Slide in Upstream Slope of Lake Shelbyville Dam

Lake Shelbyville Dam is located in central Illinois. It is a 108-ft (33-m) high earth dam that was 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 by constructing a rockfill berm at its toe. No plausible explanation of the failure could be established until 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. Movements were gradual because the well-compacted clay soil tended to dilate when sheared, which induced negative excess pore water pressures that temporarily increased the strength in the failure zone. However, 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.