What is a Slurry Wall?

A slurry wall, also known as a slurry cut-off wall, is a type of ground engineering technique. It is primarily used to provide an underground low-permeability barrier, preventing the flow of groundwater and other contaminants. [1]

The slurry wall technique was founded in the 1950s by an Italian company called ICOS during the evacuation of the Red Line of the Milan Metro. Over the decades, slurry wall construction has been demonstrated in many well-known engineering projects such as the World Trade Center in New York City. [2]

Aarsleff’s single-phase, self-hardening slurry trench cut-off walls are constructed by excavating a trench under a cement-bentonite slurry. The slurry remains fluid during excavation, and in this state allows the formation of a filter cake of bentonite on the side walls enabling the hydrostatic pressure of the slurry to prevent the collapse of the trench. Due to the cementitious content, the slurry hardens within about 24 hours to form the low permeability barrier. The hardened slurry has a low permeability, typically 1×10-8 to 1×10-10 m/s and the strength properties of a stiff clay but it does not possess the plastic properties of clay.

Most slurry trench cut-off walls built in the UK are 0.6 m wide. The practical maximum width is 1.5 m, although a more than 1.0 m width is unusual. Trenches can be excavated to about 12-15m deep in most ground conditions using hydraulic back-actors (backhoes). For depths below this and in particularly difficult ground conditions, diaphragm wall grabs or cutters are used. Slurry walls are normally keyed into an aquiclude, an underlying layer of low-permeability material.

The cost of a slurry wall can vary from project to project. It can depend on a variety of factors:

1. Wall Dimensions

Deeper and thicker walls such as diaphragm walls require more materials and specialised equipment, which can increase costs.

2. Ground Conditions

Sites with complex or challenging ground conditions such as high groundwater levels require specialised techniques or additional testing, raising slurry wall costs.

3. Materials

The type of materials used to build the slurry wall can influence the overall cost. While cement and bentonite or ready-to-mix blends are reasonably priced, higher-quality materials can drive costs. Soil-bentonite slurry walls are generally more cost-effective than cement-bentonite slurry walls as the soil excavated on-site is used instead of cement.