Protecting roller-compacted concrete armored emergency spillway against uplift with geocomposite drain: Loxahatchee Reservoir example

ABSTRACT: When designing a roller-compacted concrete (RCC) spillway for a reservoir, seepage-induced uplift pressures need to be considered. This case study describes the design, construction and post-construction performance of a drainage system consisting of a geocomposite drain immediately beneath a stair-stepped RCC spillway at the Loxahatchee Reservoir in western Palm Beach County, Florida, USA. The installation comprised 2.8 km of RCC and a geocomposite drain to construct the fixed-weir emergency spillway. The primary stability issue for the spillway was the development of excessive pore water pressure beneath the RCC in the event of rapid drawdown of the reservoir. Transient seepage analysis predicted that installation of the geocomposite drain would eliminate excess pore pressures beneath the RCC, and dramatically increase the factor of safety against uplift (increased from 0.5 to a value of the order of 6), preventing structural instability of the spillway. Since placing the reservoir in operation, no structural instability has been observed in the spillway. Also, the absence of excess pore pressure is predicted by post-construction transient seepage analysis using reservoir operational-stage data.