Supplemental Report for Appraisal Assessment: Geology and Hydrogeology, Right Abutment, Black Rock Damsite. A Component of Yakima River Basin Water Storage Feasibility Study, Washington. Technical Series No. TS-YSS-18

摘要

The Black Rock damsite was investigated by a private engineering consulting firm in 2002 and 2003. The study suggested that the right abutment bedrock at the damsite is highly fractured and permeable, and recommended further studies to determine the suitability ofthe right abutment for a dam foundation. The ridges ofthe Yakima Fold Belt are generally asymmetrical, with the south limb gently inclined while the north limb is steeply folded with a thrust fault near the base ofthe fold. This configuration exists at the Black Rock damsite, which lies between the Yakima Ridge anticline on the north and HorsethiefMountain anticline on the south. The current investigations were undertaken to obtain additional geologic and hydrogeologic data on HorsethiefMountain, which forms the right (south) abutment at the damsite. The specific location was chosen with the purpose of encountering a thrust fault mapped at the base ofHorsethief Mountain, and to characterize the engineering and hydraulic properties of the basalt rock in and around the thrust fault zone. Two deep holes were drilled on the abutment slope. The first hole was drilled to obtain core samples and the second hole was drilled to perform hydraulic conductivity tests. The drilling and borehole testing program was initiated in December 2005 and continued until May 2006.