K-117, K-88, K-118, C-55, K-89, C-60, K-90, and C-66. These borings were completed in 1940 utilizing auger and bailer. Sampling was conducted with an auger and drive tube. Most of the borings are completed from El 198 to El 95. From El 200 to El 190 a clay blanket approximately 5 feet in thickness can be expected, followed by a silty clay or silty loam. El 190 to El 180 consists of silty clay to silty loam. El 180 to El 170 consist of silty loam interbedded with poorly graded sand. El 170 to El 130 consists of medium to coarse grain sands, interbedded with gravel in a fining upward sequence. El 130 to El 120 consists of approximately 10 ft of clay strata that increases in thickness north of the centerline of the outlet works. South of the outlet works centerline the clay layer is either thinned or nonexistent and lithology consists of fine to medium sands. El 120 to 100 consists of fine to medium grain sands.

Foundation borings 2-72, OSB-1-78, and OSB-2-78U are located adjacent to the north and south wing walls at the outlet works, downstream of the dam. The location of the proposed relief wells falls in the vicinity of these foundation borings. These borings were completed in 1972 and 1978 utilizing mud rotary drilling method and samples were collected with 2.5-inch drive tube and 5-inch vacuum tube. The borings and their locations are added as attachments to this drill plan (Appendix A). Borings were completed from El 200 to El 100. El 200 to El 190 consists of a clay blanket less than 5 ft in thickness underlain by medium to coarse grain sands. El 190 to El 170 consists of medium to coarse grain sands interbedded with silt and clay. El 170 to El 100 consists of fine to medium grain sands with interbedded gravel. Lens of clay and silt are also present. Additionally, an appendix to the 1994 Periodic Inspection report for Arkabutla Dam, Appendix G – Arkabutla Outlet Works Emergency Relief Wells, includes useful documentation on the installation of the existing relief wells located adjacent to the outlet works. The existing wells, N Outlet and S Outlet, were

Drilling and Invasive Program Plan 15 Arkabutla Lake Dam installed at total depths of 80 to 90 ft. Materials encountered during drilling included similar materials as mentioned above. A relatively thin clay layer was noted (as seen in Boring 2-72 at El 130) as being the basal unit of the contact with the underlying Tertiary sands. The thickness of the clay layer may increase downstream considering the notes in Appendix G that discuss the clay layer being closer to 8 ft. in thickness (El 130 – 122). The relief wells will be tipped in the alluvium foundation sands overlaying the thin clay layer encountered at depths greater than El 130. The screened interval will approximate 30 ft.

Borings P-1-19 and P-2-19 are located on the upstream side of the dam. These borings were completed in 2020 in order to install piezometers and were completed using hollow stem auger and mud rotary drilling method. Samples were collected with split spoon and Shelby tube. These boring logs and locations are found in Appendix A. Both borings were completed from El 266 to El 166. El 266 to El 246 consists of reddish tan sandy clay. El 246 to El 181 consisted of tan silty clay. El 181 to El 170 consisted of a grey silty clay. El 170 to El 166 consisted of grey silty sand fining upward into a fine to medium grain, poorly graded sand, that continues to TD (El 166).

Water surface elevations measured in backfill piezometers located adjacent to the outlet works are consistently below tailwater by approximately 7 to 10 ft. Foundation piezometers and existing relief wells at the outlet works have water surface elevations that consistently exceed tailwater but are still well below the ground surface. Locations of these piezometers are found on the instrumentation plate, Figure 12, on the subsequent page. Piezometers at the outlet works are being monitored and recorded daily.

Boring logs of the two pilot borings drilled in June 2023 are shown on the subsequent page, Figure 10, respectively. These borings were collared in pervious backfill at EL 200 on both the North and South wing walls and terminated at EL 100 in Tertiary sands. Continuous samples were collected once encountering foundation material. Samples were processed and sieved by MVK Geotechnical laboratory staff to inform relief well design specifications and add resolution to anticipated subsurface site conditions. These holes identified a critical geologic unit near EL 130. It is imperative that the contractor consider the following subsurface conditions and consult the logs in Figure 10.

The critical geologic unit is a very fine poorly graded Tertiary sand (SP) underlying a discontinuous Tertiary clay. This clay caps the critical Tertiary SP sand north of the outlet works centerline typically around EL 130.

The clay can be a good indicator of approaching this critical sand but is unfortunately absent and discontinuous at the new relief well locations proposed in this DIPP. It is essential that the contractor avoid this fine SP. The relief well design specs were not designed to be placed in this material. In both the north and south pilot borings this Tertiary sand was encountered at a depth of 74’ or EL 126. For this reason, all relief well drilling proposed in this DIPP shall be terminated no deeper than EL. 130.5 to avoid the Tertiary sand. Figures 9 and 10 on the following pages illustrate consistency of this depth between the two borings. Percent fine sand content is regularly 90% or higher at depths below 74’ at both locations.

Relief well screen slot size and engineered filter pack material were designed to be emplaced in foundation sands and gravels. Comparison of Figures 9 and 10 to Figure 11 illustrate a clear distinction between typical foundation sand and the Tertiary sand. In Figure 9 and in Figure 10 it is evident that drilling reached the Tertiary sand unit at the 74 – 76’ interval. Percent fine sand becomes dominant from this interval to the total depth of the borehole. It regularly occurs with concentrations over 90% fine sand. The particle size distribution report in Figure 11 indicates a normal foundation sand in the area with fine sand content consistently remaining in the 30% range.

Drilling and Invasive Program Plan 16 Arkabutla Lake Dam

Figure 9. North pilot boring particle size distribution report. Graphic indicates depth of the critical geologic unit at 74’.

Drilling and Invasive Program Plan 17 Arkabutla Lake Dam

Figure 10. South pilot boring particle size distribution report indicating depth of the critical geologic unit at 74’.

Drilling and Invasive Program Plan 18 Arkabutla Lake Dam

Figure 11. Particle size distribution report for the south pilot boring illustrating a typical SP foundation sand.

All subsurface electrical and water utilities will be located by project personnel using existing utilities maps and marked using colored paint prior to the drilling activity. Figure 6 found in Section 3.1 above depicts all known utilities at the project location. All other utilities (telephone, etc.) will be marked by outside contractors, such as Re-engineered Business Solutions (RBS).

Drilling and Invasive Program Plan 19 Arkabutla Lake Dam

Figure 12. Outlet works piezometer locations.

Drilling and Invasive Program Plan 20 Arkabutla Lake Dam

Figure 13. Boring log view of Pilot Borings drilled in June 2023. Red outline is the proposed screened interval for the relief wells.

Drilling and Invasive Program Plan 21 Arkabutla Lake Dam

Figure 14. Plan view of Borings 2-72 and OSB 2-78.

5.3 Drilling and Sampling Methodology

Drilling operations will be conducted in accordance with ER 1110-1-1807 and the best management practices outlined in EM 1110-1-1804. Sampling and logging methodology will be conducted using appropriate ASTM standards and best management practices outlined in EM 1110-1-1804.

5.3.1 Drilling Methodology

Relief Wells: Drilling and installation of six 12-inch diameter relief wells.

The drilling contractor will have a valid and up to date water well driller’s license applicable to the state of Mississippi and shall have a resume showing experience with drilling on at least 2 dams or levees per ER 1110-1-1807 Drilling and Invasive Activities on Dams and Levees. Drilling and sampling activities of the subsurface materials associated with the relief well installations will be accomplished by an experienced and qualified USACE contractor.

Due to the potential for large gravels being present in the substratum – as evidenced during toe ditch relief well installation and to maximize relief well efficiency- the use of reverse circulatory rotary drilling has been determined optimal for completing the wells. The area to be affected during drilling operations is located at the outlet works which is located downstream of the dam embankment. Casing will be set within

Drilling and Invasive Program Plan 22 Arkabutla Lake Dam the top 15 feet of each boring to provide borehole stability as well as reduce the risk of fracturing pervious backfill materials. Ideally, clean water will be the only drilling fluid utilized during initial stages of borehole advancement. However, a commercial drilling polymer can be used in the drilling fluid if needed to maintain borehole integrity. The drilling fluid will have a sufficient consistency and weight to prevent caving of the borehole and minimize intrusion of the drilling fluid into the overburden. The drilling fluid will also be light enough to minimize risk of potential hydraulic fracture. Ideally, the drilling fluid weight will be no more than 70 to 72 pounds per cubic foot, unless it is determined that a higher weight is necessary to stabilize the borehole from heave and/or control artesian conditions. It is not anticipated that heave and/or artesian conditions will be encountered. The drilling fluid mixture will be adjusted, as needed, to ensure that the hole is filled and remains stable at all times. Moreover, as per ER 1110-1-1807, the drilling equipment utilized during drilling operations must be equipped with a floating needle pressure valve to monitor and record maximum pressure spikes.

Reverse Circulation procedures listed below:

Flooded RC offers the best method for relief well efficiency and it is unlikely that the drilling fluid, if conditions warrant its use, will need to be significantly thickened.

This method was chosen given that it lowers potential for clogging behind the bit since drilling fluid is not being introduced into the hole via drill stem.

Drilling fluid will work its way down hole by gravity.

All drilling activities will be in unconsolidated soils not rock.

MVK made the decision to lower reservoir water levels as an interim risk reduction measure (to elevation 204 ft.); therefore, there is no anticipation of artesian pressures while drilling at the toe of the dam.

Piezometers: Drilling and installation of all piezometers. The drilling methodology will consist of using hollow stem auger (HSA) or other dry methods in accordance with ER-1110-1807 through the embankment and into the foundation materials. Mud Rotary drilling can be employed once the depth to the embankment-foundation interface is exceeded. Moreover, as per ER 1110-1-1807, the drilling equipment utilized during drilling operations must be equipped with a floating needle pressure valve to monitor and record maximum pressure spikes.

Locations can be found on attached plan included as Appendix B.

Mud Rotary procedures listed below:

All fracture calculations were made with this method since it yields the most conservative FS values.

Tools will be sized and designed to minimize the likelihood of the return flow clogging.

Fluid discharges from the bit will be upward.

Drill rods and tools will be raised and lowered slowly to avoid pressure changes in the drillhole.

Drilling feed rate will be slow enough to avoid overcrowding the bit.

Drilling media properties, pressure, and return will be continuously monitored, and a floating needle pressure valve utilized to record live and maximum pressures.

HSA will serve as casing advance through embankment. Below the embankment-foundation interface, drilling mud will be used during drilling operations to mitigate borehole collapse, and after completion of drilling activities and installation of the piezometers, the remaining borehole annulus will be backfilled using the appropriate procedures outlined in the piezometer construction section.

All drilling activities will be in unconsolidated soils not rock.

MVK made the decision to lower reservoir water levels as an interim risk reduction measure (to elevation 204 ft.); therefore, there is no anticipation of artesian pressures while drilling at the toe of the dam.

Drilling and Invasive Program Plan 23 Arkabutla Lake Dam

Hollow Stem Auger procedures listed below:

Hollow stem augers will be advanced in 5-foot increments.

Augers will be advanced 35 feet and serve as temporary casing for backfill material during pilot hole drilling.

All drilling activities will be in unconsolidated soils not rock.

The Vicksburg District has lowered the reservoir water levels to decrease elevation head across the structure and to mitigate inundation impacts in the case of a dam breach. Therefore, our current plan will be based on current conditions that exist today, a reservoir elevation of 204 ft. Backfill piezometers, mentioned in Section

4.2 above, are being read daily at the current reservoir level and are consistently below tailwater. Foundation piezometers, also read daily, are reading above tailwater, but are still well below the ground surface. The existing two wells at the outlet works are also being monitored and are available to be used for pumping to reduce pressures in the vicinity of the proposed drilling operations.

ER-1110-1807 Drilling and Invasive Activities on Dams and Levees 6-C states that in cases where specific details may be unknown at the time of the DIPP (contractor personnel, specific methodology, etc.), contingent approval can be provided, pending submission and approval of final details after contract award.

For this reason, the contractor may propose different drilling methodologies not listed in this document that are safer and conform more readily to ER-1110-1807. Any drilling method must be approved by USACE before operation can begin and methods must be validated by district staff with RMC approval. All drilling methodologies submitted must conform to ER-1110-1807.

5.3.2 Sampling Methodology

Relief Well Installation.

The drilling contractor will be directed to grab drill cutting samples for borehole logging purposes at each relief well site.

5.3.3 Drill Log Requirements

A complete and accurate drill log will be completed for all emergency-related drilling activities done onsite.

All logging activities will be completed by a Professional Engineer (PE) or Professional Geologist (PG) with at least five years’ experience. In addition, water well completion reports and drill logs will be completed by the contractor. Drilling activities will be accomplished by a licensed operator with a minimum of three years’ experience with the utilized drilling method. Subsurface data generated by drilling activities will be archived in the USACE authoritative boring log database. The contractor will provide data documentation within 30 days of drilling in digital format.

5.4 Relief Well Construction

MVK geotechnical branch used the results from the sieve analysis that was conducted on the samples collected at the two pilot borings to inform the engineering design of the new passive relief well system.

Each relief well will be completed with 12-inch diameter, rod based, stainless steel screen. Refer to Figures 15 and 16 on the subsequent pages for relief well schematics. The well screen will be commercially manufactured and consist of a non-clogging wire wrapped, continuous slot strainer. The continuous wire wrapping will be triangular toward the screen center. The width of the clear space between the wire wrapping is 0.030 inch. The outside diameter of the screen will be 12 inches. The relief well riser pipe material, along with all couplings, will also consist of new commercially manufactured stainless steel.

Figures 15 indicates the lateral tie-in elevation and Figure 16 illustrates the thicknesses, in elevation, of the

Drilling and Invasive Program Plan 24 Arkabutla Lake Dam well screen and associated filter pack. It can also be referenced for target bentonite plug and grout cap thicknesses. It is important that these elevations and thicknesses are targeted so that there is enough space to tie in and weld the lateral tie in for passive pressure relief.

Material for the filter pack around the riser pipes and screen will be a dry process sand, composed of hard, tough, and durable particles free from adherent coating. Limestone, dolomite, or carbonate materials will not be acceptable material. The filter pack shall meet the following gradation requirements:

U.S. Standard Sieve No. Percent by Weight Passing No. 8 100 No. 10 100-98 No. 12 98-90 No. 14 80-65 No. 16 60-45 No. 18 38-20 No. 20 9-0 No.25 4-0 No. 30 1-0

Filter pack materials will be uniformly graded between the limits specified above. For each relief well, the filter pack material and its gradation will be approved by USACE-Vicksburg District, before its placement.

During installation, the filter pack will be tremied into place to prevent segregation and ensure uniformity.

The filter pack will have a minimum thickness of 4 inches between the outside of the well screen and the natural formation. The contractor will be responsible for placing the filter pack, developing the filter pack, re-establishing the top of the filter pack as needed during development, testing the well by performing adequate pumping tests, and then sealing and grouting the well.

A minimum, 2-foot-thick bentonite seal, intended to keep cement grout from entering the filter pack, will be installed above the filter pack. The bentonite seal will consist of hydrated granular chips, or pelletized, sodium montmorillonite furnished in bags or buckets from a commercial source and will be free of impurities which adversely affect water quality. Powdered bentonite shall not be used. The bentonite seal will be allowed to fully hydrate for a minimum of 4 hours prior to cement grout placement.

A neat cement-bentonite grout will be placed via tremie pipe to fill the remaining annular space, above the bentonite seal, of each relief well. The cement-bentonite grout will consist of a mixture of Portland cement (conforming to ASTM C 150, Type I or Type II). One 94-pound bag of Portland cement will be mixed with six (6) gallon of water. Five (5) pounds of bentonite (or 5 percent by weight of cement) will be added to the mixture to reduce shrinkage and to hold the cement in suspension until the grout has set.

Following the placement of the filter pack materials, the contractor will, upon installation of each relief well, conduct a step-drawdown pump test shall be performed on each relief well to develop the wells and to provide critical efficiency data of each new well. The minimum pumping duration for each step of the drawdown tests shall be at least 30 minutes. The overall pump test shall include a minimum of 4 distinct steps for each well pumping test at 200 gpm, 400gpm, 600gpm and 800gpm plus or minus 20gpm. Specific language on step drawdown tests will be covered in the specification documentation.