Compaction grouting for sinkhole repair at WAC Bennett dam

WAC Bennett dam is a zoned earth embankment slightly greater than 600 ft (183 m) in height. Two sinkholes, of which the deepest descended about 400 ft (120 m) from the crest, were discovered in the core of the dam in 1996. Compaction Grouting was chosen as the best remedial method, and was used. Because even the slightest chance of hydraulic fracturing of the core could not be tolerated, grouting was carefully designed and controlled including: precision drilling of the deep holes to within a verticality tolerance of one degree; continuous analytical evaluation of the grouting based on real time computer monitoring; careful control of the grout mix and especially its aggregate component; and, rigid control of the grout injection rate. The dam core was also monitored during grouting using a broad range of instrumentation, both on and within the embankment. Procedures were verified by a full-scale test before working on the dam.