US Army Corps of Engineers
Little Rock District

Clearwater Major Rehab

Clearwater Major Rehab Location and Description: Clearwater Lake is located on the Black River in Wayne and Reynolds Counties in southeast Missouri. Clearwater dam was classified as a Dam Safety Action Class – 1 (DSAC-1) dam by the Corps of Engineers and confirmed by an independent panel of consultants. Since, the detection of a sinkhole in the upstream face of Clearwater Dam in January 2003, efforts were undertaken to complete foundation exploration and pre-treatment 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 full length double line grout curtain that was completed in 2009. The grouting project was a down-stage high-mobility grouting effort involving approximately 840 bore holes in two lines along the 4225 foot long embankment. In addition, the project included treatment of an extensive epikarst zone and placement of low-mobility and sanded grout mixes in selected problematic locations. Pervasive solution enhanced bedrock fractures resultant from repeated tectonic uplift and sub-aerial exposure set forth challenging conditions in the face of the pre-treatment objectives. The purpose of the grouting project was to reduce the foundation rock permeability in order to successfully install a seepage cutoff wall utilizing a slurry backfilled excavation method.

The Clearwater Dam Major Rehabilitation Project, Phase II was awarded to Bencor, Inc. in 2008 to construct a seepage cutoff wall approximately 4,200-feet long, 200-feet deep and 3-feet wide through the core trench and both abutments. Bencor mobilized on site in 2009 while the grouting phase was still under construction. Bencor began construction of the work platform and the required Test Section on the right abutment after completion of the grouting in that area. Upon completion of the successful Test Section and the grouting project, Bencor began excavating with two hydromills and soon added a third hydromill to excavate the cutoff wall. The final cutoff wall panel was placed on 6 December 2011. The overall project will be completed by the end of 2013 at a cost of approximately $240 million.

Construction Status: The contractor completed the cutoff wall construction for this high priority dam on Dec. 6, 2011. The remaining on-going work is installation of the clay blanket and repair of overflow spillway damages, which was caused by construction of the haul road. Pool restrictions and other interim risk reduction measures will remain in place through the end of calendar year 2013 to allow for completion of the remaining work and to ensure the modifications are performing as designed.