wa-ef-gifpn2020-1-1_final_GEO_Memo.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: September 30, 2024

TO: Carrie Warren Project Manager

FROM: Eric Lim Sr. Geological Engineer

SUBJECT: GM0624 Final Geotechnical Memorandum (Revision 1)

WA ERFO FS GIFPN603 2020-1(1)

Gifford Pinchot Repairs 2020, Phase 1

INTRODUCTION

This geotechnical memorandum provides our final recommendations for the 2020 Gifford Pinchot Emergency Relief for Federally Owned Roads (ERFO) repair project. Geotechnical work under this contract is to support the United States Forest Service (USFS) and included a field reconnaissance. Subsurface explorations were not performed as part of this ERFO project.

PROJECT DESCRIPTION

During the Winter of 2020, a major storm damaged several recreational and administrative access roads within the Gifford-Pinchot National Forest, and the ERFO program is being used to restore these roads to their original conditions. The USFS has been making ongoing, minor repairs to several of the roads to prevent worsening of their current conditions.

Work under this ERFO project will repair the roads to their pre-storm conditions, replace failed culverts, and construct reinforced fill slopes to stabilize or rebuild the embankments. The WFL will provide PS&E (PS&E) and administrative services from preliminary engineering through construction and project closeout.

Figure 1 - Oblique View of Project Sites Facing North

SUMMARY OF FINDINGS

The site locations are shown in Figure 1 and summaries of the recommended repair work are described in Table 1.

Table 1 – Site Summary ROAD NO. and

MILEPOST

NAME DESCRIPTION OF REPAIR

25 MP 13.1 Randle-Lewis River Remove and replace embankment failure with deep patch and culvert repair.

28 MP 0.9 Yellowjacket Reset culvert, recondition roadway. No geotechnical component.

28 MP 1.3 Yellowjacket Reset culvert, recondition roadway. No geotechnical component.

59 MP 0.6 Copper Creek Reinforced Soil Slope with minor cut slope cleaning. Add Special Rock Embankment foundation where needed to aid construction on steep slopes.

ROAD NO. and

MILEPOST

NAME DESCRIPTION OF REPAIR

59 MP 0.9 Copper Creek Reinforced Soil Slope with minor cut slope cleaning. Add Special Rock Embankment foundation where needed to aid construction on steep slopes. New 36” culvert crossing.

59 MP 1.0 Copper Creek Reinforced Soil Slope with minor cut slope cleaning. Add Special Rock Embankment foundation where needed to aid construction on steep slopes. New 36” culvert crossing.

59 MP 1.1 Copper Creek Clear debris and deep patch road, do not debuttress landslide in road cut. Recondition road and embankment.

59 MP 1.2 Copper Creek Reinforced Soil Slope on bedrock. Add Special Rock Embankment foundation where needed to aid construction on steep slopes. Replace 24” culvert with 36” culvert.

85 MP 3.3 Catt Creek Remove and replace embankment failure with deep patch and 24” culvert replacement.

The culvert repair work on Road 28 at MP 0.9 and 1.3 will not have a geotechnical component. General recommendations for the Road 28 culvert reset should ensure the pipe is installed with backfill material meeting FP-14 704.03 requirements. A separation fabric should be placed between the reset culvert and finished road surface gravel. Inlet and outlet armoring is large and unstable rounded boulders that appear to have been end dumped and not mechanically placed to create an interlocking rockery. Armoring at the inlet and outlets should also be mechanically reset with rock meeting the requirements for angular rip rap.

Washouts on Road 59 exposed evidence of the original construction from circa 1955, which was built with “cut and sidecast” methods. Eroded fills exposed buried stumps and deep fill sections up to 15 feet deep, with the toe catch using buried trees anchored against the stump as a rudimentary anchor key. Fully reestablishing the road width in the original alignment will be difficult considering the deep erosion into the slope, remaining debris in the drainages, and the overall unstable slope conditions.

In particular, repair of Road 59 may require some realignment into the slope to shift the fill away from the newly eroded drainage slope, if feasible. This shift will require some excavation into the cut slope, but should be kept to a minimum to prevent removal of buttressing materials for the slopes above the road.

SITE GEOLOGY AND FIELD RECONNAISSANCE

On January 27, 2022, a field reconnaissance was performed to observe the damages by walking the sites, traversing the slopes, and noting visible features such as slope heights and inclinations, scarps, cracks, hummocks, vegetation, . Additionally, on February 9, 2024, the cross functional team returned to Road 59 to reassess the current site conditions and design assumptions.

Road 25 Conditions

The United States Geological Survey (USGS) maps the Road 25 site and surrounding area as underlain by Quaternary landslide (Qls) deposits, with a bedrock bedding dipping between 14 and 16 degrees, generally towards the east (Figure 2). Based on our field observations, these landslide deposits are unstable and intermittently moving, likely associated with seasonal increases in precipitation and surface runoff. Evidence of slope movement includes hummocky terrain, curved and leaning trees, and pavement distress outside of the project site. No bedrock outcrops or exposures were observed during the field visit because the site is a large fill which is situated mid-slope of the landslide.

Figure 2. Road 25, Greenhorn Butte Quad, (Swanson, 1989).

Road 59 Conditions

The USGS maps the Copper Creek Road 59 as underlain by Tertiary Puget Group Formation (sandstone, siltstone, mudstone, and coal) with a regional southward plunging anticline as shown in Figure 3. Based on field observations of exposed outcrops, the bedrock is gray (fresh) to reddish brown (weathered), medium strong (R3)1 sandstone with mica lenses forming localized weak failure planes. The combination of medium strong bedrock and adverse bedding with weak mica layers have created large-scale, prehistoric (older than recorded/documented history) landslides regionally and rock slope failures locally.

As the LiDAR shows in Figure 4, multiple prehistoric landslide deposits on the hillside are crossed by Road 59. Portions of these landslides remobilized in the failures at MP 0.6 and MP 1.1, resulting in losses of the road fill and cut slope. The overlying regolith soils along the landslide margins appear stable, with some indications of minor slope creep.

1 Per ISRM, 1978 & 1981

Figure 3. Road 59, Mount Rainier Quadrangle (Schasse, 1987) .

Over the two site visits, the team observed saturated soil conditions at the repair locations and runoff into the roadway prism between the MP 0.6 and MP 0.9 sites. The ongoing erosion since the 2020 storm damage has scoured areas within the failures to bedrock. Based on comparisons of the photos from the initial 2020 failure and our subsequent site observations, the landslides along the roadway are continuing to move, as evidenced by vegetation loss and minor cut slope failures into the ditch.

Significant runoff was observed at the MP 0.6 landslide (Figure 5) and the conditions appear similar to Washington Department of Transportation (WSDOT) photos of the failure taken in 2020 (Figure 6). The observed runoff flows are sufficient to require a culvert in the MSE wall as well as an underdrain system to prevent the fill wall becoming saturated and failing.

Additionally, the runoff is eroding into the landslide debris, creating an incised channel and oversteepening the slopes above the roadway. At a minimum, the debris above the road should be graded to a 1V:1.5H (vertical:horizontal), where possible, or up to 1V:1H in limited areas slope up to a natural catch point above. However, this steep of a slope will be prone to continued sloughing of the saturated landslide debris and ongoing maintenance will be needed to clear the roadway.

Figure 4. WA DNR LiDAR, Lewis East Sthelens 2020 Hillshade.

Interpreted Landslide Margins

Figure 6. Road 59 MP 0.6 failure photographed by WSDOT (Feb 10, 2020).

Figure 5. Road 59 MP 0.6 actively flowing water sourcing from landslide debris (Feb 9, 2024).

Beyond the MP 0.6 failure is a small slump caused by continued erosion of the roadway, which undermined the cut slope and the blocked culvert up station from the MP 0.6 site is directing flow into the roadway. This stream was flowing during our recent site visit in the eroded channel and discharging into the eastern lateral flank of the 2020 landslide. Repairs for the eroded channel include installing a drainage pipe and replacing the lost roadway fill.

The MP 1.1 site is within the debris flow field where the road crosses an unmapped landslide which deposited soil, rock, and timbers, filling the drainage. This repair work should be limited to removing the near surface, loose debris and should not cut into the slope due to the risk of debuttressing the slope, causing additional failures. Vegetation loss was observed in the roadway cut slope between MP 1.1 and MP 1.2, ranging from minor slumps to entire trees falling into the road.

Excavation into prehistoric landslide materials will encounter non-uniform material with the appearance of an uncontrolled fill, as evident in Figure 7 below. Landslide deposit appears to be a mixture of weathered sandstone clasts with entrained debris. However, exposed bedrock was observed at MP 1.2 site and should provide adequate foundation surface for the reinforced soil slopes.

Figure 7. Exposed landslide deposit shows a chaotic mixture of weathered sandstone clasts and unconsolidated overburden soil.

Road 85 Conditions

The USGS maps the Catt Creek Road 85 as Tertiary Ohanapecosh Formation (volcanic rock and lahar deposits) with regional bedrock layers dipping between 23 and 45 degrees, generally towards the south (Figure 8). No bedrock outcrops were observed during the field observations because the site is a large fill, which appears to be situated mid-slope of the regional bedding structure. The slope failure appears to be limited only to the road fill.

Figure 8. Road 85, Mount Rainier Quadrangle (Schasse, 1987)

DESIGN RECOMMENDATIONS

The following design recommendations will not fully stabilize the pre-existing landslides impacting the different sites. In several cases, the existing pavement distress and slope along and surrounding the roads are the continued movement of the unstable ground the road traverses. Due to the limited time frame for repair under the ERFO program and low volume nature of these roadways, subsurface explorations are not within the scope of work for this project. A risk-based approach to building across unstable slopes will assume that the road fills will incrementally move with the larger landslide complexes, and that future catastrophic failures of the larger unstable features cannot be fully mitigated by ERFO delivered projects.

Subsurface material properties are estimated based on our site visit observations of the exposed soil and rock and the published geologic maps. For design purposes, we are assuming the landslide debris is a chaotic mixture of non-plastic silty sand with gravel, likely derived from the underlying sandstone and volcanic bedrock. Estimated soil and bedrock properties for drained conditions are listed in Table 2 below.

Table 2 – Estimated Soil and Bedrock Material Properties, Drained Conditions

Silty SAND Embankment FILL /

RSS Backfill

SANDSTONE

Dry Density (pcf) 125 125 150 Friction (deg) 32 32 25

Cohesion (psf)* 100 0 1,000 Compressive Strength (psi)

___ ___ 1,500 – 3,500

* Apparent cohesion included for in-situ silty sand only to match oversteepened, existing conditions.

Deep Patch Subexcavations

The deep patch designs were completed using the USFS Deep Patch Road Embankment Repair Application Guide 0577 1204-SDTDC publication (Musser and Denning, 2005). We recommend reinforcement geogrids with a minimum nominal long-term breaking strength of 1,500 lb/ft, with imported select borrow for the deep patch backfill. Minimum design recommendations for the patches are listed in Table 3 below.

Table 3 - Gifford Pinchot ERFO Deep Patch Design, Reinforcements at 1-ft lifts.

Forest Road, Milepost L (ft) Depth

(ft) Treq

(lbs/ft) Tnom

(lbs/ft)

Reinforcement Geogrid

(Table 714-6)

Reinforcement Length (ft)

25 Rd, MP 13.1 180 4 2136 712 Type II 8 (half width) 85 Rd, MP 3.3 70 3 1005 503 Type II 18 (full width)

Reinforced Soil Slopes and Special Rock Embankments

The remaining sites on Road 59 at MP 0.6, MP 0.9, MP 1.0 and MP 1.2 can be repaired with reinforced soil slopes (RSS) with a facing angle of 2V:1H. These RSS’s should provide a repair structure with a higher tolerance for future movements than an MSE wall or more vertical structural wall. RSS’s can also have a lower quality backfill, that will help in making more local materials available over processed imported aggregates, and are therefore typically lower in cost than traditional walls. If needed, the RSS can be constructed over a special rock embankment (SRE) foundation to aid in construction over the wet steep slopes.

The SRE foundation also provides a free draining support base for the RSS. A filter fabric is recommended to be placed below and above SRE to prevent migrations of fine materials and associated loss of ground due to soil piping.

The RSS will use a welded-wire mesh (WWM) facing form with gabion rock backfill that typically fits in well on Forest Service roadways. The reinforcement can be either a wrap around geosynthetic reinforcement with adequate overlap with WWM facing form, or an inextensible, integrated steel reinforcement mat. Geosynthetic reinforcement vertical spacing should be a maximum 18-inches and reinforcement lengths should be 8-feet (minimum).

Based on the estimated subsurface information, we modeled the RSS’s in Slide 2 limit equilibrium software by Rocscience. Typical design factors of safety for the stability of new retaining structures are a minimum of 1.5 for static conditions and 1.1 for seismic conditions, where applicable, per the Federal Lands Highways Project Development Design Manual and accompanying Geotechnical Technical Guidance Manual. Global stability with a minimum static factor of safety of 1.3 is typically acceptable for retaining structures not supporting building or bridge structures. However, the ERFO program typically funds in-kind repairs and replacements, with some allowance for low to moderate cost stability and constructability improvements, such as the use of geosynthetics and imported rock materials. For many ERFO repairs and other landslide repairs, these ideal design factors of safety cannot be obtained without significant additional investment, which would be considered betterments under the ERFO program. Therefore, we typically design for lower factors of safety, or an improvement in stability, as part of the emergency repair. The owner should be aware of the level of improvement and associated risk for future movements of these structures. Also, it should be noted that with these lower factors of safety and simple improvements in stability may not be resilient under a design level earthquake.

CONSTRUCTION CONSIDERATIONS

The project repair sites are located on narrow, steep forest roads which will limit equipment access and staging. The Road 59 sites will require either completing sequential repairs, starting with the lower sites (e.g., MP 0.6) before accessing the sites further along the road.

or a temporary construction path could be built across the failures to access multiple sites for repair.

Excavation into the sandstone bedrock will require ripping or impact hammer attachments with heavy duty equipment. If required, blasting for the rock embankment embedment will be within the existing roadway corridor, and not extend beyond cleared project limits. Blasting may not be allowed by environmental restrictions yet to be determined.

The Contractor may need to use temporary shoring to support open excavations for repairs on Road 59. For estimating this project, include the FP 208 Shoring Item as Lump Sum for a temporary soil nail wall. Temporary soil nail walls do not require a 75-year design life and can be constructed without corrosion protection coated elements. Excavations into the slopes should not advance if the ground is saturated and/or indications of slope movements occur.

Sites that encounter wet subsurface conditions may need to be excavated and drained, either by removal and replacement with free-draining backfill or allowed to drain.

On the Road 59 at approximately MP 2.7, a sixth washout was not included in the USFS ERFO application. No recommendations have been provided in this memo to implement a potential repair of that site.

CLOSING

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

CC: Hassan Mohammed, DES James Burke, DES Michael Madar, DES James Neighorn, HYD

Benjamin White, ENV Mark Swank, GEO

Geotechnical File

REFERENCES

Musser, S. and Denning, C. (October 2005). Deep Patch Road Embankment Repair Application Guide. USFS 0577 1204 – SDCTC, 21 p.

Natural Resources Conservation Service, USDA. (January 2012). NRCS National Engineering Handbook, Part 631 Geology, Chapter 4 Engineering Classification of Rock Materials.

210=VI-NEH, Amend. 55, January 2012, 61 p.

Schasse, H. W., compiler, (1987), Geologic map of the Mount Rainier quadrangle, Washington:

Washington Division of Geology and Earth Resources Open File Report 87-16, 43 p., 1 plate, scale 1:100,000

Swanson, D.A. (1989). Geologic maps of the French Butte and Greenhorn Buttes quadrangles, Washington. USGS Open-File Report 89-309, 25 p., 3 plate, scale 1:24,000