Oahe Dam: Geology, Embankment, and Cut Slopes

Study of the bedrock geology and slide experiences at both Fort Peck and Oahe dam projects shows that the solution of embankment and high cut slope stability problems on a Pierre shale foundation, depend primarily on the degree and extent of geologic processes that have occurred to weaken the firm bedrock. Design studies at the Oahe dam site reveal that, although the basic, intact, firm shale bedrock has more than adequate strength, the closely spaced faults, joints, and large displacements have allowed natural weathering processes to weaken the formation greatly. The embankment has been designed by use of semi-empirical methods based on the soil mechanics science and data from the Oahe and Fort Peck slides. For cut slope design, the use of a slope curve based on unexplored natural slopes is not recommended. The following additional factors should be considered: Foundation geology; depth of weathering; shear strength of weaker strata or zones; pore water and seepage forces; stability analyses; experience with similar materials at other projects; and the importance of the slope in regard to over-all safety of the project.