Teton Dam - Cause of The Collapse

Cause of The Collapse

Study of the dam's environment and structure placed blame for the collapse on the permeable loess soil used in the core and on fissured (cracked) rhyolite in the foundations of the dam that allowed water to seep under the dam. The permeable loess was found to be cracked. It is postulated that the combination of these flaws allowed water to seep through the dam and led to internal erosion, called piping, that eventually caused the dam's collapse.

An investigating panel had quickly identified piping as the most probable cause of the failure, then focused its efforts on determining how the piping started. Two mechanisms were possible. The first was the flow of water under highly erodible and unprotected fill, through joints in unsealed rock beneath the grout cap, and development of an erosion tunnel. The second was "cracking caused by differential strains or hydraulic fracturing of the core material." The panel was unable to determine whether one or the other mechanism occurred, or a combination:

The fundamental cause of failure may be regarded as a combination of geological factors and design decisions that, taken together, permitted the failure to develop.

A wide-ranging controversy ensued from the dam's collapse. According to the Bureau of Reclamation, BOR engineers assess all Reclamation dams under strict criteria established by the Safety of Dams program. Each structure is periodically reviewed for resistance to seismic stability, internal faults and physical deterioration. The dam safety program identified two other dangerous dams - Fontenelle, which very nearly failed like the Teton Dam when it was filled and again in May of 1985 and the Jackson Lake Dam which would have failed during an earthquake on the nearby Teton Fault.