Slurry walls, in general, are specially constructed underground walls that have broad applications for control of groundwater around underground structures, such as during construction of building foundations. However they have found special value for control of groundwater flow around contaminated soils as in the current application at the CFAC site.

Above – Design and construction of a slurry wall.

A slurry, in general terms, is a loose, relatively low viscosity mixture of solids in a liquid. Imagine a mixture of a small amount of fine soil in water. After mixing, the muddy, suspended solids can easily flow and can even be poured or pumped from a container. A special solid is used for slurry walls, called bentonite. It is a unique, absorbent, swelling type of clay that is finely divided and remains suspended in water during construction operations. The slurry has enough density and strength to support the surrounding earthen walls of a deep trench during digging operations. The bentonite slurry in the trench is then replaced with a denser soil-bentonite (S-B) mixture which is the final wall composition.

Slurry walls for contaminant containment. The S-B mixture is a special engineered composition of various particle sizes of stone, gravel, sand, and soil along with Bentonite (which fills the smallest voids in the mixture). The resulting wall is packed tightly and has a very low water permeability, effectively inhibiting contaminants in groundwater behind the wall from passing through into the surrounding areas. The S-B wall doesn’t set up like concrete – it is strong but has a little give which allows it to flex without breaking over time. Thus, slurry walls have found valuable application in Superfund remediation applications that require containing contaminants in soils or landfills. Note that it is important for the S-B to remain hydrated for it to retain its strength and low permeability to work properly. Thus they are not recommended in arid climates.

The slurry wall construction, illustrated at the top of the page, involves a semi-continuous process in which the trench is excavated while being simultaneously filled with the bentonite slurry. As the excavation proceeds, the Soil-Bentonite backfill replaces the bentonite slurry, slowly flowing to the bottom, on the end where the excavation began. Excavating and backfilling proceeds until the trench is completed, in this case, until the Cell is encircled.

The following diagram shows what the construction process and the resulting Containment Cell at the CFAC site would look like. From “CFAC Public Engagement Presentation Boards” document (2025).

The overall Containment Cell includes other components of the remediation design in addition to the slurry wall, such as a cap to prevent inflow of water from the top, a base determined to have a low permeability, and monitoring/extraction wells.

Post-construction performance testing. After construction of the Containment Cell, its performance is monitored in two ways:

1. Measurement of groundwater levels inside and outside of the Cell. If interior water levels are found to be increasing that indicates groundwater is entering through the bottom or sides or from the top via the cap. An increase in water levels inside the Cell may lead to lower performance.

2. Measurement of the concentration of contaminants of concern, such as cyanide and fluoride, in surrounding groundwater are also measured to confirm that permeation through the wall is indeed inhibited as designed.

If performance is below design standards, water may be removed from inside the cell using an auxiliary process involving on-site treatment and disposal.

References: Soil-Bentonite Slurry Walls.

1. Geo-Solutions, Inc. https://www.geo-solutions.com/services/slurry-walls/soil-bentonite/

2. Menard-Group. https://www.menard-group.com/soil-expert-portfolio/slurry-wall/

3. Soil-Bentonite Slurry Walls: What We Have Learned The Last 50 Years. https://www.youtube.com/watch?v=uo154HyaTto.