Slurry Wall

A slurry wall is a non-structural barrier built under ground to prevent the flow of groundwater.^[1] The initial application of slurry walls was first limited to civil construction projects namely hydraulic barriers used in and under dams, levees, and other similar structures. More recently, however, slurry walls, cutoff walls, or slurry trenches are being widely used as a ground remediation tool for underground waste and groundwater containment.^[2] It is now common to use slurry walls in the construction and building of tunnels. Over 26,000 linear feet (7,925 m) of slurry walls equivalent to five miles (8 km), for example, were used in the Central Artery Tunnel Project in Boston.^[3]

[edit] Process

The construction of a slurry wall begins with the “slurry excavation technique.” Developed in Europe, the technique has been widely used in the U.S. since the 1940s.^[4] A narrow trench is excavated and filled with slurry, a clay-like mixture. The slurry mixture exerts hydraulic pressure against the trench walls during the excavation process essentially acting as support preventing the collapse of the wall even below groundwater.^[5]

Trenches for slurry wall construction are dug using hydraulic excavators and can be dug to a depth of 1.5 to 5.0 feet (0.5 to 1.5 m). Trenches dug over a 100 feet (30 m) require the use of a special crane with a clam bucket or other specialty equipment.^[6] The trench is usually dug in a stratum that has low permeability such as clay or bedrock so that leakage can be minimized as much as possible.

Slurry placed in the trench is typically a mixture of bentonite and water combined in a mixer. The slurry is pumped directly from the mixer through a pipe into the trench. In addition to just stabilizing excavation, bentonite slurry forms what is called a “filter cake” on the slurry trench walls, reducing the wall’s final soil permeability.^[7]

Once excavation is completed, it is common the backfill the trench with a mixture of excavated soil, dry bentonite, and bentonite slurry. The backfill eventually has the consistency of wet concrete and is placed into the end of the slurry trench, displacing the other slurry. The process of excavation and backfill is carried out until the slurry wall is completed.^[8]

The slurry may also eventually be replaced with concrete. In the Central Artery Tunnel Project, for example, concrete was pumped into the trenches after the slurry was pumped away and recycled.^[9] Sometimes tieback anchors are used to provide additional support to concrete walls. These are rods or wire that reinforce and stabilize retaining walls. In the construction of the World Trade Center slurry walls and tieback anchors were used in the construction of the building’s huge bathtub shaped basement foundation. It was only the third time slurry walls had been used in the U.S. and one of the earliest occurrences of a single large-scale project using so many tieback anchors.^[10]

[edit] References

1. ↑ Slurry Walls. Geo-Solutions, 2008-09-29.
2. ↑ Slurry Wall, Cutoff Wall, Slurry Trench Technology Overview. Slurrywall.com, 2008-09-29.
3. ↑ Slurry Walls. Massachusetts Turnpike Authority, 2008-09-29.
4. ↑ Slurry Wall, Cutoff Wall, Slurry Trench Technology Overview. Slurrywall.com, 2008-09-29.
5. ↑ Slurry Walls. Geo-Solutions, 2008-09-29.
6. ↑ Slurry Wall, Cutoff Wall, Slurry Trench Technology Overview. Slurrywall.com, 2008-09-29.
7. ↑ Slurry Wall, Cutoff Wall, Slurry Trench Technology Overview. Slurrywall.com, 2008-09-29.
8. ↑ Slurry Wall, Cutoff Wall, Slurry Trench Technology Overview. Slurrywall.com, 2008-09-29.
9. ↑ Slurry Walls. Massachusetts Turnpike Authority, 2008-09-29.
10. ↑ Tamaro, George J. World Trade Center "Bathtub": From Genesis to Armageddon. National Academy of Engineering, 2008-09-29.