Once panels are fastened to the towers by the corbel extensions, the area between the towers and panels will be filled with tremie concrete. Typical construction would use several 10-foot thick horizontal lifts of concrete. With the extended lock walls complete, new miter gates will be installed. This graphic depicts the placement of the float-in extensions. Since the lift-in approach can be accomplished underwater, there is considerable savings in eliminating a cofferdam.
Cofferdams are steel sheet walls that form a dam around a construction area in the river.
The water behind the walls is pumped out so construction can take place in the dry. More important, a cofferdam would close both chambers for two to three years, causing severe impact to the shipping industry. The only remaining alternative is to build a new lock chamber landward of the existing chambers. After each closure, the backed-up barges will be locked through before the next shutdown. These walls extend beyond the lock chambers to shield tows from river currents and to help align the tows so they can enter the lock chambers.