Band drains
Geocomposites formed of a hollow cored, geotextile wrapped drainage element (geonet) inserted vertically into soft ground to speed up the consolidation process. They are also known as Prefabricated Vertical Drains (PVDs).
Band drains are typically used in constructing embankments over compressible and water saturated soils (e.g. silty clays and alluvial deposits) to improve stability and accelerate settlement.
These soils have a weak porous structure, usually filled with water (pore water). When such soils are loaded by a superimposed embankment or structure the water is “squeezed” out. However, because of the impermeable nature of some soils, this could take a long time, hence causing instability and long term settlement problems for any structure built on top.
Band drains are installed vertically from the working platform deep into the ground and act as wicks providing a route for the water to drain out quickly either to the surface or a more permeable stratum at depth. This process speeds consolidation of the soil and minimises settlement of the superimposed embankment or structure.
Band drains can be used in combination with vibro stone and vibro concrete columns for strengthening purposes and/or drainage layers and horizontal drains to dissipate the pore water pressure. They are also commonly used in conjunction with basal reinforcement, which provides short term stability of the embankment while the band drains contribute to accelerate consolidation of the soft deposits underneath.
Band drains were used in the approach embankment either sides of the River Bann in the construction of the Toome bypass in Northern Ireland.
Band drains and basal reinforcement were utilised in the construction of a distributor road over embankments for a new housing development at Dering Way near Gravesend, in the Thames Gateway. The band drain solution was selected because it was economically advantageous. The faster consolidation settlement enabled the embankment to be used as the main site haulage road during the construction phase.
- BS EN 15237: 2007 Execution of special geotechnical works - vertical drainage
- CIRIA Publication C573 Guide to ground treatment, London 2002
- TRL Project Report PPR341 Drainage of earthwork slopes, Crowthorne 2008
