The paper describes the results of site investigation and laboratory testing and the analysis performed for the determination of foundation shear strength at the Oldman River Dam site in southwestern Alberta, Canada. Horizontally bedded claystones, siltstones, and sandstones at the site contain relatively weak bedding-plane shears that adversely affect foundation stability. Data on the bedding-plane shear characteristics were collected by mapping, borehole coring, shaft exploration, and large-diameter sampling. Shear planes of structure-wide continuity were identified. Numerous laboratory direct shear tests were done to measure in situ and residual shear strengths. The design angle of shearing resistance of selected continuous bedding-plane shears was evaluated by summing the representative residual angle of shearing resistance and components of the angle of shearing resistance due to in situ state, roughness, and thickness of the bedding-plane shears. Relatively flat dam slopes were found to be required for stability. The methods used at the Oldman River Dam should be applicable at other sites located in flat-lying mudrock sequences. Key words: Oldman River Dam, foundation shear strength, sedimentary rocks, bedding-plane shear, residual angle of shearing resistance, in situ state, roughness, thickness.
