A slurry wall is a reinforced-concrete diaphragm wall used to build tunnels, to open cuts, and to lay foundations in areas of soft earth close to open water or with a high ground water table.
Slurry walls are typically constructed by starting with a set of guide walls, typically 1 metre (3.3 ft) deep and 0.5 metre (1.6 ft) thick, constructed on the ground surface. A special clamshell-shaped digger is used to excavate the slurry trench guided by the guide walls. The trench is kept filled with slurry (usually a mixture of bentonite and water) at all times to prevent collapse. Once the first trench is completed to design depth, or bedrock, an adjacent trench 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 from the bottom up using tremie pipes. The concrete displaces the bentonite slurry which is pumped out and recycled.
On completion of concreting, digging within the now concrete wall-enclosed area can proceed. To prevent the concrete wall from collapsing into the newly open area, temporary supports such as tiebacks are installed. When completed, the structure built within a walled-off area prevents the wall from collapsing, so that tiebacks and/or other temporary bracing may be removed.
Slurry wall construction was used to construct the "bathtub" that surrounded most of the World Trade Center site. Slurry walls were also used heavily in Boston's Big Dig tunnel project.
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