• Use of Cement-Bentonite Slurry Wall in the Closure of an Oil Refinery Sludge Basin
• Advantages of Cement-Bentonite Slurry Trench Methods Over Soil-Bentonite Trench Methods
• FAQ: What low-permeability ranges are possible with cement-bentonite slurry walls?

Slurry Walls Used to Manage Waste

A slurry wall is a non-structural underground wall that can serve as a barrier to the horizontal flow of groundwater. A slurry is a watery mixture of insoluble matter. Cement-based slurry walls are self hardening mixtures that may include clay, soil, and slag cement in addition to portland cement. Slurry walls are constructed by placing the mixture into a trench excavated to the desired depth. The cement-based slurry remains to set up and form a permanent underground cut-off wall to control groundwater movement.

Slurry walls are used in waste management to control the flow of groundwater at contaminated sites. One of the earliest uses of slurry walls in EPA’s Superfund program was at Brodhead Creek in Stroudsburg, Pennsylvania. The 12-acre (4.9 ha) site is the location of a former coal gasification plant. In 1981, EPA took steps to stop the seepage of coal tar from the site into the creek by constructing an underground cement-bentonite slurry wall. The slurry wall was installed between the toxic coal tar deposits and the creek. The 12-inch (30mm) wide, 638-foot (194-m) long trenched wall ranged in depth from 18 to 23 feet (5.5-7m).

Although some coal tar as source material of contamination was pumped from the site, EPA determined that complete removal of the residual coal tar would be technically unworkable due to site-specific constraints. Both the first 5-year review (1999) and second 5-year review (2004) of the remedy completed by EPA indicated that the remedy including the slurry wall continues to be protective of human health and the environment. More.

Work was completed in early 1992 to close a 5.5 acre (2.2ha) stormwater sludge basin for an oil refinery company. The basin, which had served as a stormwater collection and settling pond for over 40 years, contained oily sludge with concentrations of hazardous constituents including metals and organic compounds. The closure plan included two portland cement applications; (a) installation of a cement-bentonite slurry wall and jet grouting to contain the site and (b) solidification/stabilization treatment of the basin’s contents.

The most important properties of slurry walls are low permeability and compatibility with site conditions including groundwater. Both soil-bentonite (SB) and cement-bentonite (CB) slurry mix designs were tested. Testing indicated that both material mix designs SB and CB were compatible with the site’s groundwater. Permeability tests were performed on both SB and CB trial mixtures using the site’s groundwater as the leachate. In general the CB mix produced an initial permeability on 1X10^-6cm/sec which gradually decreased as the CB continued to cure. Soil-bentonite trial mixes generally produced permeabilities in the range of 1X10^-7cm/sec but held steady for the three pore volume duration of the test. Both mix designs were determined to be acceptable. The SB mix design required considerable more material handling, mixing and clean-up compared to a CB mix design.

Cement-bentonite was selected for its acceptable permeability at initial installation and demonstrated decrease in permeability during extended cure, compatibility with site groundwater, and cost.

The basin’s closure plan design included installation of the CB slurry wall and jet grouted wall down to a depth of 40 to 45 feet (12-14 m) into underlying clay. The underlying clay was approximately 70 feet (21 m) thick.

The slurry wall was installed by trenching by backhoe, filling the trench with the CB slurry as trenching progressed. Jet grouting involving jetting a mixture of cement, bentonite, and native soils between two parallel rows of steel sheet piling along a narrow portion of the property. The sheet piling and jet grouting were also keyed into the underlying clay layer of the site.

At the end of the project the basin’s contents were surrounded by a low permeability wall keyed into the low permeability clay beneath the site. Other work for the closure included cement-based solidification/stabilization of the basin’s contents and installation of a cover and cap system.

The closure is monitored. No issues involving the closure have been reported by the State regulator.

More information:

Cement-Bentonite Slurry Trench Cutoff Walls (IS227)

"Remediation of Oil Refinery Sludge Basin" (PDF)

"RCA Closure of Refinery Sludge Basin" (PDF) from the Proceedings of the Fourth Great Lakes Geotechnical/Geoenvironmental Conference on in-Situ Remediation of Contaminated Sites, Editor: Reddy, K.R., University of Illinois at Chicago, May 1996".

Advantages of Cement-Bentonite (CB) Slurry Trench Methods Over Soil-Bentonite (SB) Trench Methods

• The CB method is not dependent on the availability or the quality of soil for backfill.


• The CB method is more suitable in trenching through weak soils where trench stability may be a concern. The CB slurry has a higher density than SB slurry and begins to set within hours after excavation, thereby reducing the chance of failure.


• The CB slurry sets up to a stiff claylike consistency. Trenches may be cut through the wall without sloughing. Construction traffic may cross the trench after a few days.


• The construction sequence is more flexible. The CB method permits trench construction in sections to meet site constraints. It adapts to hilly surfaces where a step-type construction can be performed. With the SB method, the long open trench necessary to accommodate the flat slope of the backfill normally requires trenching continuously in one direction at a constant elevation.


• With a CB slurry trench, construction may proceed during subfreezing temperatures. With the SB method, special precautions are required to keep the backfill from freezing.


• The width of a CB trench is generally less than for a SB trench. For the SB method, the trench must be wide enough to permit free flow of the backfill material.


• With the CB method an area adjacent to the trench is not required for mixing, making it more suitable on projects with space limitations such as the crest of a dam. Also, cleanup is easier with the CB method.

More information on slurry wall applications, design, construction methods and specification can be found in PCA’s publication Cement-Bentonite Slurry Trench Cutoff Walls, IS227.

FAQ: What low-permeability ranges are possible with cement-bentonite slurry walls?

Permeability is one of the most important factors in the design of a slurry wall. Both laboratory and field tests indicate that the permeability of cement-bentonite slurry walls range from 1X10^-6 cm/sec to 1X10^-7 cm/sec. Compatibility with site conditions is also very important. Permeability tests should be run with the subject site’s groundwater or leachate.

 
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