Wick Drains or Prefabricated Vertical Drains

Wick drains or Prefabricated Vertical Drains (PVD’s) are typically used in combination with preloading to accelerate the consolidation of fine grained soils, and therefore minimize post-construction settlement for proposed improvements. Once loaded by new structures such as buildings or embankments, the rate at which settlement of saturated soft ground occurs is based on the rate at which water can be squeezed out and drain off from those soils. The consolidation settlement of such fine grained soils may take 10 years or more. Wick drains work by decreasing the drainage path distance through the soil by inserting closely spaced artificial vertical drainage paths in a grid pattern, to which soil pore water can flow, thus decreasing the consolidation time to a matter of months. Once the wick drains and a drainage blanket are installed, soil is piled in the area to be preloaded to increase stresses in the soil and squeeze soil water into the wick drains. The wick drains consist of a central plastic core surrounded by a thin geosynthetic filter jacket. The drains functions as free-draining water channels conducting water up or down to high permeability sand layers or a constructed surficial drainage blanket. Wick drains can be installed to depths exceeding 200 feet. A typical wick drain is approximately 4 inches wide and comes in rolls up to 1,000 feet in length. Wick drains are installed with equipment called stitchers, which are mounted on either backhoes or cranes, and consist of a vertical mast housing an installation mandrel. The mandrel containing the wick drain is hydraulically pushed or vibrated into the ground to the desired treatment depth. The mandrel is then withdrawn into the mast leaving the installed wick drain in place.

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