CONTRIBUTING FACTORS FOR THE MITIGATION OF LANDSLIDE-TYPE HAZARDS ALONG OREGON DEPARTMENT OF TRANSPORTATION HIGHWAY CORRIDORS

The Oregon Department of Transportation (ODOT) is responsible for maintaining a highway system that traverses dynamic topography and complex geology. Consequently, ODOT’s highways are subject to geologic hazard related damage every year that results in road closures and substantial repair costs. Maintaining the highway system requires not only awareness of the geologic hazards but also familiarity with the non-geologic limitations of the transportation corridors. Determining the appropriate mitigation to repair a landslide, rockfall or retaining wall failure is often governed by environmental issues, right of way, utility, roadway, and mobility limitations. In addition, engineering geology and geotechnical engineering within a transportation corridor demand the understanding of schedule, access, and constructability issues which typically drive the repair design.

During November 2008, a rockfall on US30, west of Clatskanie, Oregon, resulted in over five weeks of lane closures while ODOT engineering geologists worked with ODOT Maintenance and a rock scaling contractor to remove remaining loose rock and vegetation from the slope. Because US30 is a primary mobility and emergency route between US101 along the Oregon Coast and I-5, scaling efforts were conducted during 20-minute road closures and consequently lasted over one month and cost nearly $100,000 in emergency contracting. For rock fall mitigation along major transportation routes, mobility considerations which limit the duration of lane closures will drive the schedule and cost of operations. For the public transportation system, maintaining safe mobility is a top priority.

To begin evaluation and design, ODOT engineering geologists and geotechnical engineers coordinate internally with environmental, utility and right of way specialists, as well as roadway and traffic engineers. Simultaneously, the engineering geologist and geotechnical engineer must begin developing a preliminary design, to include a subsurface exploration program when appropriate, in order to complete a fast-track repair that reduces the risk to travelers. In most cases it is also necessary to provide ODOT Maintenance with recommendations for temporary mitigation measures to ensure that the traveling public is protected prior to the final repair.