The International Information Center for Geotechnical Engineers

Vertical Impermeable Barriers (Cutoff Walls)

 

2.2.     Grouted Barrier Walls

Theoretical Background/Applicability

In general, grouting technology consists of injecting a fluid material at a certain pressure into soil or rock in order to decrease the permeability and/or strengthen the formation (USEPA, 1984). Grouted barriers have higher permeability than other techniques such as slurry and geomembrane walls, and therefore have been used less frequently at hazardous waste sites. They are capable of extending the key of other types of barriers, like slurry walls, through the bedrock in situations were standard excavation equipment is not adequate or is too costly (USEPA, 1984; USEPA, 1998).

Grout walls are typically formed by overlapping grout columns which form a single row.  Additional rows can be added to ensure the effectiveness of the wall (Pearlman 1999).

The ability of the soil to receive the grout is known as groutability. This parameter depends on the permeability of the soil and the viscosity of the grout. Higher viscosity grouts are more suitable for high permeability soils (Pearlman, 1999). Table 7 shows the relationship between the permeability of the soil and its groutabiltiy.

Table 7: Relationship Between Soil Permeability and Grouting (Karol 1990 in Voss et al., 1994 as presented by Pearlman 1999)
table 8.1

 

Advantages (Sharma & Reddy, 2004)

  • Depths greater than 200 ft can be achieved;
  • Ideal for situations involving fractured rock;
  • Little waste material is generated.

Disadvantages (Sharma & Reddy, 2004)

 

  • This configuration involves more cost;
  • Relatively high hydraulic conductivity values are obtained.

 

            2.2.1.      Jet grouting

Theoretical Background/Applicability

According to Pearlman (1999) the methods for jet grouting can include a single-rod system (injection grout alone), a double-rod system (injecting grout and air) or a triple-rod system (injecting grout, air, and water). Usually the grout consists of Portland cement or cement-bentonite and may include other clays and silicates (CPEO website; Pearlman, 1999). Due to the fact that these materials may not be chemically compatible with the contaminants they may crack and/or may not be durable. Therefore, polymer grouts have been used. Polymer grouts are resistant to radiation, as well as acidic and alkaline environments (CPEO website).

In general this technique injects the grout mixture at very high pressures and velocities (5,000-6,000 lbs/in2 and 800-1,000 ft/s, respectively) directly into the pore spaces of the soil or rock. The grout being injected cuts, replaces, and mixes the in situ soil, destroying the soil structure and forming a homogeneous mass. As the rod is being removed and the grout is being injected, a column is formed (Pearlman, 1999). Each time the rod is inserted in such a way that each column intersects the next, therefore forming a continuous wall or curtain (CPEO website).

Pearlman (1999) suggests that this method can be applied to soil ranging from gravel to clay, however the soil properties can alter the diameter of the grout column and the efficiency as jet grouting in clay is less efficient than in sand. According the CPEO website, high-pressure jetting appears to be a cost effective means to place physical barriers in unstable soils, near foundations, and around underground obstructions.

Advantages (Mutch et al., 1997; Dwyer, 1998 as presented by Pearlman, 1999)

  • Versatile – jet grouting can stabilize a wide variety of soil types ranging from gravel to heavy clays
  • Starting from a small borehole, large diameter columns or panels can be created
  • Can install wall (inject) in confined places that might otherwise limit installation – for instance, cut-off walls can be constructed beneath buildings without disrupting the structure
  • Installed at depths up to 150 – 200 ft
  • Can drill at any angle forming both vertical and horizontal water control barriers
  • Jet grout unit is mobile, permitting drilling with rotation and percussion
  • Down-the-hole (DTH) percussion hammer coupled with the drill string results in more reliable drilling alignments (straight and parallel), faster drilling rates, and a quieter operation
  • Innovative equipment allows injection of multiple fluids or gasesCan be used in coordination with treatment
    • DTH percussion hammer
    • Multi-nozzle grout injection unit increases the efficiency of injection

 

Disadvantages (Rumer et al., 1995 as presented by Pearlman, 1999)

  • Difficult to ensure panel continuity (verticality is critical to ensure that gaps will not occur between panels)
  • Boreholes can become misaligned
  • Obstruction of jet nozzle can be a problem
  • Different soil types and densities affect ability to grout
  • Gaps between panels or thin spots may lead to cracking
  • Separation or tears may occur as barriers harden
  • Potentially large amount of spoils to clean up
  • Injection pressure and volume must be closely monitored

Field Setup/Process Involved

As previously mentioned there are three main types of jet grouting, single, double and triple rod. Single rod jet grouting injects only grout into the soil and is capable of creating columns that can be up to 1.2 m in diameter in granular soils and up to 0.8 m in diameter in cohesive soils. In double rod jet grouting air is injected along with the grout in order to clear the jet stream of soil and groundwater. Double rod jet grouting columns can potentially have twice the diameter as single rod columns but tend to be more permeable due to the added air content. Triple rod jet grouting injects water and air in a separate jet which cuts and lifts the in-situ soil. The grout is injected through another jet so that a column comprised of almost pure grout is formed as the drill rod is lifted.  These columns can have diameters as large is 3 m in granular soils and 1.5 m in cohesive soils. A triple rod jet grouting setup injecting grout, water and air into the soil is depicted in Figure 8 (USEPA, 1998).

 

figure 8

Figure 8: Jet Grouting Wall Construction Setup (Evans, 1995)

Cost (as presented by Pearlman, 1999)

Cost ranges from $15 – $30/vertical ft2.

            2.2.2.      Permiation Grouting

Theoretical Background/Applicability

Unlike jet grouting, permiation grouting injects a low-viscosity grout into the soil at low pressure. The grout fills the soil voids, achieving low permeability without significantly changing the structure or volume of the soil (Pearlman, 1999).

In general, a hole is drilled into the ground and a fluid is injected under pressure into the soil or rock, permeating the voids (Ryan, 1987). The two main methods of permiation grouting are point injection and sleeve pipe injection. For point injection, the casing is first driven to full depth then as the casing is withdrawn the grout is injected. For the latter, as the name suggests, a sleeve pipe is inserted in the grout hole and grout is then injected through the holes in the pipe. An advantage of the sleeve pipe injection method is that grout can be re-injected if there is a problem (Pearlman, 1999).

Permiation grouting is a useful technique to achieve low permeability barriers in semi- arid unconsolidated alluvial soils and where a barrier is needed in a rock formation (Pearlman, 1999; Ryan 1987).

Advantages (as presented by Pearlman, 1999)

  • In situ technique means lower costs because there is no excavated soil
  • Directionally drilled boreholes allow access without disturbing the waste
  • Can be used to emplace vertical or horizontal barriers for complete containment
  • Appliable for short-term or long-term applications
  • Barriers may enhance the effectiveness of in situ remediation while containing the volume of waste
  • Applicable to rock

Disadvantages (as presented by Pearlman, 1999)

  • Limited to formations with moderate to high permeabilities
  • Hard to ensure continuity
  • Difficult to direct the flow of grout in heterogeneous soils because the grout tends to follow the path of least resistance
  • Hard to predict grout penetration radius

Field Setup/Process Involved

The two main methods used for permeation grouting are point injection and the tube-a-manchette method. The point injection method simply involves advancing the grout casing to the required depth and then, as the casing is withdrawn, grout is extruded through the end of the casing. The tube-a-manchette method involves grouting a sleeve pipe into a borehole with a weak grout. The pipe contains small holes at 1-ft intervals which are all covered by rubber sleeves (manchettes) which acts as a one-way valve. A double packer is placed in the pipe so that it straddles the manchettes so that pressurized grout can be injected in to the soil. Because the tubes can be left in the ground, this method allows for further addition of grout of other additives at a later date. A depiction of a tube-a-manchette apparatus is shown in Figure 9 (USEPA, 1998).

 

figure 9.1

Figure 9: Tube-A-Manchette Permeation Grout Apparatus (From USEPA 1998)

 

 

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