Dewatering Case History in Florida

A project required lowering the water 15 ft in a 100-ft x 110-ft excavation in sand in Florida. A wellpoint system finally reaching 452 points pumping 8,000 gpm from three stages with tips in the 16 ft. of sand below final grade, lowered the water only 11 ft. Large boils then warned of trouble. Piezometers, flownets, boring recharge tests, and a simple and dual pumping test from 12-in wells proved that a high-permeability limestone under the sand supplied the water. Because of a potential 50% cost saving over alternate methods, the engineers tried an inverted, weighted filter scheme requiring removal of all wellpoints, flooding and then shaping the excavation, and finally covering the sand with a 7-ft blanket of fine and then coarse gravel. The scheme also required pumping from a single sump fed by a 12-in perimeter pipe drain within the blanket. The scheme succeeded, but an unexpected low-permeability silty layer lining the natural sand first required puncturing by a field of 120 short, buried, self-pumping, and unconnected wellpoints.