4005 - Attachment V - GDSM Anchors Phase 2 DIPP_20250318.pdf

CONTROLLED UNCLASSIFIED INFORMATION (CUI)

Drilling and Invasive Activities Program Plan (DIPP)

Garrison Dam (ND00145) Missouri River, North Dakota Embankment, Outlet Works, Powerhouse, and Spillway Omaha District, Northwestern Division

Garrison Dam Spillway Modification Spillway Modification Anchor Investigation Phase 2

Date: March 2025 Status: Attachment for Investigation RFP

For Official Use Only

Garrison Dam Drilling and Invasive Program Plan (DIPP)

Anchor Investigation Phase 2 CUI

Page is intended to be blank.

Anchor Investigation Phase 2 CUI

CONTENTS

1. Introduction

2. Background Information

2.1 Garrison Dam

2.2 Spillway Structure

2.3 Site Geology

3. Investigation Justification and Objectives

3.1 Garrison Dam Spillway Modification (GDSM) Program

3.2 Previous GDSM Investigations

3.3 Anchor Investigation Phase 2 (This Investigation)

4. DIPP Scope (Anchor Investigation Phase 2)

4.1 Investigation Scope

4.2 Utilities

5. Risk Evaluation

5.1 Hydrofracture calculations

5.2 Other identified risks

APPENDICES

Appendix A: Geologic Profiles

Appendix B: Hydrofracture Analysis for Risk Evaluation

Notes on Document Intent

• This DIPP document is attached to the Anchor Test Investigation Phase 2 RFP “For Information Only”, to support the Contractor’s development of proposals and submittals.

• The Contractor submittals described in the PWS, primarily the “Anchor Test Plan” submittal, are scoped to include all investigation information required by ER 1110-1-1807. USACE will internally update and manage the comprehensive Garrison Dam Spillway Modification DIPP with no additional work required by the Contractor.

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1. INTRODUCTION

A Drilling and Invasive Activities Program Plan (DIPP) for Garrison Dam has been prepared in accordance with ER 1110-1-1807 (June 2023 initial published date). This exploration program comprises multiple investigations including drilling, sampling, installation and testing of pumping wells, piezometers, and anchors.

Garrison Dam is characterized as DSAC 2 (High Urgency) in the USACE portfolio of dams. A dam safety modification study was completed in 2023 and a recommended plan for risk reduction was approved by the USACE dam senior oversight group. The recommended plan addresses multiple potential failure modes relating to the project spillway. Project engineering and design (PED) for the Garrison Dam Spillway Modification (GDSM) initiated in 2023 and is scheduled to continue through approximately 2030. This DIPP relates to early field investigations intended to better inform the design.

Specifically, this drilling and field testing will be utilized to improve understanding of anchor constructability and field performance, allowing for refinement of the final design.

The Garrison Dam Spillway Modification PED has multiple investigation components including:

• FY23 Phase 1 Geotechnical Investigation (Initial DIPP Approval July 2023)

• FY24 Pumping Test Program

• FY24 Anchor Investigation Phase 1

• FY25 Anchor Investigation Phase 2 (Focus of this DIPP Document)

Relevant data collected from the previous investigations are provided in a separate Geotechnical Data Report.

2. BACKGROUND INFORMATION

2.1 Garrison Dam

The Garrison Dam – Lake Sakakawea Project was authorized by the Flood Control Act approved 22 December 1944, as part of the comprehensive plan for flood control and other purposes on the Missouri River Basin. The authorized purposes include flood risk management, hydroelectric power, water supply, water quality control, irrigation, recreation, navigation, and fish and wildlife. The dam was constructed primarily between 1947 and 1954. Major project components include the earth-fill dam embankment, hydroelectric generating plant, and the concrete spillway as well as those shown in Figure 1.

Figure 1. Overview of Garrison Dam Project.

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2.2 Spillway Structure

The spillway is cut in the left (east) bank of the river leaving about 1,800 feet of undisturbed material between it and the left abutment of the dam. The chute type spillway consists of an approach channel, crest structure including Tainter gates and bridge, abutments, control building, chute paving, chute walls, stilling basin, and return channel.

The spillway crest has a curved length between the abutment faces of 1,336 feet and an over-all length of 1,576 feet (including abutments). The weir, chute, and stilling basin have a combined length of about 3,000 feet. Various relief wells and subdrains are located beneath areas of the spillway to provide pressure relief from several different lignite layers throughout the east abutment.

Approach Channel: The spillway approach channel is cut in the left bank of the river just upstream from the dam and curves to become tangent to the centerline of the spillway about 240 feet upstream from the crest of the weir.

The bottom of the cut is at elevation 1,810 feet and the width varies from about 2,200 feet at the bank of the reservoir to 1,356 at the spillway crest structure.

Crest Structure: The crest structure consists of 27 independent monoliths laid out on 48-foot chords of a curve with a radius of 2420.75 feet. Each monolith is made up of a pier in the center with 20 feet of ogee type weir on either side to form a monolith 48 feet 3-1/2 inches wide at the upstream face, 46 feet 10-7/8 inches wide at the downstream face and 72 feet 1-1/4 inches long parallel with the flow. The piers support 28 tainter gates 40 feet wide by 29 feet high.

The weir is a mass concrete structure with shrinkage reinforcing in the face to control cracking and structural reinforcing in the bottom. The upstream face slopes up from the approach channel at elevation 1,810 feet on a batter of 3 to 1 to elevation 1,822.97 feet where it intersects the curved agee section. The crest is at elevation 1,825 feet from whence it curves down through a reverse curve to the point of tangency with the horizontal at elevation 1,802.5 feet and continues horizontally for about 7 feet to the joint with the articulating slab. A seat one foot wide is provided for support of the upstream edge of the articulating slab. The base of the crest structure is at elevation 1,792 feet and a heavy key (ten feet wide at the bottom) extends 23 feet below the base at the upstream edge. The downstream face of the key slopes 1 on 1.35 to intersect the base 31 feet from the downstream edge.

Chute: The spillway chute extends downstream of the crest structure from station 39+95.63 at the centerline of the spillway to station 66+50 where it intersects the horizontal section forming the stilling basin. The chute varies in width from approximately 1,290 feet to 800 feet. The chute slab is generally 18 inches thick with thickened edges to Station 62+50 from which it transitions to a 5-foot thickness at spillway station 63+00. A 12-foot-wide articulation slab is provided downstream of the weir. The entire slab is anchored to the foundation and where the slab is less than 5 feet thick an insulating layer of pervious material is provided to give a total thickness of 5 feet to protect the foundation from frost action. The chute walls are semi-gravity type varying in height from 20 feet to 83 feet 6 inches. From the crest structure to station 40+40 the slab is flat at elevation 1,802 feet. From there to station 58+06.43 it slopes at 0.75% and beyond station 58+06.93 it slopes at 20%.

Stilling Basin: The stilling basin is 800 feet wide and extends downstream of the chute from station 66+50 to station 68+60 at elevation 1,620 feet and includes baffles and an end sill. The stilling basin walls are a continuation of the chute walls with a top elevation of 1,704 feet. The stilling basin slab is 5 feet thick and anchored to the foundation.

A single row of 10 feet high by 8 feet wide baffles spaced at 16 feet on centers are provided at station 67+40. The end sill is 18 feet thick and steps up in three 4-foot steps to a height of 12 feet.

2.3 Site Geology

The materials that form the foundation of the dam consist of valley alluvium, glacial till, slopewash, and Fort Union Group (Tongue River Formation) bedrock as shown in Figure 2. Because the spillway is located in the abutment area, there is generally little or no alluvium overlying Fort Union in the vicinity of the spillway.

The Fort Union forms the bedrock of both the abutments and the valley floor below the valley fill and specifically acts as the foundation for concrete structures associated with the emergency spillway. With the exception of the lignite beds (low-grade coal) and sand deposits, the Fort Union is relatively impervious silts and clays (USCS of CL, ML, CH). The various sediments which make up the Fort Union are well consolidated but have no cementing material, with the exception of the harder lignite, sandstone, and limestone horizons.

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The regional dip of the rocks in western North Dakota is northwestward. The Fort Union, however, is nearly flat-lying and local irregularities and small structures, which can be discerned from a gentle undulation of the lignite beds, are common. The total thickness of the Fort Union Group and underlying Hell Creek Formation (which is of similar material) at the site is about 1000 feet. These formations lie above the Fox Hills sandstone which is 200 feet thick, and the Pierre Formation, which is over 1000 feet in thickness.

Primary historical references for geology, foundation testing and properties include:

• 1946 Definite Project Report; Including Appendix II: Geology, Appendix III: Embankment and Foundation Design

• 1948 Analysis of Design, Excavation and Main Embankment

• 1971 Earth Dam Criteria Report

• 1981 Embankment Criteria Report

• 1983 Construction Foundation Report Portions of these documents are included as needed in the attached Geotechnical Data Report.

The profile of the original ground surface along the dam axis from east to west (Figure 6) Note the distinction between overburden (green) and Fort Union group “bedrock” (blue) in the figure.

Figure 2. Geologic profile along dam axis facing upstream.

Figure 3. Outcrop exposure of Fort Union Group on-site

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Deposition: The dam site and most of the reservoir area lies in an upper portion of the Fort Union Group known as the Tongue River Formation (Earth Dam Criteria Report, 1971). The formation is highly compacted from glacial ice and previously eroded sediments with an estimated pre-consolidation under a load of 80 to 100 tons per square foot.

Prominent throughout the area are beds of lignite which are jointed and cracked, and usually water bearing. The sandy horizons also carry water. The thicker beds of lignite serve as correlative beds and may be traced for miles around. The formation is a lignite-bearing, non-marine, lacustrine delta deposit located in western North Dakota.

According to Jacob (1973), the deposits represent a basic cyclic unit related to the formation and filling of flood basins on an alluvial plain (Figure 7.

Figure 4. Basic cyclic unit in Tongue River Formation and variation of basic unit from Jacob, 1973)

Three basic lithologic types are locally present in the formation from bottom to top: (1) gray clay and silt, (2) lignite, and (3) sand. The gray clays and silts are interpreted as flood-basin deposits and are lignitic and fossiliferous. As the flood basins filled, swamps became established and lignite beds formed. The upper sand bodies (west and east abutment sands) consist of linear sand bodies which show deeply eroded bases and likely represent low-sinuosity channel deposits. Thin beds of limestone are also present in deposits, but are typically scarce in the gray clay and silt unit which makes up the majority of the bedrock foundation at the Garrison Dam Site. The Tongue River Formation is typically very calcareous, and the sand in the formation contains large amounts of primary clastic carbonate grains (up to 40 percent).

Regionally, crevassing and expansion of natural levees across the flood basins destroyed the swamps and deposited yellow silt and sand over the lignite. The yellow silt and sand (not observed at the Garrison Dam site) commonly show climbing ripples and remnants of vertical tree trunks, indicating high rates of deposition characteristic of natural levees and crevasse splays. The yellow silt and sand deposits represent natural levee or crevasse supply deposits where depositional rates were highest (Jacob, 1973).

Engineering Properties (Other than lignites): The clay and silt beds in the Fort Union make up the majority of the deposit at the dam site. According to Jacob (1973), these gray clay and silt units probably were deposited in flood basins on a floodplain. Units consisting of beds of gray clay and silt in the Tongue River Formation range from a few inches to more than 15 feet thick with varying shades of gray to tan color. Testing indicates that the clay and silt has properties intermediate between a “rock” and a “soil” similar to a soft shale. It differs from “rock” in that is not cemented, deriving its cohesion from compaction alone. It is believed that the consolidation is due to strata that has since been removed. The clay and silt beds typically classify as one of the following: fat clay (CH) lean clay (CL), silty clay (CL-ML), clay loam (CL), silty clay loam, (CL), or silty loam (ML).

The dominant materials in the Fort Union are claystone, siltstone, lignite, and sandstone, with prevalence in the order listed. The Fort Union sediments have no cementing material and are typically described using soil terms. The stability of the deposits is due to compaction alone. The material may be broken down into their constituent grains using mechanical analysis (Figure 8). More than 70 percent of the Fort Union materials at the dam site have an

5 of 20 Anchor Investigation Phase 2 CUI effective grain size of less than 0.005 millimeters (Ref: DPR). The range in gradation of these materials is shown in Figure 2-16.

The Fort Union varies between 95 and 115 pounds per cubic foot dry density and moisture content ranges from 16 to 24 percent. As a general rule, the in-situ Fort Union can be considered to have a dry density of about 104 pounds per cubic foot and a moisture content of' 21 percent (Ref: DPR).

Figure 5. Core recovered of typical Fort Union Group clay.

Figure 6. Fort Union gradation bands

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Mechanical analyses tests indicated a general range of soil types from fat clays to fine sands; however, the predominant soil type is a lean clay. Liquid limits range from 19 to 108 and plasticity indices range from non-plastic to 70.

Table 1. Atterberg limits of Fort Union gray clay and silt samples

The unconfined compressive strength of the Fort Union results almost entirely from its cohesion, not from cementation of the constituent particles. Tests indicate that the compressive strength varies from 2 to over 20 tons per square foot with strength between 2 and 6 tons per square foot occurring more frequently.

The 1952 Report on Stability of the Spillway Crest Structure (p.4) discusses that the design strength of C = 0.70 tsf, ɸ = 20° is applicable to permanent slopes in which a shear plane would cross multiple layers of stratification, but not for stability of the spillway crest structure where a failure plane is more likely to develop along a stratification or slickenside.

The 15- to 20-foot thick zone of Fort Union between the 1U and 1 1/2U lignites consists of fine clayey sand, is more pervious than the majority of the Fort Union materials, and is continuous beneath the crest (Ref: Spillway AD 6-01).

For these reasons, additional strength testing was completed along the planned base for the spillway crest structure (17 CU tests, 10 CD tests, and 1 undrained test). Six of the test specimens were from samples with visible slickensides, but only in two of the specimens were the slickensides still visibly apparent. The report selected strength parameters of C’ = 0, ɸ’ = 29° for the general Fort Union foundation in stability calculations for sliding/uplifting of the weir structure. For postulated slickensided zones, C’ = 0, ɸ’ = 25° was assigned. (1952 Stability Report plate 6).

Fat clay layers were observed immediately above and beneath both the 1 1/2U and 1U. Slickensides are common in the clays of the Fort Union; however, the fatter clays are generally more slickensided than the more silty clays, especially where they are adjacent to lignite. The slickensides are very discontinuous and lack any regular orientation. The slickensides in fat clays have spacings of 1 to 2 inches, and in silty clays are separated by 1 foot or more (Ref: 1952 Spillway Stability p2). Two field tests on a slickensided clay in the powerhouse test tunnel found that the strength of a slickensided clay may be as low as 60% of an intact clay. However, the lesser strength reduction to ɸ’ = 25° was considered appropriate to the designers due to the random and discontinuous nature of the slickensides (1952 Spillway Stability p4).

A total of 52 consolidation tests were performed on the Fort Union material and the results generally supported the assumption that the material had been previously consolidated by glaciation to loads much higher than any that would be imposed by the dam or structures. Results of the consolidation tests and seven absorption-pressure tests

7 of 20 Anchor Investigation Phase 2 CUI indicated to the designers that the theory of consolidation would not be applicable to the Fort Union foundation in terms of time rate of consolidation or to degree of pore pressure response to load conditions (Ref: 1983 CFR Chap 3 Section 9.3.6). Modulus of deformation of the Fort Union material was determined by results of consolidation and triaxial compression tests. The average modulus of deformation from the 52 consolidation tests was about 500 to 600 tons per square foot. The range of modulus as determined by the slope of the tangent of the stress-strain curves of 124 triaxial compression tests was found to be about 338 to 14,400 tons per square foot and as a whole was generally substantially above the values from consolidation testing. It was thought that the difference was due to the inability to develop lateral pressures equal to those in the triaxial compression tests. Consequently, only the triaxial compression tests were used for selection of the design parameters (Ref: 1983 CFR Chap 3 Section 9.3.7).

Engineering Properties (Lignite): The lignite is firm to hard in its unweathered state and is very dark brown in color.

The lignite layers are composed of plant accumulations, possibly from a swamp-like forest and have been metamorphosed to the degree of lignite coal classification. According to Jacob (1973), many of the lignite beds in the Tongue River are continuous for significant distances laterally. The lateral continuity of the lignite beds indicates that the flood basins in which they were deposited were extensive.

The lignite beds are the only horizons on which any stratigraphic correlation or structural continuity has been attempted (Figure 2-17 and Table 2-3). The lignite deposits on the east and west abutments were given different designations but can be correlated across the valley. Those beds are easily recognized in cores and there is little question as to their correct identity over limited areas. Variations in the elevations of lignite beds are mainly due to slumping of oversteepened bluffs, the burning out of lignite beds, the compression and unequal settlement of all the beds, or by original variations in the thickness of the beds from one place to another.

Figure 7. Correlation of lignite beds on east and west abutments.

Table 2. Lignite bed correlations

The water content of these lignites is higher and the specific gravity is lower than that of the clays, silt, and sand phases of the Fort Union (Ref: DPR), while harder than most of the other portions of the formation, are often

8 of 20 Anchor Investigation Phase 2 CUI fractured and contain numerous slickensides. The 1971 Earth Dam Criteria Report states that 3 direct shear tests (“S” type test, drained strengths) were completed on lignites resulting in C=2,500 psf and Φ=26.1°

At the dam site, it was believed that lignites and occasional thin sand strata are the only horizons in the Fort Union which could carry appreciable amounts of water. The heavy clays, when associated with lignite often causes them to be water bearing. Frequently the drilling mud was lost when a lignite seam was encountered by the drill and pressure testing indicated that many lignite beds would take water readily. The water carrying capacity of the lignites is due to small cracks and fissures rather than uniform porosity and is greatly variable. Flow through the lignites occurs primarily through a network of vertical cracks with a typical width of ¼” or less. For lignite layers that are less than 3 ft thick, the cracks are smaller and less continuous. This increased the difficulty of grouting these thinner lignite layers (Ref: 1951 Meeting of Board of Consultants, p.6). Pressure tests documented in the “1952 Report on Stability of Spillway Crest Structure” (p2-3) also lead to the conclusion that thicker lignites are open jointed and highly permeable while thin lignites are often closed to grouting. Seepage is typically noted from thin lignites in natural or excavated slopes, indicating that joints are present, but the joints are less continuous and often not found by borings.

The 2U lignite (6 ft) is highly cracked and jointed, and caused significant water loss during drilling operations. The 1U and 1 1/2U lignites together (1 ft and ½ ft) were found to be tight in 9 of 12 pressure tests, indicating varying amounts of seepage beneath the spillway. The 2U and 1 1/2U lignites were fully cut off by the spillway crest key (Ref: 1952 Report on Stability of Spillway Crest Structure p.3). Observations made of the rate of seepage from a thin lignite seam into a calyx hole in the powerhouse area indicated a permeability of 0.0045 ft/sec (or 0.137 cm/sec) for that particular seam (Ref: ECR). A pumping performed in the BB lignite seam indicated a permeability ranging from 0.082 to 0.110 cm/sec. Estimates of the permeability of lignites in Mercer County (Croft, 1973) were 120 ft/day (0.042 cm/sec) (Ref: CFR).

Fort Union Geologic Structure: The bedrock at the dam site is essentially flat lying with only a slight regional dip to the northwest (dip equals about 15 feet per mile). Small flexures and local irregularities are common in the lignite beds. Little other structure has been observed in the Fort Union beds, with the exception of slickensides, which are often found associated with the fatter clays adjacent to the lignite horizons.

No system of jointing was observed at the site although joints were apparent in the lignites which were recovered in rotary core samples. Two sets of joints, nearly vertical and at approximately right angles to each other, have been observed in the lignites. These are quite pronounced in the lignite and probably account for the high permeability of the lignite horizons which is of the same magnitude as the valley sands. Some indications of joint patterns were noted but there did not appear to be a well-defined system. Joint measurements in the lignites excavated in the spillway showed two predominant sets striking N35° to 50° E and N 30° to 50° W. In general, most joints appear to be in a conjugate set and dip near vertical.

Only minor faulting was noted at the dam site. No evidence of deep-seated faulting exists in the Garrison Dam area.

The faults which were observed in the excavations are of the gravity or normal type. The faults were believed to be a result of differential compaction during periods of superincumbent sediment loading or ice loading in the valley during Pleistocene glaciation. The only faulting of any importance was in the spillway area. Two well-pronounced faults, SF-l and SF-2 were detected by exploratory borings, but were not visually observed in the foundations. Based on boring data as shown on the geologic section, Fault SF-1 had a displacement of less than 20 feet within the spillway crest excavations but the throw increased to about 100 feet in the area outside of the spillway excavations and downstream of the dam axis. Fault SF-1 was at first thought to be hinged near its intersection with the spillway crest structure, but during curtain grouting along the spillway structure, it was found to continue beyond the structure and served as a cutoff of the lignite beds at this location. Because it was not visually observed, the primary evidence for this fault is the contours of the 1U lignite. A similar structure was observed in the overlying 1 1/2U, 2U, and 3U lignites. Similar folding was potentially observed in the deeper LL and 1L lignites, but there were not enough sufficiently deep borings to decisively conclude that the fault extended to this depth (Ref: 1952 Report on Stability of Crest Structure p.2). Fault SF-2 had a relatively small displacement of five to ten feet and was not investigated in detail. The fault was believed to dissipate rapidly with depth.

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Figure 8. Location of faults SF-1 and SF-2 in spillway area (Plate 64, CFR).

Figure 9. Folds in silty clay and fat clay lenses in powerhouse excavation

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Site Geology, Spillway Excavation: The foundations for the spillway approach channel, crest structure, chute, stilling basin, and discharge channel were founded on Fort Union deposits varying from clay shale to silty sands ranging from a few inches thick to 15 feet thick. The foundation for the spillway crest structure was excavated to a depth of approximately 33 feet with the key portion of the structure extending to 41 feet. Excavations for the foundation encountered the 2U, 1-1/2U and the 1U lignites interbedded in Fort Union clay shale.

Excavated slopes in the spillway were good and encountered only a minor amount of slope instabilities, such as sloughing and raveling of lignite beds in areas where the lignite was allowed to dry out, and minor raveling of air dried, fat clay exposures. Occasionally, small overbreaks occurred in the clay, which excavated very blocky.

Vertical cuts for the rest of the structure foundations and vertical cuts for drains were accomplished with a mechanical shale saw which assured a competent finished face.

The construction foundation reports for the spillway structures stated that no unusual foundation conditions were encountered. A small amount of groundwater seepage occurred at the base of the 2U and 1-1/2U lignite beds on the upstream side of the excavation in an area extending from the centerline of the spillway to 100 feet west of the centerline. The seepage varied from 30 gallons per minute in the spring to 3 gallons per minute in the fall. The seepage was collected in a sump and pumped as required. The bottom and side slopes of the crest structure key were thoroughly cleaned of loose and wet materials prior to placing concrete. Other than this routine procedure, no special foundation treatment was required.

Figure 10. Spillway Excavation

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3. INVESTIGATION JUSTIFICATION AND OBJECTIVES

The purpose of the broader Garrison Dam Spillway Modification project is discussed, followed by discussion of how this anchor investigation supports design of the spillway modification.

3.1 Garrison Dam Spillway Modification (GDSM) Program

Justification: Garrison dam was assigned a DSAC 2 (high urgency) rating following the 2013 Periodic Assessment No. 1 due to the extreme life safety consequences which would result from a dam failure. An issue evaluation study was completed in 2020 which identified that incremental risks are primarily driven by the potential for hydraulic jacking of the spillway chute slabs. The following dam safety modification study included additional risk assessment and determined that there was a total of six risk-driving potential failure modes relating to the spillway, with total project risk above tolerable risk guidelines.

This led to development of risk management plans (RMP) and a recommended plan endorsed by the dam senior oversight group in October 2022. The recommended plan primary components consist of a chute overlay with anchoring, new drain system, armoring outside chute walls, abutment monolith raise, and Tainter gate modification.

Implementation of the recommended plan is intended to address all risk-driving potential failure modes and reduce project risk to within tolerable guidelines.

Project engineering and design (PED) was initiated in 2023 with recommendation to complete the following investigations:

• Phase 1 Geotechnical Investigation

• Dewatering Test Program

• Anchor Testing Program (this DIPP)

• Concrete Materials Testing

• Concrete Mix Designs

• Hydraulic CFD Modeling

• Hydraulic Physical Modeling

• Trunnion Hub Bolt Evaluation

3.2 Previous GDSM Investigations

Figure 11 depicts the completed GDSM PED investigation drilling to date. The previous investigations are summarized below with additional data provided separately in the Geotechnical Data Report.

2023 Phase 1 Geotechnical Investigation As noted previously information collected from the exploration program will be utilized in the project engineering and design (PED) phase for the modification of the emergency spillway. Specifically, drilling and field testing will be used to refine geotechnical and hydro-geologic information which will then be utilized to improve understanding of:

• Spillway crest monolithic stability (“S”)

• Future Anchor System Design (“A”)

• Replacement and Construction of Drainage System (Cutslope stability, “C”)

All borings included basic laboratory testing for soils including Atterberg’s limits, sieve analysis, and water content.

• BH-23-S1

• BH-23-S2

• BH-23-S3

• BH-23-C2

• BH-23-C8

• BH-23-A1

• BH-23-A2

• BH-23-A3

• BH-23-A5

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Figure 11. Garrison Spillway Modification Investigation Borings (2023-2024)

(Full-size drawing provided in the Appendix A)

Borings BH-23-A1 to BH-A3 and BH-A5 (4 borings) were drilled primarily to characterize the geological conditions that will impact anchor design specifically that associated with active strand anchors located in the upper chute and stilling basin. These boring length ranges from 240 feet; 120% of the DSMS anchor design length. In field testing will include down-hole optical and acoustical logging the length of each hole to characterize any bedding, fractures or slickensides which may exist within the transitional lithology. In addition, five of these borings will be pressure tested fully to better characterize the hydraulic conductivity of the Fort Union Group and specifically associated lignite zones. This information will be utilized to develop and designed grouts for pre-grouting and grouting of anchor bond zones. Testing on core samples for unconfined compressive strength, poisson’s ratio, elastic modulus, ultimate and allowable bond strength. Laboratory pull-out testing will be conducted prior to a future field anchor study. Grout utilized in pull-out testing will be evaluated for strength and bond as well. A singular observation well will be installed in BH-23-A6 to monitor uplift not associated with lignite layers at the stilling basin.

Borings BH-23-S-1 to S-4 are being drilled primarily to characterize the geological conditions that will impact monolithic stability. This may impact concrete overlay thickness and design of the anchorage system. This drilling will attempt to better characterize potential weak sliding planes associated with slickensides associated with both the general Fort Union Group as well fat clays associated with the top and bottom of lignite layers and/or failure planes within the lignites. This will be accomplished through logging of core and geophysical logging in each hole as described previously. Pressure testing will be conducted in S-2 and S-4 to characterize hydraulic conductivity of the foundation of the upper foundation of the spillway. Similar geotechnical properties will be evaluated similarly to the A-series borings, however a greater emphasis will be placed on shear strength testing. All samples associated with tri-axial or direct samples utilized to determine the existing shear strengths for critical failure planes are from the

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1950’s or prior. Modern testing equipment and procedures are different for determining shear strength along failure planes (specifically with rock), thus results may differ.

Borings BH-23-C-1 to C-8 are being drilled primarily to characterize the geological conditions that will impact excavation and stabilization of slopes for replacement of the current drainage system. Similar to the previous programs coring, logging as well as a variety of laboratory tests will be conducted. Pressure testing and geophysical logging will not be conducted for these holes

2024 Pumping Tests and Anchor exploratory borings The USACE drill crew completed a field investigation to support conceptual dewatering design for the stilling basin and collector drainage excavations. Boring locations included:

• BH-24-SWT-01. Determine inflow and piezometric level immediately behind the spillway crest and in the 1U lignite, in an area where the top of the collector drain will meet lateral drain row #1.

• BH-24-SWT-02. Determine inflow and piezometric level immediately behind the SF-1 fault, along the collector drain alignment.

• BH-24-SWT-03. Determine inflow and piezometric level relating to the LL lignite.

• BH-24-MWT-01. Determine inflow and dewatering effort relating to the 1L lignite group interval.

Includes two adjacent observation wells.

• BH-24-MWT-02. Determine inflow and dewatering effort relating to the full soil profile including LL lignite, 1L lignite group, and clay layers in between.

2024 Anchor Investigation Phase 1 The USACE drill crew completed a field investigation to inform location and depths of test anchor installation.

These borings include

• BH-24-ATP-01

• BH-24-ATP-01A

• BH-24-ATP-03

• BH-24-S-01

• BH-24-S-01A

• BH-24-S-03

• BH-24-S-04

• BH-24-NWO-SP-01 (east side of spillway).

An AE Services task order was awarded to Stantec, with Coastal Drilling as subcontractor, to install and test three under-reamed end-bearing anchors downstream of the spillway. These three anchors are numbered with “CD” for Coastal Drilling:

• BH-24-CD-SP-01

• BH-24-CD-SP-02

• BH-24-CD-SP-03

3.3 Anchor Investigation Phase 2 (This Investigation)

This drilling and field testing will be utilized to improve understanding of anchor constructability and field performance, allowing for refinement of the final design.

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Figure 12. Anchor Test Area Geologic Profile, with example anchor depths shown.

(Full-size drawing provided in Appendix A)

4. DIPP SCOPE (ANCHOR INVESTIGATION PHASE 2)

4.1 Investigation Scope

Per the PWS, the major work activities for the GDSM Anchor Investigation Phase 2 includes pre-fieldwork submittals, mobilization of equipment to Garrison Dam, material procurement, construction of twelve (12) reinforced concrete test pads, construction of twelve (12) foundation anchors (including underream, bell, and Conventional Bond Zone (CBZ) type anchors) varying generally from 50 ft to 160 ft depth, anchor field testing, demobilization, and all incidental activities required to fulfill the requirements of the PWS.

All 12 anchors will be constructed in the “Anchor Testing Area” downstream of the right side of the spillway stilling basin. The PWS and attachments include the specific anchor locations and depths.

The Contractor shall meet all ER 1110- 1-1807 guidance, per the PWS requirements. Relevant drilling methodology will be documented in the required PWS submittals, primarily the Anchor Test Plan and QCP. USACE will internally update the program DIPP with the relevant information including the exploration team, required personnel safety preparations, USACE POC’s, schedule; description of operations including drilling, water-tightness testing, anchor installation, grouting, and testing.

4.2 Utilities

The Contractor will contact the North Dakota One Call service prior to commencement of drilling operations, in order to have public utilities marked. USACE engineering and operations staff is responsible for marking any project utilities that are not covered by the One Call service. Utility clearances and a field-check (review with site USACE Staff) of each borehole location will occur prior to commencement of drilling operations.

An existing utilities map is also provided as an attachment to the PWS.

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Figure 13. Anchor Testing Area Location.

5. RISK EVALUATION

5.1 Hydrofracture calculations

The primary considered risk is hydrofracture of the foundation. The “Hydraulic Fracture Risk Screening Level Assessment Tool for Vertical Borings” developed by USACE MVR was utilized for calculations which are provided in Appendix A. The Appendix A calculations relate specifically to the drilling of boring BH-23-A3, completed in 2023 immediately outside of the stilling basin. Three scenarios are analyzed: Drilling, backfilling, and packer (pressure) testing. The calculations conservatively assume groundwater level or drilling fluid to be at the ground surface.

The previously completed calculations are believed to capture the worst-case pressures (in terms of hydro-fracturing potential) that may be applied at any time during the Anchor Phase 2 investigation, including drilling of pilot borings, water-tightness testing of bond zones, installation of anchors, and testing of anchors. If any higher-risk activity appears to be required, during submittal review or field coordination, USACE will update and assess the hydrofracture calculations accordingly prior to the work being executed.

Per the PWS, to further mitigate hydrofracture potential, the Contractor will monitor while drilling noting changes in drilling effort resistance, torque, advance rate, drops, water level, changes in cuttings, and drill water, including these observations in daily reporting. Specifically, they are expected to report any losses or gains of water that would indicate issues within the foundation, such as artesian pressures or hole collapse.

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Figure 14. Geologic profile showing relative conductivity along boring BH-23-A5, 5 ft pressure testing intervals.

These tests initially intended for BH-23-A3 but were re-assigned to A5 during field execution.

In terms of hydrofracture analysis, these two borings are equivalent.

Boring logs are available in the Phase 1 Geotechnical Investigation report (See GDR).

5.2 Other identified risks

The following risks were identified by the Anchor Phase 1 Investigation contractor with relevant mitigation methods. These discussions are provided for the benefit of the Anchor Phase 2 Investigation Contractor.

Drilling Test Anchors:

1. Drilling through the anchor pads. Care will be taken to avoid drilling through new concrete by installing an 18-inch pipe that will extend from the bottom of the excavation to the top of the pad. Per the PWS, the Contractor is responsible for ensuring concrete pads have achieved the required design strengths and that drilling activity loads do not exceed the concrete strength requirements.”

2. Duplex drilling pilot hole. Spikes in water pressure are a concern during the drilling process and could result in possible hydrofracturing. Using steel casing fitted with cutting teeth and a claw bit or roller bit attached to the inner drill string, water will be used as the drilling medium to assist with the clearing of the cuttings from the bore hole. Tolerable water pressures will be monitored and maintained duringbthe drilling. The drill operator will constantly monitor the pressure for spikes or significant loss of pressure.

Any notable spikes will be an indication to stop drilling operations. Drill crew will attempt to determine if there are any plugs in discharge lines or issues with the drill bits down the hole. Loss of pressure or water return would be a sign of significant voids or communication with an aquifer. Options to repair may be to inject grout and redrill or to advance casing beyond the “loss zone” and see if water return is eventually restored.

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3. Under-reaming process. At the start of the underreaming process the tool will be set at the top of the bond zone and rotated with little feed. The driller will monitor the rotation pressure and once a drop in pressure is constant, the underreaming tool should be fully open to the desired diameter. The feed will then be increased and the underream will be advanced to the bottom of the bond zone. Care will be taken to note rotation pressures, feed rates and water pressure throughout the drilling of the bond zone to note any noticeable deviations normal readings.

4. Under-reaming and bell drilling objectives. Per the PWS, the Phase 2 Contractor will consider the best equipment and process to achieve the desired anchor bond zone geometries and control any un-intended scour of the boring wall. Ensure continued verticality of the tooling during operation and that it does not travel laterally.

Installing Test Anchors:

1. Corrosion Protection. USACE intends to monitor these test anchors over a 10-year period, therefore the corrosion protection of the test anchors is vital to their longevity. Care will be taken when handling, preparing and installing the bar anchors into the bore holes. Crews will have an onsite epoxy repair kit to make touch-ups to the epoxy on the anchor bars and hardware. Grease and sheathing will encapsulate the free length of each anchor to provide an additional layer of protection during the handling and installation process.

2. Installation of Bars. There is a potential to “lose” bars in the hole during the installation process requiring a challenging process to retrieve them from the hole. Several anchors will require the use of couplers to build the complete anchor to the design length. The first 50-ft section of the bar will have the tip plate and associated hardware, centralizers and a coupler attached to the top of the bar and will be inserted into the borehole. The bar will be suspended with a U-wrench designed to fit the coupler. The subsequent section (up to 50-ft) will then be lowered to the coupler and threaded into the coupler. The bar will be lifted enough to remove the U-wrench and then lowered to tip elevation or to the next coupler. This process will be repeated until the anchor is set to final elevation. The U-wrench or blocking may be used to suspend the anchor bar at the designated elevation until the bond zone is grouted and initial set of grout is achieved.

Water Testing and Grouting Anchors:

1. Water Tightness Testing (Hydrofracture). Concerns during watertightness testing are related to potential for hydrofracturing the foundation materials. Water tightness testing will be completed using a packer that is inserted into the casing and extended approximately 2-foot beyond the casing and inflated to create a seal. Water will be injected to fill the bond zone and then using a valve. Water will be injected at a constant pressure of 5 psi over a 10-minute period to determine the conductivity of the bond zone. The injection pressure for the water testing is significantly low and should not cause any hydrofracturing.

Crews will constantly monitor the pressure throughout the test for any spikes or anomalies.

2. Grouting Anchors (Hydrofracture). Concerns may exist during the grouting of the anchors for hydrofracturing of the foundation materials. All grouting, whether consolidation grouting or anchor grouting, will be accomplished using tremie placement methods. Since the grout is free to travel once it exists within the tremie pipe, the pressure used to deliver the grout is dissipated immediately and should not be a concern for hydrofracturing the rock. Pressure will be constantly monitored, and unnecessary spikes will be an indication to stop grouting and evaluated grout lines and mix for any concerns. Repairs and replacements will be made as necessary prior to any return to operations.

3. Pre-Grouting (Casing). Concerns surrounding the grouting of casing into the hole during the pre-grouting process. For Test Anchor holes that fail to pass the watertightness testing criteria, consolidation grout must be injected into the hole and subsequently redrilled. To avoid grouting the steel casing into the hole, the Contractor will inject grout to the proper elevation and monitor the volume and depth in relation to the casing. If necessary, the drill operator will reconnect to the casing and pull the casing up the hole to a location a few feet above the consolidation grout level. CDE will constantly monitor grout level with

18 of 20 Anchor Investigation Phase 2 CUI respect to casing tip elevation and react accordingly to avoid getting casing grouted into hole.

4. Anchor Grouting (Casing). Concerns surrounding grouting the casing in the hole. Like the pre-grouting process, which may or may not be required on each test anchor, the injection of anchor grout has the same risk of getting casing stuck in the bore hole. During the injection of the anchor grout into the bond zone, the crew will monitor the targeted grout elevation with respect to the casing tip elevation. If necessary, the drill operator will reconnect to the casing and retract the casing a few feet above the bond zone grout elevation to avoid getting casing stuck in the hole. Casing can be lowered, if necessary, after the grout has achieved its initial set.

Anchor Testing:

1. Anchor Pad settlement. There is the possibility of the anchor pad settling during the testing process. To assist with alleviating that concern, the Contractor is installing four survey monuments at the corners of each anchor pad to be monitored throughout the testing of each Test Anchor.

2. Test Beam Rotation. Utilizing a test beam to perform the testing of the Test Anchors inherently has a risk of rotating during the testing process. During the setup of the test beam, care will be taken to properly setup and level the test beam, cribbing the stressing jack to minimize potential for any rotation during the testing process. One individual will be dedicated to observing the behavior of the test beam at each incremental increase in load and should immediately stop operations should there be any concerns with the test setup. Since these are bar anchors, it is relatively easy to stop the stressing operation and restart as needed to make any adjustments to the test setup.

3. Anchor Lock Off. Since the Test Anchors are being tested multiple times, the anchors will be loaded and unloaded frequently. Once the testing is completed the anchors are to be locked-off at a substantially lower load than the test loads and the overall capacity of the anchor bars. The Contractor will take care when performing the final lock off of the Test Anchors to maximize a consistent load transfer at the specified Lock Off Load. The Contractor will perform lift off tests to confirm final load.

4. Anchor Instrumentation. There is concern that the instrumentation designated for the Test Anchors may not be able to provide information throughout the duration of the intended monitoring period. Each Test Anchor is designated to receive a load cell and strain gages to monitor long term load in the anchors and to identify loading in the bond zone. The Contractor will take extra care when installing instrumentation and necessary wiring. The contractor will also provide a weatherproof box to house any necessary instrumentation components to protect from damage. Even with the special care and protection, the vendors cannot provide guarantees over the intended monitoring duration.

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Appendix A: Geologic Profiles

• Map of spillway boring locations

• Map of spillway boring locations, filtered to GDSM PED investigations 2023-2024

• A-A’ Spillway chute right side profile

• G-G’ Spillway terrain profile outside of right wall

• I-J’ Spillway stilling basin cross-section

• Anchor test pad geologic profile with example anchor depths

• Example shallow depth anchor geologic log – bond zone above LL lignite – SP-1

• Example intermediate depth anchor geologic log – bond zone near top of 1L lignite group – SP-2

• Example deep anchor geologic log – bond zone above 2L lignite group – SP-3

TEST ANCHOR GEOLOGIC PROFILE

Profile scale 1:1

SP-1, 2, 3

0 ft depth 20 ft 40 ft 60 ft 80 ft 100 ft 120 ft 140 ft 160 ft

SP-1 BORING

SP-2 BORING

SP-3 BORING

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Appendix B: Hydrofracture Calculations

• BH-23-A3: Drilling scenario

• BH-23-A3: Backfill scenario

• BH-23-A3: Pressure-testing scenario

Geologic profile showing relative conductivity along boring BH-23-A5, 5 ft pressure testing intervals.

These tests initially intended for BH-23-A3 but were re-assigned to A5 during field execution.

In terms of hydrofracture analysis, these two borings are equivalent.

Boring logs are available in the Phase 1 Geotechnical Investigation report (See GDR).

Reference Borings:

Calculations Date:

1710 ft 0 ft

62.4 pcf

62.4 pcf

0 ft 0 psi

240 ft

Material I 115 20 0.658 500 Material II 95 26 0.562 500 Material III 145 45 0.293 500 Material IV 115 30 0.500 Material V 1.000

Material Depth (ft) Elevation (ft) γ (pcf) γ' (pcf) Minor Principal

Confining Stress, σ'3 at Depth (psi)

Major Principal Confining Stress, σ'1 at Depth (psf)

Shear Strength from Friction, τ' at

Depth (psf)

Shear Strength from Cohesion, τ' at Depth (psf)

Shear Strength, τ' at Depth (psi)

Resisting Pore Water Pressure at

Depth (psi)

Minor Principal + Shear at Depth +

Resisting Pore Water Pressure (psi)

Slurry/Grout Hydrostatic

Pressur…