Analysis of Sheffield Dam Failure

Abstract
This paper describes the circumstances leading to the failure of a 25-ft high earthfill dam during a magnitude 6.5 earthquake in 1925. Photographs and descriptions of the dam after the earthquake indicated that failure occurred by sliding on a nearly horizontal plane near the base. Pseudo static methods of analysis using soil strengths determined by static loading tests indicated a theoretical factor of safety between 1 and 1.2. However, the position of the most critical sliding surface did not correspond with the observed mode of failure. Dynamic response analyses are described including test data obtained by cyclic loading triaxial and simple shear tests and stress distributions determined by finite element methods; these lead to improved predictions of both the failure mechanism and the calculated factor of safety. The study suggests that failure was initiated by liquefaction of a zone of soil below the center and at the base of the dam; as shaking continued the zone of liquefied soil spread laterally until after 10 to 15 sec the base became so weakened that the dam was pushed downstream by the force of the reservoir water.

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