UNSTABLE SLOPE MANAGEMENT FOR OREGON HIGHWAYS: USING COST-BASED RISK ASSESSMENT IN CONJUNCTION WITH SITE CHARACTERISTICS TO MANAGE LANDSLIDE AND ROCKFALL EFFECTS ON THE TRANSPORTATION SYSTEM

Landslides, rockfalls, and debris flows are endemic to the transportation infrastructure in Oregon. Unstable slopes are hazardous to system users and expensive in terms of direct costs for repair and maintenance as well as costs related to service disruptions and liability. Approximately 4,000 unstable slopes affecting state routes have been identified to date. The Oregon Department of Transportation expends nearly $7 million annually for unstable slope maintenance alone and less than $6 million is available per year to permanently address known sites. The typical cost for individual site mitigation approaches $3 million.

This magnitude of problem and limited repair funds have driven the development of a system to 1) effectively manage the limited construction budget dedicated to unstable slope repair, 2) identify sites receiving the greatest benefit from repair funding, 3) create a prioritized list of unstable slope repair projects, 4) be adaptable to risk assessment and life-cycle cost analysis from a system-wide to site-specific level, and 5) provide accessible information to decision-makers.

Previous unstable slope management has focused on frequently complex geographic and geologic conditions that contribute to slope instability for specific cases. To address unstable slopes on a state-wide level, an asset management-based approach was chosen. Rather than rating individual sites based on the characteristics that predispose a site to instability and assigning a probabilistic score, the new system evaluates only known unstable slopes and scores criteria based on a site’s current impact to the system. Scoring criteria are based on relative hazard, infrastructure affected, failure consequence, site history, and system use. Further scoring modification is based on the cost-benefit of repair and route hierarchy.

Site data collection is supplemented by data research and personnel interviews. Information is provided with existing, common software and accessed via database and GIS interface. This system is currently in use for project selection, scoping, and life-cycle cost analysis for proposed mitigation projects. It is also used as a tool to facilitate decisions based on project goals, available funding and desired level of service in addition to “what if” evaluation of mitigation alternatives.