Conceptual Designs And Cost Estimates: A Critical Step In Managing Unstable Slopes Along Washington State Highways

摘要

The Washington State Department of Transportation (WSDOT) has developed a proactive and rational approach for identifying, categorizing, and prioritizing for mitigating unstable slopes along their 6,835-mile highway system. Early evaluation and scoping of unstable slopes as it relates to the geologic problem, mitigation and cost issues are critical in planning, budgeting and prioritizing unstable slopes for mitigation. In 1993, WSDOT developed the Unstable Slope Management System (USMS) to address known slope hazards adjacent to WSDOT's highway system. The objectives of the program are to (1) rationally evaluate more than 2500 unstable slopes, (2) perform early scoping, conceptual designs and cost estimation, (3) conduct cost-benefit analysis of unstable slopes, and, (4) prioritize the mitigation of known unstable slopes according to the expected benefits. Utilizing a matrix-based numerical rating system, the USMS includes not only rockfall hazards but also landslide, settlement, and erosion problems. Most of the 2,500 unstable slopes have been rated and entered into the USMS. Presently WSDOT Geotechnical Division and their consultants are performing early scoping of the slopes by developing conceptual designs and preliminary cost estimates for mitigation. Because of the large number of sites, WSDOT based prioritization of the slopes for evaluation on (1) highway functional class, (2) USMS numerical rating, and (3) average daily traffic (ADT). Senior-level engineering geologists field inspect the unstable slope and develop a problem definition and conceptual design for each slope. Each unstable slope conceptual design slated for mitigation includes location, problem definition, problem correction and cost estimate supported by detailed field notes. Once the conceptual design has been completed, cost estimates for the work are developed. Unit costs are based on average construction bid tabulations for similar type work in the last five years. The conceptual designs and cost estimates to mitigate the slope hazard are then entered into the USMS and become a permanent record in the database. The WSDOT Geotechnical Division uses the cost estimate for each unstable slope in the cost-benefit analysis for mitigation of the slope hazard. Cost-benefit analysis for slope stabilization considers anticipated cost of traffic impacts resulting from a slope failure and the annual maintenance costs over 20 years divided by the cost to mitigate the slope hazard. Because of limited funding, only those slopes with cost-benefit ratios above 1.0 are considered for mitigation. Since the USMS program has begun, WSDOT can demonstrate accurate and conservative results between the conceptual design, engineer's estimate and the contractor's low bid. Typically, cost estimates in the conceptual design have been accurate and conservative, and higher than the Engineer's estimate and the Contactors low bid. The results of the USMS are a rational and proactive program for mitigating geologic hazards along the Washington State highway system.