Big Creek Lake Dam, Mobile, Alabama

Abstract Only - Big Creek Lake Dam is located approximately 18 miles west of Mobile, Alabama and impounds a raw water reservoir that is the sole source of drinking water for the City of Mobile. The facility was built in the early 1950’s and is an earth embankment, approximately 5,000 feet in length and 75 feet in height. The dam impounds approximately 69,000 acre-feet of raw water. Since 2001, Malcolm Pirnie has provided a comprehensive array of geotechnical, hydraulic, and structural related dam engineering services at Big Creek Lake Dam. An initial inspection of the dam and appurtenant works, combined with a review of historic documents, provided the basis for establishing short, intermediate, and long term goals for modernizing the 50-year old dam. Although Alabama remains as the only state without a dam safety program, the dam owner (Mobile Area Water and Sewer System) has been very proactive in maintaining the dam and implementing improvements to bring the dam into compliance with anticipated dam safety criteria. Reservoirs such as Big Creek Lake Dam are critical components of the nation’s water supply system and this presentation is intended to highlight some of the typical problems with aging dams along with the requisite analyses and remedial improvements to ensure long-term operation of these reservoirs.
In late 2004 and early 2005, sinkholes developed along the top of the spillway chute training walls and sand was observed eroding out from the spillway chute underdrainage system. After a significant rain event in April 2004, the Owner noticed movements of the slabs at three locations and loss of large amounts of material from beneath the slabs. Malcolm Pirnie performed an emergency inspection and discovered voids and caverns four or five feet deep and approximately 15 feet in diameter beneath some concrete slabs and determined that the drain pipes that extended through the concrete chute slabs were not properly filtered and that soil had been eroding for some time. Based on our recommendations, emergency construction repairs were initiated consisting of 18-hour days, seven days a week for several weeks. Voids were backfilled and grouted, the drain pipes were fitted with slotted PVC pipe inserts (grouted in-place) to stop erosion, and several cracked spillway slabs were reinforced with soil anchors to avoid uplift during spillway releases. Emergency repairs were all done in advance of the hurricane season to temporarily stabilize the spillway chute. Cont.