Filling a Void Under an Existing Spillway Structure

This paper describes the filling of a large void below a concrete spillway structure on a sand foundation. The precautions taken, action, design and construction are described. Webber Dam, on the Grand River, Michigan, is comprised of a concrete spillway structure 32-ft high between earth embankments. The dam forms a reservoir for hydroelectric power generation for Consumers Power Company. During a November, 1993 inspection for rehabilitation of the spillway, a large void was detected under the concrete spillway slab. The void was suspected to have been formed when the spillway was in operation and high water flows over the spillway slab caused suction at the pressure relief holes through the slab resulting in wash out of the sand foundation. This paper discusses the justification for this hypothesis. Probes through the relief holes indicated a void approximately 60 ft by 80 ft by up to 7 ft deep. Analysis of the structure by Acres International showed that the structure had a very low factor of safety with the void under the structure, and plans were immediately prepared to fill the void. Water levels in the reservoir were lowered 5 ft to improve the stability of the structure prior to void filling. Various filling materials were examined including concrete, cement grout, pea gravel with post-cement grouting, and flowable fills consisting of various mixes of flyash, cement, sand, and water. Key criteria for the fill placement were that it be economical, could be batched locally, and did not require a specialist contractor. Also, the material was required to be flowable to facilitate placement and should achieve a strength equal to or greater than the original soil formation. A main concern in filling the void was, that during the placement, the existing pressure relief holes should not be blocked causing a buildup of high uplift pressures. A method was devised to install pressure relief wells into the sand foundation prior to placing the fill, and to drill through the fill as soon as it achieved sufficient strength to re-establish pressure relief. To ensure complete filling, secondary cement pressure grouting was performed. A monitoring system of piezometers was installed prior to void filling and monitored throughout construction to ensure stability of the structure. The remedial design included the reinstallation of permanent pressure relief wells. Stainless steel sleeves and well screens were used to ensure protection of the underlying soils against erosion. The void was successfully filled in March 1994 with 158 cubic yards of flowable fill and fully grouted.