Containing waste; cut-off walls
Cut-off walls and engineered barriers, using cement-based grouts or concrete, can be formed using a range of ground engineering technologies to confine contamination, permitting redevelopment or re-use of a site.
Slurry cut-off walls are formed by excavating a trench through or around contaminated ground into an underlying low-permeability layer. The trench is filled with a cement-based slurry, which hardens to form a continuous low-permeability barrier.
An alternative approach is to construct concrete diaphragm walls. The trench is excavated as before, but filled with a highly workable concrete in place of the grout. Reinforcement is incorporated within the wall. The resulting wall is stronger that the grout wall, and can be used structurally.
Concrete secant piling can be used to form a solid cut-off wall. Initially closely-spaced bored piles are constructed. At an early age, a second series of piles is constructed in the gaps between the initial set, cutting into them to form a continuous interlocking barrier with good structural stability.
For further information see Containing and encapsulating waste, published in CONCRETE in January 2003, pp 49–50. Copies are available as a free download from the Members Area of the Concrete Society web site.