Undermining of Spillway Chutes

This paper includes discussion of undermining of chute spillways, including several case histories. Loss of foundation materials beneath a spillway which remains undetected and in a state of disrepair can potentially lead to a failure of the structural spillway, followed by headcutting erosion through the embankment dam and an uncontrolled release of the reservoir. The identification of the failure mechanisms and suspect construction details that could lead to undermining is critical to the inspection and rehabilitation of existing spillways. The paper will include a review of failure mechanisms (structural collapse and hydraulic jacking) identified by Trojanowski in several recent publications, which focused primarily on high velocity flow entering open joints in chute spillways. Additional failure mechanisms include: (1) erosion of foundation materials beneath chute spillways due inadequate filtering of spillway drainage material and (2) significant seepage through foundation materials due to inadequate cutoff at the spillway control section. Several cases studies of chute spillways with undermining problems will be presented, and the poor construction details or design oversights which contributed to the undermining at each site will be highlighted. In addition, current state-of-the practice approaches for preventing chute undermining will be presented and inspection, detection, and rehabilitation approaches for existing chute spillways will be reviewed. 22 pp., 7 references.