containment walls in environmental management

containment walls in environmental management Vertical containment walls have been used for several decades to provide low-permeability barriers to depths of over 30 m below ground surface. Typical applications of these walls include containing sanitary and hazardous waste landfill contaminants, sealing dams and dykes, enclosing oil and chemical tank farms to prevent releases, dewatering structural excavations, and hydraulically isolating lagoons.

Containment walls, also known as ‘cutoff walls’ or ‘slurry walls’, are constructed either of soil-bentonite or of cement- bentonite slurry. For most applications, the soil-bentonite slurry provides the most economical and impermeable wall. Containment walls can be constructed using several methods according to depth and permeability requirements. Slurry walls are built by excavating a narrow trench 0.5 to 1 m wide while pumping in the slurry and maintaining its level at or near the top of the trench during excavation. The excavated soil is mixed with additional clay (the quantity is determined from tests) and bentonite slurry to a consistency similar to that of wet concrete. The mixture is used to fill the trench and sets to form a low-permeability containment wall. The bottom of the trench is set 30 cm to 1 m into an underlying low-permeability formation. The wall, in conjunction with the low-permeability formation, provides containment of the material within the wall.

Containment walls can be constructed in situ by using other techniques such as deep soil mixing (DSM). The DSM system uses a set of leads supported by a crane that guides a series of mixing shafts each 1 m in diameter. Three to four shafts are advanced vertically into the soil while bentonite slurry is injected through their hollow stems. A combination of auger flights and mixing blades along the shafts lift the in situ soil and mix it with the slurry. Continuity of the wall is ensured by using multiple sets of shafts overlapping the previously drilled columns. Containment walls typically reuse most of the excavated material, and the quantity of bentonite used varies from 5 to 10 per cent. Cement-bentonite slurry walls are used where self-hardening, stability-enhancing walls are required.

Permeability is the critical design parameter for containment walls; it varies from 10⁻⁸ m s⁻¹ to 10⁻¹¹m s⁻¹. The permeability is reduced as the natural clay content or bentonite content increases. The percentage of coarse-grained materials in the soil affects the strength and compressibility of the wall. The strength increases as the percentage of coarse-grained material increases. As the water content in the soil-bentonite slurry reaches equilibrium with that of the surrounding soil, the strength of the wall is approximately equal to the strength of the surrounding soil.

The development of high-density polyethylene (HDPE) products led to the installation of vertical HDPE liners underground without excavation. Since 1991, some manufacturers in the USA have installed HDPE containment walls to depths of about 10 m below ground without excavation. These walls consist of a plastic barrier of HDPE liner panels locked together with a watertight joint.

V. Rajaram