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Take Control Over Contaminated Soil and Groundwater: Four geotechnical techniques you should know about

Posted on Tuesday, October 23rd, 2012

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These days it's the rule rather than the exception that contaminants in the soil and/or groundwater will be present within an excavation on an urban jobsite. In such circumstances, the solution to contaminated groundwater is often straightforward: install dewatering wells or wellpoints to draw the groundwater table down and treat the pumped water in an on-site mobile treatment plant. But there are other circumstances where the desired–or specified–solution is not to remove the contaminant but prevent its movement on or off site. Specialty geotechnical contractors have a number of ways of achieving this objective.

Slurry Trenches
Soil-bentonite or cement bentonite slurry trenches form an impermeable subsurface barrier that prevents the flow of groundwater. They can be installed up gradient or down gradient of the contaminant source to prevent off-site migration. Slurry trenches can be keyed into a deep impervious barrier or "hanging" to stop movement of floating contaminants (LNAPL).

Jet Grouting
Jet grouting has applications for both groundwater barriers and the encapsulation of contaminated soils.

  • Barrier Walls
    The movement of contaminant plumes can be effectively impeded by the installation of a jet curtain wall. The grout is formulated to meet a specified permeability requirement, with the jetting            pinpointed to a specific subsurface stratum. While the curtain can be composed of either interlocking columns or panels, panels are the more cost effective option.    
        
  • Encapsulation
    Jet grouting is effective in the mass stabilization of soils in situ to prevent the release of contaminants. Depending on the project objective, jet grouting can create a low permeability perimeter containment barrier around a contaminated zone, or encapsulate in place the entire impacted soil mass.

Permeation Grouting
Permeation grouting is an effective method of sealing gaps and windows in excavation support systems to ensure watertightness. Grout injection can be performed through sleeve port pipes or lances. Sodium silicate permeation grouting has also successfully sealed windows in temporary cut-off walls to prevent the movement of contaminated water.

For long-term use, microfine cement, acrylate and acrylamide grouts offer superior permanence and resistance to breakdown in contaminated soils .

Ground Freezing
Installation of a temporary frozen ground cofferdam to rock or an impervious soil stratum provides watertight excavation support for the removal and disposal of contaminated soils or to isolate an area from the surrounding hydrological regime and thus minimize the amount of groundwater that needs to be pumped.

Ground freezing is a relatively low impact process involving only the drilling of small diameter holes for freeze pipe installation. Soil disturbance and waste is thus minimized, even for large frozen walls. Once the project objective has been met, and freezing discontinued, the ground reverts to its original state.

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