GM_25-17_Cowlitz_Valley_Geotechnical_Reconnaissance.pdf

Memorandum Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661-3801

TO: Susan Ellis Project Manager

Ninh Phan Roadway Designer

In Reply Refer to: HFL-17

FROM: Diann Morehouse Geotechnical Engineer

DATE: September 30, 2017

SUBJECT: Geotechnical Site Reconnaissance Geotechnical Memorandum No. GM25-17 and Preliminary Recommendations Cowlitz Valley Road Repairs, Phase 2, WA FS ERFO 2016(1)-22(2)

INTRODUCTION

This memorandum provides preliminary recommendations for Cowlitz Valley Emergency Relief for Federally Owned roads (ERFO) Phase 2 project. This project is located in Gifford Pinchot National Forest, Skamania County, Washington. Figures 1 and 2 shows the project location map.

SCOPE OF WORK

The scope of work for this project is to repair thirteen roadway sites that were damaged during a large storm event in 2016. The project is using ERFO funding to repair the sites. The following thirteen Forest Services (FS) sites are included in this project:

• FS Road 2324, MP 0.6;

• FS Road 2325, MP 3.5;

• FS Road 25, MP 25;

• FS Road 29, MP 7.6;

• FS Road 29, MP 9.3;

• FS Road 29, MP 10.2;

• FS Road 29, MP 10.4;

• FS Road 54; MP 26.9,

• FS Road 88, MP 19.7,

• FS Road 90, MP 11.2,

• FS Road 90, MP 11.5,

• FS Road 90, MP 28.0, and

• FS Road 90, MP 45.0.

However, only the following nine sites are discussed in this memorandum:

• FS Road 25, MP 25;

• FS Road 29, MP 7.6;

• FS Road 29, MP 9.3;

• FS Road 29, MP 10.2;

• FS Road 29, MP 10.4;

• FS Road 88, MP 19.7;

Cowlitz Valley Road Repairs

WA FS ERFO 2016(1)-22(1)

Geotechnical Site Visit Observations and Recommendations Page 2 September 2017

• FS Road 90, MP 11.5;

• FS Road 90, MP 28.0; and

• FS Road 90, MP 45.0.

The Damage Survey Report(s) (DSR) are attached.

Three sites, FS Road 2324, FS Road 2325, and FS Road 90, MP 11.2 consist primarily of hydraulic elements and are not discussed in this memorandum. FS Road 54, MP 26.9 site was not found during the August 20, 2017 site reconnaissance. The picture in the DSR did not resemble any of the features in the field along the west end of the FS Road 54.

Figure 1: Project Vicinity Map, North Half.

Mt. St.

Helens

Mt. Adams

Geotechnical Site Visit Observations and Recommendations Page 3 September 2017

Figure 2: Project Vicinity Map, South Half.

OBSERVATIONS AND RECOMMENDATION

Observations made during site visits on June 5, July 9, and August 20, 2017 are summarized below along with a recommended mitigation. Geotechnical subsurface explorations were not performed due to the time and budget constraints for these emergency repair projects. Subsurface conditions were estimated from observable conditions at the ground surface and may vary during construction.

FS Road 25, MP 25; 46.26298°N -121.96436°W An embankment failure resulted in the loss of the roadway shoulder about 150 feet long (Picture 1). The disturbed materials remaining on the slope below the toe of the embankment exhibit debris flow characteristics extending more than a 1000 feet down the slope and in-place at slope angles between 15 and 34 degrees. The embankment is about 60 feet high with post failure slope angles as steep as 50 degrees. Just below the east lateral scarp, there is wet area that indicate ground water close to the surface, even though the site was visited in July (Picture 2). Materials found in the existing embankment and in the disturbed debris material include clayey, silty sand with gravel and cobbles.

Cut slopes are standing at about 45 degrees and range in height between 40 to 60 feet. Water seeps from a discontinuity in the volcanic derived bedrock (Picture 3). These slopes are exhibiting slope erosion and vegetation in the area between the discontinuity and the ditch. The erosion is resulting in debris filling the ditch. Closer to the culvert, water about 2-inches deep was noted in the ditch. The soil in the cut slopes are also clayey, silty sand with gravel. It is assumed that the embankment was constructed from material in the cut slope.

Mt. St.

Helens Mt. Adams

Geotechnical Site Visit Observations and Recommendations Page 4 September 2017

U.S. Geological Survey (USGS) Geologic Map of the French Butte Quadrangle, Swanson, 1989, identifies the rock unit at this site as Miocene-aged andesite and basaltic andesite flows. The existing pavement shows signs of frequent patching in the outside lane. An asphalt patch across the road is in-line with a 24-inch diameter corrugated metal pipe (CMP). At the time of the field reconnaissance the pipe was partly filled with soil.

The DSR included two Williamson Drive Probe (WDP) results performed by the Forest Service.

WDP2 was performed near the toe of the current embankment and WDP1 was performed about 67 feet down slope from WDP2 on the disturbed slide material. Unlike Standard Penetration Test that blow counts that can be correlated with published engineering index properties of a soil, the WPD provides information about density of the material relative only to that test. The WDP2 extended into the ground 14 feet and WDP1 extended into the ground 14.5 feet. Water was observed from measuring the extracted rods where they exhibit moisture on the rods. Ground water was measured in WDP2 at 7.5 below the ground surface and at 8.5 feet in WDP1.

The software program SLIDE 7.0 by Rocscience Inc. was used to perform global stability analyses of the landslide. The landslide was evaluated by reviewing the slide conditions, assumed groundwater conditions as noted during field reconnaissance, subsurface materials, and surface topography to produce generalized cross-section for analysis. Two material types were used in the analysis, one layer modelled as the volcanic derived bedrock and the second layer as the embankment soil from the cut slope.

Volcanic derived bedrock

Embankment Soil

Moist Unit Weight (pcf) 130 120

Friction/ Residual Friction Angle (deg) 35 32

Cohesion (psf) 50 0

This site has been unstable for some time. The Unstable Slope Management Program (USMP) rated this site in 2014, citing slump and erosion past edge of pavement.

A special rock embankment that is mechanically placed is recommended for this site. A detail is included as Attachment A. The face of the rock embankment is no steeper than 1V:1.25H. Install Class 1, non-woven, Type B geotextile across the entire excavation. Choke the top of the special rock embankment and place geotextile between the rock for special rock embankments and the bottom of the pavement structure. Based on the WDP results, it appears undisturbed ground is about 12 feet below the ground surface. Embed the special rock embankment a minimum of 12 feet below the existing ground surface measured at the toe of the embankment.

Geotechnical Site Visit Observations and Recommendations Page 5 September 2017

Picture 1: Failure Area

Picture 2: Red arrow pointing to wet soil where ground water is emerging.

Geotechnical Site Visit Observations and Recommendations Page 6 September 2017

Picture 3: Cut slope exhibiting ground water seeps and slope erosion.

Picture 4: Picture from USMP of the site in October 2014.

FS Road 29, MP 7.6; 46.3622°N -121.7998°W

This site was a large embankment fill that crosses Camp Creek that failed in 2016. A debris flow deposited logs and boulders against the embankment and culvert inlet, washing out both of them.

The previous culvert was a 72-inch diameter culvert.

USGS Geologic Map of the McCoy Peak Quadrangle, Swanson, 1992, identifies the underlying rock unit at this site as volcaniclastic rocks with sulfide mineralization with an unknown age.

During the field reconnaissance, possible exposed bedrock outcrops were noted near the bottom of the drainage on the west (downslope) side of the road. The overlying soils consist of silty sand with gravel. Cobbles and boulders up to 6 feet in diameter are also present.

Geotechnical Site Visit Observations and Recommendations Page 7 September 2017

Western Federal Lands (WFL) Hydraulics Group is recommending to replace the culvert in kind.

The slope of the drainage is steep and becoming steeper on the west side. In order to catch this west side slope, the DSR is recommending a 20-foot high mechanically stabilized earth (MSE) wall. A survey activity was performed and cross sections have not been issued. Once cross sections are provided a design can be determined.

Picture 5: Photo taken from the north side of the washout.

Picture 6: Photo taken from the north side of the washout.

Possible bedrock outcrop

Geotechnical Site Visit Observations and Recommendations Page 8 September 2017

Picture 7: Photo taken from the south side of the washout.

Picture 8:P Photo taken upstream from the washout.

FS Road 29, MP 9.3; 46.3464°N, -121.7998°W

This site was a large embankment fill that crosses Scamp Creek. The site is similar to MP 7.6 site except has two creek channels or tributary to Scamp Creek that their confluence is just up the drainage from the centerline of the road. A debris flow deposited logs and boulders against the embankment and culvert inlet, washing out both of them. There are two previous culverts, a 48-inch and 60-inch.

Geotechnical Site Visit Observations and Recommendations Page 9 September 2017

USGS Geologic Map of the McCoy Peak Quadrangle, Swanson, 1992, identifies the underlying rock unit at this site as volcaniclastic rocks with sulfide mineralization with an unknown age. No bedrock outcrops were noted during the field reconnaissance. Soils in the exposed slopes under the road consist of silty sand with gravel. Cobbles and boulders up to 4 feet in diameter are also present.

Recommendations

WFL Hydraulics Group is recommending to replace both culverts with two 72-inch culverts. The DSR recommends a soil embankment to repair this site. The slope of this drainage is not as steep as the drainage at Camp Creek. A survey activity was performed and cross sections have not been issued. Once cross sections are provided a design can be determined. WFL Hydraulics and Geotech Groups will collaborate if an open bottom culvert is recommended for this site.

Picture 9: Photo taken from the north side of the washout.

Geotechnical Site Visit Observations and Recommendations Page 10 September 2017

Picture 10: Photo downstream from the north side of the washout.

FS 29 Road, MP 10.2; 46.3348°N, -121.7896°W

This site was an embankment fill about 15 to 20 feet high that crosses Sunrise Creek. A debris flow deposited primarily logs, cobbles, and boulders against the embankment and culvert inlet;

and washed out the embankment. The creek channel has incised back into an old channel north of the previous culvert and is relatively clear of debris. The previous culvert was a 96-inch diameter CMP culvert.

Recommendations

USGS Geologic Map of the McCoy Peak Quadrangle, Swanson, 1992, identifies the rock unit at this site as volcaniclastic rocks with sulfide mineralization with an unknown age and Quaternary aged-alluvium consisting of unsorted, unconsolidated deposits of slope wash and local open-work talus, mostly along sides of major streams.

WFL Hydraulics Group is recommending to replace the 96-inch CMP culvert with an 84-inch CMP culvert installed in the north incised channel. The DSR recommends a soil embankment. A soil embankment with a 1V:2H slope ratio at this site is an appropriate mitigation.

Geotechnical Site Visit Observations and Recommendations Page 11 September 2017

Picture 11: Photo from the north side of the failure, incised channel.

FS 29 Road, MP 10.4; 46.3328°N, -121.7886°W

This site was a large embankment fill that crosses Jumbo Creek. A debris flow deposited logs and boulders against the embankment and culvert inlet, washing out the road embankment.

USGS Geologic Map of the McCoy Peak Quadrangle, Swanson, 1992, identifies the rock unit at this site as volcaniclastic rocks with sulfide mineralization with an unknown age and Quaternary aged-alluvium consisting of unsorted, unconsolidated deposits of slope wash and local open-work talus, mostly along sides of major streams. The exposed bedrock is moderately weathered with sulfide staining. The bedrock on the north side of the junction of the road and Jumbo Creek is steeply dipping at about 60 to 70 degrees to the south and is exposed on both sides of the road.

The bottom of the creek is about 28 feet below the existing road. On the south side of Jumbo Creek bedrock is exposed several hundred feet at road elevation up and down Jumbo Creek but is assumed to be under the south side road embankment.

Recommendations

WFL Hydraulics Group is recommending to install a fish passage culvert with a span of 17 to 19 feet. It is assumed the rock quality in Jumbo Creek will show signs of being completely to highly weathered near the surface and improving weathering condition with depth. The rate that is weathering changes is unknown. Bedrock and boulders should be expected during excavation.

WFL Hydraulics and Geotech Groups will collaborate if an open bottom culvert is recommended for this site.

Geotechnical Site Visit Observations and Recommendations Page 12 September 2017

Picture 12: Jumbo Creek from the north side looking up stream.

Picture 13: Jumbo Creek from the north side looking down stream.

Geotechnical Site Visit Observations and Recommendations Page 13 September 2017

Picture 14: Jumbo Creek from upstream looking west (downstream). Road is located at the arrows.

FS Road 88 MP 19.7; 46.1825°N, -122.6874°W

This failure is about 45 feet long and has effected almost full width of the road. It appears that a 1V:2H embankment can be reconstructed at this site. The Hydraulics Reconnaissance Report dated July 20, 2017, states this site requires an Aquatics Organism Passage (AOP) culvert.

Hydraulics and Geotech Groups will collaborate if an open bottom culvert is recommended for this site.

90 Road MP 11.5; 46.0621°N -122.0865°W

The DSR states heavy rains have over saturated the cut slope causing the slope to fail. The slide blocked both lanes of the road, resulting in road closure. (Picture 15 and 16) The Forest Service cleared one lane.

The slope failure affects about 140 feet of road and extends up the slope about 74 feet high. A stream and an 18-inch CMP culvert is located along the western limits of the slide. The culvert inlet is partially damaged and blocked.

The slide material consists of clayey silty sand with gravel and cobbles. The material under lying the clayey, silty sand is volcanic-derived rock unit. It appears globally stable. During the field reconnaissance, no seeps in the slope were noted. A field reconnaissance drawing is provided in Attachment B-1

The software program SLIDE 7.0 by Rocscience Inc. was used to perform global stability analyses of the landslide. The landslide was evaluated by reviewing the slide conditions, assumed groundwater conditions as noted during field reconnaissance, subsurface materials, and surface topography to produce generalized cross-section for analysis. Two material types were used in the analysis, one layer modelled as the volcanic derived bedrock and the second layer as the embankment soil from the cut slope.

Geotechnical Site Visit Observations and Recommendations Page 14 September 2017

Volcanic derived bedrock

Embankment Soil

Moist Unit Weight (pcf) 130 120

Friction/ Residual Friction Angle (deg.) 35 32

Cohesion (psf) 50 0

Recommendations

A mechanically place rock buttress is recommended for this site. A detail is provided as Attachment B-2. Clean all the loose material from the slope. Remove trees that exhibit exposed roots or are leaning towards the road. Place Class 4 rock for buttresses, 5-feet thick against the slope at a 1V:1.25 face slope. Install Class 1, non-woven, Type B geotextile between the rock and the slope. Choke the top of the rock with small pieces of buttress rock. Lap over the geotextile over the top of the choked buttress and place a layer of small buttress rock over the geotextile covering it. Clean the ditch near the culvert inlet and verify the culvert is functioning.

Picture 15: Site 90 Road, MP 11.5.

Geotechnical Site Visit Observations and Recommendations Page 15 September 2017

Picture 16: Site 90 Road, MP 11.5.

90 Road MP 28; 46.1399°N -122.9029°W

WFL received two DSRs for this site with the inspection dates of December 12, 2015 and April 24, 2017. Ryan Cole, Zone Geotechnical Engineer, Mt. Hood National Forest and Jonathan Heyl, Forest Engineer, Gifford Pinchot National Forest provided a box of background documentation about this slide dates back to 1971.

This slide is known as the Crab Creek Slide. Mapping completed by the Forest Service shows that this slide has affected this section of road for about 3000 feet. In 1988 the Gifford Pinchot National Forest Geotechnical Engineer identified 7 sites affecting this section of road for about 3000 feet. Six geotechnical subsurface borings were drilled in 1985 and 1988. One boring had a piezometer installed, three borings had slope inclinometer casing installed, and two had stand pipes installed. Figure B-1 shows a Site Plan from 1989 with the locations of the borings and zones of distress. Also on the Site Plan are two additional sites of distress and settlement that were noted during the 2017 site reconnaissance.

FS Road 90 is a paved two lane road; however, for about 3500 feet this section of FS Road 90 has a gravel surface. The DSR dated April 24, 2017 discusses six areas of settlement with lateral scarps exhibiting vertical displacement of 1 to 2 feet. The 2017 site reconnaissance noted four distinct zones of settlement and road distress. One area is only wide enough to pass one direction, and the other sites exhibit a reduced roadway width. Seeps from the cut slope, non-existent ditches, areas where water collects in the ditch, and leaning trees are all present through this corridor.

Geotechnical Site Visit Observations and Recommendations Page 16 September 2017

In the DSR dated April 24, 217, the first picture shown is also shown below as Picture 17. In contrast, Picture 18, taken on July 9, 2017, was taken very near to the location of Picture 17. The arrows show reference points. Picture 19 is a close up of the reference point on the outboard shoulder.

This is a large landslide that appears to have instabilities within larger instabilities and will require time to study and organize the information to understand the characteristic of this area.

Understanding prior mitigations to stop or slow these instabilities will also be useful during a geotechnical analysis. What mitigations have the Forest Service installed that have or have not worked in the past and how long did they work? An updated geologic mapping of the site will provide additional information for the analysis and a subsurface investigation may be needed to verify limits of the instabilities.

Recommendations

Below are some possible mitigation alternatives; however, an analysis is required to determine the most effective mitigation for this area. The geotechnical mitigation selection should consider 1) cost, 2) length of service, and 3) required maintenance. Possible mitigation alternatives include:

• Horizontal drains;

• Shear key below the road embankment;

• Shear key under the road;

• Trench Drains;

• Soldier pile wall; and

• Realignment

Picture 17: Picture of one of the site provided in the April 2017 DSR for FS Road 90 MP 28.

Geotechnical Site Visit Observations and Recommendations Page 17 September 2017

Picture 18: Picture taken of the same site in July 2017. Arrows correspond with reference points on Figure 13

Picture 19: Close up of the lateral scarp crossing the road, July 2017.

90 Road MP 45, 46.2426°N, -121.6347°W

The culvert became plugged and water overtopped the road eroding the outboard embankment slope. Picture 20 shows about 4 feet of pavement surface was lost for a length of 120 feet. The soils in the roadway embankment and the surrounding ground at the toe of the adjacent embankment slopes consist clayey, silty sand with gravel and cobbles. Small boulder sized rocks were noted in the failed embankment slope. Occasional larger boulders were noted in the slopes adjacent to the failure and may also be present within the failure area.

Geotechnical Site Visit Observations and Recommendations Page 18 September 2017

Recommendations

Reconstruct the damaged embankment with 1V:2H slope. The Hydraulics Reconnaissance Report states that the culvert will be replaced with a AOP culvert. WFL Hydraulics and Geotech Groups will collaborate if an open bottom culvert is recommended for this site.

Picture 20: Damaged embankment at the 90 Road MP 45 Site.

CLOSING

Recommendations contained in this memorandum are based on observations made during site visits on August 22 and 23, 2016. The mile post stationing listed were provided in the DSRs and should be considered approximate.

Questions regarding this memorandum should be addressed to Diann Morehouse at (360) 619- 7609 or Diann.Morehouse@dot.gov.

ATTACHMENTS

• A 25 Road, MP 25 - Special Rock Embankment Detail

• B-1 90 Road, MP 11.5 - Field Reconnaissance Drawing

• B-2 90 Road, MP 11.5 - Rock Buttress Detail

REFERENCES

Swanson, D.A., (1992) Geologic Map of the McCoy Peak Quadrangle, Southern Cascade Range, Washington: U.S. Geologic Survey, Open-File Report OFR92-226, 1:24,000 scale.

Swanson, D.A., (1989) Geologic Map of the French Butte and Greenhorn Buttes Quadrangle, southern Cascade Range, Washington: U.S. Geologic Survey, Open-File Report OFR89-309, 1:24,000 scale.

mailto:Diann.Morehouse@dot.gov

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