SLIDE IN UPSTREAM SLOPE OF LAKE SHELBYVILLE DAM.

Lake Shelbyville Dam in central Illinois is a 108-ft high earth dam completed in 1970. A slide occurred in the steeper portion of its upstream slope immediately after construction and before filling of the reservoir was begun. The slide was stabilized in 1972. Re-examination of construction records revealed a layer of more plastic, weaker fill in the closure section at the elevation of the base of the slide. As drainage occurred, the effective stresses acting on the failure surface decreased with time and the shear strength was reduced from that corresponding to undrained shear. Accordingly, total stress stability analysis, using undrained shear strength, is not a reliable index of stability in well-compacted, silty clay fills.