Dewatering Cofferdam for Construction of Olmsted Locks

Twin 110-ft×1,200-ft (SI unit conversions are provided in the Appendix) locks and a dam are being constructed in the Ohio River near Olmsted, IL. Construction of these locks required digging a 1,200-ft-wide and 2,200-ft-long excavation in the Ohio River to 100 ft below the design river stage. The size of the cofferdam required for lock construction, and the depth of excavation, made control of groundwater of paramount importance to excavation safety. The work included removing and/or controlling all ground and surface water within the cofferdam; controlling seepage through or beneath the coffercells and from the open cut along the Illinois bank; reducing the artesian head in an underlying stratum of highly jointed, indurated clayey silt and/or siltstone; and lowering the water level in the coffercells. The dewatering was accomplished by installing and pumping deep wells with submersible pumps, with vacuum, within the cofferdam area. The dewatering system was pump tested and evaluated before unwatering the excavation. Evaluation of the pump test data showed that the dewatering system as designed and installed would be adequate for the maximum design river stage with the addition of a few wells in the bottom of the excavation.