Slurry trenches are low permeability, vertical barriers constructed in the ground for the purpose of reducing the transport rate of liquids. Most commonly groundwater is the liquid in question, although at hazardous waste remediation sites almost any liquid might be encountered. The terms slurry trench and slurry wall are used interchangeably here although slurry wall implies a more permanent structure which may stand on its own when the soil around it is partially excavated. This distinction will become more apparent when construction techniques are discussed later.
Slurry trenches are commonly used in dewatering operations for excavations and at construction sites, and although this section will focus on trenches at hazardous waste sites, the same principles hold for all slurry trenches. There are several ways slurry trenches might be beneficial in remediating a site:
Figure 1: Plan view of slurry trenches with wells: (a) containment, (b) upgradient, and (c) downgradient.
Slurry trenches used to contain contaminants are not usually designed for permanent retention, but rather as temporary barriers to allow some cleanup of the waste without it spreading through the groundwater. These structures can greatly increase the efficiency of a pump and treat system by reducing the volume of water that will enter the wells. The trenches are typically keyed into a confining layer like clay or bedrock to prevent migration under them.
Slurry walls placed upgradient of the waste serve to redirect the groundwater around the contaminant, retarding its spreading. The groundwater flow is locally reduced and this also assists in recovering any plume that has moved downgradient from the site.
Downgradient slurry walls act like slurry walls which circumvent the entire spill by preventing movement of the plume. They are different however because clean groundwater is allowed to flow into the site which helps to flush the waste into the pumping wells. This type of geometry may be useful for LNAPL spills where long term pumping may be part of the remediation process.
Several types of slurry walls are commonly used for various reasons. The discussion here is kept to soil-bentonite slurry walls which are by far the most common type used for remediation of hazardous waste sites. Other wall types include cement-bentonite, lean or plastic concrete, and vibrating beam.
Soil-bentonite slurry walls are constructed by excavating a trench with a backhoe or a clamshell. The trench is kept open by filling it with a bentonite-water slurry which prevents the native soil from caving into the excavation. The slurry level must be kept above the water table to provide enough support for the sides of the trench. As the excavation progresses, a soil-bentonite backfill is placed in the trench, in turn displacing the bentonite-water slurry.
Figure 2: Soil-bentonite slurry wall construction in profile.
Placement of the backfill is the most critical part of the construction process. It is during this stage that the integrity of the wall is most at risk. If the backfill is simply dumped into the trench, some of the bentonite-water slurry may get trapped within the soil-bentonite forming "windows". Windows are areas of higher permeability which compromise the intent of the wall.
Another concern which must be addressed to prevent windows in the finished slurry wall is the sloughing of the native soil into the trench. This is of special concern at the bottom of the trench where the soil will settle out and could cause large windows if it is not removed. Therefore, cleaning of the trench bottom is required. Rough cleaning can be performed with a backhoe or clamshell and for finer cleaning, an airlift pump system can be used.
The slurry trench is normally keyed into some confining layer such as clay or bedrock to prevent water or contaminant from flowing beneath it. If a slurry trench is being used for the cleanup of an LNAPL it may not be keyed into a confining layer but simply installed to a depth below the water table.
Bentonite (a montmorillonite) is a clay mineral which is commonly obtained from natural deposits in Wyoming (sodium montmorillonite). It has a very high surface area of about 900 m2 per gram and when it is fully hydrated (adsorption of water onto the particles) it becomes very viscous. Full hydration of the bentonite is the main key to a good slurry.
Complete hydration may take anywhere from 30 minutes to 24 hours depending on the process used. Batch mixing is common for small jobs where the bentonite and water are mechanically mixed in a tank and full hydration usually occurs in 30-40 minutes. Flash mixing provides quick exposure of the bentonite to water in a venturi nozzle system and the mixture is pumped to detention ponds where it will hydrate in about 24 hours. Hydration is typically checked in the field with a Marsh Funnel or by a filtrate loss test.
Bentonite-water slurry usually contains 2-4% bentonite by weight and its typical unit weight (gamma) is 64-65 pcf (gamma water = 62.4 pcf). Soil-bentonite slurry is made by mixing the bentonite-water slurry with a well-graded soil. If possible, the native soil which was just excavated is used in order to minimize the cost of the project. It is typically mixed with a bulldozer right next to the open trench and then pushed into it so it slides down the previously placed slurry. The final product usually has a permeability around 10^-6 cm/sec, but even 10^-8 cm/sec can be reached with the proper mix.
Care must be taken to ensure that any chemicals present in the groundwater will not compromise the integrity of the wall. Some compounds may break down the slurry mix and create zones of higher permeability. This is obviously of great concern at hazardous waste sites and may dictate whether or not slurry walls will be effective.
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Daniel Gallagher, dang@vt.eduCopyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
