Prefabricated vertical drains (PVD)/wick drains
Prefabricated vertical drains, also known as wick drains or band drains are used for accelerating the consolidation of marine deposits or soft soils.
In general, prefabricated band drains consist of a central core that functions primarily as a free drainage channel and a non-woven filter jacket that prevents the soil surrounding the drain from entering the central core but allows water to flow in.
Band drains are commonly used because of they are easily fabricated, facilitate quality control, are economical and only slightly disturb the surrounding soil during installation.
The drain sheathing can be inserted into the ground to depths of as much as 60 m. The drain itself is inserted in a metal mandrel pipe that is driven hydraulically into the ground. The end of the drain is connected with an anchoring plate. When the drain band reaches the required depth, it is lifted by the insertion device and the drain is left anchored (with the plate) at the required depth. The drains are inserted in a 1-7 m² grid. Higher density creates shorter pathways for the drainage of pore water above hydrostatic pressure, causing acceleration of the consolidation phenomenon in the clay layer.
The use of vertical drains and pre-loading of saturated clay soil brings about a condition in which most consolidation settlement occurs during the construction phase before the structure – the roadway embankment, reservoir embankment, large tank infrastructure or warehouse structure – reaches its service phase.