K20676_Alder Slope Rockfall Criteria Memo.pdf

GEOTECHNICAL MEMORANDUM

ODOT Region 1 Geo/Hydro/HazMat Unit

Date: April 16, 2020

To: Magnus Bernhardt, Project Leader

From: Michael Zimmerman, PE, GE, CEG; Region 1 Senior Engineering Geologist

RE: Rockfall Catchment at Alder Slope Historic Columbia River Highway State Trail Viento to Mitchell Point, Trail Segment E Hood River County Key No. 20676

An approximately 200-ft-long section of the Historic Columbia River Highway (HCRH) is located adjacent to the base of Alder Slope. This abandoned segment of the historic highway will be re-opened to trail users as part of the HCRH State Trail (State Trail). The consultant design team of David Evans and Associates and Cornforth Consultants evaluated several rockfall mitigation alternatives for the Alder Slope, as presented in the Preliminary Rockfall Evaluation Report (Cornforth Consultants, 2018).

This Geotechnical Memorandum briefly discusses the alternatives considered by the consultant team for rockfall mitigation at the Alder Slope section of the State Trail and provides our recommendation for a preferred alternative considering safety, operations, aesthetics, construction cost, and meeting overarching State Trail project goals.

SITE DESCRIPTION

Alder Slope is a northeast-facing slope about 100 ft tall located between about State Trail station 386+25 and 388+25, about 150 to 350 ft west of Perham Creek. The portion of the slope with rockfall hazard considerations is about 200 ft long. The hillsides to the east and west are lower, less steep, and heavily vegetated with trees. The trail alignment follows the historic highway grade along the base of Alder Slope. As described in the Preliminary Rockfall Evaluation report for the State Trail Segment E, the majority of Alder Slope is a natural slope. An approximately 100-ft-long cut up to about 30 ft tall is present at the base of Alder Slope. The cut has an average inclination of 71° (about 0.3H:1V) and the natural rock slope above the cut has an average inclination of 65° (about 0.5H:1V) (Cornforth Consultants, 2018).

As discussed in a meeting with the consultant design team regarding the rockfall mitigation criteria for Alder Slope, this section of the HCRH has been closed to traffic and unmaintained since construction of the highway along the I-84 alignment in the early 1950s. However, very little rockfall debris has accumulated on the

Department of Transportation Region 1 Geo/Hydro/HazMat Unit

123 NW Flanders Street

Portland, OR 97209‐4012

Phone: (503) 731‐8200

Rockfall Catchment at Alder Slope Historic Columbia River Highway State Trail

Viento to Mitchell Point Section, Trail Segment E Hood River County

Key No. 20676 historic highway pavement adjacent Alder Slope. Considering this section of the HCRH has been unmaintained for nearly 70 years, the rate of rockfall from Alder Slope appears to be relatively low (Cornforth Consultants, 2018).

Engineering geologists from ODOT Region 1 visited the Alder Slope site on December 30, 2019 to confirm the observations provided by Cornforth Consultants. The rock slope is heavily vegetated with ferns or is moss-covered. Relatively few pieces of rockfall debris are visible on the historic highway and most of the observed rockfall debris is covered by a thick growth of moss. All the rockfall debris observed within the limits of the proposed State Trail alignment was covered with moss. ODOT observed rockfall debris in the 6-inch-diameter size range, but no obvious rockfall debris in the 2-ft-diameter range was visible.

DISCUSSION

In 2013 ODOT geotechnical engineers and engineering geologists assessed the rock slopes along and 8 mile section of the HCRH from Wyeth to Hood River and developed design guidelines for rock cuts and recommendations for rockfall mitigation measures along the HCRH State Trail based on 4 primary criteria:

safety, operations, aesthetics, and cost (Gullixson and Peltz, 2013). The 2013 study recommended rockfall catchment areas should be designed to contain 99% of all initial rock impacts with the ground adjacent the base of the rock slope and 90% of all rocks rolling or bouncing across the ground after impacting the area adjacent the base of the slope. These design guidelines were primarily developed to address safety (99% impact retention) and operations and maintenance (90% roll out retention), but do not account for the rate of occurrence of rockfall. The rate of occurrence should be considered in rockfall hazard evaluations. For example, rockfall hazard rating systems, such as the widely used Rockfall Hazard Rating System (RHRS) (Pierson, et al., 1993) and the Unstable Slopes Management Program (USMP) recently developed by the Federal Highway Administration (Beckstrand et al., 2017), include the frequency of rockfall in their respective hazard rating criteria.

Three alternatives were considered for rockfall mitigation at Alder Slope: relocation, catchment barrier, and catchment ditch. Relocating the trail away from the slope would move the trail beyond the limits of the historic highway pavement and would require a minor fill slope extending outside the work limits established for this project. A low rockfall catchment barrier consisting of rock-filled wire mesh gabion baskets would improve rockfall retention to meet the design guidelines. However, the gabion wall might also be seen as benches that encourages trail users to linger at the base of the rock slope. Additionally, the gabion basket barrier would have a negative impact on the aesthetics, cost to construct the rockfall mitigation, and maintenance of the ditch at this site.

Rockfall modeling completed for the Alder Slope site (Cornforth Consultants, 2020) indicates retention rates for rolling rocks can be substantially increased compared to the current slope geometry if the accumulated rockfall debris is removed from the base of the slope to create a catchment ditch with a 4H:1V backslope away from the trail. Based on the rockfall modeling completed for two locations, the edge of the trail will be located about 12 to 14 ft away from the base of the slope. With this geometry, rockfall modeling for 6-inch-diameter

Viento to Mitchell Point Section, Trail Segment E Hood River County

Key No. 20676 rocks indicates 96 to 98% of the impacting rocks and 74 to 79% of the rolling rocks are contained within the catchment area. The rockfall modeling indicates approximately 99% retention of impacting rocks and 74 to 78% retention of rolling rocks for 2-ft-diameter rockfall debris for the proposed catchment ditch geometry.

RECOMMENDATIONS

Our observations and the observations of the consultant design team for the project suggest rockfall occurs infrequently at the Alder Slope site. Based on the apparent low frequency of rockfall occurring and the amount of rockfall retention that can be achieved with a catchment ditch; and considering the aesthetics in the National Scenic Area and the project goal to connect the State Trail using the alignment of the historic highway, we recommend the rockfall catchment ditch as the preferred alternative for rockfall mitigation at Alder Slope.

LIMITATIONS

The recommendations presented in this memorandum are intended solely for use on this site for this project.

The rates of geologic hazards can vary from past observations. If rockfall rates different from the conditions stated in this memorandum are identified or encountered at this site, promptly advise the ODOT Region 1 Geo/Hydro/Hazmat Unit so we may observe these conditions and revise our recommendations if necessary.

Any interpretation or evaluation of the information provided by this memorandum for use outside the project described in this memorandum is done so at that individual’s sole risk.

Prepared by:

Michael J. Zimmerman, PE, GE, CEG Region 1 Senior Engineering Geologist

Renews: 11/30/2020

Reviewed by:

Thomas Braibish, PE, GE Region 1 Senior Geotechnical Engineer

Viento to Mitchell Point Section, Trail Segment E Hood River County

Key No. 20676

References:

Beckstrand, D., Stanley, D., Thompson, P., Bilderback, E., Wittie, M., Kanewala, U., Cuelho, E., and Anderson, D.A., 2017, Unstable Slope Management Program for Federal Land Management Agencies. Federal Highway Administration Report No. FHWA-FLH-18-00x

Cornforth Consultants, 2020, Alder Slope Rockfall Simulations, Historic Columbia River Trail – Viento to Mitchell Point, Hood River, Oregon.

Cornforth Consultants, 2018, Preliminary Rockfall Evaluation Report, Historic Columbia River Highway State Trail, Segment E, Hood River County, Oregon.

Gullixson, C.F. and Peltz, T.R., 2013, Historic Columbia River Highway Rockfall Hazard Study. Oregon Department of Transportation, Region 1 Geo/Hydro/HazMat Unit

Pierson, L.A., and Van Vickle, R., 1993, Rockfall Hazard Rating System – Participants’ Manual. Federal Highway Administration, Report No. FHWA-SA-93-057

Pierson, L.A., Gullixson, C.F., and Chassie, R.G., 2001, Rockfall Catchment Area Design Guide. FHWA Final Report SPR-3(032)