1998 – WAC Bennett Dam – The Sinkhole Crisis

Raymond A. Stewart

On I7 June 1996 while investigating a small pothole on the crest 183 m high Bennett Dam an unexpected crest collapse occurred resulting in a large sinkhole. Following this incident the safety status of the dam was uncertain. The reservoir was lowered by 2 m over a six week period by spilling up to 5,000 m 3 over the spillway and through the turbines.

An unprecedented dam investigation commenced immediately and was completed December 1996. During drilling a second sinkhole was discovered at another location on the dam.

A sophisticated compaction grouting technique was developed to remediate the sinkholes to the depth of 5 m and the work was successfully completed by 1997. -The reservoir was returned to service in time to collect the freshet in spring 1997. This event was the most dam safety concern in the history of BC Hydro operations.

This paper describes how B.C. Hydro managed the crisis, and the subsequent safety assessment.