The Clearwater Dam in Wayne County, Missouri, is a 154 foot (46.9 m) high zoned earth-fill structure constructed on Cambrian and Ordovician carbonate rocks, primarily for flood control. A sinkhole, 10 feet (3 m) across and 10 feet (3 m) deep, formed in January 2003 on the upstream face of the dam at an elevation of 570 feet (173.7 m). The sinkhole development was most likely a result of sediment loss through piping due to the epikarst solution features at the base of the dam. Epikarst is defined as the uppermost weathered zone of karstified rock with enhanced porosity and permeability compared to the underlying bulk rock. Epikarst formation progresses over time through stress release, weathering, and dissolution. The near surface weathered zone acts as a recharge zone for the underlying karst. The epikarst at the Clearwater Dam site is a highly variable zone. The zone spans a thickness of up to 100 feet (30.5 m) in the foundation of the dam. Upstream, the epikarst is exposed in an outcrop where it formed along the bedding planes, joints and top surface of the Potosi dolomite, roughly 30 feet (9.1 m) thick. These solution features and joints create serious seepage and piping problems for the dam.
The dam has undergone remedial measures to install a cutoff wall to block the flow of water through the open cavities.
This study investigated the nature and causes of seepage problems in light of geologic conditions present at the Clearwater Dam site by evaluating outcrops and borehole logs provided by the U.S. Army Corps of Engineers and Gannett Fleming, Inc. It also characterized the epikarst layer in the dam foundation in terms of its geologic, hydrologic, and engineering properties. A representative site was chosen upstream from the right abutment of the dam which contained an outcropping of the Potosi dolomite and the overlying epikarst material. This site is locally known as “epikarst beach”. A detailed line survey was conducted to measure the orientation and other aspects of the joints. Epikarst material from the site was collected to perform absorption tests, specific gravity tests, unconfined compressive strength tests and direct shear tests. Laboratory testing revealed a much weaker and more porous rock than typical dolomite. Epikarst porosity is roughly 50%, specific gravity is half of standard dolomite, and absorption is twice that of dolomite. The unconfined compressive strength and shear strength parameters were significantly lower than dolomite, proving the epikarst is at least 50% weaker. However, there was some variation of strength between the different samples that may be attributed to the amount of chert and voids in the samples.