GM 18-22 Giff Pinchot Geotech Memo_Revised Final.pdf

FINAL

FOR OFFICIAL USE ONLY

Revised Final Geotechnical Memorandum

Gifford Pinchot Repairs 2017, Phase 2

WA ERFO FS GIFPN603 2017-1(2)

Lewis County

Western Federal Lands Highway Division

Contract No. DTFH7015D00001

Task Order No. 69056719F000046

GM 18-22

June 23, 2022

Revised Final Geotechnical Memorandum FINAL Gifford Pinchot Repairs 2017, Phase 2

Contract No. DTFH7015D00001 Task Order No. 69056719F000046

Prepared for: Federal Highway Administration – Western Federal Lands Highway Division J:\Dcs\Projects\Trn\60601124_Wfl_Giff_Erfo_17\400_Technical\435_Geotechnical\Report\05 Final Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

AECOM Technical Services, Inc.

Page i

Prepared for:

Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661-3801

Prepared by:

1111 Third Avenue, Suite 1600 Seattle, Washington 98101

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Page ii

Quality information

Prepared by Checked by Approved by

Mahendra Shewalla, P.E., PMP Dan Meier, C.E.G. Bryan Huschka, P.E.

Revision History

Revision Revision date Details Authorized Name Position

0 Nov, 2019 Preliminary Geotech Memo

Nov, 2019 Bryan Huschka Project Manager

1 Mar, 2020 Final Geotech Memo

Mar, 2020 Bryan Huschka Project Manager

2 May, 2022 Revised Final Memo

May, 2022 Bryan Huschka Project Manager

3 June, 2022 Revised Final Memo 2

June, 2022 Bryan Huschka Project Manager

Distribution List

# Hard Copies PDF Required Association / Company Name

0 X Carrie Warren (Project Manager)/ WFLHD

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Page iii

Table of Contents

Part 1. Introduction

Part 2. Project Description

Part 3. Geotechnical Scope of Work

3.1 Field Inspection and Data Collection Methods

Part 4. Regional Geologic Setting

4.1 Geologic Units

4.2 Geologic Hazards

4.2.1 Debris Flow

4.2.2 Landslides

4.2.3 Earthquakes

Part 5. Field Reconnaissance Summary

5.1 Field Reconnaissance Team and Summary

5.2 Field Reconnaissance Dates

Part 6. Field Reconnaissance Observations and Design Recommendations

6.1 Repair Site 1 – FR 90, Mile Post 44.8

6.1.1 Site Conditions

6.1.2 Design Recommendations

6.2 Repair Site 2 – FR 88, Mile Post 19.8

6.2.1 Site Conditions

6.2.2 Design Recommendations

6.3 Repair Site 3 – FR 88, Mile Post 19.7

6.3.1 Site Conditions

6.3.2 Design Recommendations

6.4 Repair Site 4 – FR 90, Mile Post 28.0

6.4.1 Site Conditions

6.4.2 Design Recommendations

6.5 Repair Site 5 – FR 90, Mile Post 11.5

6.5.1 Site Conditions

6.5.2 Design Recommendations

6.6 Repair Site 6 – FR 54, Mile Post 26.9

6.6.1 Site Conditions

6.6.2 Design Recommendations

6.7 Repair Site 7 – FR 63, MP 12.0

6.7.1 Site Conditions

6.7.2 Design Recommendations

6.8 FR 99, All Repair Sites

Part 7. Limitations and Closure

Part 8. References

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Page iv

Figures

Figure 1 - Site Vicinity Map Figure 2 – Map of Each Repair Site Figure 3 – Repair Site 1 – FR 90, MP 44.8 – Damaged Area Plan View Figure 4 – Repair Site 1 – FR 90, MP 44.8 – Existing Conditions and

Temporary Embankment Excavation Figure 5 – Repair Site 1 – FR 90, MP 44.8 – Section A-A’ Proposed Repair Figure 6 – Repair Site 2 – FR 88, MP 19.8 – Site Plan and Profile of Existing

Conditions Figure 7 – Repair Site 2 – FR 88, MP 19.8 – Section B-B’ and Section C-C’

Proposed Repair Figure 8 – Repair Site 3 – FR 88, MP 19.7 – Site Plan of Existing Conditions Figure 9 – Repair Site 3 – FR 88, MP 19.7 – Section A-A’ Existing Conditions and Proposed Repair Figure 10 – Repair Site 3 – FR 88, MP 19.7 – Section B-B’ Existing

Conditions and Proposed Repair Figure 11 – Repair Site 4 – FR 90, MP 28.0 – Site Plan of Existing Conditions Figure 12 – Repair Site 4 – FR 90, MP 28.0 – Section A-A’ ’ Existing

Conditions and Proposed Repair Figure 13 – Repair Site 4 – FR 90, MP 28.0 – Section B-B’ and Section C-C’

Existing Conditions and Proposed Repair Figure 14 – Repair Site 5 – FR 90, MP 11.5 – Site Plan of Existing Conditions Figure 15 – Repair Site 5 – FR 90 MP 11.5 – Section A-A’ Existing Conditions Figure 16 – Repair Site 5 – FR 90, MP 11.5 – Section A-A’ Proposed Repair Figure 17 – Repair Site 6 – FR 54, MP 26.9 – Section A-A’ Proposed Repair Figure 18 – Repair Site 6 – FR 54, MP 26.9 – Section A-A’ Proposed Repair Figure 19 – Repair Site 7 – FR 63, MP 12.0 – Existing Site Plan Figure 20 – Repair Site 7 – FR 63, MP 12.0 – Design Recommendation, Section A-A’ Figure 21 – FR 99 Repair Sites –Proposed Repair

Tables

Table 1. Repair Sites Identified by FHWA and USFS

Table 2. Field Reconnaissance Team Members

Photos

Photo 1: Complete Washout of Embankment at Repair Site 2, viewing north

Photo 2: Complete Washout of Embankment at Repair Site 2, viewing north

Photo 3: Complete Washout of Embankment at Repair Site 2, viewing north

Photo 4: Embankment material, debris, and cobbles at Repair Site 2, viewing south

Photo 5: Embankment Washout at Repair Site 2. Geotextile fabric beneath roadway, viewing north

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Page v

Photo 6: Pavement crack north of eroded area at Repair Site 3, viewing north

Photo 7: Dip and lateral deflection in the unpaved road at Repair Site 4, viewing south

Photo 8: Photo showing rock placement at Repair Site 4, viewing east

Photo 9: Leaning trees at Repair Site 4, viewing north

Photo 10: Existing Conditions at Repair Site 5, viewing north

Photo 11: Existing Conditions at Repair Site 6, viewing east

Photo 12: Existing Embankment Material at Repair Site 6

Photo 13: Google Earth image of Existing Conditions at Repair Site 6

Photo 14: Smaller road prism slump, 95 feet south of cut slope failure, viewing northwest

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Page vi

Acronyms and Abbreviations

AECOM AECOM Technical Services, Inc.

AOP Aquatic Organism Passage

CAB Crushed Aggregate Base

CMP Corrugated Metal Pipe

CSZ Cascadia Subduction Zone

DSR Damage Survey Report

ERFO Emergency Relief for Federally Owned Road Program

FHWA Federal Highway Administration

FR Forest Road

H:V Horizontal to Vertical

MP Mile Post

OSHA Occupational Safety and Health Administration

FLH PDDM Federal Lands Highway Project Development and Design Manual

PGA Peak Ground Acceleration

USDOT United States Department of Transportation

USFS Gifford Pinchot National Forest

USGS United States Geological Survey

WFLHD Western Federal Lands Highway Division

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Part 1. Introduction

This report presents the results of our geotechnical studies for the Gifford Pinchot Repairs 2017 project [WA ERFO FS GIFPN603 2017(1)-2] within the Gifford Pinchot National Forest (USFS) in Lewis, Skamania, and Klickitat Counties. AECOM Technical Services, Inc. (AECOM) performed these services for the Federal Highway Administration (FHWA), Western Federal Lands Highway Division (WFLHD) in accordance with the Statement of Work under Contract Number DTFH7015D00001, Task Order 69056719F000046.

Part 2. Project Description

The USFS experienced a heavy rain event during the spring of 2017. This event caused major flooding and damage to the Forest Roads (FR) throughout the USFS. Following the storm, FHWA and USFS personnel identified 14 Repair Sites on USFS property within Lewis, Skamania, and Klickitat Counties, as shown in Table 1 below. At each Repair Site, the FHWA and USFS personnel completed Damage Site Reports (DSRs) with descriptions of observed roadway damage, preliminary mitigation recommendations, and estimated construction costs.

Table 1. Repair Sites Identified by FHWA and USFS

Forest Road & Mile Post Forest Road & Mile Post

FR 54 MP 26.9 FR 99 MP 4.9

FR 63 MP 12.0 FR 99 MP 5.1

FR 88 MP 19.8 FR 99 MP 5.3

FR 88 MP 19.7 FR 99 MP 5.4

FR 90 MP 11.5 FR 99 MP 7.5

FR 90 MP 28.0 FR 99 MP 8.4

FR 90 MP 44.8 FR 99 MP 8.69

Source: compiled by AECOM in 2019

The purpose of this project is to develop a 70% design for construction of each of the 14 Repair Sites to pre-disaster conditions. The 70% designs are based on the DSRs where appropriate. This memorandum presents the geotechnical reconnaissance observations and the geotechnical recommendations for each of the Repair Site visited. The Site vicinity map of each Repair Site visited during the site reconnaissance is shown in Figure 1, and map of each Repair Site is shown in Figure 2.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Part 3. Geotechnical Scope of Work

3.1 Field Inspection and Data Collection Methods

The scope of work for this project consists of a geotechnical reconnaissance of each of the accessible identified sites, review of existing DSRs, and preparation of this geotechnical memorandum. The higher elevation sites associated with FR 63 MP-12 and all of the Road 9900 sites were inaccessible during the May 16, 2019 geotechnical reconnaissance due to excessive snow.

Geotechnical reconnaissance tasks include:

• Completing a visual geotechnical reconnaissance;

• Locating geological hazards or any other geologic or geotechnical constraints;

• Identifying and locating landslides, fill slope failures or slumps, areas of wet or soft soils, oversteepened or undercut slopes;

• Collecting information for design recommendations of roadway repairs; and

• Considering site access constraints for equipment access, if necessary.

Reporting tasks include:

• Description of site conditions observed during reconnaissance;

• Review of conceptual designs in DSRs;

• Providing design recommendations to update or amend conceptual designs in the DSRs for development of 70% design plans;

• Identifying geotechnical constraints (geologic hazards, etc.) and providing recommendations to avoid or mitigate constraints; and

• Identifying available active commercial material sources within a one-hour (one-way) distance from the Repair Site.

Part 4. Regional Geologic Setting

4.1 Geologic Units

The Repair Sites are located in the Cascade Range physiographic province of Washington, part of an active volcanic arc characterized by extensive surface exposures of volcanic rocks and volcanic-derived sedimentary rocks generally younger than about 40 million years old (Schuster, 2005; Korosec, 1987; Philips, 1987).

Bedrock units are typically mantled by geologically recent deposits of alluvium (river-emplaced), colluvium (gravity-emplaced on hillsides), landslide (mass-wasting), debris flow (debris torrent-emplaced), alpine glacial deposits (ice-deposited), and fill (man-made) deposits. Alluvium is typically located in the valleys of significant rivers in the region (Cispus, Lewis, etc.) and typically consists of roughly horizontally stratified and sorted accumulations of silt to boulder-sized sediments emplaced in active channels and floodplains, sometimes by glacial meltwaters.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Colluvium is present above bedrock at many hillside locations, and typically consists of poorly sorted mixtures of clay to boulder-sized sediments emplaced by weathering and gravity processes on hillsides.

Fill sediments are man-made accumulations of soil and other materials. We observed fill materials in roadway embankments at sites visited. On low-volume forest roads, fills are typically derived from various local sources of various quality and may include clay- to boulder-sized materials. Site geology details for each individual site are discussed in Part 6.

4.2 Geologic Hazards

4.2.1 Debris Flow

The Repair Sites are located in an area of generally high topographic relief, adjacent to Mount Adams and Mount Saint Helens, two cascade strato volcanos that are greater than 12,000 feet and 8,000 feet in elevation, respectively. The area is traversed by several deeply incised stream valleys that serve as tributaries to the Lewis and Cispus Rivers. The steep terrain, combined with widespread deposits of poorly consolidated volcaniclastic sediments and high annual snow and rainfall amounts, provide prime conditions for debris flow activity.

A debris flow is a viscous, flowing mixture of water, mud, rock fragments, and organic debris transported downslope during heavy rainfall or snowmelt runoff events. Debris flow deposition areas, or debris fans, form through repeated debris flow deposition events, most often at the base of steep intermittent drainages or tributary drainages.

Debris flow events commonly bury roadways and/or plug culvert inlets. Where culvert inlets are plugged, flood waters can overtop and wash out the embankment.

During the reconnaissance, evidence of debris flow activity was observed at Road 5400 MP 26.9, FR 88 MP 19.8, FR 88 MP 19.7, and FR 90 MP 44.8. These debris flows may have contributed to the observed roadway damage, though they were not necessarily the main causative factor. Repairs at all sites should consider possible effects of future debris flows, as debris flows are a common geologic hazard on USFS property.

4.2.2 Landslides

Landslides are mapped in the region, and consist of unstable, poorly sorted mixtures of clay to boulder-sized sediments that slide down hillsides in discrete masses. Large, deep-seated landslides, sometimes measured in miles, are common in the project vicinity. It is common for smaller slumps and translational landslides to occur within these larger complexes. The damage at FR 90 MP 28.0 is due to a smaller slump feature located within a large landslide complex.

4.2.3 Earthquakes

Regionally the site lies within the influence of the Cascadia Subduction Zone (CSZ). In the CSZ, the Juan de Fuca Plate is being subducted underneath the North American plate. The subduction is occurring in the coastal region between Vancouver Island and

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx northern California. The Pacific Northwest has four types of seismic sources due to the presence of the CSZ. These sources include (Wong and Silva, 1998):

1. The subduction zone megathrust, which represents the boundary (interface) between the subducting Juan de Fuca plate and the overriding North American plate;

2. Faults located within the Juan de Fuca plate (referred to as the intraplate or intraslab region);

3. Crustal faults, principally in the North American plate; and

4. Volcanic sources beneath the Cascade Range.

Based on probabilistic seismic hazard deaggregation from the United States Geological Survey (USGS) Earthquake Unified Hazard Tool maps, the nearest source-to-site distance of a M = 9.0 CSZ source is approximately 100 miles from the project site, and numerous M = 5.0 to 7.5 sources are located less than 5 miles from many of the various Repair Sites. Based on USGS mapping for the 2% in 50-year earthquake event (2,475-year mean return period), the Peak Horizontal Ground Acceleration (PGA) exceeds 0.3g throughout the project site, which represents relatively high-intensity earthquake shaking.

On this basis, the project site lies within an active seismic area, and may be subject to strong earthquake ground motions. Accordingly, earthquake-induced ground instabilities (liquefaction, lateral spreading, fault rupture, landslides, etc.) are possible in the event of an earthquake.

The repair strategies described herein are not intended to mitigate earthquake-related hazards. Earthquake mitigation is beyond the scope of this evaluation. Further, earthquake hazards mitigation for the extensive National Forest Road system is expected to be economically infeasible given current funding availability. However, it is important to note that future earthquake activity could initiate instability at areas where repairs have been made and/or areas where instability has not been observed to date.

Part 5. Field Reconnaissance Summary

The following discussion presents an overview of the reconnaissance investigation summary and date of investigation.

5.1 Field Reconnaissance Team and Summary

The team members for the field reconnaissance are listed in Table 2. AECOM performed the investigation using a multi-disciplinary team of qualified personnel.

Representatives from the WFLHD and USFS accompanied the AECOM field team. The investigation was performed by driving to each Repair Site and inspecting the accessible and visible areas of the site.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Table 2. Field Reconnaissance Team Members

Role Name Organization

WFLHD Project Manager Brandon Stokes, P.E. WFLHD

WFLHD Environmental/Permitting Cyrus Bullock WFLHD

AECOM Project Manager Tom Gut, P.E. AECOM

Hydraulics Engineer Stephen Blanton, P.E. AECOM

Geotechnical Engineer Mahendra Shewalla, P.E. AECOM

Environmental/Permitting Linda Howard AECOM

USFS Representative Mike Dolan USFS

USFS Representative Rithy Bein USFS

Source: compiled by AECOM in 2019

The geotechnical engineer made visual observations of geologic conditions, potential geologic hazards, and existing structures. While onsite, the geotechnical engineer evaluated the preliminary design recommendations provided in the individual DSRs.

The reconnaissance team also made general observations regarding site access constraints for construction equipment if AECOM determined that subsurface explorations would be required for final design. Reconnaissance observations and geotechnical design recommendations for each site are discussed in Part 6 of this report.

5.2 Field Reconnaissance Dates

Field reconnaissance of six (6) Repairs Sites was completed on May 16, 2019. The Repair Sites are given below in the order the sites were visited.

• Repair Site 1 – FR 90 MP 44.8

• Repair Site 2 – FR 88 MP 19.8

• Repair Site 3 – FR 88 MP 19.7

• Repair Site 4 – FR 90 MP 28.0

• Repair Site 5 – FR 90 MP 11.5

• Repair Site 6 – FR 54 MP 26.9

All FR 99 Repair Sites were not accessible for reconnaissance due to winter snow. The snow on FR 63 does not melt until end of June. Site visit for geotechnical reconnaissance was performed on July 11, 2019 after the USFS personnel confirmed site accessibility. The site visit on July 11, 2019 was performed by Mahendra Shewalla and Stephen Blanton of AECOM. WFLHD and USFS Representatives did not accompany the AECOM field team.

Snow on all FR 99 Repair Sites does not melt until late June or early July, and the field reconnaissance will not be performed for these sites as a part of this project. According

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx to the DSRs, all Road 9900 Repair Sites are pavement repairs. AECOM will rely on the measurements in the DRSs to provide the design recommendations.

Part 6. Field Reconnaissance Observations and

Design Recommendations

6.1 Repair Site 1 – FR 90, Mile Post 44.8

6.1.1 Site Conditions

Damage to Repair Site 1 consists of failed roadway embankment on the downstream side (Photo 1).

Site reconnaissance suggests that the road failure was likely due to structural failure of the downstream 24-inch-diameter Corrugated Metal Pipe (CMP) culvert, which caused piping of the embankment material on the downstream side of the road.

Approximately 40 feet of embankment (parallel to the roadway) to a depth of about 15 feet below the existing road surface washed out on the downstream side (Photo 1).

Figure 3 shows the site plan of the existing conditions. Figure 4 shows the profile view of the existing conditions. It appears that the culvert separated before or during the high-flow event, leading to the internal erosion of the embankment and subsequent failure. The existing intact culvert under the embankment on the downstream side is covered by debris and could not be seen.

The exposed embankment material varies from low plasticity Sandy Silt to non-plastic Silty Sand with gravel and cobbles. The existing asphalt is approximately 7.5 inches thick. Crushed Aggregate Base (CAB) was not observed under the asphalt.

The inlet of the existing 24-inch-diameter CMP culvert is covered by woody debris and is not visible, but the flow appears to be reaching the inlet (Photo 2). The debris obscuring the culvert inlet may restrict full function of the culvert during high flows events.

Available geologic mapping at the site (Korosec, 1987), indicates that the area is underlain by Quaternary alluvial deposits and volcanic rocks of the Miocene age (approximately 5.3 to 23- million-year old) Council Bluff volcanic group.

Photo 1: Complete Washout of Embankment at Repair

Site 2, viewing north

Failed 24-inch-diameter culvert

Approx. 40 feet wide and

15 feet deep

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

6.1.2 Design Recommendations

The DSR for this Repair Site recommends:

• Excavating the existing road down to the 24-inch-diameter CMP culvert at 1 Horizontal to 1 Vertical (1H:1V) temporary slopes;

• Removing and replacing the existing culvert with a 36-inch-diameter CMP culvert sized for the 100-year event; and

• Reconstructing the embankment to match existing adjacent slopes (1.25H:1V slopes on upstream side and 1H:1V slopes on downstream side) and road to pre-failure conditions.

The preliminary recommendations and material quantities provided in the DSR are generally appropriate for this site. However, AECOM recommends constructing a reinforced soil slope on the downstream side due to steep downstream slope, which is discussed later in this section. The temporary excavation slopes as steep as 1H:1V are not recommended based on the observed soil types. The U.S.

Department of Transportation (USDOT) and FHWA, 2018, Federal Lands Highway Project Development and Design Manual (PDDM), Section 6.4.4.7 –

Temporary Cuts and Shoring, require that maximum temporary un-shored slope heights and ratios be based on observations, experience, and representative limit equilibrium slope stability analysis (when observations and experience are not conclusive). In general, and particularly if personnel are required to enter excavations, temporary excavations should comply with the Occupational Safety and Health Administration (OSHA) guidelines contained in 29 CFR Part 1926, Subpart P. Given the low cohesion and sandy nature of the materials observed in the exposed embankment, AECOM anticipates the soils will be classified as OSHA Type C soils, which have maximum permissible temporary slope inclination of 1.5H:1V for cuts higher than 4 feet. Flatter slopes may be necessary if groundwater seepage is encountered and/or heavy construction surcharges are required near the top of the excavation (equipment, stockpiled materials, etc.). A competent professional (i.e. geotechnical engineer or engineering geologist, licensed in the State of Washington), should inspect the cut slopes prior to any personnel entering the excavations. If the slopes are deemed unsuitable by the competent professional during and/or after embankment excavation, we recommend flattening the excavation slopes. Accordingly, the estimated material

Photo 2: Complete Washout of Embankment at Repair

Water entering the

Inlet. Inlet not visible

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx quantities should be updated to account for the additional excavation and backfill volumes.

The exposed subgrade should be compacted to attain a dense and unyielding surface in accordance with FHWA (2014) Standard Specification Section 204.09. We recommend placing bedding material prior to placement of the new culvert.

Based on visual inspection of embankment material exposed in the failure area, the excavated embankment material will be suitable for reuse, as long as the material is free from organics or any unsuitable material.

For additional embankment backfill material, the native material available near the site is not appropriate for use as embankment fill due to high content of organic material and debris. We recommend importing free draining borrow material in accordance with FHWA (2014) Standard Specification Section 704.06, for embankment construction.

Use of free draining borrow would minimize the effort to achieve embankment compaction, improve embankment stability (especially in areas where permanent embankment slopes are steeper than 1.5H:1V), and reduce the likelihood of elevated pore water pressures within the embankment by providing a free-draining fill medium.

The nearest developed rock pit to the site is the Baby Shoe Rhyolite pit located approximately six miles north of Repair Site 1 via Roads 90, 23, and 335 (USFS, 2013).

This is considerably closer than the nearest developed sand and gravel pit, which is the Blue Lake Creek Alluvium Pit approximately 20.5 miles north via Roads 90, 23, and 2801 (USFS, 2013). The relative proximity and resultant decrease in haul times may offset higher unit costs associated with processing the rock material.

As mentioned earlier, we recommend constructing the downstream side of the embankment with reinforced soil slope due to 1H:1V slopes on the downstream side.

AECOM presents following design recommendations for reinforced soil slope construction on downstream slopes in accordance with FHWA (2014) Standard Specification Section 261. For the repair required to reopen FR 90, MP 44.8, AECOM recommends construction of reinforced soil slopes with 1H:1V, as shown in Figure 5 and as described below:

• Remove the failed embankment and clear the debris material and unwanted vegetation along the failed section to expose the native material.

• If soft areas are encountered in the exposed native material, they should be compacted to attain a dense and unyielding surface in accordance with FHWA (2014) Standard Specification Section 204.09.

• Reconstruct the upstream and downstream of the embankment, as shown in Figure 5. Downstream side of the embankment should be reinforced soil slopes in accordance with FHWA (2014) Standard Specification Section 261.

• Embankment should be constructed using available embankment material, if debris free, and imported free draining borrow material. Embankment should be compacted over compacted subgrade and benched into the existing subgrade in accordance with FHWA (2014) Standard Specification Sections 204.11 and

204.13 respectively. The bench height should be 3 feet and the bench width will vary based on the existing ground slope or cut slope.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

• The reconstructed embankment be benched into the existing embankment in accordance with FHWA (2014) Standard Specification Sections 204.13.

• For downstream side of the reconstructed embankment, Class 2, riprap in accordance with Table 705-1 of FHWA (2014) Standard Specification) should be used for reinforced soil slope construction. The thickness of placed riprap should be 3 feet.

• 12”x12” – W5.5xW5.5 fabricated steel welded wire reinforcement in accordance with FHWA (2014) Standard Specification Section 709.01(h) should be used in the riprap and compacted embankment at a vertical spacing of 3 feet along the reinforced soil slope.

• Type 4 stabilization geogrid in accordance with FHWA (2014) Standard Specification Section 714.04 and Table 714-6 should be wrapped at the face of the compacted embankment with a vertical spacing of 1.5 feet. The geogrid should be embedded a minimum of 20 feet into the reconstructed embankment on top and bottom of the wrap.

• Separation-stabilization geotextile, class I, type A (unwoven) in accordance with FHWA (2014) Standard Specification Section 714.01 and Table 714-1 should be wrapped inside the wrapped primarily and secondary stabilization geogrid to prevent erosional undercutting of compacted embankment below the engineered angular riprap.

We recommend removing the woody debris from the inlet and armoring the embankment around the inlet and outlet of the culvert with riprap in accordance with FHWA (2014) Standard Specification Section 251.04 to reduce erosion potential during future high flow events. As indicated above, the nearest developed rock pit to the site is the Baby Shoe Rhyolite pit located approximately six miles north of the Repair Site 1 via Roads 90, 23, and 335 (USFS, 2013).

The new embankment should be seeded in accordance with FHWA (2014) Standard Specification Sections 713.04 to minimize erosion along the constructed embankment slopes. New pavement and edge areas should be sloped to provide rapid drainage of surface water. Water allowed to pond on or adjacent to the pavement areas could cause saturation of compacted fill and subgrade material resulting in reduced strength which could contribute to premature pavement deterioration.

Erosion control efforts during construction should be adopted in accordance with WFLHD PDDM (2018) Section 9.5.6. We do not anticipate that subsurface explorations will be required for design of this repair.

6.2 Repair Site 2 – FR 88, Mile Post 19.8

6.2.1 Site Conditions

The DSR for this site, which was based on initial site reconnaissance, performed by USFS and FHWA personnel in 2017, does not indicate a likely cause of the partial failure of the upstream embankment slope. Based on descriptions and photos of the damage provided in the DSR, it appears that saturation of the upstream embankment fill

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx and / or internal piping of fill material caused a circular slump failure involving over 50% of the roadway width. The failed mass buried the upstream (inlet) end of the 30” CMP culvert. At the time of the original inspection, the downstream embankment and outlet culvert remained intact, as indicated by the DSR photographs.

Subsequent to the original reconnaissance, additional damage has occurred at the site.

At the time of the most recent reconnaissance (May, 2019), the entire embankment prism along approximately 25 feet of roadway was completely washed out to a depth of 15 to 20 feet below the existing intact road level.

Approximately 40 feet of the road south of the washed out embankment is partially washed out or displaced (Photo 3). Figure 6 shows the existing conditions at the Repair Site. The culvert outlet was not visible during site reconnaissance and is mostly likely buried by the failed embankment material.

The exposed embankment material varies from low-plasticity Sandy Silt to non-plastic Silty Sand with gravel and cobbles up to 8 inches (Photo 4). The existing asphalt is approximately 0.5 inches thick, and CAB is about 4 inches thick.

Available published geologic mapping (Korosec, 1987) indicates Repair Site 2 is underlain by Lower Miocene-age (less than approximately 23-million-year-old) volcanic-derived sedimentary rocks of the Eagle Creek Formation. Bedrock was not observed at this site.

6.2.2 Design Recommendations

The DSR recommends replacing the damaged culvert with an Aquatic Organism Passage (AOP) culvert and reconstructing the failed upstream portion of the embankment. The DSR recommendations may have been appropriate for this site at the time the DSR was written. However, as described in Part 6.2.1, the site conditions have further deteriorated since the DSR was written. In addition to the erosion that was initially observed on the upstream side of the embankment, approximately 25 feet of road is completely washed out and an additional 40 feet of the road to the south is completely or partially damaged. For this reason, the material quantities provided in the DSR are not accurate.

The FHWA Geotechnical Memorandum (2019) for this Repair Site recommends rebuilding the roadway to pre-storm conditions with embankment slopes of

Photo 3: Complete Washout of Embankment at Repair

Embankment washed out

Damaged Pavement and Subgrade

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx approximately 1.75H:1V. AECOM believes the final embankment slope recommendations are appropriate for this site.

Removal of disturbed material and replacement of the existing culvert will require excavation of temporary slopes. In general, and particularly if personnel are required to enter excavations, temporary excavations should comply with the OSHA guidelines contained in 29 CFR Part 1926, Subpart P. Given the low cohesion and sandy nature of the materials observed in the exposed embankment, AECOM anticipates the soils will be classified as OSHA Type C soils, which have maximum permissible temporary slope inclination of 1.5H:1V for cuts higher than 4 feet. Flatter slopes may be necessary if groundwater seepage is encountered and/or heavy construction surcharges are required near the top of the excavation (equipment, stockpiled materials, etc.). A competent professional (i.e. geotechnical engineer or engineering geologist, licensed in the State of Washington), should inspect the cut slopes prior to any personnel entering the excavations (i.e. for preparation and installation of the new culvert). If the slopes are deemed unsuitable by the competent professional during and/or after embankment excavation, we recommend flattening the excavation slopes. Accordingly, the estimated material quantities should be updated to account for the additional excavation and backfill volumes.

The exposed subgrade should be compacted to attain a dense and unyielding surface in accordance with FHWA (2014) Standard Specification Section 204.09. We recommend placing bedding material prior to placement of the new AOP culvert. A sketch of the proposed repair is shown in Figure 7. AECOM recommends that the compacted embankment fill material be placed over compacted subgrade and benched into the existing embankment in accordance with FHWA (2014) Standard Specification Sections 204.11 and 204.13 respectively.

Based on visual inspection of embankment material exposed in the failure area, the excavated embankment material (which will be limited in volume) will be suitable for reuse, as long as the material is free from organics or any unsuitable material.

For additional embankment backfill material, the native material available near the site is not appropriate for use as embankment fill due to high content of organic material and debris. We recommend importing free draining borrow for embankment construction.

Use of free draining borrow would minimize the effort to achieve embankment compaction, improve embankment stability (especially in areas where embankment slopes are steeper than 1.5H:1V), and reduce the likelihood of elevated pore water

Photo 4: Embankment material, debris, and cobbles at

Repair Site 2, viewing south

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx pressures within the embankment by providing a free-draining fill medium. The nearest rock quarry is the Island Shelter pit approximately 3.2 miles west along Road 88 (USFS, 2013). No developed sand and gravel pits are known within a one-hour drive of Repair Site 2. The relative proximity of the Island Shelter pit, and resultant decrease in haul times, may offset higher units costs associated with processing the rock material.

We also recommend armoring the embankment around the replacement culvert inlet and outlet with riprap in accordance with FHWA (2014) Standard Specification Section

251.04 to reduce erosion potential during future high flow events. Riprap can likely be derived at least in part from the deposits near the site. The nearest developed rock pit to the site is the Island Shelter Andesite pit located approximately 3.2 miles north of the Repair Site 2 via Roads 88 and 32 (USFS, 2013).

New pavement and edge areas should be sloped to provide rapid drainage of surface water. Water allowed to pond on or adjacent to the pavement areas could cause saturation of compacted fill and subgrade material resulting in reduced strength which could contribute to premature pavement deterioration.

Erosion control efforts during construction should be adopted in accordance with FLH PDDM (2018) Section 9.5.6.

We do not anticipate that subsurface explorations will be required for design of this repair.

6.3 Repair Site 3 – FR 88, Mile Post 19.7

6.3.1 Site Conditions

The DSR and recent site reconnaissance of this Repair Site indicates a slump and/or possible piping erosion around a 30-inch-diameter CMP culvert removed most of an approximately 25-foot-high roadway embankment (Photo 5). A schematic plan of the existing conditions is shown in Figure 8.

The asphalt-paved roadway is approximately 28 feet wide. We observed geotextile fabric beneath the roadway base course. Embankment slopes to the upstream side of the culvert are about 1.25H:1V, and downstream embankment slopes adjacent to the erosion area are about 2H:1V. The 30-inch-diameter CMP culvert may be damaged or separated in the subsurface. Observed discharge from the culvert appeared to be less than observed inflow.

We observed a pavement tension crack with up to 6 inches of offset extending about 60 feet to the north of the erosion area (Photo 6). A small slump or surface erosion area was present on the downstream embankment slopes adjacent to this tension crack. A 1986 USFS “Road Management Objective Report” included in the DSR recommends monitoring a “mass movement area”, indicating a history of ongoing movement in this area.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Available published geologic mapping (Korosec, 1987) indicates Repair Site 3 is underlain by Lower Miocene-age (less than approximately 23-million-year-old) volcanic-derived sedimentary rocks of the Eagle Creek Formation. We did not observe indications of bedrock at this site.

Embankment fill soils exposed within the slump consist of moist, low plasticity Sandy Silt with gravel and cobbles. The embankment appeared to overlie colluvium soils where visible away from the slumped materials. The slump appears to be confined to embankment fill materials.

6.3.2 Design

Recommendations

The DSR for Repair Site 3 recommends removal of unsuitable failed materials in the slump scar and replacement of the downstream 40 feet of the 30-inch-diameter CMP culvert. The DSR recommends embankment reconstruction with suitable material compacted in lifts, and placement of a 6-inch-thick aggregate driving surface on the reconstructed embankment fill.

The DSR recommendations are generally appropriate. Removal of disturbed material and replacement of the damaged portion of the existing culvert will require excavation of temporary slopes. In general, and particularly if personnel are required to enter excavations, temporary excavations should comply with the OSHA guidelines contained in 29 CFR Part 1926, Subpart P. Given the low cohesion and sandy nature of the materials observed in the exposed embankment, AECOM anticipates the soils will be classified as OSHA Type C soils, which have maximum permissible temporary slope inclination of 1.5H:1V for cuts higher than 4 feet. Flatter slopes may be necessary if groundwater seepage is encountered and/or heavy construction surcharges are required near the top of the excavation (equipment, stockpiled materials, etc.). A competent professional (i.e. geotechnical engineer or engineering geologist, licensed in the State of Washington), should inspect the cut slopes prior to any personnel entering

Photo 5: Embankment Washout at Repair Site 2.

Geotextile fabric beneath roadway, viewing north

Photo 6: Pavement crack north of eroded area at Repair

Site 3, viewing north

Tension Crack

Geotextile Fabric

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx the excavations (i.e. for preparation and installation of the new culvert). If the slopes are deemed unsuitable by the competent professional during and/or after embankment excavation, we recommend flattening the excavation slopes. Accordingly, the estimated material quantities should be updated to account for the additional excavation and backfill volumes.

Following removal of the unsuitable failed materials, an inspection of the full length of the culvert should be conducted to determine whether partial or complete replacement of the culvert is required.

The exposed subgrade should be compacted to attain a dense and unyielding surface in accordance with FHWA (2014) Standard Specification Section 204.09. We recommend placing bedding material prior to placement of the new culvert. AECOM recommends that the compacted embankment fill material be placed over compacted subgrade and benched into the existing embankment in accordance with FHWA (2014) Standard Specification Sections 204.11 and 204.13 respectively. Sketches of the proposed repair are shown in Figure 9 and Figure 10.

Onsite materials will not be suitable for use in the embankment repair. AECOM recommends importing free draining borrow material in accordance with FHWA (2014) Standard Specification Section 704.06 for embankment construction. Use of free draining borrow material would minimize the effort to achieve embankment compaction, improve embankment stability (especially in areas where embankment slopes are steeper than 1.5H:1V), and reduce the likelihood of elevated pore water pressures within the embankment by providing a free draining fill medium. The nearest rock quarry is the Island Shelter pit approximately 3.3 miles west along Road 88 (USFS, 2013). No developed sand and gravel pits are known within a one-hour drive of Repair Site 3. The relative proximity of the Island Shelter pit, and resultant decrease in haul times compared to conventional fill sources, may offset higher unit costs associated with processing the rock material.

Within the tension crack area north of the slump, we recommend that the upper three feet of embankment fill be removed and replaced with free draining borrow material in accordance with FHWA (2014) Standard Specification Section 704.06, compacted in accordance with FHWA (2014) Standard Specification Sections 204.11. We also recommend placing riprap (1) in the slump or surface erosion area on downstream embankment slopes adjacent to the tension crack, and (2) at the culvert inlet and outlet.

The nearest developed rock pit to the site is the Island Shelter Andesite pit located approximately 3.3 miles north of the Repair Site 3 via Roads 88 and 32 (USFS, 2013).

No developed sand and gravel pits are known within a one-hour drive of Repair Site 3.

Materials could potentially be screened from crushed rock aggregate. The nearest rock quarry is the Island Shelter pit approximately 3.3 miles west along Road 88 (USFS, 2013). Alternately, the Pine Pit is located about 28 miles south of Repair Site 3 via Roads 25, 90, and 88 (USFS, 2013). The Pine Pit site is reportedly cut into lahar and mudflow deposits and will need to be investigated to determine if it can provide sound material for use as RSS backfill.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

We do not anticipate that subsurface explorations will be required for design of this repair.

6.4 Repair Site 4 – FR 90, Mile Post 28.0

6.4.1 Site Conditions

Localized activity within a large mapped landslide complex has caused the unpaved roadway to settle up to four feet vertically and a few feet laterally at three locations along approximately 800 feet of the roadway alignment (Photo 7).

Multiple culverts have been placed to drain low spots on the upstream side of the road. Mass wasting material is accumulating in multiple locations within the ditch on the upstream side of the road, creating ponding. Ditch maintenance was being conducted during the site visit to remove material and restore the flow line.

The removed material was placed upslope of the ditch.

Slopes upstream of the roadway stand at angles between 1.5H:1V and 2H:1V. Slopes downhill of the roadway vary from 4H:1V to 2H:1V. We observed riprap/rock placement at two uphill slope locations (Photo 8), indicating historic ground movement at these locations.

Available geologic mapping at this Repair Site (Korosec, 1987) indicates a large landslide measuring approximately 6,000 feet long (perpendicular to the road) and 1,700 feet wide (parallel to the road). The current roadway settlement is interpreted as a smaller slump / translational slide occurring within the larger mapped landslide unit.

Observed surficial conditions at the site include wet, poorly sorted soils consisting of clayey sand with angular gravel to boulder-sized blocks consistent with landslide debris.

Photo 7: Dip and lateral deflection in the unpaved road at Repair Site 4, viewing south

Photo 8: Photo showing rock placement at Repair Site

4, viewing east

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

Several mature trees, located within the failure area above and below the roadway are leaning (Photo 9), suggesting long-term downhill soil creep consistent with landslide movement. A demarcation between the leaning trees and generally vertical, mature trees further upslope from the roadway is an indicator of the upslope limits of the most recent movement. Hydrophilic vegetation covered the downhill roadway shoulder, suggesting shallow groundwater in the area.

6.4.2 Design Recommendations

The DSR for the site recommends:

• Reconstructing the embankment through the slide area to match adjacent elevations by compacting “existing material” in 6-inch-thick lifts.

• Constructing a 6-inch-thick roadway subgrade composed of imported pit run material on a 95-foot geotextile fabric layer.

• Placing crushed aggregate roadway surfacing, and

• Cleaning and re-profiling the uphill roadway ditch.

The DSR design recommendations are generally appropriate to backfill the slide to restore sight distance and improve safety without the need for stabilizing the landslide.

However, the material quantities are outdated. Based on site observations, the vertical movement up to 4 feet and few feet of lateral movement was observed at three locations along the 800 feet of roadway inspected by the reconnaissance team. The three locations are shown in Figure 11.

For embankment reconstruction, the embankment fill material will likely be obtained from the downhill roadway shoulder. We observed that the material placed at downhill shoulder in the slide area may be excessively wet and difficult to compact without moisture conditioning and may become unworkable in wet weather conditions. Figure 12 and Figure 13 show the recommended repair at the three locations.

If additional material is required at this Repair Site, the nearest sand and gravel pit is the Pine Pit, located about 13.5 miles southwest of Repair Site 4 via Roads 25 and 90 (USFS, 2013). This site is reportedly cut into lahar and mudflow deposits and will need to be investigated to determine if it can provide sound material for use as backfill. In our opinion, placing backfill material at the Repair Site will likely add more weight to the head of the slump, and could thereby cause further movement of the slide.

There are multiple relief culverts at this repair site that were observed to be partially blocked, causing ponding of water on the upstream side of the roadway. We recommend that the cut ditches be cleaned, and relief culverts be reestablished.

repair to backfill the slide to restore sight distance and improve safety.

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx

This Repair Site is located within the larger Crab Creek landslide, and the site has been repaired multiple times. The above improvements do not include any recommendations for stabilizing the landslide, and this section of roadway is likely to continue moving in the future. Further studies (including subsurface investigation) may be required to perform a detailed analysis of the landslide and present recommendations on stabilizing the landslide and the road.

6.5 Repair Site 5 – FR 90, Mile Post 11.5

6.5.1 Site Conditions

Heavy rains saturated the cut slope, causing the slope to fail onto the roadway. During site reconnaissance, slope gradients were observed to vary from nearly vertical at the failure headscarp to 1.25H to 1V at the toe of the failure (Photo 10).

The landslide debris varies from low plasticity Silt with sand to low plasticity Sandy Silt with gravel and cobbles up to 8 inches maximum diameter. Figure 14 shows the site plan and Figure 15 shows the profile view of the existing conditions.

Available geologic mapping at the site (Phillips, 1987), indicates that the area is underlain by volcanic and sedimentary rocks of Oligocene-Eocene age (approximately 23- to 56- million-year-old) Goble Formation.

6.5.2 Design Recommendations

The DSR for this Repair Site recommends:

• Clear and grub trees. Remove timber at top of the excavation/failure;

• Excavate and waste unsuitable material along the failed slope;

• Flatten cut slope to 1.25H to 1V; and

• Re-vegetate slope.

The preliminary recommendations and material quantities provided in the DSR are generally appropriate for this site. However, onsite materials observed in the headscarp may be marginal for planned cut slopes of 1.25H to 1V, and the failure of the original cut slope at similar inclination indicates that flatter permanent slopes may be required for long-term stability. Consequently, future maintenance may be needed if 1.25H:1V permanent cut slopes are implemented. Additionally, the new cut slope may be excavated in variable soil and / or rock materials and may encounter softer zones that

Photo 9: Leaning trees at Repair Site 4, viewing north

Rev2\Giff Pinchot Geotech Memo_Finalrevised.Docx are not suitable for a 1.25H:1V slope gradient. It may be necessary to cut the slope at a flatter angle, or to remove lower portions of the cut and replace with rock fill to provide buttressing.

The nearest rock quarry is Ranger Station Basalt, located about 3.0 miles east of from Repair Site 5 along FR 90.

Figure 16 shows the proposed repair at FR 90, MP 11.5. In addition to the above described preliminary recommendations set forth in the DSR, AECOM recommends the following:

• For the fill portion of the embankment, the embankment material should be benched into the existing native material in accordance with FHWA (2014) Standard Specification Sections 204.13. The bench height should be 3 feet and the bench width will vary based on the existing ground slope or cut slope.

• Type 4 stabilization geogrid in accordance with FHWA (2014) Standard Specification Section 714.04 and Table 714-6 should be placed at a vertical spacing of 1.5 feet. The geogrid should be embedded into the fill portion of the embankment along the entire width of the bench.

• Construct 3-foot deep, 6-foot wide, and approximately 100 feet long v-shaped ditch next to the road to provide drainage and catchment for future rockfall and or sloughing of the cut slope.

Erosion control efforts during construction should be adopted in accordance with FLH PDDM (2018) Section 9.5.6.

We do not anticipate that subsurface explorations will be required for design of this repair.

6.6 Repair Site 6 – FR 54, Mile Post 26.