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Vertical Impermeable Barriers (Cutoff Walls)

 

2.            Technologies

2.1.      Slurry Walls

Theoretical Background/Applicability

Slurry walls are used to contain or divert contaminated groundwater from drinking water intake, divert uncontaminated groundwater flow from contaminated sites, and/or provide a barrier for the ground water treatment system (Van Deuren et al., 2002).

In general, slurry walls consist of a vertical trench excavated along the perimeter of a site, this trench is then filled with bentonite slurry for support and then backfilled with a mixture of low-permeability material, 1x10-6 cm/s or lower (USEPA, 1998). The three main types of slurry walls are soil-bentonite, cement-bentonite, and soil-cement-bentonite (see sections 2.1.1-3 for specifics). Depending on which type, the backfill may contain a mixture of bentonite, other clays, cement, fly ash, groundblasted furnace slag, among others (Pearlman, 1999).

Slurry walls are often used in cases that the waste mass of the contaminant is too large for treatment or where soluble and mobile constituents pose an imminent threat to a source of drinking water (Van Deuren et al., 2002).

Slurry walls have been used as a long-term solution for seepage control for over 50 years and have demonstrated its effectiveness to the point that they are considered baseline barriers. Therefore, the requirements, equipment, and practices for design and installation are well established (Pearlman, 1999; Van Deuren et al., 2002). In terms of pollution control, they have been used since the 1970s; the issue with this application is that specific contaminant types may degrade the slurry wall and therefore reduce the long-term effectiveness. Therefore, even though the design and installation criteria may be established, the process of choosing the proper wall materials for that specific contaminant is less developed (Van Deuren et al., 2002).  Ressi and Cavalli (1985) adds that some suggest that this technology on its own should not be the final measure for remediation due to the fact that long-term performance of these walls when chemicals are present is not known.

The most effective vertical configuration of slurry walls for site remediation or pollution control is keyed-in, where the wall is keyed 2-3 feet into a low permeability layer, such as clay or bedrock, providing a foundation with minimum leakage potential (Van Deuren et al., 2002).

The following list contains factors, according to Van Deuren et al. (2002), that must be assessed prior of designing the slurry wall:

  • The maximum allowable permeability;
  • Anticipated hydraulic gradients;
  • Required wall strength;
  • The availability and grade of bentonite to be used;
  • Boundaries of contamination;
  • Compatibility of wastes and contaminants in contact with slurry wall material;
  • Characteristics of backfill material;
  • Site terrain and physical layout.

Advantages (Pearlman, 1999; USEPA, 1998)

  • Can reach hydraulic conductivities values less than 10-7 cm/s;
  • This is the most common type of cutoff wall;
  • Due to the fact that requirements and practices are well understood they are installed quickly;
  • Depths of up to 200 ft can be reached;
  • This is the only method that permits visual inspection of the key material and therefore assurance of the key-in depth during construction.

Disadvantages (Sharma & Reddy, 2004)

  • Depth above 50 ft require specialized equipement;
  • Large excavation site, excavated soil storage, slurry mixing, material storage, etc.;
  • It is hard to ensure integrity of the wall.

Field Setup/Process Involved

One of the most significant disadvantages of slurry trench construction is the extensive field setup required.  The major construction process involved in the installation of a slurry trench include “preconstruction planning and mobilization, preparation of the site, slurry mixing and hydration, excavation of soil, backfill preparation, placement of backfill, cleanup of the site and demobilization” (USEPA, 1995).  In order to accomplish all of this, a large site is typically required to accommodate the various mixing areas, storage of soils excavated, heavy machinery, etc.  The components of construction of a typical slurry trench are depicted in Figure 5.  Types of typical physical constrains affecting slurry wall construction are listed in Table 2. 

figure 5 

Figure 5: Soil Bentonite Cutoff Wall Operation (Rumer and Ryan, 1995 as presented by Evans, 1995)

 

Table 2: Types of Physical Constraints and their Effects on Slurry Wall Construction (USEPA 1984)
table 3.1

 

 

            2.1.1.      Soil-Bentonite Slurry Walls

Theoretical Background/Applicability

Soil-bentonite (SB) slurry walls are the most widely used technique for containment in the U.S. (Katsumi et al., 2009; Pedretti et al., 2012; USEPA, 1998). As the name suggest, they are constructed by mixing a bentonite slurry with the excavated soil (USEPA, 1998; Van Deuren et al., 2002). USEPA (1998) reported that additional borrow material or dry bentonite might be added to the mixture in order to meet design requirements and that specialty additives have been added to the backfill in order to increase the sorption capacity.

Bentonite provides a high sorptive capacity, thixotropic nature, high dispersibility, sufficient deformability, and low-permeability (Garving & Hayles, 1999; Katsumi et al., 2009). Figure 6, shows they hydraulic conductivity as a function of percent fines and coarse fraction of the backfill. However, Katsumi et al. (2009) suggest that in order to promote the application of soil-bentonite walls there are several issues to be solved such as “achieving the higher construction quality and understanding the chemical compatibility.”

The hydraulic conductivity of bentonite-based material is affected by the chemical components of the permeant. It has been observed that bentonite does not swell and/or shrinks in the presence of inorganic solutions or some organic compounds. In the case of inorganic solution containing polyvalent cations the hydraulic conductivity can be increased, even at low concentrations (Katsumi et al., 2009). Table 3 shows a qualitative list of hydraulic permeability increase of certain contaminates through the SB wall. Therefore, its chemical compatibility should be assessed under the given conditions in the field.

figure 7

Figure 6: Hydraulic Conductivity of Soil Bentonite Backfill as a Function of Percent Fines and Coarse Fraction of the Backfill (Benson, 2002)

 

 

Table 3: Soil-Bentonite Permeability Increases due to Leaching with Various Pollutants (USEPA 1984)
table 7

 

Advantages (Pearlman, 1999; Pedretti et al., 2012; USEPA, 1998)

  • Among the slurry walls this is the most economical one;
  • Most cases allow reuse of all or most of the material excavated during trenching;
  • Construction techniques are well understood, practiced, and accepted;
  • Typical hydraulic conductivity are around 10-7 cm/s, but can be as low as 5 x 10-9 cm/s.

Disadvantages (Pearlman, 1999)

  • Installation requires excavation, therefore produces substantial quantities of spoils that must be disposed of, and requires a mixing area;
  • Wet/dry cycles and freeze/thaw cycles can cause deterioration;
  • This comfiguration is limited to vertical orientation;
  • Assessment of performance is difficult;
  • It is difficult to ensure proper emplacement;
  • May degrade over time due to contaminants in the soil, for exaple:
    • Silica and aluminum in the bentonite and/or soil may dissolve in the presence of strong organic and inorganic acids (pH < 1) and bases (pH > 11) increasing the porosity of the barrier;
    • Inorganic salts and some neutral polar and nonpolar organic compounds result in shrinkage of bentonite clay particles.

 

Cost (as presented in Pearlman, 1999)

In 1991, the cost ranged from $5 – $7/ft2, however these costs do not include cost needed for chemical analyses, feasibility, or compatibility test. Therefore, cost varies depending on site conditions, type of slurry/backfill, depth, among others. 

            2.1.2.      Cement-Bentonite Slurry Walls

Theoretical Background/Applicability

Cement-bentonite (CB) slurry walls are a common form of vertical barriers in Europe, especially for seepage control in the UK (Garvin & Hayles, 1999; USEPA, 1998). They were initially used for water exclusion but now their use has extended to control migration of contaminants from industrial or landfill sites (Garvin & Hayles, 1999). Originally for the CB walls, cement was mixed with the bentonite slurry before refilling the trench. The first trials with CB slurry walls had problems since bentonite and cement start to react when mixed together, the slurry became nonhomogeneous and unstable, and due to flocculation and sedimentation, the solid and liquid phase separated. Therefore, now the bentonite and cement are mixed together as powder. These powders are available as commercial products (Koch, 2002).

CB walls are used if greater structural strength is needed, if there is chemical incompatibility between bentonite and the site contaminants, if there is a lack of soil for backfill, if insufficient space is available for mixing of backfill, and/or for applications in steep slopes where shear strength of the cutoff walls is am issue (Pearlman, 1999; USEPA, 1998; Van Deuren et al., 2002). The most common cement used is Portland (Garvin & Hayles, 1999; Pearlman, 1999). Even though the CB imparts strength to the wall, it also increases the permeability of the backfill up to 10-5-10-6 cm/s, this becomes a problem since the typical required permeability is of 10-7 cm/s (Pearlman, 1999; USEPA, 1998). This behavior can be observed in Figure 7. However, additives such as ground-blast slag can be incorporated to the cement in order to reduce the permeability to 10-7-10-8 cm/s (Pearlman, 1999). Another concern with the CB walls is that contaminants can affect its long-term durability and performance (Garvin & Hayles, 1999). Pearlman (1999) suggests that adding fly ash can reduce the degradation of the concrete. Table 4 shows the typical composition of a cement-bentonite slurry. Table 5 and 6 compare some properties of soil-bentonite and cement-bentonite for the slurry and backfill, respectively.

Applications of CB walls include trench excavation adjacent to an existing structure or through soft or unstable soil (USEPA, 1998). 

 

figure 6

Figure 7: Hydraulic Conductivity of Soil Bentonite (Mixture A) and Cement Bentonite (Mixture B) (National Research Council, 2007)

 

Table 4: Typical Compositions of Cement-Bentonite Slurries (USEPA 1984)
table 8

Table 5: Specified Properties of Soil-Bentonite and Cement-Bentonite Slurries (USEPA 1984)
table 3

 

Table 6: Properties of Soil-Bentonite and Cement-Bentonite Backfills (USEPA 1984)
table 7.1

 

 

Advantages (Pearlman, 1999; USEPA, 1998)

  • This configuration provides higher strength than SB walls;
  • Self-hardening slurries do not require backfill, therefore CB walls can be constructed in limited access areas and at a lower cost;
  • Can be used on steep slopes with unstable soil;
  • Little or no slurry is displaced.

Disadvantages (Pearlman, 1999)

 

  • It is difficult to ensure panel continuity;
  • Some mixes can have undesirable high permeability (e.g., Portland cement can adversely affect the swelling of bentonite clay);
  • Often difficult to achieve sufficiently low permeability;
  • Cracking due to shrinkage, thermal stress, and wet/dry cycling can occur. 

 

Cost (as presented by Pearlman, 1999)

Cost ranges from $10 – $20/vertical ft2 for a 2-ft wide barrier of less than 100 ft.

            2.1.3.      Soil-Cement-Bentonite Slurry Walls

Soil-cement-bentonite (SCB) slurry walls are combination of the SB and CB walls. An advantage of SCB wall is that they provide similar strength to CB wall while providing similar hydraulic conductivity to SB walls (Pearlman 1999).

 

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