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Slurry Trench Cut-Off Walls
Slurry walls are non-structural barriers (Cutoff Walls, Slurry Trenches) that are constructed underground to impede groundwater flow. Slurry walls have been used for decades to provide cost-effective, long-term solutions for many groundwater control and groundwater remediation problems.
Slurry wall construction starts with the “slurry excavation technique”, which was developed in Europe and has been used in the United States since the 1940s. The slurry wall construction technique involves excavating a narrow trench that is kept full of an engineered fluid or “slurry”. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Slurry trench / slurry wall / cutoff wall excavations can be performed in all types of soils, even below the groundwater table.
Slurry trench /slurry wall / cutoff wall excavations are normally performed with hydraulic excavators and their widths can vary from 1.5 to 5.0 feet. Excavation depths greater than 100 feet require the use of a crane and clam bucket or other specialty equipment. In most cases, the excavation will “key” 2.0 to 3.0 feet into a low permeability stratum such as clay or bedrock to assure minimal leakage under the final wall. In some cases, “hanging” slurry walls / slurry trenches/ cutoff walls, which penetrate the groundwater table, are used to stop the movement of floating contaminants or gases. After an excavation segment is completed, the excavator backs up and begins a new overlapping segment to create a continuous trench. Once sufficient excavation is complete, trench backfilling begins.
Bentonite slurry is the most common excavation fluid used in a slurry trench. Bentonite clay and water are combined in a colloidal mixer and the resulting slurry is pumped, as required, through a pipe to the excavation site. In addition to stabilizing the excavation, bentonite slurry forms a “filter cake” on the slurry trench walls that reduces the slurry wall’s final soil permeability.
It is common for the slurry trench to be backfilled with a mixture of excavated soil, dry bentonite, and bentonite slurry. Walls of this composition provide a low cost barrier with low soil permeability and good chemical resistance. Excavated soil is placed on the work platform adjacent to the trench. A bulldozer tracks and blades the material to produce soil-bentonite backfill, which has a consistency of wet concrete. The backfill is placed into the end of the slurry trench, in a manner that displaces the slurry forward toward the ongoing excavation. The excavation/backfill routine continues until the slurry wall is complete.
Slurry wall construction requires the use of heavy construction equipment and involves excavation; therefore, workroom is a factor that must be considered. However, there are variations of the technique that can make construction more feasible in limited workroom. These variations include: “remote backfill mixing”, which requires the excavated soil be transported to a remote location, mixed, and returned to the excavation as backfill; and “cement-bentonite slurry walls”, which are excavated under a slurry that solidifies to create the final backfill. In addition, there are alternate techniques such as jet grouting that can be used to install a barrier wall in limited working areas.
A second factor that should be considered is compatibility. Bentonite slurry and soil-bentonite backfills may not be able to withstand the attack of some chemicals. However, alternate slurry materials including attapulgite and treated bentonites are available to satisfy specific project compatibility requirements; similarly, other backfill compositions such as soil-attapulgite and soil-bentonite with geomembrane inserts can be used. Additionally, if a project requires greater wall strength, cement-bentonite and soil-cement-bentonite can be used.