Slurry wallA slurry wall is a type of wall used to build tunnels, open cuts and foundations in areas of soft earth close to open water or with a high water table. A special clamshell-shaped digger is used to excavate a hole. The hole is kept filled with slurry (a mixture of bentonite and water) at all times. Once the first hole is completed to bedrock, an adjacent hole is dug in the same manner. Eventually, once a particular length is reached, a reinforcing cage is lowered into the slurry-filled pit and the pit is filled with concrete. The concrete displaces the slurry which is recycled. On completion, digging within the slurry wall-enclosed area can proceed. To prevent the slurry wall from collapsing into the newly open area, tiebacks are installed. These are steel cables drilled through the slurry wall out into bedrock on the other side. Concrete is poured so that the cables become attached to bedrock. Once excavation is complete, construction of the structure inside the wall can begin. When completed, the structure itself prevents the slurry wall from collapsing and so the tiebacks can be removed. Slurry wall construction is the type used to construct the "bathtub" that surrounds most of the World Trade Center site.