GR 12-21 WA NPS NOCA 10(7) FINAL Geotech Recommendations Report.pdf

Reroute Stehekin Valley Road at Mile 5.5 June 2021

Table of Contents 1-i

Geotechnical Report No. 12-21

Reroute Stehekin Valley Road at Mile 5.5

FINAL GEOTECHNICAL

RECOMMENDATIONS REPORT

WA NPS NOCA 10(7)

Chelan County, Washington

June 2021

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Table of Contents i

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Project Description

1.2 Purpose and Scope

2 REGIONAL SETTING

2.1 Average Temperatures and Precipitation

2.2 Geology

2.2.1 Geologic Hazards

3 SITE CONDITIONS

3.1 General

3.1.1 Nearby Cutslopes

3.2 Subsurface Conditions

3.2.1 General

3.2.2 Groundwater

3.2.3 Laboratory Testing

4 GEOTECHNICAL RECOMMENDATIONS

4.1 Topsoil

4.2 Cut and Fill Slopes

4.3 Soils and Pavement Structure

5 CONSTRUCTION CONSIDERATIONS

5.1 Excavation

5.2 Excavated material reuse

6 LIMITATIONS

7 REFERENCES

List of Figures ii

LIST OF FIGURES

Figure 1-1. Location map of the Reroute Stehekin Valley Road at Mile Post 5.5 project

Figure 1-2. Map showing the reroute design centerline in relation to Stehekin Valley features

Figure 1-3. Photographs of the Stehekin River cut bank washout

Figure 3-1. Map displaying the reroute design features in relation to the subsurface investigation sites

Figure 3-2. Photograph of a boulder located on a debris flow levee

Figure 3-3. Photograph displaying representative forest

Figure 3-4. Photographs showing relatively stable and unstable cut slopes

Figure 3-5. Map showing locations of topsoil thickness measurements

LIST OF TABLES

Table 2-1. Climate Data Summary

Table 3-1. Hand soil pit and dynamic cone penetrometer (DCP) sites

Table 3-2. Hand dynamic cone penetrometer (DCP) results

Table 3-3. Soil Classifications from hand soil pits

APPENDICES

APPENDIX A

Material Laboratory Results, 13 pages

APPENDIX B

Hand Soil Pit Photographs, 7 pages

APPENDIX C

Hand Dynamic Cone Penetrometer (DCP) Data, 13 pages

APPENDIX D

Final Pavement Structure Recommendations Memorandum, 2 pages

Introduction 3

1 INTRODUCTION

This report provides geotechnical investigation results and final geotechnical recommendations for the “Reroute Stehekin Valley Road at Mile 5.5” project (SVR). Final recommendations are based on thorough literature research, a subsurface investigation, laboratory testing of soil samples, surficial assessments of proposed design options, and a review of historical aerial imagery.

The SVR provides local road access for residents of Stehekin and the surrounding community as well as visitors. It is the primary means of access for all people from the lower Stehekin Valley to the upper reaches of the valley within the Lake Chelan National Recreation Area (LCNRA) and North Cascades National Park (NOCA). Both units are managed by the National Park Service (NPS). Mount Baker-Snoqualmie National Forest managed lands are also accessed by this roadway.

Stehekin Village is at the end of Lake Chelan and is only accessible by boat, air, or foot trails.

Most supplies, including construction equipment and materials, are delivered to Stehekin by barge. Figures 1-1 and 1-2 provide project location and vicinity maps.

1.1 PROJECT DESCRIPTION

The project is an approximately 1,100 feet long reroute located within the Lower Stehekin Valley in the eastern North Cascade mountains of north-central Washington State (Figure 1-1). The existing road is a two-lane, rural roadway with portions of aggregate and chip seal asphalt pavement surfacing. The roadway was undermined and eroded by the Stehekin River along private property in December of 2018 near mile post 5.5 (Figures 1-2 and 1-3).

The project proposes to reroute the road away from ongoing cut bank erosion from the Stehekin River and, in the process, move the road off and around private property. Access to and from the private property is part of this project and includes utility access on the up-valley and down-valley sides of the property. The project begins at approximate latitude, longitude coordinates 48.3600°, -120.7304° (World Geodetic System [WGS] 84) and ends at about 48.3609°, -120.7346°.

1.2 PURPOSE AND SCOPE

The purpose of this report is to provide geotechnical design recommendations and construction considerations for this project. The geotechnical scope of work included a visual assessment and reconnaissance along the proposed alignment, a subsurface exploration program consisting of hand soil test pits, conducting hand dynamic cone penetrometer (DCP) testing, submitting near-surface soil samples for laboratory testing, and preparation of a final geotechnical recommendations report. Soil pits were dug by hand and logged by a Western Federal Lands Highway Division (WFLHD) engineering geologist. Material laboratory testing was completed by the WFLHD Materials Laboratory. No subsurface geotechnical drilling or geophysical studies were completed, and no other geotechnical memorandums were prepared and submitted as part of this project.

4 Introduction

Figure 1-1. Location map of the Reroute Stehekin Valley Road at Mile Post 5.5 project. A) a regional overview showing the project location in relation to Seattle, Chelan County, and the city of Chelan. Map B is shown in the black box. B) map of upper Lake Chelan and lower Stehekin River Valley. Figures 1-2, 3-1 and 3-5 have their extents indicated in the red boxes. Basin 16a is delineated as defined in Riedel and Sarrantonio, 2021.

Introduction 5

Figure 1-2. Map showing the proposed reroute centerline in relation to locations of basin 16a and its associated debris cone (Riedel and Sarrantonio, 2021), Bear Trap Spring, the cut bank road wash out (red circle and in Figure 1-3), and nearby cut slopes conditions (at sites of Figure 3-4). Figures 3-4 A, B, C, and D are at the arrow tips, which are pointing in the direction photographs were taken. Figures 3-1 and 3-5 have their extent delineated in the black box. The base layer 315 degrees azimuth hillshade map was derived from 2015 LiDAR elevation data.

6 Introduction

Figure 1-3. Photographs of the Stehekin River cut bank washout that undermined and eroded the Stehekin Valley Road at mile post 5.5. A) Looking up valley. B) Looking down valley. Location of Figure 1-3 is on Figure 1-2. Photographs were taken April 28, 2021.

A

B regional Setting 7

2 REGIONAL SETTING

2.1 AVERAGE TEMPERATURES AND PRECIPITATION

The Stehekin Valley is a mountainous environment at somewhat low elevation (approximately 1,200 to 1,300 feet elevation) in the Cascade Mountain climate. Summers are typically warm and dry and winters cool and wet. From December to March the valley is most susceptible to rain-on-snow events. The Stehekin Valley typically experiences an average of 34 inches of precipitation and 123 inches of snowfall annually. Micro-climates associated with the steep, high elevation terrain within the North Cascade Mountains can often produce conditions conducive to short duration, high intensity precipitation events in summer and early fall.

Table 2-1 shows the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data for the period from 1906 to 2016 for the Stehekin 4 NW, Washington (45805) weather station (Western Region Climate Center, 2021).

Table 2-1 Climate Data Summary at the Stehekin 4 NW, Washington (45805) weather station.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Average Max.

Temperature (F)

33.2 39.0 47.7 58.9 68.3 74.9 83.0 81.7 72.0 57.5 42.2 34.2 57.7

Average Min.

Temperature (F)

22.8 25.1 29.2 35.3 42.2 48.4 53.1 52.4 45.0 36.7 30.1 25.3 37.1

Average Total Precipitation (in.)

6.01 3.90 2.82 1.39 0.97 0.82 0.50 0.62 1.10 3.06 6.04 6.82 34.05

Average Total Snow Fall (in.)

43.0 21.2 7.3 0.3 0.0 0.0 0.0 0.0 0.0 0.3 12.1 39.0 123.1

Average Snow Depth (in.)

25.0 27.0 18.0 2.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 12.0 7.0

Notes:

1. Percent of possible observations for period of record; 01/20/1906 to 06/10/2016.

2. Max. Temp.: 97.1% Min. Temp.: 97% Precipitation: 97.1% Snowfall: 97.1% Snow Depth: 96.3%

2.2 GEOLOGY

The Stehekin Valley is a classic, U-shaped, glacially carved valley with the Stehekin River flowing through the valley bottom. Steep metamorphic rocks derived from igneous intrusive rocks are the back bone of the high mountains and ridges are source material for numerous large alluvial-debris fans and cones along the valley’s edges. Haugerud and Tabor (2009) have mapped the bedrock geology in the vicinity of the project as a gneissic hornblende-biotite tonalite (map symbol TKso), which is technically a high grade metamorphic rock that formed at high pressure and temperatures from granitic rocks.

8 regional Setting

Haugerud and Tabor (2009) mapped sediments covering bedrock as alluvial deposits (water transported deposits). However, along the valley edges are remnants of glacial activity in the form of interspersed, lateral glacial moraines. Haugerud and Tabor (2009) did not map glacial deposits at the project’s location, but lateral alpine glacial moraines are interpreted to be present near the site during this investigation.

2.2.1 GEOLOGIC HAZARDS

Geologic hazards observed in the valley appear to be driven by precipitation, the high relief, and the steep slopes discussed above. Debris flows can be a common occurrence, initiating along confined channels of the steep ridges and depositing in the lower gradient valley. The debris flows typically result from intense rainfall events, rain-on-snow events, spring snowmelt, and post wildfire-related hazards. The Stehekin River Valley’s debris flow magnitude and frequency is well characterized by Riedel and Sarrantonio (2021). They determined debris flow magnitude and frequency within the Stehekin River Valley is linked to post wildfires conditions, and are more likely to occur on southwest facing slopes (Riedel and Sarrantonio, 2021). The project area is below the southwest facing basin 16a, as described and discussed by Riedel and Sarrantonio (2021) (Figure 1-2).

Erosion of banks by the Stehekin River is common as well. The combination of having a low gradient valley bottom with a high frequency of debris flows transporting material to the valley bottom gives the Stehekin River a heavy sediment load that facilitates frequent channel migration and braiding. Combined with heavy flows and flooding from large precipitation events, channel migration can often pose challenges to the Stehekin Valley’s adjacent infrastructure.

No deep-seated landslides are mapped or were observed in the vicinity of the project.

Site Conditions 9

3 SITE CONDITIONS

3.1 GENERAL

Site conditions were observed during field visits by Western Federal Lands Highway Division (WFLHD) engineering geologist, Orion George on April 23, 26, 27, and 28, 2021. WFLHD design engineers, Hunter Goguen and Y-Thao Truong, were on site April 26, 27, and 28. Private property owner Jonathon Scherer was on site April 23, 26, and 27. North Cascades National Park personnel, Vicki Gempko and Maria Cook, were also on site April 27. Reconnaissance included visual inspection of road surface conditions, review of the proposed alignment and the upslope areas. Digging soil pits combined with soil probing and conducting hand DCP tests to characterize in situ soil strengths and resistance. Figure 3-1 is a map of the project site.

The site is in an open Douglas fir (Pseudotsuga menziesii) dominated forest with a decent population of mountain dogwood (Cornus nuttalii) and bigleaf maple (Acer macrophyllum). Under-brush includes mock orange (Pjiladelphus lewisii), mountain serviceberry (Amelanchier alnifolia), and Oregon grape (Mohania aquifolium) among others. Grasses were observed as well. Areas of diseased Douglas fir, from root rot, were noticeable while visiting the site. Figure 3-2 is a representative photograph of the typical forest conditions in this area.

The proposed alignment is located approximately mile post (MP) 5.5 and climbs away from the Stehekin River’s edge onto an alluvial fan deposit. An alluvial fan is a triangle-shaped deposit of gravel, sand, cobbles, and boulders intermixed with fine-grained (primarily silt) sediments. The sediments are typically transported, deposited, and usually created when water and soil and debris material flows from a steep, confined channel to a less steep slope, where the channel becomes less confined, or unconfined. The change in channel steepness and confinement reduces the carrying capacity of the debris flow and it begins depositing materials. This depositional environment is prone to dramatic changes in channel location as episodic debris flows and high-water storm events produce large amounts of debris that often initiate from steep drainage channels above.

The proposed alignment crosses an alluvial fan that is debris-flow dominated from approximately station 17+20 to 24+20, meaning the fan has a network of mostly inactive channels in the upper fan with smaller debris flow fan deposits located on the lower-level of the fan deposit (Riedel and Sarrantonio, 2021). The channels also appear to be filled by subsequent debris flows. Based on what we know about these landforms and site observations, it should be anticipated that finer sediments are deposited lower on this fan. The level of vegetation growth on the alluvial fan suggests most of the fan has not experienced recent debris-flow activity, particularly in the lower portion of the fan, where the proposed alignment is located.

10 Site Conditions

Figure 3-1. Map displaying the proposed reroute design features in relation to the subsurface investigation sites (HP= hand pits, DCP = hand dynamic cone penetrometer sites) and Figures 1-3, 3-2, and 3-3. Proposed reroute design stations are labeled in black. Figures 3-2 and 3-3 are at arrow tips, which is pointing in the direction photographs were taken. The base layer 315 degrees azimuth hillshade map was derived from 2015 LiDAR elevation data.

Site Conditions 11

Debris flow levees were observed along, and directly uphill of, the proposed alignment at approximately stations 20+50 to 22+50. Large boulders measuring up to four feet in dimension were observed on these levees (Figure 3-3). In general, boulders were concentrated within the mapped debris cone landform between approximate stations 17+20 to 24+20 (Figures 3-1 and 3- 5). Again, lichen and algal growth on boulders and levee materials suggest this material has been in place for some time. Also, a discussion with private property owner Jonathon Scherer, who has lived close to this site since 1978, said he had never seen a debris flow come down the fan, or reach the proposed alignment location.

Figure 3-3. Photograph displaying representative forest along the up valley section of the proposed reroute. Figure 3-1 displays the location of this photograph. Photograph was taken April 28, 2021.

Figure 3-2. Photograph of a boulder located on a debris flow levee. The soil probe is 36 inches tall and dimensions of boulder were measured at 44 inches by 49 inches. Note the heavy lichen growth on the boulder, which suggests a large amount of time has passed since placement during a debris flow. Location of the photograph is displayed in Figure 3-1. Photograph was taken April 28, 2021.

12 Site Conditions

A historical aerial photograph review was conducted of the debris flow fan where the new alignment is proposed. The photographic record included aerial photographs from 1953, 1958, 1968, 1974, 1998, 2006, 2009, 2011, 2013, 2015, and 2017. No significant visual impacts to the area of the proposed alignment were noted, such as exposed, non-vegetated ground, that would typically indicate a recent debris flow event. It must be stated that alluvial fans exist due to periodic, and rapid, channel avulsion (channel course change). As such this investigation does not try to predict or mitigate for a debris flow hazard associated with this drainage system.

3.1.1 NEARBY CUTSLOPES

Cut slopes up-valley of the project along the roadway were observed to have similar geologic material and also assess slope performance at different slope angles (Figures 1-2 and 3-4). The slopes observed are southwest facing, a similar aspect as those proposed for this reroute of the alignment project. Materials were noted as unconsolidated, silty SAND with gravel supporting cobbles and boulders. These slopes were typically vegetated up to an angle of 38 degrees, but anything above was poorly vegetated or had signs of instability and past failures (Figures 1-2 and 3-4).

3.2 SUBSURFACE CONDITIONS

3.2.1 GENERAL

During the site visit, a subsurface investigation was conducted to support design for the project.

It consisted of digging seven soil pits (HP) and conducting five DCP tests. Grab samples were collected for material index and subgrade testing and to assess natural moisture content of encountered subsurface soils. Note moisture content is specific to test locations and conditions at the time of sampling. Testing locations are included in Figure 3-1, tabulated in Table 3-1, and material laboratory test results are provided in Appendix A. Photographs of the hand pits are included in Appendix B.

Hand pits ranged in depth from 17 to 29 inches below ground surface. Predominantly granular materials were encountered at all hand pits. Varying amounts of silt and gravel were minor constituents, but almost all soils logged were considered SAND except a silty GRAVEL with sand (GM) observed at HP-014 from 3 inches to 29 inches below ground surface. Using the AASHTO soils classification system soils are classified as A-1-b and A-2-4. All soils encountered during the near surface investigation were considered natural and in place materials, and stem from the alluvial debris fan depositional environment described in Section 3.1. Table 3-3 provides descriptions of soils observed in the hand pits.

In addition to hand soil pits, topsoil depth was measured along the proposed alignment in six additional locations. Figure 3-5 is a map of the proposed alignment with the measured thickness of topsoil at 13 locations.

Site Conditions 13

Figure 3-4. Photographs showing relatively stable and unstable cut slopes in similar material observed along the proposed reroute. These slopes are up valley of the reroute with locations identified on Figure 3-1. A) and B) Relatively stable, vegetated cut slopes without evidence of shallow failures. C) Slope has failed within the last 10 years. Exposed soils and little vegetation. D) An oversteepened cut slope with some exposed soils and little vegetation. A 1982 one door Toyota Tercel SR5 is for scale. Photographs were taken April 28, 2021.

B

C

D

14 Site Conditions

Table 3-1 Hand soil pit and dynamic cone penetrometer (DCP) sites with approximate station, latitude and longitude coordinates topsoil thickness, hand pit depth, material sample ID and depth.

Hand Soil Pit Site ID

DCP ID Approximate Station1

Coordinates2 Topsoil Thickness,

in. (ft.

bgs3)

Hole Depth,

in. (ft bgs3)

Samples

ID

Sample Depth,

in. Latitude Longitude

HP-002 DCP-002 22+90 48.36087 -120.73411 5 (0.4) 26 (2.2) GB21-01 15 - 24

HP-005 - 20+25 48.36098 -120.73307 5 (0.4) 24 (2.0) GB21-02 15 - 24

HP-008 DCP-008 18+50 48.36086 -120.73240 3 (0.25) 25 (2.1) - -

HP-009 - 17+95 48.36075 -120.73221 2 (0.2) 17 (1.5) - -

HP-010 DCP-010 16+60 48.36069 -120.73166 5 (0.4) 25 (2.1)

GB21-03 5 - 11

GB21-04 11 - 20

GB21-05 20 - 25

HP-012 DCP-012 14+00 48.36024 -120.73086 6 (0.5) 27 (2.25) GB21-06 7 - 10

HP-014 DCP-014 13+154 48.36013 -120.73054 3 (0.25) 29 (2.4)

GB21-07 3 - 19

GB21-08 19 - 29

Notes:

1. Stationing is approximate and based off plans dated April 7, 2021. Locations may be offset up to 15 feet from centerline.

2. Coordinates are approximate from a hand held GPS and in world geodetic system (WGS) 1984.

3. Below ground surface = bgs.

4. Test sites HP-014 and DCP-014 are offset approximately 35 feet right of center line.

Table 3-2 Hand dynamic cone penetrometer (DCP) test depth, residual modulus estimation, and notes.

Site DCP Depth, in (ft)

Mr1 (psi) Notes In Situ

Soil

DCP-002 32.0 (2.67) 2,745

DCP-008.1 10.2 (0.85) 3,154 Ended in refusal DCP-008.2 15.5 (1.29) 2,879 Ended in refusal

DCP-010 31.7 (2.64) 3,739

DCP-012 20.8 (1.74) 3,890 Ended in refusal DCP-014 22.4 (1.87) 3,379 Ended in refusal

Average 3,298 Notes:

1. Mr is the residual modulus.

Site Conditions 15

Table 3-3. Soil Classifications. Approximate stationing of boreholes is in Table 4-1.

Hand pit ID

Depth (in)

Soil Classifications based on laboratory tests (USCS group symbol)1

Soil Classifications based on the Visual/Manual Method (ASTM D2488)

Notes

HP-

0 - 5 --- Poorly graded silty SAND with gravel Topsoil

5 - 14 --- Poorly graded SAND with gravel ---

14 - 26 Poorly graded SAND with silt and gravel (SP-SM) --- ---

HP-

0 - 5 --- Silty SAND with gravel Topsoil

5 - 15 --- Poorly graded silty SAND with gravel

Cobbles (9"), boulders likely

15 - 24 Silty SAND with gravel (SM) --- Cobbles, boulders likely

HP-

0 - 4 --- Poorly graded SAND with silt and gravel

Topsoil; cobbles, boulders

4 - 6 --- Poorly graded SAND with gravel ---

6 - 21 --- Poorly graded SAND with silt and gravel Cobbles (8")

21 - 25 --- Poorly graded silty SAND with gravel ---

HP-

0 - 2 --- Poorly graded SAND with silt and gravel

Topsoil; cobbles, boulders

2 - 6 --- Poorly graded SAND with silt and gravel ---

6 - 17 --- Poorly graded silty SAND with gravel Cobbles, boulders likely

HP-

0 - 5 --- Silty SAND with gravel Topsoil 5 - 11

Silty SAND with gravel (SM) --- Depth 5" - 25" combined, R-value = 67

11 - 20 20 - 25

HP-

0 - 6 --- Poorly graded silty SAND with gravel Topsoil

6 - 20 Silty SAND with gravel (SM) --- Charcoal layer at 18" 20 - 27 --- Silty SAND with gravel Cobbles

HP-

0 - 3 --- Poorly graded SAND with silt Topsoil 3 - 19

Silty GRAVEL with sand (GM) --- Depth 3" - 29" combined 19 - 29

Notes:

1. Test results are located in Appendix A.

16 Site Conditions

Figure 3-5. Map showing locations of topsoil thickness measurements (depth below ground surface) in relation to the proposed reroute alignment. The base layer 315 degrees azimuth hillshade map was derived from 2015 LiDAR elevation data.

Site Conditions 17

Hand DCP tests were also conducted to assess the strength and resistance of the in situ subsurface soils. The DCP test is designed to correlate depth per blow of the approximately 18 pound hammer to the residual modulus of base and subgrade soils below pavements. The DCP test was not conducted below a pavement, rather in native, in situ soils. The DCP test results were used to give a relative understanding of soil strength and resistance of the in situ soils below the proposed pavement structure. The DCP sites ranged in depth from 10.2 to 32.0 inches below ground surface. The shallower holes ended in refusal on large roots, or more likely cobbles or boulders. Results are displayed in Table 3-2 with the related strength and resistance correlation calculations are presented in Appendix C. Overall, results of the DCP testing are fairly consistent and are considered a good representation of the residual modulus of the native, in-place soils.

It was communicated in the field by Jonathon Scherer that a buried water line conveys water from Bear Trap Spring to his property via a buried polyethylene water line. Jonathon identified the location at approximately station 16+80, the site of a planned culvert, and stated the water line was buried approximately 2 to 3 feet below ground surface. Bear Trap Spring is located on Figure 1-2, and the location of the planned culvert and waterline crossing the proposed realignment is shown on Figure 3-1.

3.2.2 GROUNDWATER

No groundwater was encountered in any of the hand-dug soil pits. However, water was observed seeping along roots at site HP-012 at a depth of approximately 6 to 20 inches (Figure 3-1). It was uncertain if the water was seeping from the roots or surrounding soils at the time of the investigation. It is anticipated groundwater will be seasonally shallower when snow melt is infiltrating in the spring, but is otherwise not considered near surface along the proposed alignment year-round.

3.2.3 LABORATORY TESTING

Select soil samples were submitted to the WFLHD materials testing laboratory for index tests that include soil classification, grain size distribution, apparent specific gravity, Atterberg limits, natural moisture content, and R-value. Note that not all tests were performed on all samples that were tested. Remaining samples were classified visually using the Visual/Manual Method, Standard Practice for Description and Identification of Soils, ASTM D2488. Laboratory tests classify materials as mostly silty SAND with gravel with some variation of more or less gravel. Resulting classifications from both methods are shown in Table 3-3. Note that the Unified Soil Classification System (USCS) group symbols and group names are provided only where laboratory tests for classification are performed.

An R-value test was performed on three samples that were combined at site HP-010. The samples were combined to obtain a large enough sample size for the test method. Soils were considered relatively similar and representative to other samples collected from sites nearby. The resulting R-value was 67 at 5 to 25 inches below ground surface (Appendix A).

Moisture conditions were observed to be dry at some locations in topsoil, but were moist in all deeper soils encountered.

18 Geotechnical Discussion and Recommendations

4 GEOTECHNICAL DISCUSSION AND RECOMMENDATIONS

4.1 TOPSOIL

Topsoil along the proposed alignment was observed at 13 separate locations along the proposed alignment (Figure 3-5). Results of the investigation suggest the average topsoil depth is approximately 4-inches along the proposed alignment.

4.2 CUT AND FILL SLOPES

Cut slopes were evaluated in the area near the proposed alignment. No large cuts or fills are anticipated, but small cuts and fills will be constructed. Based on the similarity of materials observed on site, and peripherally to the project, we recommend cut slopes no steeper than 1V:1.5H (~34 degrees) and fill slopes no steeper than 1V:2H (26.6 degrees). Steeper slopes will have difficulty revegetating. Depending on weather conditions during the construction season, flatter slopes will improve chances of vegetation becoming established quicker. Establishing vegetation on freshly excavated slopes is critical to short term surficial stability.

4.3 SOILS AND PAVEMENT STRUCTURE

Near-subsurface materials were evaluated for strength and resistance to support the pavement design in the subgrade and base layers. Materials observed and tested are categorized as granular with most materials classified as silty SAND with gravel (SM) and some poorly graded SAND with silt and gravel (SP-SM) or silty GRAVEL with sand (GM) using the Unified Soil Classification System. Using the AASHTO soils classification system soils were classified as A- 1-b and A-2-4. Cobbles and boulders up to 4 feet in dimension were observed on the surface and are likely present in the subsurface. It is challenging to determine where cobbles and boulders will be encountered, but based on surface observations, they should be anticipated.

The WFL Materials Engineer has provided a Final Pavement Structure Recommendations memorandum, provided as Appendix D. The recommended pavement structure is synthesized here:

• Chip seal – Type 2B (Item 40702-1200)

• 6-inches – Roadway Aggregate , Method 2 (Item 30202-2100)

• 12-inches – Unclassified Borrow (Item 20404-000)

We recommend in place, native soils can be built upon after excavation and compaction.

Roadway excavation materials along the length of the alignment are anticipated to meet the

704.06 Unclassified Borrow (FP-14), and can be used for embankment construction and final subgrade. Use of excavated material is addressed below in Section 5 Construction Considerations.

Construction Considerations 19

5 CONSTRUCTION CONSIDERATIONS

5.1 ROADWAY EXCAVATION

Conform to Section 204 of the FHWA Standard Specifications FP-14 for all excavations.

Bedrock or boulders are anticipated to be encountered during excavations associated with this project. If boulders are encountered, excavate larger material and fill voids with 704.06 Unclassified Borrow or 704.07 Select Borrow (FP-14, as quantities allow) and compact in accordance with Section 204 (FP-14). Subgrade soils should be proof-rolled or otherwise compacted prior to placement of embankment fill material.

5.2 ROADWAY EXCAVATION MATERIAL REUSE

In place natural soils are expected to consist primarily of granular, non-plastic soils. Laboratory tests performed on samples collected from onsite show fine content (passing Sieve no. 200) ranged from 8.2 to 27.3 percent. This suggests existing natural soils will meet specification requirements of 704.06 Unclassified Borrow (FP-14). However, cobbles and boulders larger than 24 inches dimension will need to be removed to meet the 704.06 Unclassified Borrow specifications.

If suitable soils are encountered and the Contractor should desire to use onsite materials, it will be the Contractor’s responsibility to ensure those materials meet contract requirements for specific reuses.

20 Limitations

6 LIMITATIONS

This report was prepared to provide geotechnical investigation results and recommendations for the Reroute Stehekin Valley Road at Mile 5.5 project. Professional judgments and recommendations are presented in this report. They are based partly on evaluation of technical information gathered for this project and partly on the Geotechnical Team’s general experience with subsurface conditions in the area. It should be noted that the near-surface hand-dug soil pits might not represent potentially unfavorable subsurface conditions away from the pits. Observed near-surface conditions indicated, are at the time of investigation. Groundwater conditions may vary considerably, with season/time, changes in climate, rainfall, and other factors and are otherwise dependent on the duration of, and methods used in, and interpretations made from the investigation program.

If soil conditions are encountered during construction that vary from those discussed in this report, or if configurations change, the WFLHD Geotechnical Team should be notified immediately in order to evaluate what effects, if any, the changes have on recommendations contained in this report. The recommendations presented in this report are applicable only to this specific site.

These data are not to be used for other purposes.

Questions regarding this report should be addressed to Orion George, WFLHD Engineering Geologist, at (360) 619-7634, or Orion.George@dot.gov.

mailto:Orion.George@dot.gov

Limitations 21

GEOTECHNICAL REPORT NO. 12-21

FINAL GEOTECHNICAL RECOMMENDATIONS REPORT

Reroute Stehekin Valley Road at Mile 5.5

WA NPS NOCA 10(7)

Chelan County, Washington

June, 2021

Prepared by: _________________________________________ Orion George, Engineering Geologist

Reviewed and Approved by: _________________________________________ Douglas A. Anderson, GEO Team Functional Manager

22 References

7 REFERENCES

Haugerud, R.A. and R.W. Tabor, 2009, Geologic map of the North Cascades Range, Washington, USGS Scientific Investigations Map 2940, 2 sheets, 29 pg.

Riedel, J.L., S. Dorsch, and J. Wenger, 2012, Geomorphology of the Stehekin River watershed:

Landform mapping at North Cascades National Park Service Complex, Washington, National Park Service Natural Resource Technical Report NPS/NCCN/NRTR -2012/566, 49 pg.

Riedel, J.L. and S.M. Sarrantonio, 2021, Debris flow magnitude, frequency, and precipitation threshold in the eastern North Cascades, Washington, USA, Natural Hazards, v. 106, p 2519 – 2544, https://doi.org/10.1007/s11069-021-04553-8

Western Regional Climate Center (2021), Cooperative Climatological Data Summaries, Stehekin 4 NW, Washington (45805): https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?wa8059, accessed https://doi.org/10.1007/s11069-021-04553-8 https://wrcc.dri.edu/cgi-bin/cliMAIN.pl?wa8059

APPENDIX A

Materials Laboratory Test Results, 13 pages

APPENDIX B

Hand Soil Pit Photographs, 7 pages

Figure 2. Site HP-004.

Figure 1. Site DCP-002. No photographs of hand soil pit HP-002 were taken. The hand driven dynamic cone penetrometer is located in the photograph after having been driven to ending depth.

Figure 3. Site HP-004.

Figure 4. Site HP-005.

Figure 5. Site HP-008.

Figure 6. Site HP-008.

Figure 7. Site HP-009.

Figure 8. Site HP-010.

Figure 9. Site HP-010 overview.

Figure 10. Site HP-012.

Figure 11. Site HP-012. Pencil is pointing out water seeping along roots at 6 inches to 20 inches depth.

Figure 12. Site HP-012 overview.

Figure 13. Site HP-014.

APPENDIX C

Hand Dynamic Cone Penetrometer (DCP) Data, 13 pages

* baseline height was 172mm, initial height is immediate penetration

DCP-002 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 204 0 0.00 32 1.26 *Seating

1 212 0.3 0.03 8 0.31 8

1 312 4.3 0.35 100 3.94 100

1 346 5.6 0.47 34 1.34 34

1 374 6.7 0.56 28 1.10 28

1 397 7.6 0.63 23 0.91 23

1 444 9.4 0.79 47 1.85 47

1 569 14.4 1.20 125 4.92 125

1 719 20.3 1.69 150 5.91 150

1 853 25.6 2.13 134 5.28 134

1 928 28.5 2.38 75 2.95 75

1 1018 32.0 2.67 90 3.54 90 Ended

G.S. reading ACP:

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 74.0 0.3 2,745 1.8

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate.

This material is in situ and not under a pavement.

DCP-008.1 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 248 0 0.00 76 2.99 *Seating

1 410 6.4 0.53 162 6.38 162

1 468 8.7 0.72 58 2.28 58 hit rock 1 481 9.2 0.76 13 0.51 13

1 507 10.2 0.85 26 1.02 26 refusal 1 507 10.2 0.85 0 0.00 0 Ended

G.S. reading ACP:

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 51.8 0.3 3,154 2.1

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate.

DCP-008.2 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 237 0 0.00 65 2.56 *Seating

1 396 6.3 0.52 159 6.26 159

1 492 10.0 0.84 96 3.78 96

1 520 11.1 0.93 28 1.10 28

1 616 14.9 1.24 96 3.78 96

1 621 15.1 1.26 5 0.20 5

1 630 15.5 1.29 9 0.35 9 Ended

G.S.

reading ACP:

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 65.5 0.3 2,879 1.9

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate.

DCP-010 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 228 0 0.00 56 2.20 *Seating

1 427 7.8 0.65 199 7.83 199

1 470 9.5 0.79 43 1.69 43

1 503 10.8 0.90 33 1.30 33

1 542 12.4 1.03 39 1.54 39

1 580 13.9 1.15 38 1.50 38

1 611 15.1 1.26 31 1.22 31

1 647 16.5 1.37 36 1.42 36

1 670 17.4 1.45 23 0.91 23

1 700 18.6 1.55 30 1.18 30

1 727 19.6 1.64 27 1.06 27

1 747 20.4 1.70 20 0.79 20

1 763 21.1 1.76 16 0.63 16

1 777 21.6 1.80 14 0.55 14

1 790 22.1 1.84 13 0.51 13

1 808 22.8 1.90 18 0.71 18

1 821 23.3 1.95 13 0.51 13

1 833 23.8 1.98 12 0.47 12

1 850 24.5 2.04 17 0.67 17

1 868 25.2 2.10 18 0.71 18

1 895 26.3 2.19 27 1.06 27

1 926 27.5 2.29 31 1.22 31

1 962 28.9 2.41 36 1.42 36

1 1000 30.4 2.53 38 1.50 38

1 1032 31.7 2.64 32 1.26 32 Ended

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 33.5 0.3 3,739 2.5

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate.

DCP-012 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 206 0 0.00 34 1.34 *Seating

1 282 3.0 0.25 76 2.99 76

1 319 4.4 0.37 37 1.46 37

1 351 5.7 0.48 32 1.26 32

1 384 7.0 0.58 33 1.30 33

1 416 8.3 0.69 32 1.26 32

1 440 9.2 0.77 24 0.94 24

1 462 10.1 0.84 22 0.87 22

1 496 11.4 0.95 34 1.34 34

1 529 12.7 1.06 33 1.30 33

1 562 14.0 1.17 33 1.30 33

1 591 15.2 1.26 29 1.14 29

1 625 16.5 1.37 34 1.34 34 hit rock 1 646 17.3 1.44 21 0.83 21

1 662 18.0 1.50 16 0.63 16

1 699 19.4 1.62 37 1.46 37

1 717 20.1 1.68 18 0.71 18 hit rock 5 735 20.8 1.74 18 0.71 4 ENDED

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 30.3 0.3 3,890 2.6

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate. This material is in situ and not under a pavement.

DCP-014 Blows height, mm penetration, in

Depth

(ft) penetration, mm penetration, in mm/blow

- 201 0 0.00 29 1.14 *Seating

1 349 5.8 0.49 148 5.83 148

1 404 8.0 0.67 55 2.17 55

1 515 12.4 1.03 111 4.37 111

1 592 15.4 1.28 77 3.03 77

1 632 17.0 1.41 40 1.57 40

1 667 18.3 1.53 35 1.38 35

1 692 19.3 1.61 25 0.98 25

1 707 19.9 1.66 15 0.59 15

1 721 20.5 1.71 14 0.55 14

1 737 21.1 1.76 16 0.63 16

1 748 21.5 1.79 11 0.43 11

1 760 22.0 1.83 12 0.47 12

2 771 22.4 1.87 11 0.43 6 Ended

Starting Depth (in) Area:

Average mm/blow Cf Mr (psi)

Base 0.62 --- CBR

Subgrade 43.4 0.3 3,379 2.3

Pavement DCP to Mr Correlation Beta042012EJL from 3/14/11 email by Justin Moderie at ODOT (see last sheet), excerpt from yet to be published Pavement Design Guide

Mr = Cf * 49022.76 * (DCP)-0.39 where: - Must be multiplied by correction factor if used as input into AASHTO, see discussion

- DCP is mm / blow

- Mr is in psi used a Cf = 0.3 for a conservative estimate.

APPENDIX D

Final Pavement Structure Recommendations Memorandum, 2 pages

158377846

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661

DATE: June 3, 2021

TO: David Arena, Project Manager Hunter Goguen, Designer

FROM: Nassim Sabahfar Assistant Materials Engineer

SUBJECT: Final Pavement Structure Recommendations Stehekin Valley Road Emergency Reroute

WA NPS NOCA 10(7)

Background

This memorandum provides final pavement structure recommendations for construction of 0.25 mile long reroute of the Stehekin Valley Road at vicinity of MP 5.5 in Lake Chelan National Recreation Area. The purpose of construction of the reroute is to maintain safe vehicular access on the Stehekin Valley Road by resolving the threat of erosion along the existing road. The road was previously damaged by a winter storm event and has been secured temporarily for the traffic through the use of bank stabilization measures.

The pavement structure recommendation is provided based on geotechnical investigations conducted in May 2021. Geotechnical investigations included hand dug soil test pits (tested at WFL laboratory) and Dynamic Cone Penetration (DCP) testing. No subsurface geotechnical drilling was conducted.

Should new information develop that impacts the information and assumptions made in this recommendation, the pavement design should be reexamined

Subsurface Investigation and Traffic Data

A geotechnical investigation was conducted in May 2021 with DCP test conducted at five locations, five samples tested for soil classification, and one soil sample tested for R-value.

The materials tested were classified as A-1-b and A-2-4, using the AASHTO soil classification system. The R-Value was reported as 67 for the soil sample tested. Based on soil classification and R-Value and DCP test results, the resilient modulus for subgrade soil is assumed as 6,000 psi.

There is no explicit traffic data available for the road segment. The segment is used by local residents and power distribution company when necessary. The average daily traffic (ADT) is assumed as 200 with 5% truck traffic and no growth during the performance period.

Recommended Pavement Structure

The recommended structural pavement section is:

Chip Seal – Type 2B (Item 40702-1200)

6 inches – Roadway Aggregate, Method 2 (Item 30202-2100, estimated at 1.97 tons/cuyd)

12 inches – Unclassified Borrow (Item 20404-0000, estimated at 1.85 tons/cuyd)

Specifications

Materials will provide any required SCRs for inclusion in the plan-in-hand package.

cc: Materials/Pavement File