What it is
This techniques involves the installation of artificial drainage paths or vertical drains into soft compressible soils with a lowering of the atmospheric pressure inside the drains to accelerate the consolidation process. The prefabricated vertical drain (PVD) used by Cofra for vacuum consolidation are specially designed to work effectively both under high settlements (i.e. under buckled or kinked conditions) and under the vacuum pressure.
How it works
Highly permeable soils like sand can adjust rapidly to loads and the corresponding compression / reduction in voids due to the ability to easily drain the excess porewater. Low permeable soils like clay and peat do not have this ability and show a delayed compression with the dissipation of the excess pore water. This process of dissipation of excess pore pressures, also known as consolidation, can take up to years depending on the thickness of the deposit. Vertical drains or also known as wick drains are used to shorten the dissipation path of the excess pore water and to accelerate the consolidation process to a few months or even weeks when the PVD is placed at close center to center distances.
Vacuum consolidation acts, in principal, similar to prefabricated vertical drains (PVD) with surcharge. However an additional load is applied to the subsoil by lowering the atmospheric pressure inside the vertical drain. The applied vacuum pressure inside the system can be seen as an equivalent surcharge load. This means that the total applied pressure (surcharge load plus vacuum load) acts at the soil in order to accelerate the consolidation. This principle is the same for all vacuum methods. The difference between the methods is their application area caused by the ability of each system to seal the vacuum drains from ingress of air or permeable layers. The liner vacuum system uses a top liner and slurry walls around the area to seal the soil completely from air. The BeauDrain system installs the complete system below the ground water in the soft compressible soil with the help of a plough. BeauDrain-S uses tubes to cross any permeable layers and connects to a PVD in the soft material below the ground water. Please refer to the image below for more information.