B08_Attach_4_22_10_06_-_Geotech_Report_for_Disposal_Field_Test_Pits_-r1.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 DATA REPORT

Stehekin Wastewater Treatment Plant Replacement Project

CHELAN COUNTY, WASHINGTON

October 6, 2022

Shannon & Wilson No: 106705-003

Stehekin Wastewater Treatment Plant Replacement Project Geotechnical Data Report

106705-003 October 6, 2022 10/6/2022-106705-003-R1r1.docx i

Submitted To: HDR Engineering, Inc.

419 Canyon Ave., Suite 316 Fort Collins, CO 80521 Attn: Mr. Matt Gough

Subject: GEOTECHNICAL DATA REPORTREPORT TYPE, STEHEKIN WASTEWATER

TREATMENT PLANT REPLACEMENT PROJECT, CHELAN COUNTY,

WASHINGTON

Shannon & Wilson prepared this report and participated in this project as a subconsultant to HDR Engineering, Inc. Our scope of services was specified in Subconsultant Services Agreement No. 1000100077724 with HDR dated August 17, 2022. This report presents the information required to assist in advancing the effluent drain field design for the Stehekin Wastewater Treatment Plant Replacement project and was prepared by the undersigned.

This revised report contains a correction to reported year in which a debris flow occurred at the site.

We appreciate the opportunity to be of service to you on this project. If you have questions concerning this report, or we may be of further service, please contact us.

Sincerely, SHANNON & WILSON

David C. Ward, PE, LEG Vice President

EAS:DCW/eas

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CONTENTS

1 Introduction

2 Site and Project Description

3 Site Geology

4 Subsurface Explorations

5 Limitations

6 References

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan

Appendices Appendix A: Subsurface Explorations Important Information

106705-003 October 6, 2022

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 Project’s effluent drain field design.

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.

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 and borings within the Project site. According to NPS staff, a debris flow in 2013 originating

106705-003 October 6, 2022 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

The subsurface explorations performed for the Project consisted of four test pits. The proposed exploration locations were determined in October 2022 by HDR and NPS during their site visit. The approximate locations of the explorations performed are shown in Figure 2.

The test pits, designated TP-1 through TP-3, were dug using a rubber-tired backhoe excavator. The equipment is owned by and was operated by the NPS. The test pit excavations were observed by a Shannon & Wilson field representative.

The test pit logs were prepared in accordance with Washington Administrative Code 256- 272B-03400. Horizons of different soils were visually determined by the Shannon & Wilson field representative. A description of the soil horizons is included on the test pit logs in Appendix A.

5 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. 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 Project’s effluent drain field design.

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.

106705-003 October 6, 2022

6 REFERENCES

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

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.

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

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PROJECT

LOCATION

Lake Chelan

Stehekin

VICINITY MAP

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

October 2022 106705-003

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Approximate Scale in Feet

FIG. 1

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Wastewater Treatment Plant

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SITE AND EXPLORATION PLAN

FIG. 2

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0 80 160

Scale in Feet

LEGEND

Test Pit Approximate Location

Stehekin Wastewater Treatment Plant Replacement Project Stehekin, Washington

October 2022

TP-1

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Appendix A: Subsurface Explorations

Appendix A

Subsurface Explorations

CONTENTS

Test Pit Logs

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Test Pit

Depth

Color Texture Structure

Rock Roots Restrictive

Layer Soil Type Comment Top Bottom

TP-1 0 0.5 Grayish-brown

VGls Granular 60 Few No 1 Very Gravelly loamy sand

0.5 3.5 Grayish-brown

VCBls Granular 70 Few No 1 Very Cobbly loamy sand

3.5 4 Brown GRls Granular 50 Common No 1 Gravelly loamy sand

TP-2 0 2.5 Grayish-brown

VCBls Granular 75 Few No 1 Very Cobbly loamy sand

2.5 4 Grayish-brown

VCBls Granular 75 Few No 1 Very Cobbly loamy sand

TP-3 0 4 Grayish-brown

EBYls Granular 90 Few No 1 Extremely Bouldery loamy sand

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N Important Information

Important Information About Your Geotechnical/Environmental Report

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CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR

SPECIFIC CLIENTS.

Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineer.

Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first conferring with the consultant.

THE CONSULTANT’S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS.

A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors. Depending on the project, these may include the general nature of the structure and property involved; its size and configuration; its historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations. Unless your consultant indicates otherwise, your report should not be used

(1) when the nature of the proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site.

Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors that were considered in the development of the report have changed.

SUBSURFACE CONDITIONS CAN CHANGE.

Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally.

Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant should be kept apprised of any such events and should be consulted to determine if additional tests are necessary.

MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS.

Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in your report. While nothing can be done to prevent such situations, you and your consultant can work together to help reduce their impacts. Retaining

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N your consultant to observe subsurface construction operations can be particularly beneficial in this respect.

A REPORT’S CONCLUSIONS ARE PRELIMINARY.

The conclusions contained in your consultant’s report are preliminary, because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report’s recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations.

The consultant who developed your report cannot assume responsibility or liability for the adequacy of the report’s recommendations if another party is retained to observe construction.

THE CONSULTANT’S REPORT IS SUBJECT TO MISINTERPRETATION.

Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative to these issues.

BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED

FROM THE REPORT.

Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and laboratory and/or office evaluation of field samples and data.

Only final boring logs and data are customarily included in geotechnical/environmental reports.

These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process.

To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report’s limitations, assuming that a contractor was not one of the specific persons for whom the report was prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that aggravate them to a disproportionate scale.

READ RESPONSIBILITY CLAUSES CLOSELY.

Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims

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N being lodged against consultants. To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports, and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant’s liabilities to other parties;

rather, they are definitive clauses that identify where the consultant’s responsibilities begin and end.

Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your questions.

The preceding paragraphs are based on information provided by the GBA, Silver Spring, Maryland