GEOLOGIC AND SEISMIC SETTING PERTINENT TO DAM SAFETY REVIEW OF DUNCANDAM

Duncan Dam is a B.C. Hydro facility constructed on the Duncan River in southeastern British Columbia, Canada, between 1965 and 1967. The dam was founded on a complex sequence of more than 380 m of glacial drift and glaciofluvial sediments, some of which are pervious and compressi ble. Some sandy units are potentially liquefiable, in particular a san d layer (unit 3c) up to 23 m thick. Current B.C. Hydro seismic guideli nes for dams require that Duncan Dam should be able to withstand the M aximum Credible Earthquake (MCE) without catastrophic release of the r eservoir. This paper describes the geologic and seismic setting of the region around the dam and the selection of seismic ground motion para meters. Probabilistic methods were applied to develop MCE ground motio ns, which were estimated to consist of a firm ground peak horizontal a cceleration of 0.12 g, which could be caused by a M 6.5 earthquake at a distance of about 50 km. Several time histories with characteristics similar to this design earthquake were selected for dynamic soil anal yses.