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Vertical Impermeable Barriers (Cutoff Walls)

 

2.5.      Geomembrane Walls

Theoretical background/Applicability

Geomembranes have been used in vertical wall applications since the 1980s. Due to the concerns of the integrity of slurry walls, geomembranes have been used as a complementary strategy to the standard slurry walls providing a more complete containment structure. However, geomembranes have also been used by themselves as a separate technology (Koerner & Guglielmetti, 1995; Thomas & Koerner, 1996).

High density polyethylene (HDPE) is the most common material used due to the fact it is highly resistant to water vapor transport and to a variety of other chemical. Moreover, it has been used extensively in other geotechnical applications and therefore is readily available (Koerner & Guglielmetti, 1995; Thomas & Koerner, 1996). However, HDPE does not provide enough stiffness in order to directly drive the material into the soil. Therefore, in cases that direct driving is required polyvinyl chloride (PVC) may be used (Koerner & Guglielmetti, 1995).

Figure 12 shows a qualitative resistivity of HDPE geomembranes to several chemicals. Concentrated hydrocarbons, especially chlorinated and aromatics, can produce a loss in tensile yield strength of up to 30%, though it has been seen that this behavior is reversible when the compounds are allowed to vent off (Thomas & Koerner, 1996). Moreover, Table 8 shows the vapor transmission rates of several organic solvents through HDPE geomembranes. Some factors that affect the rate of permeation through the membrane include chemical concentration, temperature, and sheet thickness. 

 figure 11

Figure 12: Relative Chemical Resistivity of an HDPE Geomembrane (as presented by USEPA 1992

 

Table 8:  Vapor Transmission Rates of Organic Solvents Through HDPE Membranes (Park et al., 1995 as presented by Thomas & Koerner, 1996)

table 7.5

Advantages (Koerner & Guglielmetti, 1995; Ressi & Cavalli, 1985; Thomas & Koerner, 1996)

  • Can ensure continuity of extremely low hydraulic conductivity values (as low as 10-12 cm/s), when good connection of interlocks exist
  • Added chemical resistance, including organic solvents

Disadvantages (Koerner & Guglielmetti, 1995; Thomas & Koerner, 1996)

  • Hard to assure quality installation
  • Not stiff enough to be directly driven into the soil like steel sheets

Additional advantages and disadvantages for individual geomembrane wall construction methods are listed below in Table 9.

Table 9:  General Assessment of Geomembranes as Vertical Barriers (Rumer et al., 1996 as presented Pearlman 1999)

table 9

Field Setup/Process Involved

Geomembranes can either be used alone to form a vertical barrier or in conjunction with other technologies such as slurry trenches. Geomembrane walls are usually constructed from interlocking panels but can also be rolled out in a trench to form a continuous wall. A geomembrane/slurry wall combination schematic can be seen in Figure 13. The typical methods used for the construction of geomembrane cutoff walls are listed in Table 10 and include the use of a trenching machine, vibrated insertion plates, slurry support, a segmented trench box and a vibrating beam (Evans, 1995 and Pearlman, 1999).

figure 13.1

Figure 13:  Geomembrane/Slurry wall combination (Evans, 1995)

 

 

Table 10:  Installation Methods for Geomembrane Vertical Barriers (Rumer et al., 1996 as presented Pearlman 1999)

table 10

 

 

 

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