UPPER TAUM SAUK RESERVOIR FOUNDATION PREPARATION AND GEOLOGY

At 5:20 a.m. CST, December 14, 2005, Taum Sauk Upper Reservoir located about 90 miles southwest of St. Louis, Missouri; suffered a catastrophic failure. This failure released nearly 4,350 acre-feet of water down the west side of Proffit Mountain into the Black River, in an estimated 30 minutes. The cause was an instrumentation malfunction which allowed the reservoir to fill until it overtopped a crest low point in the northwest area of the rock fill dam, washing out the embankment. This facility is a pump storage facility used to meet peak power demands for AmerenUE.

The reservoir was re-built with 2.8 million cubic yards of Roller Compacted Concrete (RCC). The materials from the original dam were used in the RCC mix. The foundation is 1.5 billion year old Precambrian Rhyolite Porphyry, which has been intruded by 1.1 billion year old diabase dikes. Along the contacts of these dikes the diabase is altered to a range of materials from sandy silt to clay. Identification, engineering analysis, and treatment of these saphrolitic “clay seams”, along with detailed foundation preparation were the focus of the engineering geologic work on the project. The project was completion in February 2010.