Following the detection of a sinkhole in the upstream face of Clearwater Dam in January 2003, substantial efforts have been completed to explore and pre-treat the dam foundation in preparation for construction of a “hanging” cutoff wall that will complete the seepage remediation program. Initial phases of pre-treatment started in 2004 and included core drilling and placement of both low- and high-mobility grouts in the immediate vicinity of the sinkhole. This work preceded the more extensive efforts that were completed in the fall of 2009. The recently completed project was primarily a down-stage, high-mobility grouting effort involving approximately 840 boreholes in two lines along the 4,225-ft–long embankment. In addition, the project also included treatment of an extensive epikarst zone and placement of low-mobility and sanded grout mixes in selected problematic locations. High initial epikarst and bedrock permeability ranging from 20 to 200 Lugeon was reduced using systematic grouting approaches focused toward achieving a pre-treatment target of 10 Lugeon within the vertical limits of the proposed cutoff wall and a 3-Lugeon target beneath the proposed cutoff wall. Pervasive solution-enhanced bedrock fractures resulting from repeated tectonic uplift and sub-aerial exposure set forth challenging conditions in the face of the pre-treatment objectives. Against this baseline condition, the recent grouting efforts have set the stage for construction of a cutoff wall and completion of the Clearwater Dam Major Rehabilitation Project.

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