WILLOW ISLAND HYDROELECTRIC PROJECT: TAMING THE OHIO RIVER TO CREATE GREEN ENERGY-Paper

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The Willow Island Hydroelectric Project required a temporary cofferdam and a 120-foot-deep excavation beside the Ohio River to build a 44 MW powerhouse. The cofferdam combined a riverside cellular structure—16 cells, each about 63 feet in diameter—with a landward soil-bentonite cutoff wall. This enabled construction in the dry while river levels could stand more than 120 feet above the excavation.<br /><br />Variable sedimentary bedrock, slickensided weak layers, and a major fault created risks of deep-seated rock failure, especially during high river levels. Stability analyses showed that some rock slopes and potential fault blocks needed additional support. Designers installed 30 high-capacity, multi-strand rock anchors, each carrying up to about 2,074 kips. Deep wells lowered groundwater and reduced pressure within the rock, decreasing the anchor loads required. The flood design level was temporarily reduced while the deepest excavation was exposed; after the powerhouse base slab provided additional support, protection returned to the 100-year flood standard.<br /><br />Groundwater control also relied on the cutoff wall, toe drains, and monitored dewatering wells. A comprehensive instrumentation system tracked movement, water levels, and anchor loads during construction. Supplemental wells addressed unexpected seepage and erosion in an upstream support berm, while wells installed in several cells provided contingency drawdown during cofferdam removal.<br /><br />The combined support and groundwater-control measures performed as intended, allowing the powerhouse and associated works to be completed safely. The cofferdam endured several high-water events, though no 100-year flood occurred. The plant was completed and began operating in 2016.

Keywords

Willow Island Hydroelectric Project

cofferdam

Ohio River

powerhouse excavation

rock anchors

groundwater control

soil-bentonite cutoff wall

deep excavation

flood protection

instrumentation monitoring