Cutting off Elusive Seepage Paths at Gaylord Dam

Gaylord Dam is a 58-foot-high dam located near Granby, Colorado. In 2019 water was observed flowing under the upstream apron of the concrete emergency spillway when the reservoir was a few feet below the spillway crest. Seepage appeared to be flowing directly beneath the spillway slab. A concrete cutoff wall and subdrain beneath the concrete spillway was designed to mitigate the apparent concentrated leak erosion between the spillway slab and the underlying bedrock. However, dye testing performed after completing the cutoff wall excavation showed no seepage flows immediately beneath the slab, and instead indicated deeper vertical water flows into the bedrock underlying the trench bottom. This paper discusses the geologic trench mapping, geophysics, and boring exploration performed to assess the bedrock seepage path, the success of curtain grouting to mitigate seepage flows, and the procedures implemented to mitigate hydrofracturing the dam embankment during curtain grouting in the underlying bedrock.