Sinkhole Development and Repairs at Willow Creek Dam, Montana

The development of an 18-foot-diameter sinkhole at Willow Creek Dam was completely unexpected for a small (6-foot-diameter) outlet tunnel excavation with up to 40 feet of bedrock cover and over 30 feet of overlying embankment. Contributing factors were the open fractures and large void in the bedrock, and the erodible nature of the embankment materials. Foundation excavation and treatment, reconstruction of the dam embankment, installation of a larger air vent, and partial lining of the tunnel with CIPP were performed using emergency dam safety funds to mitigate a potential failure, with minimal project impacts.
The proposed addition of a new trashrack structure and rehabilitation of the gate operating system will improve the reliability of the outlet works for future operations. The total construction cost for the dam safety repairs will be less than $1 million, which is below the limit authorized by Congress. An assessment of all potential failure modes and associated risks to the downstream public was made, which may result in the future construction of a downstream stability berm and filtered buttress to address both dynamic stability and internal erosion deficiencies. With these modifications, Willow Creek Dam will continue to serve the historic Sun River Project of western Montana well into the future. 11 pp.