wa-ef-mtbkr2022-1-3_GEO_4024-R1.pdf

158377846

M E M O R A N D U M

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661

DATE: May 27, 2025

TO: David Arena Project Manager

FROM: Eric Lim Senior Geotechnical Engineer

SUBJECT: Geotechnical Memo 4024 R1 Geotechnical Recommendations (Revision 1) Mt. Baker Forest Road 11 MP 23.9 Repair

WA ERFO FS MTBKR605 2022-1(3)

Mount Baker – Snoqualmie National Forest, Washington

INTRODUCTION

On October 10, 2023, an office review was held at the United States Forest Service (USFS) Mt. Baker Ranger District Forest office located in Sedro-Woolley, Washington to discuss the background of multiple failure sites in the Mount Baker-Snoqualmie (MBS) National Forest.

On October 11, 2023, MBS National Forest personnel escorted Western Federal Lands Highway Division (WFLHD) staff to the failure sites for a field review. This document presents geotechnical findings from the field and office review of the above-referenced site. This document will present the following:

• Background of failure sites;

• Geotechnical recommendations for repairs; and

• Construction considerations.

PROJECT DESCRIPTION

We understand that Forest Road 11 is designated as a Forest Service Maintenance Level 3 road, which are typically low speed, single lane roads with turnouts passable to prudent drivers in passenger cars during the normal season of use. However, Maintenance Level 3 does not consider use comfort and convenience as priorities (Apodaca, et al., 2012). The one-lane forest roads appear to be original construction with later modifications and maintenance.

We anticipate that the one-lane, limited width roadway will only allow one-way construction traffic. Closure of the road at the repair site will likely be required to complete repairs.

DESCRIPTION OF THE FAILURE SITE ON FSR 11 – MP 29.3 (2022)

Forest Service Road (FSR) was constructed with cut and sidecast methods lost a section of outboard fill from scouring of the Shannon Creek drainage. A high water flow event combined with a large wood blockage directed stream flow into the eastern bank. This scour event removed the entire bank of Shannon Creek and left a vertical face that threatens the roadway (Figure 1). This section of road is constructed into the toe of a deep-seated landslide.

The LiDAR bare earth hillshade (Figure 2) shows the Shannon Creek drainage confined by the gently sloping landslide and a large, wedge-shaped alluvial fan deposit. Upstream of the bridge at FSR 11, Shannon Creek consists of two parallel running drainages but becomes a single channel downstream of the bridge. This is likely the result of streams following the lateral extents of two landslide failures. These failures are flowing toward the center of the mapped landslide, but the stream channels roughly define the extents of the more recently active individual slides within the larger mapped landslide feature. The large, wedge-shaped alluvial fan deposit downstream of the bridge appears to be relatively flat and stable.

As mapped by the USGS (Figure 3) the site is Holocene age landslide (Qls) and Holocene age alluvial fan deposits (Qf). Beyond the site limits the bedrock is mapped as Darrington Phyllite and Bell Pass mélange, which are both metamorphic formations prone to developing unstable slopes. Underlying bedrock was not exposed at the site during our site visit;

however, bedrock is often found along the bottom of stream levels where it is more resistant to stream erosion.

Figure 1 – Damage photo from the DDIR.

LiDAR Hillshade Geologic Map

Figure 2 - WA DNR LiDAR; North Puget 2017 Figure 3 - Geologic Map of Mount Baker 30”x60” Quad, WA. 2003

DESIGN RECOMMENDATIONS

No subsurface geotechnical investigation was conducted as part of this project, which is not uncommon for emergency projects funded under the Emergency Relief for Federally Owned Roads (ERFO) program.

The intent for the FSR 11 – MP 23.9 repair is to protect the remainder of the roadway from stream flows with a root wad and riprap revetment structure. Since the revetment will only be designed near the design stream levels and the roadway itself is on landslide materials, the remaining exposed roadway soils will need to be reinforced with a deep patch type of construction. The primary intent of the deep patch is to create a composite (alternating geogrid and aggregate layers) roadway that essentially bridges the repaired area should additional scour and/or slope movement occur in the future. The sides of the deep patch reinforcement will also be wrap-returned geogrid, which will form a reinforced soil slope face that can be vegetated and planted. Figure 4 shows a conceptual level detail of the proposed repair.

Figure 4 - Deep patch concept with root wad/riprap revetment

We understand that it is the USFS’s desire to have a soft armoring with plantings over the riprap up against the face of the deep patch. Therefore, the repair will incorporate a 1V:1.25H to 1V:1.5H select borrow backfill with topsoil at the face wrap and grass seed. Willow plantings will also be added to the slope face and riprap. The desired performance expectation of the soft armoring and willow plantings is to enhance riparian habitat.

CONSTRUCTION CONSIDERATIONS

The project repair sites are located on narrow, steep forest roads which will limit equipment access and staging.

Excavation

The proposed repairs consider the variable geologic formations and slope conditions. With this in mind, this site is anticipated to encounter roadway fills, native soils and bedrock. Soils will range from fine grained materials (silt and sand) to gravel and cobbles. Boulders up to 4 feet diameter have been observed at FSR 11, and larger boulders could be encountered during construction that may require reduction for handling and hauling.

If bedrock is encountered, excavation will require ripping or impact hammer attachments with heavy duty equipment. Blasting may not be allowed due to environmental restrictions.

Excavation, sloping and benching into rock and soils should adhere to OSHA 1926 Subpart P, Appendix B based on the rock and soil types which are encountered.

Backfill Material

No samples have been tested to confirm onsite material properties. It is assumed that onsite materials do not meet Select Borrow requirements for the deep patch.

REFERENCES

Apodaca, M., M. Tippie, A. M. Verde, V. Barandino, Jr., 2012, Guidelines for Road Maintenance Levels, United States Department of Agriculture, Forest Service, National Technology & Development Program, June 2012.

USGS, 2003, Geologic Map of Mount Baker 30”x60” Quad, WA., Map I-2660, United States Geological Survey.

WA DNR, Washington LiDAR Portal, North Puget 2017 dataset, available at:

https://lidarportal.dnr.wa.gov/, accessed March 2024, Washington State Department of Natural Resources, Division of Geology and Earth Resources.

CLOSING

Please contact Eric Lim at (360) 619-7826 or eric.lim@dot.gov with any questions regarding this memorandum.

CC: Thao Truong, Highway Design Vernon Bevan, Hydraulics Ben White, Environment Mark Swank, GEO Team Functional Manager Geotechnical File