GM18-16_WA_FS_ERFO_151-2101_FDR_2922_Geotechnical_Recommendations.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

Date: April 29, 2016

In Reply Refer To: HFL-17 GM 18-16

From: Dustin Woods, PE Geotechnical Engineer, WFLHD

To: Matt Miller, PE Project Manager, WFLHD

Subject: WA FS ERFO 151-2101 FDR 2922 Geotechnical Recommendations

Geotechnical Memorandum GM 18-16

INTRODUCTION

This memorandum provides general site information and fill slope recommendations for the WA FS ERFO 151-2101 project.

PROJECT DESCRIPTION

The WA FS ERFO 151-2101 project is within Clallam County Washington located in the Olympic National Forest on Forest Development Road 2922. A project site map is available in Figure 1.

Figure 1. Project Location Map.

The ERFO site is located where a 20-120 foot high fill was constructed across a drainage. The existing or pre-damage north side of the roadway was approximately 120 feet high and built on a 1V:1.5H embankment slope. The existing south side of the embankment is approximately 20 feet high at a similar slope.

The roadway embankment failed in early 2015 after a tree fell on the downslope culvert causing it to discharge water above the energy dissipater causing erosion and shallow landsliding as material above was left unsupported. The site has not changed much from March of 2015 to April 2016, recent site photographs are shown in figures 2 and 3 below.

Figure 2. Site Photograph Looking East.

Figure 3. Site Photograph Looking West.

GEOLOGY

The Geology of the Pysht Quardrangle (Gower, 1960) shows the general project area is a mixture of quarternary aged landslide debris, alluvium, marine, glacial outwash and glacial till deposits overlying cretaceous to tertiary aged bedrock. The project appears to lie directly over the Calawah River Fault and within the borders of the mapped landslide debris and adjacent to the alluvium deposits. A geologic map of the project area is provided in Figure 4 with definitions of the mapped features directly after.

The Calawah River Fault is considered active and is a wide zone of sheared and brecciated rocks trending North 70° West. The zone has been mapped as wide as 2000 feet in this area.

Figure 4. Geologic Map of Project Area.

Tag: Argillite and greywacke sequence. Medium dark gray to greenish-gray argillite and graywacke, Tag; light-gray micaceous feldspathic and arkosic sandstone.

Tagv: Argillite and greywacke sequence. Pillow basalt, flow breccia, and diabasic basalt with grayish-red argillite and argillaceous limestone.

Tus: Undifferentiated rocks. Thick-bedded light-gray to olive-gray fine to coarse grained sandstone with laminae to thick beds of medium dark gray siltstone.

Qal: Alluvium and Recent marine beach deposits. Unconsolidated sand, gravel, and silt.

Qgd: Glacial drift. Outwash and till deposits from continental and alpine glaciers.

Qls: Landslide debris.

SITE CONDITIONS

A site visit was conducted by Hydraulics Engineer Jim Neighorn and Geotechnical Engineer Dustin Woods of Western Federal Lands on April 6, 2016. From the damage sustained in the 2015 slide, it appeared that the roadway was constructed with approximate 4 feet (+/-) of

Project Location poorly graded gravel with silt and sand. The rest of the slope appeared to be constructed with silty sand with gravel. Construction access for the toe of slope would have to come from the west side of the roadway as the eastern portion appeared to show outcrops of steeply faced bedrock.

There were no visible seeps or springs on the face of the failure plane during the site visit and it appeared that additional damage had not occurred after the initial slide.

SUBSURFACE CONDITIONS

There was no subsurface investigation for this project; however, two surface grab samples were taken with a shovel. A sample of the material that appeared to be used to construct the roadway (approximately the top 4 feet) was labeled “base and pavement layer” while a sample labeled “subgrade” was taken of the existing embankment material. These samples were transported to the WFLHD lab in Vancouver, Washington where a mechanical gradation and Atterberg limits were performed on both samples. A standard proctor was completed on the base and pavement layer sample. The test results are attached to this memorandum and summarized in table 1 below.

Table 1. Summarized Test Data.

Sample USCS

Passing #200

Liquid Limit

Plasticity Index

Max.

Density

(pcf)

Optimum Moisture Content

Base and Pavement Layer GP-GM 10.6 29 5 133.2 10.3

Subgrade SM 24.5 28 5 - -

It should be noted that the results of this testing is only representative of what was found on the surface of the slide near the visible strata difference between the layers described above. It is likely that other materials, including bedrock, will be encountered in the excavation.

Considering that this embankment crosses a drainage, both surface water and groundwater are likely to be encountered in excavations, especially during and following periods of heavy precipitation.

ANALYSIS

A slope stability analysis of the proposed new 1V:1.5H slope was completed in Rocscience SLIDE 6.0 in conjunction with the design methodology used in FHWA GEC 011, Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes. Various strength properties of the embankment material were used to recreate an existing slope with a factor of safety of slightly greater than 1.0. These strength properties are listed below in Table 2.

Table 2. Soil Parameters Used in SLIDE 6.0.

Unit Weight

(pcf) Friction Angle

(°) Cohesion (psf)

130 36 50

DESIGN RECOMMENDATIONS

Different mitigation options were analyzed in order to economize a configuration with a factor of safety of 1.25 or higher. The final configuration consisted of a 10 foot thick Class III rip-rap blanket at the bottom 30 feet (slope distance) of the toe of the slope (elevations ~105 to ~127) below 12 feet of geogrid reinforcement spaced at 1.5 foot lifts. The top 3 layers of geogrid are spaced at 1 foot and embedded 20 feet to protect the road surface from any deformations in the slope. The reinforcement should meet or exceed Type I geogrid as listed in Table 714-6 in the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14.

CONSTRUCTION CONSIDERATIONS

Select Granular Backfill was specified based on consideration of being able to incorporate material from the excavation into the reinforced soil slope. Because only a limited number of tests were performed, some additional handling of the excavation material may be required to meet the specification and all of the material that is excavated may not meet the specified gradation. Table 3 lists the recommended specification for select granular backfill.

Table 3. Select Granular Backfill Specification.

Sieve Size Percent by Mass Passing

Designated Sieve (AASHTO T 27

& AASHTO T 11)

4 inch (75 mm) 100 1 inch (25 mm) 70 – 100 No. 4 (4.75 mm) 30 – 70 No. 200 (75 µm) 0.0 – 30.0

CLOSING

Please contact Dustin Woods at (360) 619-7541 or Dustin.Woods@dot.gov with any questions regarding this memorandum.

mailto:Dustin.Woods@dot.gov

REFERENCES

Berg, R.R., Christopher, B.R., Samtani, N.C. (2009). Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, FHWA-NHI-10-024-Volume 1 and FHWA-NHI-10-025-Volume 2, Geotechnical Engineering Circular No. 11, National Highway Institute, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C.

Gower, H.D. Geology of the Pysht Triangle Quardrangle, Washington [map]. Scale 1:62500.

Washington, D.C.: United States Geological Service, 1960.

cc: Katherine Agnew, WFLHD Highway Designer Craig Sanders, WFLHD Construction Operations Engineer Jim Neighorn, WFLHD Hydraulics Engineer Dave Hilgendorf, WFLHD ERFO Coordinator Geotechnical File

Attachments: Base and Pavement Layer Test Results Subgrade Test Results