Slurry Walls

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When site constraints do not permit cheaper and more traditional excavation methods, constructing slurry walls provides an alternative method of casting concrete in place below the surface elevation. Slurry walls, used for the construction of basement walls and other below grade foundation elements, do not require formwork and excavation side bracing.  Instead, a thin trench is dug into the site soils which exactly matches the thickness and depth of the new below grade wall.

As the thin excavation is being dug out, a slurry solution, usually a mixture using an expansive clay such as bentonite, is pumped into the void space.  The clay slurry serves as the side wall bracing for the thin excavation.  The lateral pressure exerted by the clay slurry serves to hold the excavation’s perimeters in place.  Next, a prefabricated cage of reinforcing steel required for the concrete wall design can be set in place by lowering it into the slurry wall.

The next step is to place the concrete into the slurry wall through the use of a tremie.  A tremie is basically a long steel pipe, about a foot or less in diameter, with a chute at the top end and a plugging system at the bottom.  The tremie is set at the bottom elevation of the excavation and unplugged, so that the concrete can flow from it into the bottom of the slurry wall.  Placement of concrete by this method requires a more workable (less stiff) concrete mix design than the more traditional cast-in-place concrete methods.  The concrete is heavier than the clay slurry, so as the concrete is placed, the slurry is displaced and extrudes from the top of the excavation.  The slurry is expensive and can be recovered at the top of the excavation for later use.

Because slurry walls impose a vertical sheet of less permeable clay material at a desired location and depth beneath a project site, they can also be utilized to cut off the flow of water through more permeable existing site soils.  One such application is the containment of contaminated ground water, to prevent any off-site migration to adjacent properties.  A variation of this same concept is the use of slurry walls to contain desirable groundwater in a sub-surface reservoir.  Slurry walls are also sometimes used at the base of dams as a repair strategy to prevent leaks.

Last modified on Tue, Mar 30, 2010
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