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Vertical Impermeable Barriers (Cutoff Walls)

 

1.            Main Concept / Description of the Technique

Vertical barriers or “cutoff walls” are often used in geoenvironmental engineering to control the spread of contaminants. Cutoff walls in general can either be used to directly contain a contaminant by sealing it in from the outside environment or to manipulate the flow of groundwater to avoid contaminant transport through advection.Vertical barriers are often used along with surface caps or bottom barriers in order to create a more complete containment structure (CPEO website).

When groundwater flow is unknown the contaminant will most likely have to be completely surrounded by a wall that is keyed into an “impermeable layer”. Figure 1 shows a 2-dimensional view of a vertical cutoff wall completely surrounding a buried waste site with the walls keyed into a low permeability layer.  Similarly, Figure 2 shows a plan view of a circumferential cutoff wall which contains pumping wells for remediation and monitoring wells for quality control. Theoretically, there should be no flow of either contaminants out or groundwater into the contained area (USEPA, 1995).

figure 1

Figure 1:  Circumferential Cutoff Wall Keyed into an “Impermeable” Barrier (USEPA, 1995)

 

figure 2

 

Figure 2:  Circumferential Cutoff Wall Showing Divergence of Groundwater Flow (USEPA, 1992)

 

Cutoff walls can be constructed in various configurations to manipulate the flow of groundwater.  The vertical barrier can either be keyed-in into a low permeability layer, like the example shown in Figure 1, or they can be hanging, where the wall does not extend into a low permeability material. Hanging walls are typically used for floating contaminants or LNAPL’s, or to simply deflect the flow of groundwater.  Vertical barriers can also be either upgradient or downgradient, which simply specifies whether the cutoff wall is respectively in front of or behind the contaminated area in relation to the flow of groundwater (USEPA, 1998; Van Deuren et al., 2002). A summary of cutoff wall configurations can be seen in Table 1.  Figure 3 shows an example of an upgradient, hanging cutoff wall directing the flow of groundwater beneath a buried waste site.

 

   

Table 1:  Cutoff-Wall Configuration Summary (USEPA, 1984)

table 1

 

figure 3

Figure 3:  Hanging, Upgradient Cutoff Wall (USEPA, 1995)

 

Cutoff walls are often used in combination with other remediation and containment techniques such as pumping wells and gravity drains.  Figure 4 shows an example of a hanging, downgradient cutoff wall being used to stop the flow of a floating contaminant and a pumping well being utilized to remove the contaminant from the soil.

figure 4

Figure 4:  Cutoff wall, Pumping Well Combination for Remediation (USEPA, 1984)

 

Although cutoff walls are often referred to as impermeable barriers, they can never be truly impermeable (USEPA, 1995).  Depending on the type of wall and the methods of installation, the hydraulic conductivity of cutoff walls can greatly vary, but can often exceed the minimum requirement for waste containment (10-7cm/s). There are many types of cutoff walls, but the main types discussed in this paper include slurry walls, grouted barrier walls, soil mixing walls, sheet pile walls and geo-membrane (GM) walls. 

 

 

 

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