Emergency Repair of Big Creek Lake Dam Spillway

Big Creek Lake Dam was constructed in 1952 for the Mobile Area Water and Sewer System (MAWSS) to serve as the primary water supply for the City of Mobile, AL. The dam consists of a 5,000-ft long zoned embankment with a maximum height of 75 ft providing storage of 69,000 acre-ft at normal pool. Releases from the reservoir are controlled by seven 31-ft wide by 11-ft high tainter gates that discharge into a 1,000-ft long spillway. Although Alabama is currently the only state without a dam safety regulatory program, in 2002 MAWSS proactively embarked upon a phased inspection and evaluation program for the aging dam. They completed geotechnical investigations, embankment seepage and stability analyses, and had constructed new seepage control and slope stabilization features along the embankment toe. In 2005, MAWSS had just authorized the next phase of work to evaluate the spillway structure, when during a spillway release several concrete slabs in the spillway chute exhibited several inches of vertical movement (heave due to uplift pressures followed by settlement). Upon closer inspection voids were discovered beneath several slabs and a significant quantity of sand had accumulated in the stilling basin. The sand had been eroded from the spillway foundation through the underdrainage system and open joints between concrete slabs. Since the dam had been constructed without any low level outlet works (except a pump station intake), the only way to make reservoir releases was through the spillway. MAWSS immediately mobilized contractors and engineers who worked long days for several weeks to temporarily stabilize the spillway until the conditions could be analyzed and a more permanent rehabilitation design could be developed. This paper will describe all phases of the project including: the initial emergency response, the evaluation of the failure mechanism, and the performance of a multi-staged design and construction program that was completed while keeping half of the spillway in operation. Another interesting aspect of the work included the construction of overbank armoring (with Articulated Concrete Blocks) along the top of the spillway training walls to accommodate the Spillway Design Flood and convey flows back into a new stilling basin. The general conditions surrounding this type of spillway failure will be described along with recommendations on the types of evaluations that should be made when inspecting similar structures as part of any dam safety program. 29 pp., 2 tables, 57 figures, 13 references.