Non-residential Projects: Rock Island Slope Stabilization
June 1, 2007
An unstable 300-foot high basalt rock slope continuously produced falling rock onto State Road 28 in Washington State. The highway and railroad tracks occupy a thin strip of land between the base of the cliffs and the Columbia River, just below Rock Island Dam in Douglas County. DOT maintenance crews routinely were called to clear hazardous rock fall from the roadway. The stabilization project will significantly improve traveler safety. At the same time, the project is designed to accommodate future widening of the road. The project is broken into three stages.
Stage 1 removed 220,000 cubic yards of basalt rock at the southern half of the project boundaries and created two containment tiers, or “benches,” to keep rocks from reaching the roadway. The lower tier, 30 feet above the level of the highway, will serve as the roadbed for two more lanes as the road is expanded in the future.
Stage 2 constructed a retaining wall 310 feet above the roadway at the northern half of the project. The soil nail wall is more than 425 feet long, and up to 35 feet high. Its purpose is to contain rock and stabilize the loose talus rock slope above it. This will allow crews to work safely below it as they remove another large unstable rock outcropping in stage 3.
Stage 3 involves more controlled blasting and excavation of approximately 150,000 cubic yards of rock a short distance north (upstream) from Stage 1. It also involves installation of 500 feet of high-impact rockfall fence, repair of rock baskets, and railroad protection measures. Construction for this phase of the project is scheduled for construction in the spring of 2010.
When complete, the potential for falling rocks to reach the highway and railroad tracks below the huge basalt cliffs at Rock Island Dam will nearly be eliminated. Safety will be dramatically improved. DOT maintenance expenses will be reduced, as will the costs to add more lanes to SR 28 in the future.
Aside from the obvious and immediate benefits of reducing the driving hazard presented by falling rocks on the highway and the accommodations for future expansion of State Road 28, drainage and sight-distance also are improved. This further improves safety and reduces maintenance expenses. The 18,000 dump truck loads of rock removed during stage 1 were stockpiled for use in future DOT road building projects.
This project also is an example of cooperation. From an engineering standpoint, the original plan called for the work to be complete in two construction seasons. As a result of DOT’s experience during stage 1, fruit industry representatives were invited to discuss the impact of traffic delays at harvest times. The project was broken into three stages to try to minimize impacts to the early cherry harvest, soft fruit and apple harvests, which occur August through October.
Because there are no reasonable detours around the project, traffic control affecting commuters, truckers, travelers, emergency service providers and the railroad has been a top priority. Significant planning and expense resulted in a successful public information program to provide users with timely and accurate delay or closure information utilizing a number of resources.