Rebound in Redesign of Oahe Dam Hydraulic Structures

Excavation for the stilling basin of the outlet works removed a maximum overburden load of 12 tsf from Oahe Dam. Normal foundation rebound was expected; however, differential rebound occurred along pre-existing fault surface creating an unusual design problem. In situ rebound measurements were gathered rapidly and were studied and evaluated. Individual structures were designed to help control and accommodate further rebound movements. Concurrently with redesign, pilot installations of individual anchors and groups of anchors were made to determine the effect of a given retarding effort on different size areas and depths of the shale foundation. These anchor tests showed the shale in the upper 20 ft to 40 ft of the foundation could be economically and effectively restrained by use of anchors. A five year operational history (1958 to 1963) confirms the effectiveness of the anchors and indicates the redesign of the structures was a satisfactory solution to the rebound problem.