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Vertical Impermeable Barriers (Cutoff Walls)

 

2.3.      Soil Mixing Walls

Theoretical Background/Applicability

Deep soil mixing walls consist of “overlapping columns created by a series of large diameter counter rotating augers mixing” (USEPA, 1998) in situ soils with low permeability slurry creating a column of treated soil (Pearlman, 1999; Ryan, 1987; USEPA, 1998). Such slurry is injected through the augers producing a hard mass, which then acts as a barrier (Pearlman, 1999). The types of slurry will vary depending on the site conditions similar to slurry walls. Common materials for the slurry include bentonite, cement, lime, fly ash, among others (Pearlman, 1999). A continuous wall may be constructed if individual columns are overlapped.

Advantages (as presented by Pearlman, 1999; Ryan 1987)

  • These walls are capable of having hydraulic conductivitie values lower than 10-7 cm/s
  • Little waste material is generated during wall construction
  • Construction can take place in reletively confined areas
  • Any tiype of waste can be contained as long as “a chemical or physical reagent is applicable”
  • Does not require an open excavation
  • There is a low cost involved when compared to jet grouting

Disadvantages (as presented by Pearlman, 1999)

  • The bottome of the wall can not be inspected to ensure that it is properly keyed in
  • Hard ground or large boulders in the subsurface limit drilling ability
  • The continuity of the wall is difficult to ensure do to the use of many interconected holes or panels
  • The amount of bentonite that is added to the soil is limited
  • Maximum drilling depth limited to 40-100 ft

Field Setup/Process Involved

Soil mixing typically requires the use of a rig fitted with either one, or a panel of multiple, mixing augers. Figure 10 shows a panel of three counter rotating augers.  While the soil is being mixed materials such as cement and bentonite are injected to create a panel of low hydraulic conductivity.  Subsequent panels are then overlapped to create a cutoff wall. (Evans, 1995)

 figure 9

Figure 10: Deep Soil Mixing Wall Construction (Evans, 1995)

 

Cost (as presented by Pearlman, 1999)

The cost varies depending on the soil characteristics and the grout material used. For example, soil-cement is more expensive than soil-bentonite (Rumer er al., 1996). The following are some cost estimates:

  • $10 – $20/vertical ft2
  • $40 – $50/yd3 (cost does not include reagent)
  • Cost ranges from $6 – $15/ft2 for deep mixing, and $15 – $30/ft2 for DSM structure 

 

 

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