Oahe Dam: Influence of Shale on Oahe Power Structures Design

Oahe dam site is a soft-rock site. The shale bedrock is badly faulted, has bentonite sliding planes, a low modulus and low compressive strength. Power structures are located in positions relative to faulting to afford minimum construction difficulty. Structures design is controlled by influence of bedrock as much as by functional requirements. Designers develop a pessimistic attitude in working with this material. Pierre shale disintegrates rapidly when exposed to air. Exposed foundation areas are protected by bituminous sealing, guniting, and concrete slab cover. Exposed tunnel rock is protected by humidity control or by pneumatically placed mortar. Foundation areas are stitched with anchorages to prevent differential rebound. Construction sequence is carefully controlled to prevent overstressing of foundation slope faces.