Slurry Walls and Impermeable Barriers

Description

Slurry walls are subsurface barriers that impede or stop groundwater flow. They consist of a mixture of soil, bentonite clay, and water, poured into trenches as a “slurry.” The trenches form a filter cake that serves as a barrier. Slurry walls are used to contain contaminated groundwater, divert uncontaminated groundwater flow, and/or provide barriers for groundwater treatment systems. Slurry walls are placed at depths up to 200 feet and vary in thickness from 2 to 4 feet. These vertical barriers must reach down to an impermeable natural horizontal barrier, such as a clay zone, to effectively impede groundwater flow. Vertical barriers are frequently used with surface caps to produce an essentially complete containment structure.

Another impermeable wall system is the Polywall Barrier System. It consists of continuous sheets of high-density polyethylene. The Polywall is installed in one pass: The trencher cuts through subsurface strata, installs the barrier wall, and backfills soil all in one step. A waterproof interlocking joint system can be used for lengths over 300 feet. Other systems, such as the Waterloo Barrier, use interlocking sheet metal.

Limitations and Concerns

This technology contains contaminants within a specific area; it does not treat or destroy them. Therefore, further remediation is often necessary.

Sometimes the soil-bentonite mixture is not able to withstand attack by chemicals such as strong acids, bases, salt solutions, and certain organic chemicals. This hastens deterioration of the wall. As a result, the wall material should be tested prior to construction. In addition, the wall should be monitored for leakage when it is installed, and as it ages.

Physical factors, such as seismic activity and pressure buildup, may degrade or deteriorate slurry walls over time, causing them to lose their containment capacity. Thorough characterization of the subsurface is required because settling or unstable ground can limit effectiveness. This is another reason that the wall should be monitored for leakage as it ages.

The benefits of slurry walls rely on their ability to create impermeable barriers to groundwater flow. Therefore, they should be designed so groundwater does not flow underneath the wall.

Because slurry walls have been used for decades, the equipment and methodology are readily available and well-known. However, the process of designing the proper mix of wall materials to contain specific contaminants is less well developed. Excavation and backfilling of the slurry trench is critical and requires experienced contractors.

Applicability

Slurry walls contain groundwater. They provide subsurface containment for a wide variety of waste, including radionuclides, metals, and organics. They do not target a particular target group of contaminants. However, highly acidic or alkaline contaminants may not be compatible with wall material. Slurry walls often are used where the waste mass is too large for treatment and where soluble and mobile constituents pose an imminent threat to a source of drinking water.

Technology Development Status

Slurry walls and other vertical barriers are commercial.

Web Links

http://www.frtr.gov/matrix2/section4/4-53.html

http://www.sandia.gov/Subsurface/factshts/ert/ssb.pdf

Other Resources and Demonstrations

See descriptions of grout curtains.

See http://www.clu-in.org/download/techdrct/tdsubsrf.pdf for an evaluation of subsurface barriers, including vertical barriers.

See http://www.clu-in.org/products/intern/pearlman/ for a description of subsurface barriers, including slurry walls.