GM3023 WA ERFO FS Umatl614 2020-1(1) Walla Walla North Geotech 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 7, 2023

TO: Matthew Miller Project Manager

FROM: Eric Lim Sr. Geotechnical Engineer

SUBJECT: Geotechnical Memo GM3023 Site Visit Observations and Geotechnical Recommendations Walla Walla North – Umatilla Repairs 2020, Phase 1 WA ERFO FS Umatl614 2020-1(1) Umatilla National Forest, Walla Walla Ranger District, Washington

INTRODUCTION

We understand that in February 2020, a heavy rain event caused flooding and damaged roads in the Umatilla National Forest. Two sites in the northern Washington State portion of the forest were deemed eligible under Emergency Repair of Federally Owned (ERFO) roads program. These sites are designated Forest Service Road (FSR) 4712 M.P. 0.87 and FSR 4713 M.P. 0.55.

The damage at each site was embankment slope failure primarily from surface water erosion resulting in loss of road width. The proposed repairs include restoration of the embankment and roadway width using Special Rock Embankment (SRE) materials.

This memorandum documents our site observations and presents geotechnical recommendations for design and construction of repairs.

GEOLOGY

An excerpt of the regional geologic map by Schuster (1993) is shown in Figure 1.

Figure 1 - Excerpt of geologic map by Schuster (1993).

A brief summary of the information from the geologic map text by Schuster (1993) is provided in Table 1.

Table 1 - Geologic Map Info Summary

FSR 4712 MP 0.87 FSR 4713 MP 0.55

Mapping Mvgn1 pTmt

Geologic Unit Grand Ronde Basalt, lower flows of normal magnetic polarity

Pre-Tertiary Metasedimentary and metavolcanic rocks, undivided

Description excerpts Middle Miocene, generally aphyric, fine-grained flows; at least 120 flows are known,…;

entablature, columnar, and blocky jointing, but blocky jointing most common in the map area…

Argillite, greenstone, quartzite, quartzitic metasedimentary rocks, amphibolitic schist, phyllite, metagabbro, and plagiogranite; sheared, faulted, and locally mineralized…

FSR 4712–0.87

FSR 4713–0.55

SITE CONDITIONS

We visited the site on June 15, 2023, to observe current site conditions and perform a geologic reconnaissance of the sites. Below are our observations at each site.

FSR 4712 MP 0.87

At this site, we observed that the failure appeared to be relatively shallow on the southwest facing slope down from the roadway. Vegetation had grown back over much of the failed area, obscuring view of much of the surface soils. The road width here measured approximately 16 feet narrowing to an approximate 12.5-foot width in the failure area. The failure area extended approximately 31 feet parallel to the roadway edge. The height of the slope is estimated to be on the order of 40 feet. Photos of the failure area are shown in Figures 2, 3, and 4.

The existing slopes on both sides of the failure area were measured to range from approximately 35 to 40 degrees from horizontal. The slope in the failure area measured approximately 30 to 35 degrees from horizontal due to loss of soil at the roadway edge. The surficial soils appeared to be a sandy silt with gravel and cobbles. Lush plants and tall bushes were located at the bottom of the slope indicating a consistent source of water. A culvert was reportedly clogged in this location and needs to be replaced.

Figure 2 - View of FSR 4712-0.87 facing E-SE

Figure 3 - View of FSR 4712-0.87 Slide Area

Figure 4 - View of FSR 4712-0.87 slide facing NW

Figure 5 - Google Earth image of FSR 4712-0.87 with failure area outlined

Figure 5 shows an aerial photo of the failure area with the estimated extents outlined in red, with the southernmost area appearing to be runout of the slide debris. An erosional rill now appears to be bisect the failure area. The toe of the failure area is approximately 200 feet from the edge of the Tucannon River.

FSR 4713 MP 0.55

At this site, we observed that the failure appeared to be a shallow to moderate depth landslide on the southeast facing slope down from the roadway. The road width here is approximately 18 feet narrowing to an approximate 14-foot width in the failure area. The failure area extended approximately 110 feet parallel to the roadway edge. The height of the slope is estimated to be on the order of 45 feet. Photos of the failure area are shown in Figures 6, 7, 8, and 9.

The existing slopes on both sides of the failure area were measured to range from approximately 30 to 35 degrees from horizontal. The slope in the failure area ranged from approximately 30 to 45 degrees from horizontal nearest the roadway edge. The surficial soils appeared to be a sandy silt with gravel and cobbles. Sparse vegetation (mostly grasses) has grown back in the failure area.

Figure 10 shows an aerial photo of the failure area with the estimated extents outlined in red.

Note that an intact area with a large shrub partially divides the failure area. The toe of the failure area borders Panjab Creek at a bedrock bench, as shown in Figure 8. The top of the bedrock bench appears to be above ordinary high water due to the lack of alluvial soils and stream debris above the bedrock bench.

Figure 6 - FSR 4713-0.55 failure area facing SW

Figure 7 - FSR 4713-0.55 lateral scarp facing NE

Figure 8 – FSR 4713-0.55 presumed bedrock bench at toe of slope

Figure 9 - FSR 4713-0.55 bedrock outcrop across the roadway

Figure 10 - Google Earth image of FSR 4713-0.55 with failure area outlined

DESIGN RECOMMENDATIONS

We understand that the Detailed Damage Inspection Reports (DDIR) for these two site proposed placement of riprap as the repair method. Although we agree that riprap sized rock material could be used, we prefer to use the pay item term Special Rock Embankment (SRE) for placement of riprap sized stone for fill-side embankment repairs that are not subject to stream flows. Therefore, we will use the term Special Rock Embankment or SRE in this memorandum. Basic dimensions and estimated quantities of the SRE repairs are shown in Table 2.

The SRE should have a face batter no steeper than 1V:1.25H, a minimum thickness of 4 feet, and minimum benches widths of 4 feet at 10- to 15-foot vertical intervals. The size of the SRE for this application can be smaller than the standard specification in FP-14. Table 3 below provides recommended smaller sizes that can be added as Special Contract Requirement (SCR). Larger rock could also be used, if available.

Table 2 – Approximate SRE Dimensions and Estimated Quantities

FSR 4712 MP 0.87 FSR 4713 MP 0.55

Estimated Vertical Height, LNFT 25 38 to 43 Length Along Roadway, LNFT 40 110 Special Rock Embankment estimated volume, CUYD

237 1585

Table 3 - Range of Mechanically-Placed Rock

Percent of Rock Fragments by Mass

Range of Intermediate Dimensions,(1) inches

50 > 12

50 4 – 12

(1) The intermediate dimension is the longest straight-line distance across the rock that is perpendicular to the rock’s longest axis on the rock face with the largest projection plane.

We also understand that several hazard trees are located at FSR 4713 MP 0.55. Two trees (Douglas fir and ponderosa pine) are located close to the southern edge of the failure, and one tree (dead grand fir) is located close to the northern edge of the failure area.

CONSTRUCTION CONSIDERATIONS

The base of the SREs should be founded on stable ground at FSR 4712 MP 0.87 and on the bedrock shelf at FSR 4713 MP 0.55. To help reduce the chance of slippage of the SRE material on the steep slopes, benches should be constructed into the existing slopes at approximate intervals of 10 to 15 vertical feet. The benches should be a minimum of 4 feet in width.

Access to the lower portions of the slope may be difficult, and may require long-reach equipment, construction of intermediate benches, or side access roads for equipment access.

The intact slope area with the large shrub in the center of FSR 4713 MP 0.55 may need to be removed to facilitate access and construction. At this time, it is uncertain if traffic access needs to be maintained and what maximum time for temporary closures will be.

Care should be taken at FSR 4713 MP 0.55 to install temporary erosion control measures, such as silt fences, due to the close proximity of the Panjab Creek. It may be necessary to install silt fences in the sandy alluvial soils just below the bedrock shelf at the base of the slope, or place wattles directly on the rock shelf.

MATERIAL SOURCES

A Forest Service rock pit is located approximately one-quarter mile southwest of the FSR 4713-0.55 site. Some small piles of rock approximately 18 inches or less in diameter were observed during our June 2023 site visit. We visually estimate the existing piles to be on the order of 30 cubic yards. Screening or additional handling may be needed to be used for Special Rock Embankment material. Other intact basalt bedrock appears to be available at the pit, but would require ripping, blasting, and/or screening to performed. The availability of this pit for rock production or staging areas has yet to be determined.

REFERENCES

Schuster, J. E., 1993, Geologic map of the Clarkston 1:100,000 quadrangle, Washington- Idaho, and the Washington portion of the Orofino 1:100,000 quadrangle: Washington Division of Geology and Earth Resources, Open File Report 93-4, scale 1:100,000.

CLOSING

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

EJL

CC: Andrew Roseman-Hanauer, Highway Designer

Vsevolod Descov, Highway Designer Jennifer Longmore, Highway Design Manager Stephen Morrow, Environmental Protection Specialist James Neighorn, Lead Hydraulics Engineer

Geotechnical File