B.C. Hydro's Practice for Assessing the Foundations of Existing Dams on Rock

by L. J. Cornish, B.C. Hydro, Vancouver, Canada,
D. P. Moore, B.C. Hydro, Vancouver, Canada,

Document Type: Proceeding Paper

Part of: Waterpower '91: A New View of Hydro Resources


Since the early 1980's B.C. Hydro has carried out dam safety reviews which included 33 gravity and embankment dams founded on rock. The foundations include moderately jointed, but unsheared, hard, generally granitic rocks as well as moderately hard to soft, metamorphic and sedimentary rocks having weak, adversely oriented geologic features. For each dam foundation the geology, construction records and performance data were reviewed and for 19 dams, deficiencies were identified which required further investigations and in some cases, remedial measures. For dams founded on weak geological features, B.C. Hydro's practice is to base the reassessment shear strength upon the post-peak shear strength for unsheared features or residual shear strength for sheared features: often with some contribution to strength from roughness in either case. For jointed, hard rock foundations shear strength is based upon the Hoek-Brown criteria. In all cases considerable judgement is exercised to derived a reassessment shear strength from these bases. Uplift used for the assessment is based upon the observed piezometric pressures where available or on traditional assumptions where not available. This paper describes practices and rationale adopted by B.C. Hydro for review of rock foundations, and indicates key cases where new methods and criteria were used.

Subject Headings: Dam foundations | Residual strength | Earthfill dams | Rocks | Gravity dams | Foundation construction | Shear strength | Seepage

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