Subsurface Barriers

Description

There are many old unlined hazardous-waste landfill sites in the United States. Subsurface barriers are designed as a temporary or semi-permanent solution to prevent these old landfills from contaminating the groundwater. Subsurface horizontal barriers adapt vertical barrier equipment and technologies that retard the migration of contaminated leachate to create impermeable surfaces below landfills. Two types of barriers have been evaluated: permeation grouting and jet grouting. Permeation grouting injects a low-viscosity grout into the soil at low pressure. It fills the voids without significantly changing the soil’s structure or volume. Jet grouting injects grout at high pressure and velocity, destroying the soil structure and mixing grout and soil to form a homogeneous mass. Permeation grouting is only applicable to a site where a relatively homogeneous, high conductivity region bound by lower conductivity regions exists beneath the waste site.

Other techniques for placing a horizontal barrier beneath an existing landfill are in the experimental stages. A promising technology is the EarthSaw^TM, which places slurry wall trenches around a waste site, and then slices beneath the landfill using a cable, filling the void with a high-density grout. Other techniques include more complicated fabrication of hard barriers beneath a landfill.

Limitations and Concerns

Verification that the barrier forms a continuous barrier is critical to the success of this technology. Tracers and liquid flood testing may provide some information, but such geophysical techniques are limited. They cannot identify flaws in the continuity of the grouted soil.

One benefit of a subsurface barrier system is that it allows additional time to develop remedial treatments. This technology should not be considered a permanent remedy unless techniques for assuring barrier integrity are well developed.

The length of time that a subsurface barrier can contain wastes in the landfill depends on a number of variables including landfill cover, soil type, precipitation and moisture in the landfill.

Applicability

This technology is designed as a temporary or semi-permanent solution to prevent old landfills from contaminating the groundwater.

Technology Development Status

This technology is in the pilot stage. The EarthSaw^TM and construction techniques are in their experimental stages.

Web Links

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

Other Resources and Demonstrations

See http://www.netl.doe.gov/products/em/IndUnivProg/pdf/3155.pdf and http://www.netl.doe.gov/products/em/IndUnivProg/pdf/2964.pdf for descriptions of experimental systems.

See http://207.86.51.66/download/techdrct/tdsubsrf.pdf for Evaluation of Subsurface Engineered Barriers at Waste Sites, EPA 542-R-98-005, August, 1998. This document describes many different vertical barriers for controlling the subsurface flow of water into or out of a hazardous waste site. Included are slurry walls, thin walls, deep soil mixing, grout walls, sheet pile walls, and vertical liners.