Austin Dam, Pennsylvania: The sliding failure of a concrete gravity dam

The Austin Dam, a concrete gravity structure, failed catastrophically on September 30, 1911, less than 2 years after its construction. Several engineering evaluations of the failure have been made since immediately following the disaster. Engineers involved in the construction of the dam recognized early on that the dam failed by sliding on its rock foundation. The dam was constructed on interbedded sedimentary rock, and its foundation was taken to a depth of approximately 4 ft (1.2 m). Although studies of the orientation of the failed blocks of the dam and other site evidence pointed to a sliding failure, no detailed quantitative studies have been done. Recent test pit excavations showed that the critical interface for sliding was a sandstone/shale contact located at a shallow depth below the base of the dam. Stability analyses based on shear-strength tests of the different interfaces of the foundation (concrete/sandstone, sandstone/sandstone, sandstone/ shale, and shale/shale) showed that the dam was safe against a bearing capacity failure, but unsafe with respect to sliding and overturning. The lowest factor of safety of 0.6 corresponded to the sandstone/shale interface, as stated above. The major factors believed to have contributed to the failure include low shear strength of the foundation materials, inadequate provisions for reducing the uplift pressure, and weak concrete. The project stands as the sixth worst dam failure in U.S. history in terms of loss of life, and this case history provides numerous lessons with regard to dam safety.