Wick drains & Vertical Soil Drains
Wick drains, also called Prefabricated Vertical Drains, are installed in soft soils in order to accelerate consolidation of the soil by providing additional drainage paths for pore water to escape. Other names and variations of this technique are: deep drains, sand drains, sand wicks, band drains, pre-fabricated band-shaped drains. Wick drains thus enables ground improvement by consolidation of soil layers.
Deep deposits of soft cohesive soil can take a long time to consolidate even when pre-loaded. The low permeability of these soils and the long drainage path lengths produce slow rates of consolidation. The time for consolidation to occur is a function of the square of the length of the drainage path. By reducing the drainage path length, excess pore water pressures can reduce more quickly, allowing a significant reduction in time required for consolidation.
Installation of Wick drains:
Drainage paths are shortened by the insertion of a vertical wick drain into the ground. The pattern and depth of wick are determined by the consolidation properties of the soil and the desired time for consolidation to occur. Wick drains are installed by pushing a mandrel, or stitcher, through the fine-grained, soft soil layer.
The wicks can be installed by vibratory, dynamic or static means using a purpose built rig fitted with a slender mandrel which carries the drain into the ground without damage.
Typically drains are installed at 1.5 to 3m centers, and depths in excess of 35m can be achieved using special equipment. A single drain should not act as a pile, inhibiting consolidation. It should have about the same stiffness as the surrounding soil mass.
Requirements of the vertical drains(wick drains) :
- High permeability to permit rapid dissipation of pore-water pressures
- Sufficient flexibility to accept large vertical and lateral ground movements.
- Continuity over its full length and a good hydraulic connection
- an installation method that does not cause so much disturbance as to make the surrounding soil too impermeable for the drain to be effective
- Ability to function over the required period, which may be for a few months to up to two years
- Adequate drain characteristics in changing conditions of stress, usually increasing stress
- Filters that do not become clogged by the surrounding fine-grained soils.
Design of wick drains:
The rate of soil consolidation is determined by how fast the pore water can escape from the soil. The variables that control this are the spacing between the wick drains and the permeability of the soil. The amount of consolidation doesn’t depend on the presence of wick drains but is determined by the soil compressibility and weight of the fill above the wicks.
By the available information a set of design curves of drain spacing, fill height, and consolidation time, the most economical drain spacing and height of fill can be selected to achieve a given degree of consolidation in the given time period.
Applications
- Speed consolidation of embankments
- Drainage of silty soils and marine sediments
- Remove water from underneath preloads
- Consolidation of tailings deposits