Diaphragm Wall

A diaphragm wall, also called a slurry wall, is a reinforced concrete barrier built underground in soft ground or near open water where ordinary open excavation would collapse, most often to keep groundwater and loose soil out of a tunnel or foundation excavation. Builders dig a narrow trench section by section with a clamshell grab or a hydromill cutter while continuously filling it with a bentonite and water slurry whose pressure holds the trench walls open, then lower a steel reinforcing cage into the slurry-filled trench and pump concrete in from the bottom through a tremie pipe, which displaces the slurry upward for reuse elsewhere on site. The technique originated in Italy in the 1950s for the Milan Metro and later underpinned major foundations including the World Trade Center towers in the late 1960s and Boston's Big Dig in the 1990s.