Grout curtains are thin, vertical, grout walls installed in the ground. They are constructed by pressure-injecting grout directly into the soil at closely spaced intervals. The spacing is selected so that each “pillar” of grout intersects the next, thus forming a continuous wall or curtain. Typical grouting materials include hydraulic cements, clays, bentonite, and silicates. However, these materials may crack or may not be durable or chemically compatible. Polymer grouts are used for barrier applications because they are impermeable to gases and liquids and resist radiation, as well as acidic and alkaline environments. Grout curtains are similar to slurry walls although they do not require as extensive trenching. Grout is injected with grouting jets, which use a high-pressure fluid stream (i.e., slurry or water) to erode a cavity in the soil.
Limitations and Concerns
An underground injection permit may be required.
Keeping nozzles clear, maintaining air flow, and effectively creating a wall without defects have been problematic in certain environments and operating conditions. For instance, if very coarse-grained materials are encountered, defects in the curtain may occur. Therefore, the site must be well characterized to minimize unexpected geologic conditions. Additionally, if the jets are turned on and off, they may become clogged.
The orientation of the jets must be precise to ensure the curtain is continuous. At one demonstration, magnetic anomalies created problems with the orientation of the directional tool.
Grout curtains may be used up-gradient of the contaminated area, to prevent clean water from migrating through waste, or down-gradient, to limit migration of contaminants. Grout curtains are generally used at shallow depths (30 to 40 ft maximum depth). High-pressure jetting appears to be a cost effective means to place physical barriers in unstable soils, near foundations, and around underground obstructions.
Technology Development Status
Grout curtains have been widely used; they are a commercial technology. However, some of the jet grouting systems are still in the pilot stage.
http://www.pdhcenter.com/courses/c192/c192.htm (course outline on subsurface barriers).
Other Resources and Demonstrations
See http://www.clu-in.org/download/contaminantfocus/dnapl/Treatment_Technologies/Containment-1995.pdf for description of surface barriers, properties, and construction. (Chapter 2)