Slurry Walls is a generic name for below-grade installations that restrict groundwater flow. They are often also referred to as cutoff or barrier walls, and are typically constructed using soil-bentonite (SB), cement bentonite (CB) or ‘plastic’ concrete (concrete with a significant proportion of bentonite slurry included within the concrete mix constituents) backfill. The term slurry walls is derived from the fact that temporary trench stability is maintained through use of a slurry (typically bentonite). Structural diaphragm walls, occasionally also referred to as slurry walls, are below grade walls that can function as combined groundwater cut-off and soil retention systems, supporting excavations and structures, using structural grade concrete.
Traditional SB slurry walls can be installed using a number of techniques, the most common being ‘hoe-mixed’, which incorporates use of a long reach excavator to excavate out the wall/trench using a bentonite slurry to prevent collapse of the trench. The excavated soil is brought to the surface, spread out adjacent to the trench, unsuitable material is then separated out and a low permeability admixture, typically bentonite, mixed into the excavated material and blended before the mixed soil is returned to the excavated trench. In the case of CB walls, the CB slurry can be used as the temporary support medium, or the CB slurry can be introduced through a tremie to fill the trench following the excavation progression.
Pacific Ground Engineering has the capability to construct slurry walls using a variety of techniques but favour the use of the Cutter Soil Mixing (CSM) technique where quality and confidence in the long-term performance of the wall is critical. The CSM approach is an in-situ technique wherein a bentonite slurry solution is injected during both the cutting and withdrawal phases of the process. Since the technique utilizes the native soil it minimizes the requirements for a working platform area, as well as minimizing spoil handling and the clean-up efforts required. The CSM technique can be used with both bentonite slurry and also cement-bentonite slurry. The penetrations are overlapped to construct a continuous barrier with low permeability.
For structural diaphragm slurry walls, clam-shell buckets or trench cutters are typically used to excavate individual panels that will compose the wall, to design depth. During excavation, bentonite slurry is placed to prevent caving. After design depth is reached, the panel bottom is cleaned out, the verticality and dimensions of the panel checked, and the slurry is displaced with structural concrete placed through a tremie pipe that extends to the bottom of the panel and is raised incrementally as the panel is concreted. Steel reinforcement is then lowered into the fresh concrete and vibrated, as necessary, to achieve insertion to the designed elevations. The finished walls can function both as a groundwater cutoff and soil retention system during the excavation phase of a project, and then as permanent below grade external walls with load-carrying capabilities for the finished structure.