Geotechnical_Report.pdf

Fort Hall Reservation

Power County, Idaho for

HDR, Inc.

April 21, 2025

G E O T E C H N I C A L E N G I N E E R I N G R E P O R T

BUREAU OF INDIAN AFFAIRS

BANNOCK PUMPING PLANT

MODERNIZATION PROJECT

GEOTECHNICAL ENGINEERING REPORT

Geotechnical Engineering Report, Revision 1

Bureau of Indian Affairs Bannock Pumping Plant Modernization Project

Fort Hall Reservation, Power County, Idaho

CHE File No. 4000-001-00 April 21, 2025

Prepared for:

HDR, Inc.

412 E. Parkcenter Blvd., Ste 100

Boise, ID 83706

Attention: Jerry Otto

Prepared by:

CIANI & HATCH ENGINEERING, PLLC

Jacob A. Helms, PE

Geotechnical Engineer

Whitney L. Ciani, PE

Principal

JAH:MSH:WLC:jas

Disclaimer: Ciani & Hatch Engineering, PLLC (CHE) cannot guarantee the accuracy and content of electronic files. The master file of this document is stored by CHE and will serve as the official document of record.

Bannock Pumping Plant Modernization 4000-001-00

April 21, 2025 iii

Bureau of Indian Affairs Bannock Pumping Plant Modernization Project

CONTENTS

1.0 Introduction

1.1 Project Understanding

1.2 Scope of Services

2.0 Existing Conditions

2.1 Surface Conditions

2.2 Geologic Setting

2.3 Subsurface Conditions

2.3.1 Soil Conditions

3.0 Conclusions and Recommendations

3.1 Earthquake engineering

3.1.1 Seismicity

3.1.2 Ground Shaking

3.1.3 ASCE 7-22 Seismic Design Information

3.1.4 Liquefaction Potential

3.1.5 Surface Fault Rupture

3.2 Foundation Support

3.2.1 Shallow Foundations

3.2.1.1 Allowable Bearing Pressure

3.2.1.2 Subgrade Preparation

3.2.1.3 Modulus of Subgrade Reaction

3.2.1.4 Foundation Settlement

3.2.1.5 Lateral Resistance

3.2.1.6 Foundation Drainage

3.2.1.7 Construction Considerations

3.3 Debris Management Slab

3.3.1 Subgrade Preparation

3.4 Rotating Screen Slab-on-Grade

3.5 Canal Liner

3.6 Earth Retaining Structures

3.6.1 Below-Grade Walls

3.6.2 Gravity Block Retaining Walls

3.6.3 Wall Drainage

3.7 Gravel Access Road

3.7.1 Subgrade Preparation

3.7.2 Gravel Roadway Section

3.8 Earthwork

3.8.1 Clearing and Site Preparation

3.8.2 Excavation Support

3.8.2.1 Temporary Cut Slopes

3.8.3 Subgrade Preparation & Protection

3.8.4 Utility Trenches

3.8.4.1 Soil Corrosivity

April 21, 2025 iv

3.8.5 Structural Fill

3.8.5.1 Materials

3.8.5.2 Reuse of On-Site Native Soils

3.8.5.3 Fill Placement and Compaction Criteria

3.8.5.4 Weather Considerations

3.9 Sedimentation and Erosion Control

4.0 Final Design and Construction Support

5.0 Limitations

6.0 References

FIGURES

Figure Title

1 Project Vicinity Map

2 Geotechnical Exploration Site Plan

3 Permanent Below-Grade Walls

4 Recommended Surcharge Pressure

TABLES

Table Title

1 ASCE 7-22 Seismic Design Parameters

2 Gravity Block Retaining Wall Soil Properties

3 Corrosivity Testing Results

APPENDICES

Appendix Title

A CHE Field Explorations

B Laboratory Testing

C Dynamic Cone Penetrometer Analysis

April 21, 2025 v

ABBREVIATIONS AND ACRONYMS

ACI American Concrete Institute

AL Atterberg Limit

ASCE American Society of Civil Engineers

ASTM ASTM International bgs below ground surface

CHE Ciani & Hatch Engineering, PLLC

FEMA Federal Emergency Management Agency

FIRM Flood Insurance Rate Map

HVAC Heating, Ventilation, & Air-Conditioning

ISPWC Idaho Standards for Public Works Construction ksf kips per square foot

MCER Mapped Risk-Targeted Maximum Considered Earthquake

MDD Maximum Dry Density

NFPA National Fire Protection Association

PCCP Prestressed Concrete Cylinder Pipe pcf pounds per cubic foot pci pounds per cubic inch

USGS United States Geological Survey

April 21, 2025 vi

This page intentionally left blank.

April 21, 2025 1

1.0 INTRODUCTION

This report presents geotechnical engineering conclusions and recommendations based on the results of

Ciani & Hatch Engineering, PLLC’s (CHE) field investigation and laboratory testing program for the Bannock

Pumping Plant Modernization project (project) located on the Fort Hall Reservation in Power County, ID.

The general project location is shown on the Project Vicinity Map (Figure 1); the site is shown relative to existing features and infrastructure on the Geotechnical Exploration Site Plans (Figures 2-1 and 2-2).

This report was previously issued on January 16, 2025. Following confirmation of foundation loading, additional foundation support options were incorporated into this report. This report supersedes the report issued on January 16, 2025.

1.1 PROJECT UNDERSTANDING

This report was prepared based on our discussions with, and information provided by, HDR, Inc. (HDR). At the time of this report, we have been provided the 30% design drawings dated September 5, 2024. CHE understands the proposed improvements to existing facilities at the pumping plant includes expansion of the below-grade, cast-in-place valve vault and construction of a new electrical building and associated transformer, generator, and propane tank in the adjacent switchyard. At the location of the siphon, improvements include the expansion of the siphon foundation to accommodate the installation of a new rotating screen, construction of a new concrete debris management slab, and installation of a new steel catwalk structure. We understand that the new debris management slab will include a 4-foot-tall pre-cast block wall. CHE understands slope cuts will be used for the proposed site excavations and shoring recommendations are not needed for the proposed improvements.

1.2 SCOPE OF SERVICES

The purpose of our design services was to explore the subsurface soil and groundwater conditions at the site as a basis for providing geotechnical design recommendations for the project. Our scope of services has been provided in accordance with our proposal for geotechnical engineering services dated May 7, 2024. The services in that proposal were incorporated into the subcontract between HDR and CHE executed July 1, 2024, and amended August 13, 2024.

2.0 EXISTING CONDITIONS

The following sections provide a description of the existing surface conditions, the geologic setting, and a summary of the subsurface soil and groundwater conditions observed during CHE’s field investigation.

2.1 SURFACE CONDITIONS

The pumping plant site is relatively flat and vegetated with native grasses and shrubs and sits at the toe of an approximately 75-ft-tall slope immediately southwest of the pumping plant and switchyard. Topography in the immediate vicinity of the siphon is relatively flat with cultivated farmland to the northeast, southeast, and southwest. The land northwest of the siphon is predominantly undeveloped.

The pumping plant structure, located approximately 200 feet south of the intersection of the Bannock Drain and the Taghee Canal, was constructed in 1965. The age of any additions or renovations after the initial construction is unknown. This structure is constructed from conventional masonry units and houses heavy equipment including pumps and an overhead crane. Stair-step cracking is visible in the exterior of this structure, following the block path. The pumping plant structure connects under the gravel access road to the south to three 36-inch Prestressed Concrete Cylinder Pipe (PCCP) pipelines extending to the southwest

April 21, 2025 2 and one 42-inch PCCP extending to the south. The switchyard lies to the northwest of the pumping plant and connects to the pumping plant via underground electric powerlines. Overhead powerlines extend from the switchyard to the northwest.

The siphon structure, located approximately 1,600 feet to the northeast of the pumping plant, was constructed prior to 1985, based on our review of publicly available aerial imagery. A small power transformer connected to the overhead power lines and several below-grade tanks/structures lie to the south of the western side of the structure. A steel catwalk crossing over Bannock Creek connects the east and west sides of the siphon. The slopes to Bannock Creek were approximately 2H:1V (horizontal to vertical) and heavily vegetated. At the time of our exploration program and during our November 2024 site visit, the water surface in Bannock creek was approximately 9 feet below the top of the creek bank. Buried power extended from the substation through the approximate centerline of the access road to the north, across the catwalk, then south to the trash rack of the siphon. Overhead powerlines are present along the southern side of the site and continue to the east.

2.2 GEOLOGIC SETTING

Geologic information for the project area was obtained from the “Geology of the Michaud and Pocatello quadrangles, Bannock and Power Counties, Idaho” geologic map (Trimble, 1976). Surficial deposits mapped within the project areas consist predominantly of young alluvium and loess from the Holocene and

Upper Pleistocene epochs.

Alluvium in this area is attributed to Bannock Creek and is predominantly comprised of silt and sand. Loess refers to windblown deposits, predominantly comprised of silt and sand mixtures with variable degrees of cementation. Loess is typically deposited loosely, resulting in the formation of pinhole structures, which can collapse when inundated/saturated with water and subjected to loading, leading to excessive settlement.

Loess has been documented to be greater than 25 feet thick to the east of Bannock Creek and more than

200 feet thick to the west. Based on the proximity to Bannock Creek and our review of Idaho Department of Water Resources Well-Driller logs, we anticipate the loess soils extend to depths of 20 to 40 feet in the project area.

The soils encountered during CHE’s field investigation program were generally consistent with the mapped geology. In addition to loess and alluvium, we encountered variable thicknesses of fill throughout the project site.

2.3 SUBSURFACE CONDITIONS

Subsurface conditions were explored between August 7 and 8, 2024, by advancing eight test pit explorations (CHE-TP1-24 through CHE-TP8-24) to depths between 3 and 15 ft below ground surface

(bgs). The approximate locations of the explorations are shown on Figures 2-1 and 2-2.

CHE personnel monitored the field explorations, collected representative soil samples, and maintained detailed logs of the subsurface soil and groundwater conditions. Summary test pit logs and a description of the geotechnical field investigation are provided in Appendix A.

An additional site visit was completed on November 18, 2024, after the irrigation had been shut off for the season. The purpose of our visit was to evaluate subgrade soil conditions for the proposed rotating screen and to obtain subgrade soil samples. Samples of subgrade material were obtained at the new debris management slab and in the base of the Taghee Canal.

Soil samples were transported to STRATA’s geotechnical laboratory in Boise, Idaho, for further examination and testing. Test results and a description of the geotechnical laboratory testing program are provided in

Appendix B.

April 21, 2025 3

2.3.1 Soil Conditions

The soils observed underlying existing surface conditions (i.e., natural vegetation, gravel surfacing) were characterized into three general units:

Fill: Fill was encountered in every test pit except CHE-TP3-24 to depths up to 2 feet at the pumping plant and deeper at the siphon. Fill generally consisted of reworked native silt and contained gravel and cobbles as well as construction debris (such as timber or slag). Fill was typically light brown in color and ranged in consistency/density across the project site.

Loess: Loessal soils were encountered in all test pits except CHE-TP8-24. These soils consisted of soft to very stiff light brown silt. Pinhole structure was observed on the sidewalls of the test pit excavations and in the excavated soil samples. Cementation was observed in some layers, ranging from weak to moderate.

Alluvium: The alluvial deposits were encountered below approximately 5 feet bgs in Test Pits CHE-TP4-

24, CHE-TP5-24, and CHE-TP7-24, all of which were adjacent to the eastern slope at the location of the siphon structure. These deposits were a brown to dark brown mixture of silt and clay with variable sand content. Organic debris, including rootlets and twigs, were encountered within this stratum. These materials were generally soft to stiff and moist.

3.0 CONCLUSIONS AND RECOMMENDATIONS

A summary of the primary geotechnical considerations is provided below. This summary is provided for introductory purposes only and should be used in conjunction with the complete recommendations presented in this report.

• Based on laboratory test results, the loess deposits encountered at the project site are prone to collapse-consolidation settlement when loaded and saturated. For an allowable bearing capacity of 2 ksf, we anticipated up to 0.5 inch of collapse settlement. To further limit collapse consolidation settlement, an allowable bearing pressure of 1 ksf should be used.

• New structures and equipment may be supported on shallow or mat foundations, depending on the design bearing pressure. Spread footings are not recommended for structures with a bearing pressure greater than 1 ksf due to the potential differential settlement resulting from collapse consolidation settlement of loess soils.

• Due to the presence of collapsible loess soils, extra care should be taken to divert surface water away from structures and equipment pads. Infiltration facilities, if planned, should be located away from structures and in coordination with the project geotechnical engineer.

• Groundwater was not encountered in our explorations. Based on nearby, historic well logs, we anticipate groundwater is over 20 feet below existing site grades.

• The Debris Management Slab was designed as a Portland Cement Concrete Pavement (PCCP) subject to highway loading, as requested by HDR, to determine the appropriate slab thickness and base material support. The results of our analysis show that 7 inches of reinforced PCCP supported on 6 inches of crushed surfacing base course is appropriate for the debris management slab.

• Chemistry Suite testing (pH, Resistivity, Sulfates) of samples from across the project site indicate a low concern for soil corrosivity.

Specific geotechnical engineering recommendations are presented in the following sections of this report.

April 21, 2025 4

3.1 EARTHQUAKE ENGINEERING

3.1.1 Seismicity

Southeast Idaho is a seismically active region with low to moderate seismic intensity and hazard.

Earthquakes in this area are generally low in magnitude but can occasionally meet moderate levels. The seismic activity in the area is associated with the faulting in volcanic rock that occurred during the subsidence and formation of the Eastern Snake River Plain. Potential hazards from earthquakes include ground shaking, liquefaction, lateral spreading, and surface fault rupture. Our opinions regarding the likelihood of these seismic hazards occurring at the site are presented below.

3.1.2 Ground Shaking

There is a risk of earthquake-induced ground shaking at the site. The severity of ground shaking will mostly be a function of the earthquake’s magnitude and proximity to the site. In our opinion, strong ground shaking should be considered in the design of the planned structures at the site. We recommend that the seismic ground shaking at the site be evaluated in accordance with the ASCE 7-22 design code.

3.1.3 ASCE 7-22 Seismic Design Information

We understand the project is being designed in accordance with the National Fire Protection Association

(NFPA) 5000-2024 Building Construction and Safety Code, which references ASCE 7-22 for earthquake loading requirements. ASCE 7-22 requires site-specific data to a depth of 100 ft to determine seismic site class. Where soil properties are not known in sufficient detail, default site conditions should be used in accordance with Section 11.4.2.1. Spectral response accelerations shall be based on the most critical spectral response acceleration at each period of Site Class C, Site Class CD, and Site Class D. The ASCE

7-22 risk-targeted maximum considered earthquake (MCER) parameters for Site Class C, CD, and D are presented in Table 1.

April 21, 2025 5

Table 1. ASCE 7-22 Seismic Design Parameters

ASCE 7-22 Parameter

Recommended Value

Site Class C

Site Class

CD

Site Class D

Ss (percent g) Short Period Spectral Response Acceleration

0.42 0.42 0.42

S1 (percent g) 1-Second Period Spectral Response Acceleration

0.13 0.13 0.13

PGAM (percent g) Peak Ground Acceleration Adjusted for Site Effects

0.19 0.22 0.23

TL (second) Long-Period Transition Period

6 6 6

SDS (percent g)

Short Design Earthquake Spectral Response Acceleration

0.31 0.36 0.40

SD1 (percent g) 1-Second Design Earthquake Spectral Response Acceleration

0.12 0.17 0.23

SMS (percent g) MCER, 5%-damped, spectral response acceleration parameter at short periods adjusted for site effects

0.46 0.54 0.60

SM1 (percent g) MCER, 5%-damped, spectral response acceleration parameter at a period of 1-second adjusted for site effects

0.18 0.26 0.35

3.1.4 Liquefaction Potential

Liquefaction refers to a condition where vibration or shaking of the ground, usually from earthquake forces, results in development of excess pore pressures in saturated soils and subsequent loss of shear strength.

Structures supported on liquefied soils could suffer foundation settlement or lateral movement that could be severely damaging to structures. Conditions favorable to liquefaction occur in loose to medium dense, clean to moderately silty sand that is below the groundwater level. Based on our field exploration and laboratory testing, we consider the risk of liquefaction at the project site to be low.

3.1.5 Surface Fault Rupture

The nearest mapped fault systems to the project site include the Wasatch fault zone, located 33 miles southeast, and the Idaho Rift System fault, located 32 miles northwest (USGS, 2024). Based on the fault distances from the site, the risk of fault displacement resulting in ground rupture at the site is considered low.

3.2 FOUNDATION SUPPORT

The project site is underlain by loess soils, which are susceptible to collapse consolidation settlement under foundation loading when wetted. Loess soils were encountered in the depths explored in the test pit explorations completed for this study. Loessal soils form when clay and silt-sized particles are loosely deposited by wind, resulting in the formation of small voids between particles called pinholes. These soils may be relatively stable under loading, but collapse upon saturation, resulting in potentially excessive

April 21, 2025 6 settlement. Based on the design bearing elevation of the structures at the site and the anticipated thickness of the loess to depths greater than 20 feet, removal and replacement of the unsuitable soil is not feasible.

Two collapse consolidation tests were completed on site soils. The tests indicate that foundation soils have a collapse potential of 0.05 to 0.5 percent under 1 ksf and 2 ksf foundation loads, respectively.

The wetting horizon is the downward movement of water through a soil zone. The wetted depth of the soil will depend on the duration of time the soil is exposed to water head. Zones of silt within the soil become wetted at different times and exhibit different collapse potential based on their deposition and mineralogy.

CHE evaluated the hypothetical wetting horizon for our site and selected a depth of 9 ft for evaluation of collapse consolidation settlement (Li & Vanapalli, 2015). This wetted depth corresponds to an estimated collapse settlement of 0.1 to 0.5 inch for soil under 1 ksf and 2 ksf foundation loads, respectively.

The primary source of water at the project site is precipitation from rain and snow events. The site should be graded such that water flows away from structures and concrete mats to reduce potential for surface water infiltration and subsequent wetting of subgrade soils.

3.2.1 Shallow Foundations

We recommend new building and equipment pads with a design bearing pressure greater than 1 ksf be supported on mat foundations to evenly load foundation soils and reduce effects of differential settlement.

Support of the new pedestrian bridge/catwalk structure and other structures with a design bearing pressure equal to 1 ksf or less can be provided with conventional shallow spread footings. We recommend the leading edge of the pedestrian bridge footings be set back a minimum of 4 feet from the top of the slope of the canal. We recommend minimum widths of 16 and 48 inches for continuous wall and isolated spread footings, respectively. The design frost depth for Power County is 32 inches; therefore, we recommend that all foundation elements be founded at least 32 inches below the lowest adjacent finished grade.

For new structures with shallow spread footings, conventional floor slabs-on-grade are appropriate when the structures are supported on proof-rolled native soils or on structural fill placed over stable native soils, provided the subgrade soils are prepared as described in Section 3.8. Based on the test pits completed at the site, we anticipate that stiff to very stiff loessal silt will be present at the floor slab subgrade elevation.

3.2.1.1 ALLOWABLE BEARING PRESSURE

For mat foundations supported on native soils or on structural fill extending down to these soils, an allowable soil bearing pressure of 2 ksf may be used where 0.5 inch of settlement is acceptable. For shallow spread footings supported on native soils or on structural fill extending down to these soils, or where settlement must be limited to less than 0.5 inch, an allowable soil bearing pressure of 1 ksf should be used. The zone of structural fill below the foundation should extend beyond the edge of the foundation a distance at least equal to the thickness of the structural fill. The zone of structural fill should be compacted to at least 95 percent of the maximum dry density (MDD) in general accordance with ASTM International (ASTM) D1557.

The allowable soil bearing value applies to the total of dead and long-term live loads and may be increased by up to one-third for wind or seismic loads.

3.2.1.2 SUBGRADE PREPARATION

We recommend the subgrade soils be prepared and evaluated as described in Section 3.8 of this report.

Structural foundations and slabs-on-grade should be constructed on a 6-inch-thick layer of clean crushed rock to provide uniform support and to act as a capillary break. The 6-inch crushed rock layer should be underlain with a woven geosynthetic that provides enhanced lateral drainage, such as Mirafi H2Ri or approved equivalent. The woven geosynthetic should be placed on approved subgrade or structural fill

April 21, 2025 7 extending to competent site soils. Where existing fill is exposed, the subgrade soils should be recompacted to a firm condition.

If water vapor migration through the mat foundation or slab-on-grade is objectionable and/or moisture cannot be tolerated by the proposed structures, the capillary break material should be covered with a heavy plastic sheet to act as a vapor retarder. The vapor barrier should have sufficient thickness to withstand foot traffic during placement of reinforcement and concrete and be constructed in accordance with the American

Concrete Institute (ACI 302.1R). The contractor should be made responsible for maintaining the integrity of the vapor barrier during construction.

3.2.1.3 MODULUS OF SUBGRADE REACTION

For mat foundations or slabs designed as a beam on an elastic foundation, a modulus of subgrade reaction of 55 pci may be used.

3.2.1.4 FOUNDATION SETTLEMENT

We estimate that footings founded as recommended above will experience between 0.5 and 1 inch of post-construction settlement, which includes elastic settlement and the potential for 0.1 to 0.5 inch of collapse-consolidation settlement of loessal soils. Differential settlement between comparably loaded column footings or along a 25-foot section of mat foundation or continuous wall footing should be less than 0.5 inch.

We expect most of the post-construction foundation settlements will occur during construction as loads are applied. Loose or disturbed structural fill or native soils not removed from the foundation excavation prior to placing capillary break and concrete will result in additional settlement.

3.2.1.5 LATERAL RESISTANCE

Lateral foundation loads may be resisted by passive resistance on the sides of the foundation and by friction on the base of the shallow foundations. For shallow foundations supported on native soils, the allowable frictional resistance may be computed using a coefficient of friction of 0.35 applied to vertical dead-load forces.

Passive resistance may be estimated using an equivalent fluid density of 300 pounds per cubic foot (pcf) above the groundwater table. These values assume that the footings and below-grade elements are poured directly against structural fill placed and compacted as recommended.

The above coefficient of friction and passive equivalent fluid density values incorporate a factor of safety of about 1.5.

3.2.1.6 FOUNDATION DRAINAGE

We recommend that foundation drains be installed around the perimeter of the foundations for the proposed electrical building and for concrete pads supporting settlement-sensitive equipment. The perimeter footing drains should consist of at least 4-inch-diameter perforated pipe placed on a 2-inch bed of, and surrounded by, 6 inches of drainage material enclosed in a drainage geotextile that meets the requirements of Section

718.05 of the Idaho Transportation Department (ITD) Standard Specifications to prevent fine soil from migrating into the drainage material. Appropriate cleanouts for drainpipe maintenance should be installed.

We recommend that the drainpipe consist of either heavy-wall perforated pipe (JM Eagle Solvent Weld

PVC, or equal) or rigid corrugated smooth interior polyethylene pipe (ADS N-12, or equal). Perforations for the pipe should consist of two rows of holes, 0.5 inch in diameter at 5 inches on center and 120-degree angle apart. We do not recommend using flexible tubing for foundation drainpipes. The drainage material should conform to Drain Rock in accordance with Table 1 of Section 802.2.2 of the 2020 Idaho Standards for Public Works Construction (ISPWC), but with the nominal maximum aggregate size limited to 1 inch.

April 21, 2025 8

The perimeter drains should be sloped to drain by gravity to a suitable discharge point away from structures

(preferably a storm drain). We recommend that the cleanouts be covered and placed in flush-mounted utility boxes. Water collected in roof downspout lines for the new electrical building must not be routed to the footing drain lines.

3.2.1.7 CONSTRUCTION CONSIDERATIONS

Immediately prior to placing concrete, all debris and loose soils that have accumulated in the foundation excavations during forming and steel placement must be removed. Debris or loose soils not removed from the foundation excavations will result in increased settlement.

Due to the presence of moisture-sensitive loess soils, we recommend that work be completed during the dry season to limit subgrade soil exposure to moisture. If wet-weather construction cannot be avoided, we recommend that all footing subgrades be protected using concrete blankets or a lean concrete mud mat.

Concrete blankets should have sufficient overlap to prevent water migration onto the subgrade soils. The mud mats or blankets should be placed the same day that the foundation subgrade is excavated and approved for foundation support.

We recommend that all completed foundation excavations be observed by a representative of our firm prior to placing mud mat/concrete blankets, reinforcing steel, and structural concrete. Our representative will confirm that the bearing surface has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected.

3.3 DEBRIS MANAGEMENT SLAB

At the siphon structure, a debris management slab will be constructed to allow for trash and debris to be pulled from the Taghee Canal, dried, and then burned. This slab will be partially supported by a 4-foot-tall precast block wall in the northwest corner of the siphon improvement area, adjacent to Bannock Creek.

Based on our discussions with the project team, we understand that the debris management slab is to be designed as a PCCP assuming highway loading. For design of pavement structures, traffic loading is converted into equivalent single axle load (ESAL). Design of the debris management slab was completed using 4.5 million rigid ESALs and a 40-year design life following methodology for rigid pavement design provided in the 1993 AASHTO Guide for Design of Pavement Structures.

Based on the test pits we completed throughout the debris management slab area (CHE-TP4-24 through

CHE-TP7-24), we anticipate approximately 2 to 3.5 feet of undocumented fill comprised predominantly of reworked native soils with occasional cobbles and slag. Subgrade soil conditions were evaluated using the

Idaho R-Value test to evaluate the strength and load-bearing capacity of subgrade materials. We evaluated the required thickness of the slab in accordance with the 1993 AASHTO Guide for Design of Pavement

Structures rigid pavement design methodology and our experience. Based on our analysis, the debris management slab should be 7 inches thick. The PCCP should include reinforcement, which will be designed by the structural engineer. To prevent cracking or structural damage caused by temperature fluctuations, joints should be installed at intervals not exceeding 15 feet.

3.3.1 Subgrade Preparation

We recommend that concrete slabs-on-grade be constructed on a 6-inch-thick layer of clean crushed rock to provide uniform support and to act as a capillary break. The capillary break material should consist of crushed surfacing base course (CSBC) with a maximum particle size of 1 inch and negligible sand and silt.

The 6-inch CSBC layer should be placed on a subgrade separation geotextile that meets the requirements of Section 718.07 of the ITD Standard Specifications. Prior to placing the geotextile, the exposed subgrade should be prepared as described in Section 3.8 of this report. Subgrade conditions should be observed and

April 21, 2025 9 proof-rolled during construction and prior to placing capillary break to evaluate the presence of unsuitable subgrade soils. If the subgrade soils are excessively loose or soft, it may be necessary to excavate localized areas and replace them with compacted structural fill. Where existing fill is exposed, the subgrade soils should be recompacted to a firm condition.

3.4 ROTATING SCREEN SLAB-ON-GRADE

At the time of our initial exploration program in August of 2024, the Taghee Canal was carrying water and inspection of subgrade soils was not feasible. We returned to the project site on November 18, 2024, to evaluate the condition of the canal subgrade. Based on our site visit, we anticipate that the channel bed subgrade is predominantly comprised of native fine-grained soils (silt and clay) which were saturated at the time of our visit. Subgrade conditions in the base of the canal were evaluated by probing with a 0.5-inch-diameter steel probe rod. Subgrade conditions were observed to be very soft. At the time of our site visit we were able to penetrate over 30 inches into the canal subgrade using little effort.

We understand that the new rotating screen will require extension of the existing slab-on-grade, adding approximately 22 feet to the slab. We recommend that concrete slabs-on-grade for the rotating screen be constructed on an 18-inch-thick layer of clean crushed rock to provide uniform support and added capacity to site soils. The 18-inch crushed rock layer should be placed on a subgrade separation geotextile that meets the requirements of Section 718.07 of the ITD Standard Specifications. Prior to placing the geotextile, the exposed subgrade should be prepared as described in Section 3.8 of this report. Subgrade conditions should be observed prior to placing the geotextile and capillary break to evaluate the presence of unsuitable subgrade soils. If the subgrade soils are excessively loose or soft, it may be necessary to excavate localized areas and replace them with compacted structural fill.

Provided the recommendations described above are followed, the slab may be designed with an allowable bearing pressure of 950 psf.

3.5 CANAL LINER

We understand that portions of the irrigation canal channel will be lined with concrete as part of the proposed improvements. Canal embankment slopes should be shaped to at least 2 inches above the top edge of the lining, graded to a maximum slope of 2H:1V, compacted to a firm condition, and then lightly scarified. Concrete can be placed directly on native soils, provided they have been prepared as recommended.

3.6 EARTH RETAINING STRUCTURES

We understand that the transformer for the pumping plant will have a below-grade vault extending up to 4 feet below the structural mat foundation and that the debris management slab at the siphon will have a 4-foot-tall precast gravity block wall at the northwest corner, adjacent to the slope leading to Bannock Creek.

The following recommendations assume a maximum wall height of 4 feet and that vehicle traffic on or near these walls is limited to occasional passenger vehicles.

3.6.1 Below-Grade Walls

Permanent below-grade walls may be designed using the earth pressure diagrams presented on Figure 3.

Earth pressures are provided for two conditions: Not Restrained Against Rotation and Restrained Against

Rotation. Walls that are free to rotate/move laterally at the top a distance of at least one thousandth the height of the wall (H/1000) should be designed using active earth pressures provided for the Not Restrained

Against Rotation case. Walls that will be restrained from rotation/lateral movement should be designed using at-rest earth pressures provided for the Restrained Against Rotation case.

April 21, 2025 10

Foundation surcharge loads from adjacent buildings should be incorporated into the design of the below-grade walls using the surcharge pressures presented on Figure 4. If vehicles can approach the tops of exterior walls to within half the height of the wall, a traffic surcharge of 250 psf should be added to the wall pressure. Other surcharge loads, such as from construction equipment or construction staging areas, should be considered on a case-by-case basis and can be provided, if necessary, as design progresses.

In the event that walls exceeding 6 feet are planned for any structure, walls must be designed incorporating the additional seismic lateral earth pressure.

The lateral earth pressures provided on Figure 3 assume that the ground surface behind the wall is horizontal/flat. If the ground surface behind the wall is sloped steeper than 4H:1V, CHE should be contacted to provide revised earth pressures.

These recommendations assume that all below-grade retaining walls will be provided with adequate drainage. The values provided for soil bearing, frictional resistance, and passive resistance presented for foundation design are applicable to retaining wall design. Walls located in level ground areas should be founded at a depth of 32 inches below the adjacent grade.

3.6.2 Gravity Block Retaining Walls

Gravity block retaining walls may be designed using the soil properties provided in Table 2. The wall design should include a 350 psf surcharge load to account for debris and occasional traffic loading. Additionally, if the wall face is within 4 feet of the crest of the Bannock Creek slope, global stability of the wall considering the embankment slope geometry should be checked.

Table 2. Gravity Block Retaining Wall Soil Properties

Zone Friction Angle, f

(deg) Cohesion, c

(psf) Unit Weight, g

(pcf)

Reinforced Soil 34 0 130

Retained Soil 30 0 120

Foundation Soil 30 0 105

Leveling Pad 40 0 135

Notes:

a. Soil properties for reinforced soil, retained soil, and leveling pad material assume conformance with the structural fill material and placement/compaction requirements described in Section 3.8 of this report.

3.6.3 Wall Drainage

In areas where temporary cut slopes and conventional cast-in-place techniques are used to construct below-grade walls, a conventional footing drain should be located on the outside of the wall. Positive drainage should be provided behind cast-in-place and gravity block walls by placing a 2-foot-wide zone of free-draining material against the wall. Free-draining material should consist of Drain Rock in accordance with Table 1 of Section 802.2.2 of the 2020 ISPWC, but with the nominal maximum aggregate size limited to 1 inch. The zone of wall drainage material should extend from the base of the wall to within 2 feet of the ground surface. The wall drainage material should be covered with 2 feet of less-permeable material, such as the on-site silt soils that is properly moisture conditioned and compacted. A nonwoven geotextile separator should be placed between the wall drainage material and overlying cover soil. A 4-inch-diameter perforated drainpipe should be installed within the free-draining material at the base of each wall. The foundation drain recommended in Section 3.2.1.6 can be incorporated into the bottom of the drainage zone and used for this purpose.

April 21, 2025 11

The pipes should be laid with minimum slopes of 0.25 percent and discharged into the stormwater collection system to convey the water off site. The pipe installations should include a cleanout riser with cover located at the upper end of each pipe run. The cleanouts could be placed in flush-mounted access boxes. Collected downspout water should be routed to appropriate discharge points in separate pipe systems.

3.7 GRAVEL ACCESS ROAD

Site improvements include construction of an all-weather gravel surfacing roadway to support vehicular traffic around the pumping plant and switchyard. We understand that this surface will be used for maintenance and operation activities. We anticipate that the vehicles that will be utilizing this length of road will be primarily pickup trucks (up to 1 ton) and occasional tractors and dump trucks (approximately two tractors and two dump trucks per week averaged over a 1-year period).

3.7.1 Subgrade Preparation

Subgrade areas should be prepared as recommended in Section 3.8 of this report. We recommend that a proof-roll of the subgrade, compacted subbase course, and base course be observed by a representative from our firm prior to constructing the gravel access road. Soft or yielding areas observed during proof-rolling may require over-excavation and replacement with compacted structural fill.

3.7.2 Gravel Roadway Section

The gravel section recommended herein is based on AASHTO 1993, AASHTO Guide for Design of

Pavement Structures, following the design methodology for Aggregate-Surfaced Roads, our experience, and observations during the geotechnical exploration program. We designed the aggregate-surfaced road using a rutting criteria of 1 inch and a 0.5% growth rate over a 20-year design life. Based on our discussions with the design team, we approximated 8,000 ESALs for design of the gravel roadways section. This ESAL value was developed using the following estimated daily traffic loading:

• 10 pick-up trucks (FHWA Class 3 vehicle)

• 0.28 tractors (FHWA Class 6 vehicle) representing approximately two tractors per week averaged over a 1-year period.

• 0.28 dump trucks (FHWA Class 8 vehicle) representing approximately two dump trucks per week averaged over a 1-year period.

ESALs are estimated using the assumed traffic classification volume and load equivalency factors (LEFs).

We assigned a LEF of 0.004, 1.5, and 2.0 for FHWA class 3, 6, and 8 vehicles, respectively.

We recommend a roadway section consisting of a 6-inch thickness of crushed rock base course overlying

15 inches of granular subbase. The subbase and base course should be compacted to at least 95 percent of the MDD (ASTM D1557).

We understand that an asphalt surface course is not being considered. As such, roads and surfaces that do not have an asphalt surface course will require regular maintenance including re-grading of the aggregate surface.

3.8 EARTHWORK

CHE understands that earthwork will include excavating and backfilling for foundations, slabs-on-grade, below-grade walls, and utility trenches.

April 21, 2025 12

3.8.1 Clearing and Site Preparation

Construction of the proposed structures will require demolition of existing utilities, gravel access roads, and other pertinent structures. Existing debris should be excavated and removed from the site. Demolished concrete structures may be ground up and reused on-site as structural fill; otherwise, it should be hauled off site. Existing utilities and associated trench backfill should be removed from below the proposed building footprints. All excavations that extend below slabs-on-grade or foundation subgrades should be backfilled with structural fill.

Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including any debris, brush, and associated stumps and roots. Graded areas should be stripped of organic soils.

Based on our site observations, we estimate that stripping depths will typically be on the order of 8 to 12 inches to remove the rooted layer. Deeper stripping and grubbing depths will be required to remove localized deeper deposits of topsoil.

The organic soils can be stockpiled and processed for landscaping purposes or may be spread over disturbed areas following completion of grading. If spread out, the organic strippings should be placed in a layer less than 1-foot thick, should not be placed on slopes greater than 3H:1V, and should be track-walked to a uniformly compacted condition. Materials that cannot be used for landscaping or protection of disturbed areas should be removed from the project site.

3.8.2 Excavation Support

At this time, we expect that temporary open cut slopes will be suitable for excavations required for the project and understand temporary shoring is not anticipated for the site. The following sections summarize the general excavation recommendations for temporary cut slopes and permanent cut-and-fill slopes.

3.8.2.1 TEMPORARY CUT SLOPES

The stability of open cut slopes is a function of soil type, groundwater seepage, slope inclination, slope height, and nearby surface loads. The use of inadequately designed open cuts could impact the stability of adjacent work areas, existing utilities, and endanger personnel. In our opinion, the contractor will be in the best position to observe subsurface conditions continuously throughout the construction process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to use open cut slopes rather than some form of temporary excavation support, and for establishing the safe inclination of the cut slope.

Acceptable slope inclinations should be determined during construction. All open cut slopes and temporary excavation support should be constructed or installed and maintained in accordance with the requirements of the appropriate governmental agency.

For planning purposes, temporary unsupported cut slopes more than 4 feet high may be inclined no steeper than 1.5H:1V in the fill and native loess. It may be necessary to flatten the cut slopes if seepage is present on the cut face or if localized sloughing occurs. We recommend that a representative from our firm observe excavation of temporary cuts over 6 feet in height to assess stability and confirm recommended cut slope inclination.

The above guidelines assume that surface loads such as equipment loads and storage loads will be kept a sufficient distance away from the top of the cut so that the stability of the excavation is not affected. We recommend that this distance be not less than half the height of the cut or 5 feet, whichever is greater.

Water entering excavations must be collected and routed away from prepared subgrade areas. We expect that this may be accomplished by installing a system of drainage ditches and sumps along the toe of the cut slopes. Some sloughing and raveling of the cut slopes should be expected. Temporary covering, such

April 21, 2025 13 as heavy plastic sheeting with appropriate ballast, should be used to protect these slopes during periods of wet weather. Surface water runoff from above cut slopes should be prevented from flowing over the slope face by using berms, drainage ditches, swales, or other appropriate methods.

If temporary cut slopes experience excessive sloughing or raveling during construction, it may become necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing problems can be flattened, regraded to add intermediate slope benches, or additional dewatering can be provided if the poor slope performance is related to groundwater seepage.

3.8.3 Subgrade Preparation & Protection

All topsoil, existing fill, and organic soils should be removed from structure and hardscape subgrade areas.

The debris management slab and roadway section may utilize undocumented fill as subgrade. Prior to placing concrete, structural fill, and/or pavement base course materials, all subgrade areas should be evaluated by means of a 0.5-inch-diameter steel probe rod or by proof-rolling to confirm bearing capacity recommendations are met and to locate any soft or pumping soils. Proof-rolling can be completed using heavy, tire-mounted equipment such as a loaded dump truck or a 5-ton smooth-drum vibratory roller. If soft or pumping soils are observed, they should be removed and replaced with properly compacted structural fill.

Site soils contain significant fines content (silt/clay) and will be highly sensitive to increases in moisture content. The exposed subgrade soils can deteriorate rapidly in wet weather and under equipment loads.

The contractor should take necessary measures to prevent site subgrade soils from becoming disturbed or unstable. CHE recommends construction occur only during dry weather. However, if constructed during periods of wet weather, CHE recommends subgrade soils be protected with concrete blankets, a lean concrete mud mat, or other appropriate measures. The blankets, mud mat, or other appropriate measures should be placed immediately following subgrade approval. Construction traffic during the wet season should be restricted to specific areas of the site, preferably areas that are surfaced with crushed rock not susceptible to wet weather disturbance.

3.8.4 Utility Trenches

Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in the 2020 ISPWC, or other suitable procedures specified by the project civil engineer.

Utility trench backfill should consist of structural fill and should be placed in lifts of 8 inches or less (loose thickness) such that adequate compaction can be achieved throughout the lift. Sand backfill, containing less than 5 percent fines, may be compacted in loose lifts not exceeding 12 inches. Each lift must be compacted prior to placing the subsequent lift. Prior to compaction, the backfill should be moisture conditioned to within 3 percent of the optimum moisture content, if necessary.

3.8.4.1 SOIL CORROSIVITY

We understand that a series of underground pipes are planned to connect the valve vault of the pumping plant to the thrust block to the southwest and that these pipes are planned to be approximately 3 to 5 feet below ground surface. Corrosivity of site soils was evaluated through chemistry suite laboratory testing, which includes pH, resistivity, and sulfates concentration. The results of these tests are presented below in

Table 3, and in the remarks of the logs in Appendix A.

April 21, 2025 14

Table 3. Corrosivity Testing Results

Test Pit No. Depth (ft) pH Resistivity (ohm-cm)

Sulfates (ppm)

CHE-TP2-24 6.0-7.0 8.45 3,403 37

CHE-TP3-24 3.0-4.0 8.73 5,790 ND1

CHE-TP7-24 1.5-2.0 8.5 2,195 45

1. ND = non-detectable sulfate content

Soils with higher than 1,500 ohm-cm of resistivity and a pH higher than 5.5 are generally considered non-corrosive. Additionally, for structural elements, soils are considered corrosive if the sulfate concentration is higher than 1,500 ppm and/or the pH is 5.5 or less (Caltrans 2022). Based on the results of these tests, we anticipate that the subsurface soils encountered in the project area exhibit a low corrosion potential to construction materials.

3.8.5 Structural Fill

All fill that will support foundations, floor slabs, or pavements and hardscape areas, or to be placed against retaining walls or in utility trenches, should meet the criteria for structural fill presented below.

3.8.5.1 MATERIALS

Materials used to construct the building pad, placed under foundations and hardscape, and used to backfill utility trenches are classified as structural fill for the purpose of this report. Quality of structural fill material varies depending upon its use as described below:

• Structural fill placed below new buildings should consist of crushed aggregate granular structural fill, with a maximum particle size of 2 inches, as described in Section 802.2.2 of the 2020 ISPWC.

• To provide uniform support, utility line bedding material should meet the requirements of Pipe

Bedding, as described in Section 305 of the 2020 ISPWC.

• Utility trench backfill should consist of crushed aggregate granular structural fill, with a maximum particle size of 2 inches, as described in Section 802.2.2 of the 2020 ISPWC.

• Structural fill placed around perimeter footing drains, under slab drains, and cast-in-place wall drains should consist of Drain Rock in accordance with Table 1 of Section 801.2.2 of the 2020

ISPWC, but with the nominal maximum aggregate size limited to 1 inch.

• Structural fill placed as capillary break below slabs should consist of Type 1 Crushed Aggregate

¾-inch minus in accordance with Section 802 of the 2020 ISPWC.

• Base course placed as the running surface of the aggregate-surfaced road should conform to Type

1 Crushed Aggregate ¾-inch minus crushed aggregate in accordance with Section 802.2 of the

2020 ISPWC.

• Subbase course placed for the aggregate-surface road should consist of…