Blind Slough Dam, an 18 m high concrete gravity dam was constructed in 1922 and has performed satisfactorily since construction. In 1995, as part of BC Hydro’s dam safety program the dam was reviewed for the maximum design earthquake ground motions. Based on the results of dynamic structural stability analysis it was determined that anchoring the dam to the rock foundations was the optimal means of increasing the seismic withstand of the facility for current predicted ground motions. An investigation program including detailed mapping of exposures in the immediate vicinity of the dam, in-situ borehole testing, down-the-hole geophysics, and core drilling was carried out to supplement limited construction, historic drilling and instrumentation records. The rock mass strength parameters and the shear strengths on large undulating through-going discontinuities, were based on the geology and tectonic setting, detailed mapping of the dominant geological structures and data obtained from field joint-mapping and borehole testing. A carefully developed geological model based on detailed field mapping and investigations allowed realistic strengths to be selected for input to the seismic design (including earthquake and post-earthquake stability analyses) and to minimize the number and length of anchors required to achieve the desired seismic withstand.
Many dam structures around the world are reaching an age where out of necessity structural, seismic and flood withstand capabilities have been or are being reassessed. This is due in part to improved understanding of seismic and flood hazards (embodied in modern design codes, guidelines and/or standards) and in part to deterioration of the concrete structures. For existing dams approaching 50 or more years old, incomplete or limited construction records require various investigative methods to characterise the foundation and arrive at realistic failure modes and rock strengths for remedial designs.