Small Dam, Big Problems, Unique Solutions—Coal Ridge Waste Dam

Coal Ridge Waste Dam, located in Weld County, Colorado is a small homogenous earthen embankment dam. The dam is approximately 2,720 ft long, has a maximum jurisdictional height of 28 ft, and a normal storage volume of 600 acre-ft. In January 2014, during repairs to the reservoir’s control gate, contractors removed some poor quality concrete at the invert of the outlet vault structure, which allowed saturated embankment material and groundwater to flow into the vault, resulting in a significant sinkhole on the upstream slope, adjacent to the vault. To prevent further damage and possible failure of the dam, emergency measures were taken to drain the reservoir and stabilize the control vault. This paper discusses the emergency response and mitigation alternatives considered, along with the slip lining and grouting program selected to install a new outlet pipe through the dam with a new control gate on the upstream face of the dam. A construction manager at risk (CMAR) contract was selected for the design and construction of the repairs. The project was completed ahead of schedule and under budget due to the cooperative environment between the owner, designer, contractor, and regulator created by the CMAR contract.