C153705 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I , .~--.< -~\ I ,~- '^"""" Embankment and foundation sinkholes are commonly the result of seepage and piping of soil particles into and through coarse embankment zones, outlet conduits and/or coarse or fractured foundation materials. Based on review of the historical record, there is no indication of sinkhole development associated with flow into the outlet conduit from the surrounding embankment materials. The historic sinkholes have appeared in the natural foundation materials just upstream of the embankment on both abutments. These sinkholes were probably created in response to soil particles being transported by seepage of reservoir water into the natural foundation materials. Preferred seepage paths are believed to exist in the foundation. materials due to stratigraphic differences, localized faulting, areal joint and fracture patterns and a stratum of rubblized rock or "clinker" associated with an altered or burned coal seam in the foundation. Seepage in these features may ,vary as a function of time due to the plugging and unplugging of voids with transported material and dissolution of possible soluble strata and/or joint filling materials. -- --- 1.1.3 Recent Developments The most recent sinkhole activity observed at the dam consisted of five sinkholes in the reservoir floor observed ~s-the'reservoir was drawn down during the late summer of 1992. The sinkhole locations are. shown on Figure 1-4 in relation to the historic sinkholes identified on the 1971 seepage blanket drawing. Temporary repairs were implemented in Septell1ber and October of 1992 to allow storage with restrictions in 1993. The temporary. repairs consisted of excavation to enlarge and allow visual evaluation ,of thesiiikholes followed by placement of geotextile filter medium and a compacted clay cap. The technical requirements for this work are described in the September"'1~92 technical specification document by Woodward-Clyde Consultants " ' (WCe) titied. "Technical Specifications for Construction of Sinkhole Caps, Beaver ".' , Reservoir Emergency Repair Program" (WCC, 1992a). A summary of observations during construction IS provided in the October 26, 1992 construction report by WCC titled "Emergency Repair Program Construction Report, Beaver Reservoir" (WCC, 1992b). A copy of the construction report is included in Appendix D for reference. 23067/R 1.1 09-03-93(9:34am)/RPT 1-3