Cut-off walls are defined as low-permeability walls constructed within high-permeability geotechnical formations to reduce water seepage. Cut-off walls can also be referred to as water barriers.
Dams and levees that experience seepage or instabilities are generally rehabilitated by installing a cutoff wall through the entire height of the dam and anchoring it into competent bedrock or low-permeability geotechnical formations at greater depths.
Typical cut-off walls are constructed by replacing the in-situ materials with low-permeability fill materials. These materials can be cement-soil mixtures, cast-in-place concrete, or self-hardening bentonite slurry. When cut-off walls are required to accommodate larger movements or deformations, plastic concrete can be utilized to meet low permeability, medium strength (500-1500 psi), and flexibility to movement. Plastic concrete is formed by adding bentonite to tremie concrete.
Cut-off wall thickness can vary from 18 to 60 inches. It’s important to allow for sufficient verticality tolerances to achieve the required continuous seepage barrier at the bottom of the cut-off wall. Permeability in the order of 10-6 cm/sec to 10-7 cm/sec can be achieved with standard fill materials. Lower permeability can also be achieved by installing high-density polyethylene (HDPE) membranes within the cut-off wall.
Malcolm utilizes cut-off walls on almost every support of excavation project within an urban environment. The support of excavation systems serve a dual purpose, allowing for stable excavation of deep excavations and minimizing the lowering of groundwater levels (GWL) under adjacent structures. Extending the support of excavation beyond the structural requirements into a low-permeability layer allows the wall to be used as a cut-off wall as well. It’s very important to limit and control GWL outside of construction sites since long-term lowering of GWL can lead to undesired movements or settlements of adjacent structures.