MARION COUNTY, OREGON: A LOGGING ROAD LEGACY

Forest lands often contain slope stability landmines in the form of old logging roads and these problematic roads are often passed along to subsequent parties.

North Fork Road is a two-lane paved road located in the western Cascade Range foothills. The road provides residential, business, and recreational access to the North Fork Santiam River and the Opal Creek Wilderness Area and is maintained by Marion County. North Fork Road evolved from a narrow-gauge rail line to a gravel-surfaced timber haul road. In the 1950’s the road was paved and transferred to the County.

North Fork Road crosses several large landslide areas and is plagued with road failures. The Bear Trap Slide is located at milepost 3.75 where landslide movement has required continual maintenance of the road since the 1960’s. Emergency measures implemented in December, 2007 included reducing a 300-foot section of the road to one-way traffic across the landslide headscarp and imposing a maximum vehicle weight limit.

In September, 2008 we conducted a geotechnical investigation that included completion of ten borings and installation of a slope inclinometer and four vibrating wire piezometers. The inclinometer was subsequently sheared by slope movement in January, 2009 at a depth of 30 feet below the ground surface. Geologic mapping and subsurface explorations indicated road fill, colluvium, and older landslide debris were sliding on weathered volcaniclastic bedrock. The headscarp of the landslide was confined by a backstop of intrusive diorite located above the road. The slide mass extends down slope approximately 800 feet as a series of translational blocks to the bank of the North Fork Santiam River.

We considered the following mitigation options:

1. Stabilization of the entire landslide mass from the active headscarp to the toe

2. Stabilization of the headscarp area using dewatering methods

3. Stabilization of the headscarp area through construction of a retaining wall

The preferred option was to implement a down slope re-alignment of the road supported on a 30-foot tall MSE retaining wall keyed into the stable volcanic bedrock. The proposed mitigation requires removal of the landslide debris within the road alignment and isolates the road from the active portion of the landslide mass.