In October 1994, J.J. Henderson Construction Co., Gurnee, Ill., successfully completed one of its biggest challenges - a large monolithic wall pour requiring placement of 712 cubic yards of concrete in seven hours. The mass pour involved placing the main tank walls for an underground reservoir in Rosemont, Ill. The 190x120-foot reservoir walls are 34 feet tall, tapering from 3.5 feet thick and the bottom to 2.5 feet thick at the top. The pour also included several 20-foot-high walls about 2.5 feet thick, for the mechanical equipment rooms. Three different forming systems were selected: * Wood-beam gang forms backed with steel-channel walers for most of the 34-foot tank walls * Hand-set steel-framed plywood form panels for the 20-foot mechanical equipment room walls * Job-built plywood forms for some of the areas where battered walls intersect with straight walls