Tuesday, June 28, 2005

What is Diaphragm Wall?

Diaphragm Walls (U.S. Slurry Walls)

The continuous diaphragm wall (also referred to as slurry wall) is a structure formed and cast in a slurry trench (Xanthakos, 1994). The trench is initially supported by either bentonite polymer based slurries. The term "diaphragm walls" refers to the final condition when the slurry is replaced by tremied concrete that acts as a structural system either for temporary excavation support or as part of the permanent structure. This construction sequence is illustrated in Figure 1.The term slurry wall is also applied to walls that are used as flow barriers (mainly in waste containment), by providing a low permeability barrier to contaminant transport.

Slurry wall technology hinges on specialized equipment for excavating slurry trenches. The simplest type of trenching equipment is the mechanical clamshell attached on a kelly bar. Individual contractors have developed their own specialized trenching equipment like hydraulic clamshells, fraise or hydromills (sample manufacturers: Icos, Bauer, Casagrande, Case Foundation, Rodio etc). Figure 2 shows selected pictures from construction of a new subway in Boston (MBTA South Boston Transit way).

The first diaphragm walls were tested in 1948 and the first full scale slurry wall was built by Icos in Italy in 1950 (Puller, 1996) with bentonite slurry support as a cut-off wall. Icos constructed the first structural slurry wall in the late 1950s for the Milan Metro (Puller, 1996). Slurry walls were introduced in the US in the mid 1960s by European contractors. The first application in the US was in New York City [1962] for a 7m diameter by 24m deep shaft (Tamaro, 1990), that was followed by the Bank of California in San Francisco (Clough and Buchignani, 1980), the CNA building in Chicago (Cunningham and Fernandez, 1972), and the World Trade Center in New York (Kapp, 1969, Saxena, 1974). The majority of diaphragm wall projects in the US are located in six cities Boston, Chicago, Washington DC, San Francisco and New York.

Diaphragm walls are extensively used in the Central Artery/Tunnel project (CA/T) in Boston, Massachusetts (Fig. 3). Work in the CA/T involves many cut and cover tunnels constructed under the existing artery. Some of the deepest T-slurry walls, extending 120' below the surface have been constructed for the Central Artery (Lambrechts et al., 1998).

Figure 1: Typical construction sequence of slurry walls: (A) Trenching under slurry, (B) End stop inserted (steel tube or other), (C) Reinforcement cage lowered into the slurry-filled trench, (D) Concreting by tremie pipes.

Figure 2: Trenching equipment, (A) Mechanical clamshell in front and hydraulic clamshell in the back, (B) Smaller size mechanical clamshell

Figure 4: Central Artery Tunnel Project, Boston MA (Ladd et al., 1999)


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