GBR - Lowell Creek Diversion Tunnel 65percent.pdf

Geotechnical Baseline Report

Lowell Creek Flood Diversion

Seward, Alaska Alaska District, Pacific Ocean Division 12 June 2025 (65% Design)

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Lowell Creek Diversion Dam and Tunnel Upgrades 60% Design Submittal Seward, AK 10 June 2025 i

TABLE OF CONTENTS

1. Introduction

1.1. Description and Purpose of GBR

1.2. Relation to Contract Documents

1.3. Organization of Report

2. Project Description

2.1. Project Location

2.2. Existing Project Layout

2.3. Components Summary

3. Sources of Geologic and Geotechnical Data

4. Summary of Geologic Conditions

4.1. Regional Geology

4.2. Site Geology

4.3. Bedrock

4.4. Groundwater Levels

5. Ground Characterization

5.1. Equivalent Hydraulic Conductivity

5.2. Intact Rock Properties

5.3. Abrasivity

5.4. Discontinuity Orientation

5.5. Discontinuity Spacing

6. Construction Considerations

6.1. Inlet Structure and Portal Area

6.2. Outlet Structure and Portal Area

6.3. Tunnel

6.4. Diversion Dam

7. References

LIST OF FIGURES

Figure 2-1. Project Location Map Figure 2-2. New and Existing Project Features Figure 2-3: Inlet Area General Plan Figure 2-4: New Tunnel Profile Figure 2-: Outlet Area General Plan Figure 3-1. Geophysical Investigation Survey Lines and boring Locations at the Inlet from GDR (Golder, 2023) Figure 3-2. Geophysical Investigation Survey Lines and boring Locations at the Outlet from GDR (Golder, 2023) Figure 4-1. Regional geology Figure -1. Histogram of packer test hydraulic conductivities Figure -2. UCS frequency distribution and summary statistics Figure -3. Splitting tensile strength frequency distribution and summary statistics ii

Figure -4. CAI frequency distribution and summary statistics Figure -5. Lower hemisphere equal angle stereonet showing joint set poles and mean planes for inlet area Figure -6. Lower hemisphere equal angle stereonet showing joint set poles and mean planes for outlet area Figure -7. Histogram of discontinuity set spacing (apparent spacing) Figure -1. GSI frequency distribution and summary statistics for the FPF excavation

LIST OF TABLES

Table -1. Equivalent Hydraulic conductivity data statistics Table -2. Baseline Intact Rock Properties Table -3. Summary of Joint Set Baselines Table -4. Discontinuity set spacing statistics (apparent spacing) Table -1. Baseline bulk rock mass hydraulic conductivity values for the Inlet Area Table -2. Baseline GSI values for Inlet Area Table -3. Baseline values for Ground Class Percentage per Reach Table -4. Baseline Values for Steady State Inflow per Select Tunnel Reach

APPENDICES

Appendix A – FPF Excavation Cross-Sections Appendix B – Tunnel Cross-Sections iii

ACRONYMS

ADAS Automated Data Acquisition System ASCE American Society of Civil Engineers BTS Brazilian Tensile Strength CAI Cerchar Abrasivity Index cfs cubic feet per second cm/s centimeters per second Deg Degree DSC Differing Site Conditions EBU Engineering Behavioral Unit E&S Excavation and Support IDaC Integrated Design and Construction ISRM International Society of Rock Mechanics FAR Federal Acquisition Regulations FPF Fish Passage Facility GBR Geotechnical Baseline Report GDR Geotechnical Data Report GIR Geotechnical Information Report GM Geometric Mean gpm gallons per minute GSD Geometric Standard Deviation GSI Geological Strength Index HAHD Howard A. Hanson Dam ips inches per second ID Inside diameter K Fisher dispersion coefficient NGVD National Geodetic Vertical Datum psi pounds per square inch

RMR

RMRb

Rock Mass Rating Rock Mass Rating base value

ROW Right of Way RQD Rock Quality Designation UCS Unconfined Compressive Strength USACE United State Army Corps of Engineers

1. Introduction

1.1. Description and Purpose of GBR

This Geotechnical Baseline Report (GBR) summarizes geotechnical basis for construction of the Lowell Creek Diversion Dam and Tunnel Upgrades. The US Army Corps of Engineers (USACE) design team developed this GBR using data collected during multiple investigations. Supporting documentation is contained within the Geotechnical Data Report (GDR).

This GBR provides information relevant to subsurface conditions, referred to as the baseline conditions. The purpose of the GBR is to:

• Set baselines for geotechnical conditions and material behavior anticipated to be encountered during construction associated with the inlet structure, tunnel, and outlet structure.

• Identify important construction considerations, key project constraints, and selected requirements to be addressed by the Contractors.

This GBR will be used by the Government and the Contractor to evaluate claims or disputes regarding whether the character and behavior of the subsurface conditions actually encountered are materially different from what is indicated, and whether such differences constitute a differing site condition (DSC) as defined in the General Conditions. The contractual baselines do not represent warranties by the Government of the actual subsurface conditions that will be encountered by the Contractor. Rather, the purpose of the baseline is to translate the results of past geotechnical investigations and analysis, laboratory testing, various construction activities, project performance, and to provide technical interpretation of subsurface data with descriptions of subsurface conditions as a basis for bidding purposes.

This GBR pertains only to the inlet structure, tunnel, and outlet structure. The GBR is based on assumptions regarding sequence of construction, means and methods, and workmanship to be employed by the Contractor. The actual behavior of subsurface materials will be influenced by means and methods proposed by the Contractor. The Contractor must evaluate the soil, rock, and groundwater conditions described herein as they relate to, and interact with means and methods, construction sequence, and workmanship of their construction.

1.2. Relation to Contract Documents

The GBR and GDR are Attachments to the contract specifications. This GBR references the GDR, representing information obtained through all investigations for the Lowell Creek Diversion Dam and Tunnel Upgrades. If there are any inconsistencies between the GBR and the GDR, the GBR will take precedence.

The GBR is not intended to present the minimum required design and standards of construction contained in the Specifications. Nothing in the GBR should be construed as amending or altering the Specifications or applicable FAR clauses or relaxing the standards of construction. Specifications and Contract Drawings will take precedent over the GBR and GDR.

The technical concepts, terms, and descriptions in this report follow standards commonly used in geotechnical engineering/engineering geology which have specific meaning pertaining to the work. Bidders should have a qualified geotechnical engineer or qualified engineering geologist carefully review this GBR so that a complete understanding of the information presented in this report is considered prior to submitting a bid.

1.3. Organization of Report

This document broadly follows the principles contained in the ASCE document entitled “Geotechnical Baseline Reports Suggested Guidelines”, dated 2022, and is comprised of an Introduction (Section 1.0) and the following sections:

• Section 2.0 (Project Description) presents descriptions about the project location, purpose, and components.

• Section 3.0 (Sources of Geologic and Geotechnical Data) presents the sources of data used to prepare this GBR-C.

• Section 4.0 (Summary of Geologic Conditions) summarizes the geologic history and geologic units around the HAHD FPF.

• Section 5.0 (Ground Characterization) presents the physical nature of the subsurface conditions.

• Section 6.0 (Construction Considerations) presents the rational, criteria, and methodologies for the design of excavations and support systems.

• Section 7.0 (References)

2. Project Description

2.1. Project Location

The Lowell Creek Flood Diversion System is located in Seward, Alaska, 125 miles highway drive south of Anchorage. The City of Seward, with a 2019 population of 2,545, lies immediately below the flood diversion system at the head of Resurrection Bay, a deep fjord about 25 miles long on the north shore of the Gulf of Alaska, on the Kenai Peninsula. Seward has one of two ice-free ports in Alaska with road and rail connections to the state’s interior. See Figure 2-1 for project overview and typical features of the existing flood diversion system.

Figure 2-1. Project Location Map

2.2. Existing Project Layout

The project consists of an inlet, tunnel, and outlet (Figure 2-1, Figure 2-).

The existing Lowell Creek Flood Diversion System project was authorized by the Flood Control Act of 1936 (PL 74-738) with an authorized project purpose of flood risk management. The flood diversion system reroutes Lowell Creek into a tunnel through Bear Mountain and diverts flows to Resurrection Bay prior to those flows entering Seward.

Lowell Creek passes through Lowell Canyon, a rocky, rugged canyon above Seward.

The canyon is bordered by steep hillsides and talus-covered slopes. The stream, approximately 3 miles long above the existing tunnel, drains an area of about 4.02 square miles. Lowell Creek has a gradient of 1,000 feet per mile and transports large amounts of debris, often including cobbles and boulders with boulder sizes of approximately a one-half cubic yard in volume. Neither the original creek flow path nor the current flow path of the stream have levees downstream, and there are no dams upstream or downstream of the Lowell Creek constructed features.

Figure 2-2. New and Existing Project Features.

2.3. Components Summary

The major elements include the following:

• Inlet Flume and Training Walls. The inlet flume will be located upstream of the existing flood diversion structures, see The U-shaped flume extends from approximately station 7+53 to 10+00 (tunnel entrance). Training walls will be located on either side of the flume to direct flows into the flume. Backfill on the protected side of the flume walls and training walls will extend to the top of the walls. The backfill on the right side of the flume will be graded to drain runoff into the flume. On the left side of the flume, the backfill will be graded to drain towards the inlet flume upstream of the spillway, and away from the flume near the culvert outfall towards the existing tunnel. (Figure 2-3)

• Bypass Culvert. A 10x10 feet bypass culvert reinforced concrete box is located on the left side of the flume, see Figure 2-3. The culvert will be installed during construction to route the creek around the inlet work area and also form part of the cofferdam on the left side of the flume. From the face of the left training wall, the culvert extends approximately 327 feet downstream;

this portion of the culvert will become a permanent project feature. To facilitate access to the inlet flume work area, the culvert will extend roughly 200 feet upstream from the wall. This extension forms the cross for the access road along the face of the mountain to gravel road over the upstream cofferdam.

• Cofferdams. Cofferdams are needed to facilitate construction in the inlet area.

The cofferdams will be constructed using sorted in-situ random rockfill and cobbles and boulders to protect the channel bank. The upstream cofferdam is to prevent creek flows from entering the inlet flume work area.

Construction of each feature in the inlet area that involves casting of concrete below ground wall will require a cofferdam. The cofferdam that will be used to facilitate installation of the right training wall and inlet flume is a system consisting of the following: upstream cofferdam, permanent bypass culvert, and downstream cofferdam. The upstream cofferdam also provides access to the inlet area during construction. The downstream cofferdam located near the end of the culvert outfall is to protect against tailwater overtopping and inundating the inlet flume work area. In accordance with Engineering Regulation (ER) 1110-2-8152, design of the cofferdam is a Corps of Engineers responsibility. However, per this this Engineering Regulation the Contractor is to design a dewatering system that meets Corps of Engineers performance requirements.

• New Tunnel. A new 18 feet interior-diameter horseshoe, half-mile length tunnel will be constructed through Bear Mountain using drill-and-blast methods, see profile in Figure 2-4. The authorized tunnel design flow capacity is 8,500 cfs. Multiple types of rock are anticipated to be encountered along the tunnel alignment with certain areas requiring supports.

The tunnel floor will be lined with high-strength concrete to reduce wear from the large sediments passing through the tunnel during flood event, which include gravels, cobbles, and boulders.

• Outfall Structure. The tunnel outfall will transition to the outlet flume entrance, see Figure 2-. The flume entrance will be founded on rock with the rest founded on end bearing piers that are socketed in the Graywacke rock. The 10-foot diameter piers will be spaced 65 feet apart; they will stick up between 20 and 50 feet above the ground surface. The flume has a 10 feet overhang from the end of the last pier facing the bay. The top of the flume will be capped with concrete to prevent loose debris falling on the public below.

Sediment discharged from the flume outfall will flow below to the plunge pool that contains and directs the sediment to the northeast part of the bay away from the fish Canary facility. The plunge pool will be lined with a 5 feet thick layer of riprap (54-inch maximum size).

• Future Lowell Point Road. The location of the piers impacts the existing road alignment which requires re-aligning the road to pass between the piers. The re-alignment segment of the road will be founded on the alluvial fan. The road alignment will pass close to the plunge pool.

Figure 2-3: Inlet Area General Plan.

Figure 2-4: New Tunnel Profile.

Figure 2-5: Outlet Area General Plan.

3. Sources of Geologic and Geotechnical Data Multiple phases of subsurface investigations have occurred for the Lowell Creek Flood Diversion and Tunnel Updates. Most of the data is located near the inlet and outlet structures. New data collection efforts focused at the inlet. Data from these investigations are contained in the GDR included as an Attachment to the specifications.

Additional subsurface investigations are currently in progress to support design. USACE developed baselines included herein using the investigation data below which is included in the GDR:

• Phase 1 Golder and Associates Geotechnical Investigation (Golder and Associates, Lowell Creek Geotechnical Data Report (GDR), 2024): Phase I geotechnical investigations were completed in 2022 and consisted of historical background research, geologic mapping, geophysics, boring installation with in-situ testing and laboratory testing. See the GDR for the full report. Surficial geologic mapping was completed along Bear Mountain and at outcrops in the surrounding area. Seismic refraction and multi-spectral analysis of surface waves (MASW) geophysical survey lines were completed at the inlet and outlet area.

Seventeen borings were completed during Phase I investigations: 10 borings at the inlet area and seven at the outlet area

• Photogrammetry and Geologic Mapping – 2023: Two geologic mapping field efforts were completed using photogrammetry and traditional mapping methods at accessible outcrops. See the GDR for the full field report.

• Sediment Sampling – 2024: Two efforts were conducted to characterize the size, shape, hardness, and minerology of the surface deposits along the Lowell creek canyon for design of the tunnel liner. An initial effort (Phase I) was conducted in July 2024. The effort focused on characterizing the geology of the canyon, evaluate material origin, explore nearby landslides and characterize the shapes, size, and minerology of surface deposits. A second effort (Phase II) was conducted in September 2024 to collect samples for field and laboratory testing.

See the GDR for the full field report.

• Phase II Geotechnical Investigation – 2024/2025 (in progress): Phase II geotechnical investigations were completed in 2024 and 2025 and consisted of boring installation with in-situ testing and laboratory testing and aquifer pumping tests. See the GDR for the full report. Twenty-nine borings were completed during Phase II investigations: 25 borings at the inlet area and five borings at the outlet area. One additional sub-horizontal boring is currently in progress along the tunnel alignment at the inlet area.

Figure 3-1. Geophysical Investigation Survey Lines and boring Locations at the Inlet from GDR (Golder, 2023).

Figure 3-2. Geophysical Investigation Survey Lines and boring Locations at the Outlet from GDR (Golder, 2023).

4. Summary of Geologic Conditions

4.1. Regional Geology

Seward is located in southern Alaska on the Kenia Peninsula near the boundary of the North American tectonic plate and the Pacific tectonic plate. The Pacific Plate is moving northwest under the North American Plate. As a result, the region is seismically active and has experienced significant deformation of rock and sediment. The region is the most seismically active region in North America and one of the most active in the world.

Mountain building is part of these tectonic processes. Development of mountains due to volcanic activity, crustal deformation, and deposition of sediments form new land as part of an accretionary process.

The late Cretaceous to Eocene Valdez and Orca rock formations comprise the bedrock at the project site and are part of the accretionary prism complexes within Chugach- Prince William terrane (CPW) of southeastern Alaska. The Valdez Group is described primarily as complexly deformed metasedimentary graywacke (sandstone), siltstone, and shale generally considered to be deposits of turbidity currents in an oceanic trench (Wilson and Hults, 2012). Beds generally range from a few centimeters to a few meters thick to locally, massive beds as much as tens of meters thick (Wilson and Hults, 2012).

The material was originally deposited as impure sand and mud. With time and pressure, the sediments were transformed into shale, sandstone, and quartzite. The rock has been further altered during tectonic deformation (i.e., folding of the mountains) due to plate tectonics along the subduction zone. The common geologic structure now appears as hard metamorphic units such as shale, argillite, and greywacke, although locally regional metamorphism has proceeded far enough to convert the rock to phyllite, or schistose foliated metamorphic rocks interbedded with impure quartzite.

The rock formations host numerous massive sulfide deposits which are characterized as epigenetic vein deposits, replacement, and shear zone ores. These ores are directly linked to submarine volcanism. The ore deposits around Seward are associated with intercalated slate and graywacke and are analogous to Besshi-type ore deposits.

Besshi-type deposits are commonly hosted in mixed volcanic-sedimentary environments. Ore deposits of this type are commonly hosted by turbidite deposits (i.e., fine-grained sedimentary sequences deposited by currents) that have been intruded by basaltic sills. Metamorphic grade ranges from zeolite to lower greenschist to amphibolite facies (Tysdal and Case, 1979). The Valdez group has prehnite-pumpellyite to lower to middle greenschist to amphibolite mineral assemblages. The higher-grade metamorphism is a result of contact aureoles from intrusive Eocene and Oligocene sills or plutons (Nelson et al., 1985).

The Resurrection Bay Mining application lists local mining prospects adjacent to the Lowell Creek tunnel consisting of four adits driven along several quartz-calcite veins within a N50E-striking shear zone. The host rock is slate and sandstone of the Valdez Group of Late Cretaceous age (Tysdal, 1978 [MF-880-A]). Adits were driven at elevations of 10, 70, 120, and 220 feet. The 70-foot-level adit was 100 feet long, and the 120-foot-level adit was 15 feet long. The veins had a maximum width of 3 feet and generally followed jointing. Metallic minerals include arsenopyrite, chalcopyrite, galena, pyrite, and sphalerite; gold is also present (Martin and others, 1915).

CPW extends for ~2200 kilometers along southern Alaskan margin. It is a thick accretionary complex primarily composed of deep-water turbidites with abundant quartzofeldspathic and volcanic-lithic sandstones and basaltic rocks (Winkler et al., 1984; Dumoulin, 1987; Plafker et al., 1994) that was intruded by near-trench plutons of the Sanak-Baranof belt (Hudson et al., 1979).

Figure 4-1. Regional geology.

4.2. Site Geology

Glaciation has impacted the site surface and bedrock geology. The glaciers produced steep sided valley walls removing weak and weathered rock and leaving a mantle of glacial till, ice contact deposits, outwash, and lateral moraines. Glacial retreat led to isostatic rebound, which lead to river downcutting and the formation of sheet joints parallel to topography as the canyon walls adjusted to a deglaciated state.

In general, the soil stratigraphy in the project area consists of Fill, Alluvium, and Glacial deposits. Non-lithified earth materials may be differentiated based on their origins, grain size and sedimentary structures, constructional geomorphic features, stratigraphic relationships, and age. Surface deposits within the Lowell Creek canyon consist of braided stream deposits as well as talus slope deposits, upper slope glacial till and lateral moraine deposits, and localized landslide deposits. The Phase 1 subsurface investigation found a thick deposit of mostly coarse-grained alluvial soil in the valley that may exceed 130 feet deep. The draft GDR describes the soils in the valley, but this is largely based on soil samples recovered using a 2.5-inch inside diameter sampler. This biases the soil gradation by excluding particles larger than 2.5-inches. From the GDR:

“The alluvium is characterized as moist to wet, very loose to medium dense (when refusal conditions were not encountered), silty sands to clean gravels with abundant cobbles and boulders greater than eight feet in diameter. The parent material of the sediments includes graywacke, or interbedded graywacke consisting of graywacke, shale, slate, and phyllite. The alluvium ranges in thickness from approximately 20 feet on the banks of the canyon to depths greater than 100 feet as explored by drilling.

Sediments categorized as possible glacial till consist of silty sands to silty gravels with subrounded to rounded gravels, fine to coarse sands, and non-plastic silts to clays. The material is medium dense to very dense (when refusal conditions were not encountered) with subrounded to subangular gravels. There are two members, a brownish gray material with increased gravel content, and a gray to olive gray, finer grained material.”

The soils exposed in Lowell Creek have been segregated by differing flow regimes and form overlapping bodies of differing particle gradations. It is likely that these conditions persist in the subsurface creating highly variable conditions at may locally effect soil density and permeability. In addition to the alluvium in the creek channels, glacial moraine material is present upstream of the tunnel inlet, and colluvium and talus accumulations are present along the base of the mountain slopes. Figure 3-7 depicts examples of the particle size range and the segregation caused by stream activity in Lowell Creek.

Surface deposits on the bay side consist of deltaic outwash deposits from Lowell Creek.

The mountain slope deposits consist of talus, glacial till, and localized lateral moraine deposits.

4.3. Bedrock

Phase 1 investigation of the site by Golder (2022) identified three primary rock types:

graywacke, slate, and phyllite. Variations of these lithologies have historically been identified at the site and include quartzite, argillite, and shale. For the purposes of this 65% design, bedrock at the site was grouped into two informal units based on their engineering characteristics. A detailed description of the formations and their properties are presented in the GDR.

• Graywacke. The graywacke unit is considered to be massive and thickly bedded coarse-grained deposits. Graywacke is typically characterized by its hardness, dark color, and poorly sorted grains in a fine matrix. Phase I explorations described the graywacke as fresh to slightly weathered, fine to medium grained, homogeneous, medium gray to dark gray, and medium weak to strong. It is expected to be moderately jointed and moderately blocky to seamy. The graywacke generally consists of widely spaced joint sets and is often interbedded with the weaker phyllite, argillite, and shale bedrock types. During Phase I explorations, massive graywacke was encountered near the tunnel outlet, on the upper slopes of Bear Mountain and along Lowell Creek Canyon.

• Interbedded Graywacke. The interbedded graywacke unit is composed of graywacke interbedded with fine-grained deposits of varying metamorphic grade such as slate, phyllite, or schist. Argillite, an indurated mudstone, and fissile shale have also been identified at the site in the past.

• Discrete zone of slate or slate/graywacke interbeds were encountered in nearly all borings advanced into bedrock during the Phase 1 explorations with the exception of AP-218 located along the Lowell Creek canyon and AP-206 and AP-222 located near the outlet where mainly massive graywacke was encountered. The slate was generally described as foliated, very fine to fine grained, very dark gray, weak to medium weak, with joints observed along foliation planes. Figure 3-10 shows a typical rock core with slate interbeds. The slate was observed to be weaker than the graywacke and tended to break along foliations as was frequently observed during drilling. This interbed will be a controlling factor in the strength of this unit.

• Phyllite was identified in rock outcrops during the Phase 1 mapping effort along Lowell Creek canyon and the Bear Mountain Trail that generally follows the ridgeline.

Discrete zones of phyllite were also identified in the uppermost interval of borings AP-214 and AP-218, located along Lowell Creek canyon. The only identification of phyllite at depth was at boring AP-214 at 76-77 feet bgs. The phyllite was generally described foliated, dark gray and orange-gray, very fine grained, weak rock, with microfractures. Due to the lack of observed micaceous material, the phyllite identified during Phase 1 explorations could actually be weathered slate or shale.

4.4. Groundwater Levels

Groundwater occurs within the fractured bedrock and flow is primarily transmitted through interconnected fractures, joints, and/or faults. Groundwater data collected around the project site indicates that (more here) has a direct impact adjacent bedrock groundwater levels. Automated piezometers surrounding the excavation typically follow (more here). There are no piezometers along the tunnel alignment See the GDR for piezometric plots.

Groundwater monitoring wells were installed in eight boreholes (SEW-BH-2024001, SEW-BH- 2024003, SEW-PZ-2024022, SEW-PZ-2024023, SEW-PZ-2024024, SEW-

PZ-2024025, SEW-PZ-2024026, and SEW-PZ-2024027).

5. Ground Characterization USACE developed baseline properties presented in Section 5 using field and laboratory data from investigations listed in Section 3 except for data collected during Phase 1 and 2 Explorations. Data utilized are presented in the GDR. Baseline values will be updated in subsequent versions of this GBR after submittal of Phase 2 Exploration data report.

5.1. Equivalent Hydraulic Conductivity

Equivalent hydraulic conductivity of fractures was evaluated based on results from packer tests conducted in borings near the inlet. In general, fractures were classified (more here). Seepage through the bedrock is generally anticipated to be (more here).

(more here) are anticipated in highly fractured or fault zones.

Table -1. Equivalent Hydraulic conductivity data statistics.

Test Type Minimum (cm/s)

GM

(cm/s)

68% 1 (cm/s)

90% 2 (cm/s)

Maximum (cm/s)

Packer Tests (fractures)

Notes:

GM – geometric mean GSD – geometric standard deviation

68% = GM(GSD)

90% = GM(GSD)1.645

Figure -1. Histogram of packer test hydraulic conductivities.

5.2. Intact Rock Properties

Baselines for intact rock properties are summarized on Table -2. Data are separated by bedrock units.

Intact compressive strength is based upon XXX unconfined compressive strength (UCS) tests. Data are summarized on Figure -2 with the raw data located in the GDR. For baseline purposes, the 90th percentile is the maximum anticipated value at the site.

Splitting tensile strength is based upon XXX Brazilian tensile strength tests (BTS). One test is the average of [10] individual specimen tests from one sample location. The data is summarized on Figure -3 with the raw data located in the GDR. For baseline purposes, the 90th percentile is maximum anticipated value at the site.

Table -2. Baseline Intact Rock Properties

Parameter Unit Graywacke Interbedded Graywacke Geometric

Mean 90th

Percentile Geometric

Mean 90th Percentile

UCS psi

BTS psi

Figure -2. UCS frequency distribution and summary statistics.

Figure -3. Splitting tensile strength frequency distribution and summary statistics.

5.3. Abrasivity

Cerchar Abrasivity Testing was completed on XXX rock core samples to determine the Cerchar Abrasivity Index (CAI) which can be used to select excavation methods, machinery or other equipment. The data is summarized on Figure -4 with the raw data located in the GDR. There are no distinct differences between the CAI of individual bedrock units. For baseline purposes, the 90th percentile is the maximum anticipated value for all bedrock units at the site.

Parameter Unit Geometric Mean

90th Percentile

Cerchar Abrasivity Index (CAI) --

Figure -4. CAI frequency distribution and summary statistics.

5.4. Discontinuity Orientation

Discontinuity orientation data were collected in boreholes using televiewer surveys, along surface outcrops, and in the adits. Discontinuities identified during explorations demonstrate an overall scatter, likely related to the complex stratigraphy, structure, and hydrothermal alteration. Five concentrations of discontinuities were identified which seem to have preferred orientations. Data for the inlet and outlet were evaluated separately. Data plotted on Figure -5 and Figure -6 have similar pole concentrations representing (more here). Baseline ranges of rock joint orientations are summarized Table -3. The Fisher dispersion coefficient is used to baseline the range of values anticipated at the site.

Table -3. Summary of Joint Set Baselines

Structure Inlet Area Outlet Area Mean Set Plane Fisher

Dispersion Coefficient (K)

Mean Set Plane Fisher Dispersion Coefficient (K)

Dip (deg)

Dip (deg)

Dip (deg)

Dip Direction (deg)

Joint Set 1 Joint Set 2 Joint Set 3 Joint Set 4 Joint Set 5

Figure -5. Lower hemisphere equal angle stereonet showing joint set poles and mean planes for inlet area.

Figure -6. Lower hemisphere equal angle stereonet showing joint set poles and mean planes for outlet area.

5.5. Discontinuity Spacing

Set spacing of discontinuities were assessed based on mapping and televiewer data collected at boreholes along the tunnel alignment. Comparison of statistics between the two data sets shown a comparable mean spacing of XXX feet (see Table -4 and Figure -7).

Table -4. Discontinuity set spacing statistics (apparent spacing).

Spacing 10th Percentile (feet)

Geometric Mean (feet)

90th Percentile (feet)

Inlet Area

Outlet Area

Figure -7. Histogram of discontinuity set spacing (apparent spacing).

6. Construction Considerations USACE developed baseline properties presented in Section 6 using field and laboratory data from investigations listed in Section 3. Data are presented in the GDR. Baseline values will be updated after completion of the ongoing Phase 2 Explorations and included in subsequent versions of the GBR.

6.1. Inlet Structure and Portal Area

Baselines discussed in this section apply only to the excavation for the Inlet Structure and Portal. Data used to baseline conditions in this area are from borings completed in Phase 1 and Phase 2. Cross-sections at the Inlet area are shown in Appendix A.

Dewatering

Bulk rock mass hydraulic conductivity values are based on pumping tests completed in XXXX for the inlet area at boreholes XXXX. Results calculated using the Cooper-Jacob Method were used for rock mass baselines. For baseline purposes, the 90th percentile is the maximum anticipated value at the site (Table -1).

Table -1. Baseline bulk rock mass hydraulic conductivity values for the Inlet Area

Parameter Unit Geometric Mean

90th Percentile

Bulk rock mass hydraulic conductivity cm/s

Existing dewatering well flow rates during previous excavation construction activities are listed below. There are no baselines associated with existing well flow or drawdown due to the age and unknown condition of the wells and pumps.

• XXXX

Rock Mass Quality

Rock mass quality at the Inlet will be represented by Geologic Strength Index (GSI) as described by Hoek and Brown (1997); Hoek and others (2013). GSI values were calculated from borings completed around the existing FPF excavation and extension for the steep slope bypass. Data are summarized in Figure -1 and included in the GDR. For baseline purposes, the 90th percentile is the maximum anticipated value at the site (Table -2).

Table -2. Baseline GSI values for Inlet Area

Parameter Unit 10th Percentile

Geometric Mean 90th Percentile

Geological Strength Index

(GSI)

Figure -1. GSI frequency distribution and summary statistics for the FPF excavation.

Anchor Installation and Redrilling

The inlet portal slope support includes rock dowel installation. Watertightness testing is required per specification 31 68 13.20 27, Slope Support, for each dowel with grouting and redrilling for those anchor holes failing the test. For baseline purposes, the contractor shall anticipate that 40% of anchor holes will require grouting and redrilling prior to dowel installation.

6.2. Outlet Structure and Portal Area

Baselines discussed in this section apply only to the Outlet Structure and Portal Area.

Data used to baseline conditions in the Outlet Area are from borings completed during Phase 1 and 2 Design Explorations. Data from borings completed during Phase 2 Design Explorations will be incorporated into subsequent versions of the GBR.

Observations from the Phase 2 drilling are noted where applicable.

6.3. Tunnel

The characterization of the ground behavior presented in this GBR is based upon the assumption that the tunnel will be constructed using drill and blast techniques. The tunnel shall pass through the Engineering Rock Units which are classified based upon rock types with similar intact rock properties. The Engineering Rock Units are subdivided based upon rock mass behavior for this project correlated with the RMR into a series of four Ground Classes (Ground Class I, Ground Class II, Ground Class III, and Ground Class IV) and as presented in the Contract Documents.

(Add RMR Discussion)

Reach Types

Reach Types are utilized to determine the appropriate minimum ground support type as indicated in the contract drawings. Reach Types are listed below.

Reach Type 1. Inlet (Interbedded Graywacke/Slate/Shale). Reach 1 begins at the furthest downstream extent of inlet ring steel support at Sta. 09+00 and ends at Sta.

09+75, which represents 75 linear feet of tunnel. Reach 1 is defined by predominantly interbedded greywacke materials, with beds of weaker materials, such as phyllite and argillite, present. Reach 1 represents the inlet portal, which is proximate to Lowell Creek as well as a shear zone, featuring some raveling ground in phyllite.

Reach Type 2A. Representative of Interbedded Graywacke Material (Main Tunnel/Less than 40% shale). Reaches 2A and 2B are located along the tunnel alignment where predominantly interbedded graywacke materials are encountered. For project bidding and planning, the design team has estimated a total of 400 discontinuous linear feet of Reach 2A materials. Reaches 2A and 2B are defined by predominantly interbedded greywacke materials, with beds of weaker materials, such as phyllite and argillite, present. Reach 2A is defined by less than 40% shale.

Reach Type 2B. Representative of Interbedded Graywacke Material (Main Tunnel/Greater than 40% shale). Reaches 2A and 2B are located along the tunnel alignment where predominantly interbedded graywacke materials are encountered. For project bidding and planning, the design team has estimated a total of 400 discontinuous linear feet of Reach 2B materials. Reaches 2A and 2B are defined by predominantly interbedded greywacke materials, with beds of weaker materials, such as phyllite and argillite, present. Reach 2B is defined by greater than 40% shale.

Reach Type 3. Representative of Shear Zones. Reach 3 is located along the tunnel alignment where shear zones intersect with the tunnel alignment, resulting in highly fractured rock, reduced strength, and increased alteration and weathering. For project bidding and planning, the design team has estimated a total of 225 discontinuous linear feet of these materials. The shear zones are thought to exhibit extensive fracturing, resulting in a network of closely spaced discontinuities. In these zones, the rock mass strength is reduced, and the rock pieces may range from small angular fragments to larger blocks.

Reach Type 4. Representative of Graywacke Material. Reach 2 is located along the tunnel alignment where predominantly massive graywacke materials are encountered.

For project bidding and planning, the design team has estimated a total of 800 discontinuous linear feet of these materials. Reach 2 is defined by predominantly greywacke materials.

Reach Type 5. Outlet (Massive Graywacke) Sta. XX to XX. Reach 5 is located from the tunnel Sta. 25+32.5 to the outlet portal located at Sta. 26+07.5 ft, which represents 75 linear feet of tunnel. Reach 5 is defined by predominantly greywacke materials. Reach 5 represents the exit portal.

Tunnel Reaches

The tunnel was divided into five reaches based on rock type, potential shear zones, and location. Representative borings proximate to the inlet and outlet areas were used to assess the ground class percentage anticipated during tunnel construction for those areas. Borehole data was not available for the main tunnel section due to the inaccessibility of surficial drilling on Bear Mountain. For baseline purposes, the contractor shall use the reach type percentages per reach shown on Table -3.

Table -3. Baseline values for Ground Class Percentage per Reach

Reach Type Linear Feet

(Continuous or Discontinuous)

Support Category Notes

Reach Type 1: Inlet and Starter Tunnel 150 E

Reach Type 2A: Interbedded Graywacke/Shale (Main Tunnel/Less than 40% shale)

600 C

60% of overall estimation of 1000 LF of interbedded materials

Reach Type 2B: Interbedded Graywacke/Shale (Main Tunnel/Greater than 40% shale)

400 D

Reach Type 3: Shear Zones (Main Tunnel) 225 E 10% of total tunnel length

Reach Type 4: Massive Graywacke (Main Tunnel) 800 B

Reach Type 5: Outlet (Outlet Portal) 75 A

Support Categories

The following describes the recommended Support Categories:

• Support Category A: Medium steel ribs (6H20) at 4.5 to 5 foot spacing, pinned wire mesh (Contractor designed), 15 foot long, grade 75, #8 rock dowels on 6-foot centers, with shotcrete applied as necessary.

• Support Category B: 15 foot long, grade 75, #8 rock dowels on 6-foot centers.

• Support Category C: Where interbedded graywacke and shale/phyllite is encountered, 15 foot long, grade 75, #8 rock dowels on 5-foot centers, and pinned wire mesh (Contractor designed) is required.

• Support Category D: Where interbedded graywacke and shale/phyllite is encountered, where the shale/phyllite percentage per reach is greater than 40%, or as otherwise directed by the Contracting Officer, 15 foot long, grade 75, #8 rock dowels on 5-foot centers, and pinned wire mesh (Contractor designed), and shotcrete is required.

• Support Category E: Medium or heavy steel ribs (8WF31 or 8WF48) on a

2.5 to 3 foot spacing, pinned wire mesh (Contractor designed), 15 foot long, grade 75, #8 rock dowels on 5-foot centers, with shotcrete applied as necessary. Phase II Horizontal borings at the inlet will provide further stationing insight.

Tunnel Inflows

Tunnel inflow estimates are based on the equivalent hydraulic conductivity interpreted from packer tests performed at borings along the tunnel alignment. Phase 2 exploration data have not been incorporated. Test results were analyzed using the Heuer method for estimating ground water inflows into tunnels (Heuer 1995, 2005). For baseline purposes, that contractor shall anticipate that the maximum steady state inflows for the entire length of the tunnel is XXX gpm. Anticipated steady state inflows per reach are shown on Table -4.

Table -4. Baseline Values for Steady State Inflow per Select Tunnel Reach

Reach Steady State Inflow (gpm) Reach 1: Inlet and Starter Tunnel To Be Determined

Reach 5: Outlet (Outlet Portal) To Be Determined

Packer tests were completed in three boreholes (SEW-BH-202401, SEW-BH-202402, SEW-BH-202403) to assess hydraulic conductivity of the rock mass. Two geotechnical boreholes (SEW-MW-2024007 and SEW-MW-2024008) were converted to 16-inch water wells to complete pumping tests and evaluate the aquifer hydraulic properties and boundaries. Step-rate and constant rate pumping tests were performed in each and a dual-well, constant rate pumping test was performed using both wells.

Transient inflows (flush flow) at the heading can be anticipated over a maximum length of XX feet. For baseline purposes, the contractor shall anticipate encountering a singular maximum flush flow of XXX gpm that will dissipate after a maximum of X days.

6.4. Diversion Dam

Baselines discussed in this section apply only to the diversion dam. Baselines will be developed in subsequent versions of the GBR after Phase 2 Exploration data have been assessed.

Dewatering

The diversion dam will be constructed adjacent to Lowell Creek and subject to seepage through the rock mass from the creek as well as groundwater. Packer tests and piezometer installations were completed during the Phase 2 Design Explorations and will be used to baseline hydraulic conductivity values and groundwater elevations in the next version of the GBR.

7. References ASCE, 2022. Geotechnical Baseline Reports Suggested Guidelines, ASCE Manuals and

Reports on Engineering Practice No. 154, Task Committee on Geotechnical Reports, Randall J. Essex., Editor.

Ceballos, F. and Jimenez, C.O.R, 2014. Relationship between RMRb and GSI based on in situ data. Prepared for EUROCK Vigo 2014, ISRM European rock mechanics symposium.

Heuer (1995). Estimating rock tunnel water inflow, in G.E., Williamson and I.M.

Gowring, Proceedings from Rapid Excavation and Tunneling Conference, p.41- 60, 1995.

Heuer R (2005) Estimating rock tunnel water inflow—II. In: Proceedings of Rapid Excavation and Tunnelling Conference 2005, pp 394–407.

Hoek, E., and Brown, E.T., 1997. Practical estimates of rock mass strength: International Journal of Rock Mechanics and Mining Sciences, v. 34, no. 8, p. 1165-1186.

Hoek, E.; Carter, T.G.; and Diederichs, M.S., 2013. Quantification of the geological strength index chart, in 47th US Rock Mechanics/Geomechanics Symposium, San Francisco, Calif., 2013, Proceedings: Alexandria, Va., American Rock Mechanics Association, paper 13-672, 8 p.

Lynner, C., Toro-Acosta, C., Paulson, E., & Birkey, A. (June 2024). Local-S shear wave splitting along the length of the Alaska–Aleutian subduction zone. Geophysical Journal International, 1567–1574.

Melia, A., Faulkner, D. R., & McNamara, D. D. (2002). Physical property characterization of the Waipapa greywacke: an important geothermal reservoir basement rock in New Zealand.

Geothermal Energy.

Ratchkovski, N. A., & Hansen, R. A. (2002). New Evidence for Segmentation of the Alaska Subduction Zone. Bulletin of the Seismological Society of America, 1754-1765.

You, T., & Zhao, D. (August 2012). Seismic anisotropy and heterogeneity in the Alaska subduction zone. Geophysical Journal International, 629–649.

APPENDIX A

Inlet Area Cross Sections

APPENDIX B

Outlet Area Cross Sections

7. References