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A 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.