In order to control the development of excess pore water pressure, a surcharge embankment is usually constructed as a multi-stage exercise with rest periods between the loading stages. Since most compressible soils are characterized by very low permeability, depending on their thickness, consolidation may take a long time and require a very high surcharge load, which may not always be appropriate for tight construction schedules. Installation of vertical drains can significantly reduce the preloading period by reducing the drainage path radially, because, the consolidation time is inversely proportional to the square of the length of the drainage path. Due to the rapid initial consolidation, vertical drains will increase the stiffness and bearing capacity of soft foundation clays. Geosynthetic vertical drains are usually composed of a plastic core that is protected by a fabric filter with a longitudinal channel. The filter (sleeve) is made of synthetic or natural fibrous material with a high resistance to clogging. Vertical drains are most appropriate for moderate to highly compressible soils which are normally consolidated or lightly over consolidated. Pre-consolidation allows coastal structures such as transport systems, embankments and tall buildings to be more stable under large static and cyclic loads. Various types of vertical drains, including sand drains, sand compaction piles, prefabricated vertical drains (PVDs) and gravel piles have commonly been used in the past. Apart from the increasing cost of sand quarrying and strict environmental regulations in many countries, plus conventional sand drains that can be adversely affected by lateral soil movement, flexible PVD systems enjoying relatively rapid installation, have often replaced the original sand drains and gravel piles. The most common band shaped (wick) drains have cross sectional dimensions of 100mm × 4mm.
 

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