B08_Attach_3_22_07_26_-_Final_Geo-tech_Report.pdf

SUBMITTED TO:

HDR Engineering, Inc.

419 Canyon Ave, Suite 316 Fort Collins, CO 80521

BY:

Shannon & Wilson 400 N 34th Street, Suite 100 Seattle, WA 98103

(206) 632-8020 www.shannonwilson.com

GEOTECHNICAL REPORT

Stehekin Wastewater Treatment Plant Replacement Project

CHELAN COUNTY, WASHINGTON

July 26, 2022

Shannon & Wilson No: 106705-001

Stehekin Wastewater Treatment Plant Replacement Project Geotechnical Report

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CONTENTS

1 Introduction

2 Site and Project Description

3 Site Geology

4 Subsurface Explorations

4.1 Review of Existing Subsurface Explorations

4.2 Project Subsurface Explorations

4.2.1 Geotechnical Boring

4.2.2 Test Pit

4.2.3 Percolation Test Pits

5 Geotechnical Laboratory Testing

5.1 Visual Classification

5.2 Water Content Determination

5.3 Grain-Size Distribution Analysis

5.4 Ductile Iron Pipe Research Association Corrosion Testing

6 Percolation Test

7 Interpreted Subsurface Conditions and Conceptual Hydrogeologic Model

7.1 Interpreted Subsurface Conditions

7.2 Conceptual Hydrogeologic Model

8 Preliminary Geotechnical Engineering Recommendations

8.1 Seismic Design Parameters

8.2 Building Foundations

8.2.1 Foundation Bearing Capacity and Settlement

8.2.2 Lateral Resistance

8.2.3 Drainage

8.2.4 Floor Slabs

9 Preliminary Construction Considerations

9.1 Backfill and Compaction

9.1.1 General

9.1.2 Structural Fill

9.1.3 Compaction Method

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9.2 Footings

9.3 Excavation, Temporary, and Permanent Cuts

9.4 Groundwater Control

9.5 Wet-Weather Earthwork

10 Limitations

11 References

Exhibits Exhibit 6-1: Soil Type and Percolation Test Results Exhibit 7-1: Aquifer Characteristics Exhibit 8-1: Response Spectrum Parameters for Site Class D in Accordance with 2018 IBC

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Unit Extent, Surficial Aquifer & Alluvial Fan (Qa) Figure 4: Perspective Cross-Section, Surficial Aquifer Parallel to Longitudinal Axis Figure 5: Estimate Water Table Surface, Surficial Aquifer Figure 6: Estimated Thickness, Surficial Aquifer

Appendices Appendix A: Subsurface Explorations Appendix B: Geotechnical Laboratory Testing Important Information

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1 INTRODUCTION

This report presents the results our geotechnical study and analysis for the Stehekin Wastewater Treatment Plant Replacement Project (Project) located in Chelan County, Washington (Figure 1). The information presented in this report is intended to provide HDR Engineering, Inc. and the National Park Service (NPS) with the information required to assist in advancing the predesign and schematic design of the Project.

2 SITE AND PROJECT DESCRIPTION

The proposed Project is located in the unincorporated community of Stehekin in Chelan County, Washington. Stehekin is on the northwest end of Lake Chelan and is located just south of the North Cascades National Park. Stehekin is within the Lake Chelan National Recreation Area, which is administered by the NPS. The existing wastewater treatment plant is operated by the NPS. The Project is at approximately 1,170 feet in elevation (North American Vertical Datum of 1988).

We understand that the Project scope has not been fully defined, but will include increasing the size of the effluent drain field to accommodate more flow. We further understand an addition or alteration to the existing structure at the Project is also being studied.

3 SITE GEOLOGY

The bedrock of the North Cascades is composed of ancient island arcs that have been accreted to the North America Plate through the processes of plate tectonics, igneous intrusions, and metamorphic events. Subsequent intermittent glaciations from approximately 2.6 million years ago to 8,000 years ago have carved wide valleys and deep gorges creating the Stehekin River Valley and Lake Chelan (Riedel and others, 2012). Post-glaciation alluvial sediments have been deposited along the Stehekin River and along the edge of Lake Chelan.

The bedrock around the Project site is mapped as the Stehekin orthogneiss. This unit is composed of fine- to medium-grained tonolitic to granodioritic biotite orthogneiss with poor to moderately developed foliation (Dragovich and Norman, 1995). The Project site is located on an alluvial fan that formed at the end of Purple Creek along the edge of Lake Chelan overlying the bedrock. The alluvial fan is composed of variable amounts of silt, sand, gravel, cobbles, and boulders. Cobbles and boulders were observed in the test pits

106705-001 July 26, 2022 and borings within the Project site. According to NPS staff, a debris flow in 2015 originating upslope and to the north of the site traveled in a southwest direction and deposited 8 to 10 feet of debris on the lower parking lot of the site. Other debris flows were also reported to have historically occurred.

4 SUBSURFACE EXPLORATIONS

Shannon & Wilson conducted geotechnical explorations to evaluate the subsurface soil and groundwater conditions present at the Project location. The geotechnical explorations included collecting and reviewing existing subsurface explorations and completing Project subsurface explorations.

4.1 Review of Existing Subsurface Explorations

Our review of the available existing subsurface explorations in the Stehekin area identified the presence of an existing pumping well in the vicinity of the Project site. The pumping well was drilled in August 1987 by MVM Quality Drilling to a depth of 85 feet below ground surface (bgs). During drilling, the static water level was recorded at 35 feet bgs. The well report is provided in Appendix A and the approximate location of the existing pump well is shown in Figure 2.

4.2 Project Subsurface Explorations

The subsurface explorations performed for the Project consisted of one geotechnical boring, one test pit, and six percolation tests. The proposed exploration locations were determined in May 2021 by HDR and NPS during their preliminary site visit. The approximate locations of the explorations performed are shown in Figure 2.

4.2.1 Geotechnical Boring

A Shannon & Wilson field representative was present to observe the drilling and sampling of boring B-1. Holt Services, Inc. under subcontract to Shannon & Wilson, used a Mobile CME 85, truck-mounted drill rig and 6-inch-diameter hollow-stem auger to complete the boring. The hollow-stem auger drilling method advances the auger casing while the rotation transports the waste soil (cuttings) to the surface. Additional sections of the auger casing are added to achieve the desired drilling depth. This method uses applied down-pressure and rotation to the top of the continuous-flight auger. Soil samples were obtained by lowering a split-spoon sampler through the hollow stem of the auger. The hollow-stem auger drilling method had difficulty reaching the planned depth of 20 feet. A total of six slightly different locations were attempted before Holt was able to sample to a final depth of

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20.1 feet bgs. The boring log, included in Appendix A, is a composite log for two of the boring attempts. See the boring log for more details.

Soil samples were obtained in conjunction with Standard Penetration Tests (SPTs). The SPTs were performed in accordance with the ASTM Designation D1586, Standard Test Method for Penetration Test (SPT) and Split-Barrel Sampling of Soils (ASTM, 2018). The depths at which the samples were collected and the corresponding Standard Penetration Resistances (N-values) are recorded on the boring log in the Appendix A. The SPT N-value is a useful parameter for determining the relative density or consistency of the soils. The correlation of the relative density or consistency terms and SPT N-values is provided in the soil description and log key in Appendix A.

After each split-spoon sampler was removed from the borehole, the Shannon & Wilson field representative visually examined the soil sample. The field representative then described and classified the sample on the field boring log, removed it from the split-spoon sampler, and sealed the sample in a jar. Upon recovering the last sample, Holt backfilled the boring.

The jars were stored in boxes and transported to the Shannon & Wilson laboratory for further examination and testing.

4.2.2 Test Pit

The test pit, designated TP-1, was dug using a rubber-tired backhoe excavator. The equipment is owned by and was operated by the NPS. The test pit excavation was observed by a Shannon & Wilson field representative and was terminated at a depth of 6 feet when it was observed that the excavation sidewalls were marginally stable and that their collapse could potentially undermining the adjacent sludge ponds.

Horizons of different soils were visually determined by the Shannon & Wilson field representative. Sampling these horizons was performed using a shovel for the shallower material and the excavator bucket for the deeper material. A representative sample was collected from each identified horizon and placed in a plastic sample bag to bring back to the Shannon & Wilson Soils Laboratory.

4.2.3 Percolation Test Pits

Six hand-dug percolation pits were completed to perform the falling head percolation test procedure described by the U.S. Environmental Protection Agency (EPA) in the Onsite Wastewater Treatment and Disposal Systems design manual (EPA, 1980). The shallow, hand-dug percolation pits and associated infiltration tests were designated IT-1 through IT-6. While every effort was made to maintain 6-inch-diameter holes in accordance with EPA procedure, the presence of gravels and cobbles resulted in oversized holes ranging

106705-001 July 26, 2022 from 10 to 19 inches in diameter. A representative grab sample was taken from the soil derived from each hand-dug percolation test pit for geotechnical laboratory testing

5 GEOTECHNICAL LABORATORY TESTING

Shannon & Wilson performed geotechnical laboratory tests on selected samples retrieved from the boring, test pit, and percolation test pits. The geotechnical laboratory testing program included tests to classify the soil and provide date for engineering studies. Our geotechnical laboratory testing program included visual classification on all retrieved samples and water content determinations, grain-size analysis, and corrosion testing on select samples. The geotechnical laboratory tests conducted are described below and results are provided in Appendix B.

5.1 Visual Classification

Soil samples retrieved from the boring were classified using a system based on ASTM D2487-17, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) (ASTM, 2017a) and ASTM D2488-17e1, Standard Recommended Practice for Description of Soils (Visual-Manual Procedure) (ASTM, 2017b). A summary of the classification system used is presented in Figure A-1 in Appendix A. In general, visual classification assigns a Unified Soil Classification System (USCS) group name and symbol to each soil sample based on particles finer than 76.2 millimeters (3 inches).

5.2 Water Content Determination

Water content was determined on soil samples in general accordance with ASTM Designation D2216-19, Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass (ASTM, 2010). Water contents are included in with grain-size distribution plots in Appendix B.

5.3 Grain-Size Distribution Analysis

Grain-size distribution analyses separate soil particles through mechanical or sedimentation processes. Grain-size distribution analysis results are plotted per boring in Appendix B.

The plots provide tabular information about each specimen, including USCS group symbol and group name, water content, constituent (i.e., gravel, sand, and fines) percentages, personnel initials, ASTM standard designation, and testing remarks. The ASTM standard used was ASTM D422, Standard Test Method for Particle-Size Analysis of Soils (ASTM, 2007).

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5.4 Ductile Iron Pipe Research Association Corrosion Testing

Chemical analytical testing for corrosivity was completed by Norton Corrosion Limited in Woodinville, Washington, in general accordance with the Ductile Iron Pipe Research Association (DIPRA) 10-point Soil Evaluation. The evaluation includes measurements of resistivity, pH, redox potential, sulfides, and moisture to rate of the soil corrosion potential.

The DIPRA results are included in Appendix B.

6 PERCOLATION TEST

As discussed, six holes were dug to measure the rate of infiltration for the proposed infiltration field expansion. The percolation tests performed followed the procedure outlined in the Onsite Wastewater Treatment and Disposal Systems design manual (EPA, 1980).

Each hole was planned to be 6 inches in diameter. However, due to the large gravels and cobbles and running sand, the hole diameter could not be maintained at 6 inches while completing the required 12-inch depth for the test. To account for the influence of the larger than planned hole diameter on the calculated infiltration, the measured infiltration was scaled based on the ratio between the actual hole diameter and the theoretical 6-inch-diameter hole. Both the measured and calculated scaled infiltration rates are provided in Exhibit 6-1.

Exhibit 6-1: Soil Type and Percolation Test Results

Type

Soil Type, Percolation Test Designation, and Rate (minutes/inch)

IT-1 IT-2 IT-3 IT-4 IT-5 IT-6

Type 1 – Gravelly, Sandy Loam

Type 1 – Gravelly, Sandy Loam

Type 1 – Gravelly, Sandy Loam

Type 1 – Gravelly, Sandy Loam

Type 1 – Gravelly, Loamy Sand

Type 1 – Gravelly, Loamy Sand

Measured 10.00 40.00 20.00 17.78 8.89 22.86

Scaled 5.59 14.93 5.86 6.64 2.61 5.82

7 INTERPRETED SUBSURFACE CONDITIONS AND

CONCEPTUAL HYDROGEOLOGIC MODEL

7.1 Interpreted Subsurface Conditions

Based on the completed explorations, the site is underlain by medium dense alluvial soils consisting of poorly graded gravel with varying amounts of silt, sand, cobbles, and

106705-001 July 26, 2022 boulders. Fill soils, associated with previous site grading and utility work, are also present at the site. These fill soils appear to be re-used alluvial soils from the site and surrounding areas. No groundwater was observed in the explorations, which extended to a maximum depth of 20 feet bgs.

7.2 Conceptual Hydrogeologic Model

A conceptual hydrogeologic model of the surficial aquifer was constructed from the available data. We defined the maximum aquifer extent as the boundaries of an alluvial fan identified as Quaternary Alluvium (Qa) on the 1 to 100,000 scale geologic map (Washington Division of Geology and Earth Resources, 2016), and the 100-foot depth contour below the surface of Lake Chelan (Washington State Department of Ecology, 1995). The combined area covers approximately 69 acres, oriented approximately 1,500 feet in the north-south direction and 2,000 feet in the east-west direction (Figure 3).

We estimated a maximum surficial material thickness of 150 feet based on the existing 85-foot-deep pumping well (PW) and the PW's proximity relative to the interpreted center of the alluvial fan wedge (Figure 3). The PW log indicates it was completed in sand and gravel at a depth of 85 feet bgs. Using the PW log, the interpreted boundaries of the alluvial fan and bedrock surface, and our judgment, we estimated the maximum thickness of the surficial material to be approximately 100 feet at the well location and then projected it to be thicker at the center of the alluvial fan (Figure 3).

Lithostratigraphy within the surficial materials was simulated by extending the position and thicknesses of the lithostratigraphic units reported in the PW log laterally across the site (Figure 4). Four units were reported in the PW log:

Fill material extending from the ground surface to 4 feet bgs;

A sand and gravel layer extending from 4 to 18 feet bgs;

A low-permeable sequence of cobbles, sand, and gravel extending from 18 to 73 feet bgs;

and

A more permeable sequence of sand and gravel extending from 73 feet bgs to the bottom of the well.

When the PW was completed, water was encountered at 35 feet bgs, which indicates the water table is within the low-permeable sequence of cobbles, sand, and gravel. A water table elevation of 1,101 feet was estimated for the reported depth-to-water and our understanding of the ground surface elevation at the PW. A water table surface map was then created by extending a linear trend between the shoreline elevation of 1,100 feet and an elevation of 1,101 feet (Figure 5).

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The surficial aquifer was assumed to comprise the lower two lithostratigraphic units within the alluvial fan. The aerial extent of the surficial aquifer comprises the full width of the alluvial fan (north-south), but only the portion of the alluvial fan along the longitudinal axis between the assumed western extent below the lake surface and the assumed upper extent of the cobbles, sand, and gravel component of the alluvial fan (Figure 6). Based on the reported depth to water, we assume that the Purple Creek is a losing stream and a source of recharge to the surficial aquifer.

Exhibit 7-1 summarizes the characteristics of the surficial aquifer based on the assumptions and calculations described above.

Exhibit 7-1: Aquifer Characteristics

Aquifer Feature Dimensions (feet)

Width (perpendicular to flow) 1,500

Length (parallel to flow) 1,600

Maximum Thickness 149

Average Thickness 43

8 PRELIMINARY GEOTECHNICAL ENGINEERING

RECOMMENDATIONS

In the following sections, we provide our preliminary geotechnical engineering recommendations, including seismic design parameters and building foundation recommendations. These preliminary recommendations are suitable for predesign and schematic design.

8.1 Seismic Design Parameters

We developed our seismic design parameters using the International Building Code (IBC) 2018 (International Code Council, 2017), which references the American Society of Civil Engineers (ASCE) Standard ASCE/SEI 7-16 (ASCE, 2017) for seismic design. Exhibit 8-1 shows the site design ground motions and the risk-targeted maximum considered earthquake (MCER) ground motion parameters from which the design ground motions were derived. The MCER ground motion parameters correspond to a target risk of 2% in 50 years of structural collapse and are derived from probabilistic ground motions with a return period of approximately 2,500 years. The seismological inputs are the MCER short-period and 1-second period spectral accelerations, Ss and S1, respectively.

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The site soil response factors are based on Site Classification, which is determined from characterization of the soil and rock within approximately 100 feet of the ground surface.

The seismological inputs SS, S1, and peak ground acceleration (PGA) are scaled by the site soil coefficients Fa, Fv, and FPGA, respectively, that are determined based on the Site Classification and the magnitude of SS, S1, and PGA values. For the ASCE 7-16 Site Class criteria, it is our opinion that the Project site corresponds to Site Class D, based on subsurface conditions inferred from SPTs performed in boring B-1 and the anticipated depth to bedrock. The response spectra parameters for Site Class D are listed in Exhibit 8-1. The parameters were evaluated based on mapped seismological inputs and the tabulated site coefficients provided in the 2018 IBC and ASCE 7-16.

Exhibit 8-1: Response Spectrum Parameters for Site Class D in Accordance with 2018 IBC

Parameter Description Value

Ss Mapped MCER, 5% damped, short period acceleration 0.541 g

S1 Mapped MCER, 5% damped, spectral acceleration at a period of 1 second 0.209 g

SMS Mapped MCER, 5% damped, short period acceleration adjusted for site effects 0.739 g

SM1 Mapped MCER, 5% damped, spectral acceleration at a period of 1 second adjusted for site effects 0.46 g

SDs Design, 5% damped, short period acceleration 0.493 g

SD1 Design, 5% damped, spectral acceleration at a period of 1 second 0.304 g

PGA Mapped MCEG peak ground acceleration 0.238 g

PGAM Mapped MCEG peak ground acceleration adjusted for site effects 0.324 g

Mm Mean magnitude over all sources 6.48 g = acceleration of gravity

Per IBC Section 1613.2.3 and ASCE 7-16 Section 11.4.8, a ground motion hazard analysis is required for Site Class D sites where S1 is greater than or equal to 0.2 g unless the value of the seismic response coefficient, Cs, is determined by ASCE 7-16 Equation 12.8-2 for values of T ≤ 1.5TS and taken as equal to 1.5 times the value computed with either ASCE 7-16 Equation 12.8-3 for TL ≥ T > 1.5TS or ASCE 7-16 Equation 12.8-4 for T > TL. The response spectrum values in Exhibit 8-1 assume that Cs will be determined using one of the two aforementioned equations.

8.2 Building Foundations

8.2.1 Foundation Bearing Capacity and Settlement

Spread or strip footings are typically located below building columns and bearing walls and are generally designed to bear in moderate to good soil conditions. Based on the subsurface

106705-001 July 26, 2022 conditions, it is our opinion that building wall foundations could be supported on conventional footings bearing on properly prepared subgrade soils or compacted structural fill. Spread/strip footings bearing should be designed with an allowable bearing capacity of 3 kips per square foot (ksf). The allowable bearing pressure may be increased by up to one-third to account for wind, seismic, or other short-term loads, in accordance with the IBC (International Code Council, 2017). Spread footings should have a minimum width of 24 inches and strip footings should have a minimum width of 18 inches. The base of all footings should be located at least 36 inches below the lowest adjacent grade, or frost depth, whichever is greater.

We estimate that spread/strip footings designed for an allowable bearing capacity of 3 ksf would undergo a total static settlement of less than 0.5 inch. Due to the granular nature of the soil, we anticipate most of this settlement would occur during construction.

8.2.2 Lateral Resistance

Lateral loads on shallow foundations may be resisted by a combination of base friction and passive pressure against the shallow footings. In our opinion, passive earth pressures developed from structural fill should be based on an allowable equivalent fluid density of 300 pounds per cubic foot. This passive resistance value is based on the assumption that the footings extend at least 36 inches below the lowest adjacent grade and that the ground surface is horizontal for a minimum distance of 1½ times the embedment depth. The above passive earth pressure includes a factor of safety of 1.5 to limit lateral deflections. We recommend an allowable coefficient of friction of 0.35 be used between cast-in-place concrete and footing subgrade soil for calculating the resistance to sliding at the base of the footings.

8.2.3 Drainage

A subsurface drainage (subdrain) system should be installed along the outside of the perimeter footings to prevent the buildup of hydrostatic pressures against the foundation walls and route water away from structures. The subdrain system should consist of a perforated or slotted, 4-inch-(minimum) diameter polyvinyl chloride pipe bedded in ⅜-inch to No. 8-size washed pea gravel conforming to Washington State Department of Transportation (WSDOT) Standard Specifications, Section 9-03.12 (WSDOT, 2021) or similar.

Pea gravel placed in footing drain trenches should be compacted with a vibrating plate compactor in 8-inch-thick loose lifts. Pea gravel should not be used as trench backfill where future adjacent earthwork and excavations would expose the pea gravel and allow it to collapse into the excavation. Clean, crushed rock should be used as footing drain trench backfill where future adjacent excavations are anticipated.

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Cleanouts should be provided at convenient locations along installed drain lines, such as at the building corners. To promote surface water drainage, provisions should be made to direct water away from structures and to prevent water from seeping into the ground adjacent to structures. Ground surfaces should be sloped away, and surface and downspout water should not be introduced into site backfill. Surface water should be collected in catch basins and, along with downspout water, be conveyed in a non-perforated pipe (tightline) to an approved discharge point.

8.2.4 Floor Slabs

We anticipate that the floor slabs would be supported on densely compacted structural fill.

If loose, soft, or otherwise unsuitable soil is encountered during slab subgrade preparation, it should be removed and replaced with densely compacted structural fill. We recommend a vertical modulus of subgrade reaction of 200 pounds per cubic inch be used for predesign purposes.

To provide firm bearing for floor slabs, we recommend that these areas be prepared by removing existing loose, soft, or disturbed soil and organic debris. The exposed soil should be compacted, as needed, to achieve a dense, unyielding condition and evaluated by a geotechnical engineer. New fill areas under the slab-on-grade floor, including backfill for utility excavations and overexcavated areas, should consist of structural fill placed compacted to a dense, unyielding condition.

A capillary break consisting of a minimum 6-inch-thick layer of washed pea gravel (⅜ inch to No. 8 sieve size) or ¾ inch to No. 4-sized washed crushed rock (maximum size ⅝ inch and less than 2% passing the number 200 sieve) should be placed between the prepared subgrade and the slab-on-grade. A plastic vapor barrier should be placed over the capillary break in areas where moisture intrusion may be an issue. The vapor barrier should consist of 10-mil polyethylene plastic sheeting or comparable material approved by the design team.

9 PRELIMINARY CONSTRUCTION CONSIDERATIONS

9.1 Backfill and Compaction

9.1.1 General

All fill placed beneath floor slabs and footing or against belowgrade walls or other structures should be structural fill. If used to raise subgrade elevation for structural

106705-001 July 26, 2022 elements (i.e., floor slabs), the underlying existing native material should be compacted to a dense, unyielding condition prior to placement of structural fill.

9.1.2 Structural Fill

Structural fill should meet the WSDOT Specification 9-03.14(1) for Gravel Borrow (WSDOT, 2021). During wet weather or wet conditions, structural fill should not contain more than 5% fines (i.e., material passing the No. 200 sieve) by weight, based on the minus ¾-inch soil fraction. The fines should be nonplastic. Structural fill should not contain organic or other deleterious material.

9.1.3 Compaction Method

Structural fill should be placed in horizontal lifts and compacted to a dense and unyielding condition, to at least 95% of the Modified Proctor maximum dry density determined by ASTM D1557. The thickness of loose lifts should not exceed 4 inches for hand-operated compactors or 8 inches for heavy-equipment compactors. A geotechnical engineer or technician should be on site to confirm the material is compacted in accordance with our recommendations.

9.2 Footings

The recommended bearing capacity presented in this report is based on the following considerations:

Footing subgrades should be cleared of all loose soil, organic material, and otherwise deleterious material prior to constructing forms.

The footing subgrades should be compacted to a dense and unyielding condition.

If unexpected loose, soft, or otherwise unsuitable soil is encountered at or below subgrade elevation, this material should be removed and replaced with imported structural fill.

A qualified geotechnical engineer should be on site during footing excavation to evaluate the suitability of subgrade soils for bearing proposed loads.

9.3 Excavation, Temporary, and Permanent Cuts

Excavations on the Project site could be accomplished with conventional excavating equipment, such as a dozer, front-end loader, or excavator. Safe temporary excavations are the responsibility of the Contractor, depend on the actual site conditions at the time of construction, and should comply with applicable Occupational Safety and Health Administration and Washington Industrial Safety and Health Administration Standards.

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For planning purposes, we recommend that temporary, unsupported, open-cut slopes be no steeper than 1.5 Horizontal to 1 Vertical (1.5H:1V). Flatter cut slopes could be required where loose soil or seepage zones are encountered. We recommend that all exposed cut slopes be protected with a waterproof covering during periods of wet weather to mitigate sloughing and erosion. Permanent cuts should be sloped no steeper than 2H:1V.

Appropriate vegetation should be installed on permanent slopes to promote long-term stability. The exposed faces of temporary and permanent cut slopes should be compacted to prevent sloughing.

9.4 Groundwater Control

We do not expect groundwater to be encountered during relatively shallow excavations on the proposed Project site. If water is encountered at or above the depth of the footing subgrade or seeps into the subgrade during excavation, the Contractor should take measures to pump water from the surface of subgrade and then expose and stabilize the subgrade prior to placement of concrete.

9.5 Wet-Weather Earthwork

Earthwork performed during the wet weather months would cost more and take longer to complete. During these periods, surface water runoff due to heavy rain could enter site excavations and would need to be intercepted by drainage ditches or removed with sumps.

Standing water on the soil surface, along with construction activity, would result in disturbance and an unstable surface that could require overexcavation and replacement with clean crushed rock.

The following recommendations are applicable for footings, general excavation, floor slabs, or pavements:

If there is to be traffic over the exposed subgrade, the subgrade should be protected from disturbance. A lean concrete pad, about 2 or 3 inches thick (a “rat slab” or “mud slab”), could be placed immediately following excavation on the undisturbed soils. This could be done, as needed, to protect the exposed soils and act as a working surface.

Overexcavation may be necessary to accommodate this lean concrete pad.

The ground surface in the construction area should be sloped and sealed with a smooth-drum roller to promote rapid runoff of precipitation, promote surface water flow, and prevent ponding of water.

Construction should be observed on a full-time basis by an experienced geotechnical engineer to determine that all unsuitable soils are removed, suitable drainage is implemented, and an appropriate bearing surface is achieved.

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Covering work areas with plastic and/or sloping, ditching, pumping from sumps, and other dewatering measures should be employed as necessary to permit proper completion of the work.

Earthwork should be accomplished in small sections to minimize exposure to wet conditions. That is, each section should be small enough so that the removal of unsuitable soil and placement and compaction of structural fill can be accomplished on the same day.

The size of construction equipment may have to be limited to prevent soil disturbance.

Existing soil or imported fill soil that becomes wet and unstable and/or too wet to suitably compact should be removed and replaced with clean, granular soil.

Grading and earthwork should not occur during periods of heavy, continuous rainfall.

Common measures, such as silt fences and/or straw placed at the perimeter of the site, can be used to control erosion. A wheel wash or quarry spalls at the site entrance will also limit soils spread to nearby streets from site equipment.

The above recommendations apply for all weather conditions but are most important for wet weather earthwork. They should be incorporated into the contract specifications for excavations, foundation construction, and pavement construction.

10 LIMITATIONS

Within the limitations of the scope, schedule, and budget, the conclusions and recommendations presented in this report were prepared in accordance with generally accepted professional geotechnical engineering principles and practice in this area at the time this report was prepared. We make no other warranty, either express or implied. Our conclusions and recommendations are based on our understanding of the Project as described in this report and the site conditions as interpreted from the boring B-1, the test pit, and the existing pumping well log. This report does not include any environmental assessment or evaluation regarding the presence or absence of cultural resources, wetlands, or hazardous or toxic materials in the soil, groundwater, surface water or air on, below, or around the site.

This report was prepared for the exclusive use of HDR and the NPS. It is intended for use only to support the predesign and schematic design of the Project. It is not suitable for final design or construction.

Unanticipated soil conditions are commonly encountered and cannot fully be determined by analyzing existing explorations with discrete soil sampling from borings and test pits performed by others. Such unexpected conditions frequently require that additional

106705-001 July 26, 2022 expenditures be made to attain properly constructed projects. Therefore, some contingency fund is recommended to accommodate such potential extra cost.

We have prepared the included “Important Information About Your Geotechnical/ Environmental Report,” to assist you and others in understanding the use and limitations of this report. Please read this document to learn how you can lower your risks for this Project.

11 REFERENCES

American Society of Civil Engineers (ASCE), 2017, Minimum design loads and associated criteria for buildings and other structures: Reston, Va. American Society of Civil Engineers, ASCE Standard ASCE/SEI 7-16, 2 v.

ASTM International, 2007, Standard Test Method for Particle-Size Analysis of Soils (Withdrawn 2016), D422-63e2: West Conshohocken, PA, ASTM International, Annual book of standards, 8 p., available: www.astm.org.

ASTM International, 2017a, Standard practice for classification of soils for engineering purposes (unified soil classification system), D2487-17: West Conshohocken, Pa., ASTM International, Annual book of standards, 12 p., available: www.astm.org.

ASTM International, 2017b, Standard practice for classification of soils (visual manual procedures), D2488-17e1: West Conshohocken, Pa., ASTM International, 13 p., available: www.astm.org.

ASTM International, 2018, Standard test method for Standard Penetration Test (SPT) and split-barrel sampling of soils, D1586-18, West Conshohocken, Pa., ASTM International, Annular book of standards, 26 p., available: www.astm.org

ASTM International, 2019, Standard test methods for laboratory determination of water (moisture) content of soil and rock by mass, D2216-19: West Conshohocken, Pa., ASTM International, 7 p., available: www.astm.org.

Dragovich, J.D. and Norman, D.K., 1995, Geologic Map of the West Half of the Twisp 1:100,000 Quadrangle, Washington: Washington Division of Geology and Earth Resources, Open File Report 95-3, 67 p., 1 sheet, scale 1:100,000.

http://www.dnr.wa.gov/programs-and-services/geology/publications-and-data/gis-data-and-databases

International Code Council, Inc., 2017, International building code 2018: Country Club Hills, Ill., International Code Council, Inc.

http://www.astm.org/ http://www.astm.org/ http://www.astm.org/ http://www.astm.org/ http://www.dnr.wa.gov/programs-and-services/geology/publications-and-data/gis-data-and-databases http://www.dnr.wa.gov/programs-and-services/geology/publications-and-data/gis-data-and-databases

106705-001 July 26, 2022

Riedel, J.; Dorsche, S.; and Wenger, J., 2012, Geomorphology of the Stehekin River Watershed, Landform Mapping at North Cascades national Park Service Complex, Washington: Marblemount, Washington, Natural Resource Technical Report NPS/NCCN/NRTR -2012/566, 87 p.

U.S. Environmental Protection Agency (EPA), 1980, Design Manual: Onsite Wastewater Treatment and Disposal Systems: Washington DC, Office of Water Programs, 391 p.

Washington Division of Geology and Earth Resources, 2016, Surface geology, 1:100,000--GIS data, November 2016: Washington Division of Geology and Earth Resources Digital Data Series DS-18, version 3.1, previously released June 2010.

Washington State Department of Ecology, 1995, Lake Bathymetry Elevation Bands, Geospatial_Data_Presentation_Form: vector digital data.

https://fortress.wa.gov/ecy/gispublic/DataDownload/ECY_ELV_LakeBathymetry.h tm

Washington State Department of Transportation (WSDOT), 2021, Standard specifications for road, bridge, and municipal construction: Olympia, Wash., WSDOT, Manual M 41- 10, 1 v., available: https://www.wsdot.wa.gov/Publications/Manuals/M41-10.htm https://fortress.wa.gov/ecy/gispublic/DataDownload/ECY_ELV_LakeBathymetry.htm https://fortress.wa.gov/ecy/gispublic/DataDownload/ECY_ELV_LakeBathymetry.htm https://www.wsdot.wa.gov/Publications/Manuals/M41-10.htm

© 2021 Microsoft Corporation © 2021 Maxar ©CNES (2021) Distribution Airbus DS

PROJECT

LOCATION

Lake Chelan

Stehekin

VICINITY MAP

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

August 2021 106705-001

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Seattle

Washington

Project Location

0 2,000 4,000

Approximate Scale in Feet

FIG. 1

La ke

C he la n

IT-1

IT-2

IT-3

IT-4

IT-5

IT-6

TP-1

B-1

Existing Pump Well

Filename: C:\Users\jrs\CAD Group Dropbox\JDrive\_SEA\106705\001\106705-001 Explorations.dwg Layout: Figure 2 Date: 08-17-2021 Login: JRS

SITE AND EXPLORATION PLAN

FIG. 2

FIG

. 2 106705-001

0 80 160

Scale in Feet

IT-1 Perolation Test Designation and Approximate Location

LEGEND

Boring Approximate Location

Test Pit Approximate Location

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

August 2021

B-1

TP-1

106705-001 Figure 3.xlsm 8/25/2021

FIG

. 3

FIG. 3

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

UNIT EXTENT

SURFICIAL AQUIFER &

ALLUVIAL FAN (QA)

August 2021 106705-001

SHANNON & WILSON, INC.

Geotechnical and Environmental Consultants

106705-001 Figure 4.xlsm 8/18/2021

FIG. 4

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

PERSPECTIVE CROSS-SECTION

SURFICIAL AQUIFER

PARALLEL TO LONGITUDINAL AXIS

August 2021 106705-001

FIG

. 4

106705-001 Figure 5.xlsm 8/25/2021

FIG. 5

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

ESTIMATED WATER TABLE SURFACE

SURFICIAL AQUIFER

August 2021 106705-001

FIG

. 5

106705-001 Figure 6.xlsm 8/18/2021

FIG. 6

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

ESTIMATED THICKNESS

SURFICIAL AQUIFER

August 2021 106705-001

FIG

. 6

106705-001 July 26, 2022 A-i

AP

PE

ND

IX

A : S

UB

SU

RF

AC

E

EX

PL

OR

AT

IO

NS

Appendix A: Subsurface Explorations

Appendix A

Subsurface Explorations

CONTENTS

Soil Description and Log Key (2 sheets)

Log of Boring B-1

Log of Test Pit TP-1

Water Well Report, dated September 15, 1987

-0

N E

W _S

O

IL

_C

LA

S S

_K E

Y _P

G

.G P

J S

H A

N _W

IL

.G

T

/1

8/

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

Absence of moisture, dusty, dry to the touch.

Damp but no visible water.

Visible free water, from below water table.

SOIL DESCRIPTION AND LOG KEY

PI > 21

10 < PI < 20

A thread is easy to roll and not much time is required to reach the plastic limit. The thread cannot be rerolled after reaching the plastic limit. A lump crumbles when drier than the plastic limit.

A thread can barely be rolled and a lump cannot be formed when drier than the plastic limit.

Cannot roll a 1/8-in. thread at any water content.

It takes considerable time rolling and kneading to reach the plastic limit. A thread can be rerolled several times after reaching the plastic limit. A lump can be formed without crumbling when drier than the plastic limit.

Gradation

Irregular patches of different colors.

Poorly Graded

Will not crumble or break with finger pressure.Strong

Crumbles or breaks with considerable finger pressure.

Soil disturbance or mixing by plants or animals.

Nonsorted sediment; sand and gravel in silt and/or clay matrix.

Material brought to surface by drilling.

Sampler

N-Value

10 to 30 inches long Shoe I.D. = 1.375 inches Barrel I.D. = 1.5 inches Barrel O.D. = 2 inches

Plasticity2

Hard

Nonplastic

Low

Medium

PI < 4

4 < PI < 10

Moist

Wet

Dry

Smoothly curved sides with no edges.

Width/thickness ratio > 3.

Length/width ratio > 3.

Interbedded

Laminated

Fracture planes appear polished or glossy; sometimes striated.Slickensided

Fissured

Flat

Rounded

Subrounded

Subangular

Angular Sharp edges and unpolished planar surfaces.

Similar to angular, but with rounded edges.

Breaks along definite planes or fractures with little resistance.

Cohesive soil that can be broken down into small angular lumps that resist further breakdown.Blocky

Inclusion of small pockets of different soils, such as small lenses of sand scattered through a mass of clay.

Alternating layers of varying material or color with layers less than 1/4-inch-thick; singular: lamination.

Sum blow counts for second and third 6-inch increments.

Refusal: 50 blows for 6 inches or less or 10 blows for 0 inch.

Lensed

140 pounds with a 30-inch free fall. Rope on 6- to 10-inch-diameter cathead 2-1/4 rope turns, > 100 rpm. If automatic hammers are used, blow counts shown on boring logs should be adjusted to account for efficiency of hammer.

Nearly planar sides with well-rounded edges.

Structure1

Homogeneous Same color and appearance throughout.

Hammer

Elongated

Angularity and Shape1

Material that caved from sides of borehole.

Disturbed texture, mix of strengths.

Mottled

Bioturbated

Diamict

Cuttings

Slough

Sheared

Moderate

Weak

Cementation1

Additional Terms

Full range and even distribution of grain sizes present. Meets criteria in ASTM D2487, if tested.

Narrow range of grain sizes present or, within the range of grain sizes present, one or more sizes are missing (Gap Graded). Meets criteria in ASTM D2487, if tested.

Crumbles/breaks with handling or slight finger pressure.

Well-Graded

Moisture Content

Standard Penetration Test (SPT)3

Sheet 1 of 2

Shannon & Wilson uses a soil identification system modified from the Unified Soil Classification System (USCS). Elements of the USCS and other definitions are provided on this and the following page. Soil descriptions are based on visual-manual procedures (ASTM D2488) and laboratory testing procedures (ASTM D2487), if performed.

1Reprinted, with permission, from ASTM D2488 - 09a Standard Practice for Description and Identification of Soils (Visual-Manual Procedure), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM International, www.astm.org.

2Adapted, with permission, from ASTM D2488 - 09a Standard Practice for Description and Identification of Soils (Visual-Manual Procedure), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be obtained from ASTM International, www.astm.org.

3Penetration resistances (N-values) shown on boring logs are as recorded in the field and have not been corrected for hammer efficiency, overburden, or other factors.

Notes:

Alternating layers of varying material or color with layers at least 1/4-inch-thick; singular: bed.

-0

N E

W _S

O

IL

_C

LA

S S

_K E

Y _P

G

.G P

J S

H A

N _W

IL

.G

/1

8/

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

SOIL DESCRIPTION AND LOG KEY

Trace

Few

Little

Some

Mostly

< 5%

5 to 10%

15 to 25%

30 to 45%

50 to 100%

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

PT

Pounds per Square Inch Polyvinyl Chloride Rotations per Minute Standard Penetration Test Unified Soil Classification System Unconfined Compressive Strength Vibrating Wire Piezometer Vertical Weight of Hammer Weight of Rods Weight psi

PVC

rpm

SPT

USCS

qu

VWP

Vert.

WOH

WOR

Wt

ATD

Diam.

Elev.

ft FeO gal Horiz.

HSA

I.D.

in lbs

MgO mm MnO

NA

NP

O.D.

OW

pcf

PID

PMT

ppm

Sheet 2 of 2

Primarily organic matter, dark in color, and organic odor

Fine-Grained Soils

Highly Organic Soils

Gravels (more than 50% of coarse fraction retained on No. 4 sieve)

(50% or more of coarse fraction passes the No.

4 sieve)

(liquid limit less than 50)

Silty or Clayey Gravel

Relative Consistency Cohesionless Soils

Acronyms and Abbreviations

(more than 12% fines)

Sand

Silty or Clayey Sand (more than 12% fines)

Inorganic

Silty Gravel; Silty Gravel with Sand

Clayey Sand; Clayey Sand with Gravel

Surface Cement Seal

Bentonite Cement Grout

Bentonite Grout

Bentonite Chips

Silica Sand

Perforated or Screened Casing

Asphalt or Cap

Slough

Silt; Silt with Sand or Gravel; Sandy or Gravelly Silt

Inclinometer or Non-perforated Casing

Organic

Poorly Graded Gravel; Poorly Graded Gravel with Sand

Clayey Gravel; Clayey Gravel with Sand

Well-graded Sand; Well-graded Sand with Gravel

Magnesium Oxide Millimeter Manganese Oxide Not Applicable or Not Available Nonplastic Outside Diameter Observation Well

Pounds per Cubic Foot Photoionization Detector

Pressuremeter Test Parts per Million

Poorly Graded Sand; Poorly Graded Sand with Gravel

Peat or other highly organic soils (see ASTM D4427)

Silty Sand; Silty Sand with Gravel

Lean Clay; Lean Clay with Sand or Gravel; Sandy or Gravelly Lean Clay

< 2 2 - 4 4 - 8 8 - 15

15 - 30 > 30

Very soft Soft

Medium stiff Stiff Very stiff Hard

Relative Consistency

Relative Density

N, SPT,

Blows/ft

Percentages1, 2

Organic Silt or Clay; Organic Silt or Clay with Sand or Gravel; Sandy or Gravelly Organic Silt or Clay Elastic Silt; Elastic Silt with Sand or Gravel; Sandy or Gravelly Elastic Silt

Fat Clay; Fat Clay with Sand or Gravel; Sandy or Gravelly Fat Clay

Organic Silt or Clay; Organic Silt or Clay with Sand or Gravel; Sandy or Gravelly Organic Silt or Clay

Coarse-Grained Soils (more than 50% retained on No. 200 sieve)

(50% or more passes the No. 200 sieve)

Sands

Silts and Clays

Silts and Clays (liquid limit 50 or more)

Gravel

(less than 5% fines)

(less than 5% fines)

Organic

Inorganic

Typical IdentificationsSymbolMajor Divisions

N, SPT,

Blows/ft

< 4

4 - 10 10 - 30 30 - 50

> 50

Very loose Loose Medium dense Dense

Very dense

Notes:

Dual symbols (symbols separated by a hyphen, i.e., SP-SM, Sand with Silt) are used for soils with between 5% and 12% fines or when the liquid limit and plasticity index values plot in the CL-ML area of the plasticity chart. Graphics shown on the logs for these soil types are a combination of the two graphic symbols (e.g., SP and SM).

Borderline symbols (symbols separated by a slash, i.e., CL/ML, Lean Clay to Silt; SP-SM/SM, Sand with Silt to Silty Sand) indicate that the soil properties are close to the defining boundary between two groups.

No. 4 size = 4.75 mm = 0.187 in.; No. 200 size = 0.075 mm = 0.003 in.

Vibrating Wire Piezometer with Designation

Instrumentation Riser or Electrical Lead

At Time of Drilling Diameter Elevation Feet Iron Oxide Gallons Horizontal Hollow-Stem Auger Inside Diameter Inches Pounds

Cohesive Soils Relative Density

Well-graded Gravel; Well-graded Gravel with Sand

Well and Backfill Symbols

Unified Soil Classification System (USCS) Modified From USACE Tech Memo 3-357, ASTM D2487, and ASTM D2488

12.0

14.5

20.1

N on e O bs er ve d

D ur in g

D ril lin g

Medium dense to very dense, brown, Poorly Graded Gravel with Sand and Cobbles and Boulders (GP); dry; cobbles and boulders inferred from drill action and nearby test pits;

fine to coarse, subangular to angular gravel;

fine to coarse sand; nonplastic.

(Hf)

Medium dense, dark brown, Poorly Graded Gravel with Silt and Sand (GP-GM); moist; fine to coarse, subangular to angular gravel; fine to coarse sand; nonplastic.

(Qa)

Medium dense to very dense, brown, Poorly Graded Gravel with Silt and Sand (GP-GM);

fine to coarse, subangular to angular gravel;

fine to coarse sand; nonplastic; few silty sand seams.

(Qa)

BOTTOM OF BORING

COMPLETED 7/8/2021

Notes:

1. Blow counts may be overstated as a result of encountering coarse gravel and cobbles and boulders during sampling.

Drilling Method:

Drilling Company:

Drill Rig Equipment:

Other Comments:

Lo g:

E C

S

Northing:

Easting:

Station:

Offset:

SOIL DESCRIPTION

20 40 60

S am pl es

6 in.

NWJ

Automatic

Refer to the report text for a proper understanding of the subsurface materials and drilling methods. The stratification lines indicated below represent the approximate boundaries between material types, and the transition may be gradual.

August 2021 106705-001

T yp

LK

N

Geotechnical and Environmental Consultants

Sample Not Recovered

D ep th , f t.

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

D ep th , f t.

LOG OF BORING B-1

0 60

Total Depth:

Top Elevation:

Vert. Datum:

Horiz. Datum:

2.0" O.D. Split Spoon Sample

Hole Diam.:

Rod Diam.:

Hammer Type:

LEGEND

S ym bo l

1. Refer to KEY for explanation of symbols, codes, abbreviations and definitions.

2. Groundwater level, if indicated above, is for the date specified and may vary.

3. USCS designation is based on visual-manual classification and selected lab testing.

4. The hole location was measured from existing site features and should be considered approximate.

Hollow Stem Auger Holt Services

CME 85

FIG. A-1SHANNON & WILSON, INC.

20.1 ft.

~ 1165 ft.

NAVD88

SP WA N

Sheet 1 of 2

R ev

: S A

W

REV 3 - Approved for Submittal

S cr ee n

D es ig n

NOTES

CONTINUED NEXT SHEET

20 40

~ 478,259 ft.

~ 1,683,672 ft.

-1

-2

L O

G T

E M

P

5.

G

P J

S H

A N

_W

IL

.G D

T

/2 6/

PENETRATION RESISTANCE

Hammer Wt. & Drop:

(blows/foot)

140 lbs / 30 inches

% Fines (<0.075mm)

% Water Content

50/4"

50/1"

2. The first attempt of the boring was stopped at 9 feet due to encountering a boulder. In order for the boring to reach the final depth of 20 feet it was then moved over and restarted a total of 5 more times. Four more boring attempts were stopped due to encountering boulders between 3 and 6 feet. This log is a summary of all of the boring attempts.

Drilling Method:

Drilling Company:

Drill Rig Equipment:

Other Comments:

Lo g:

E C

S

Northing:

Easting:

Station:

Offset:

SOIL DESCRIPTION

20 40 60

S am pl es

6 in.

NWJ

Automatic

Refer to the report text for a proper understanding of the subsurface materials and drilling methods. The stratification lines indicated below represent the approximate boundaries between material types, and the…