Wick Drain (PVD)

How a prefabricated vertical drain works

A prefabricated vertical drain (PVD), also called a wick drain, is a band drain installed vertically into soft soil. Its polypropylene core carries water while the nonwoven PP geotextile filter keeps fines out, giving the soil short radial drainage paths to a drainage blanket at the surface. Under a preload or surcharge, excess pore pressure dissipates quickly through these vertical drains, so the soft soil consolidates and gains strength in months rather than years.

Why PVD is used for ground improvement

The ground improvement system with SIGMA PVD is efficient and cost-effective: consolidation times for soft clay foundation deposits can be cut from many years to roughly 0.5–2 years, and the undrained shear strength of the clay is increased so it can support embankment and fill loadings. SIGMA PVD conforms to internationally recognized prefabricated vertical drain specifications, making it a standard tool for soil improvement on reclamation and soft-ground projects.

Discharge capacity and design

Discharge capacity is the key design property and is measured per ASTM D4716 at a stated normal pressure and hydraulic gradient, because flow falls as confining pressure rises and the filter intrudes into the core channels. Commonly cited acceptance criteria are about 500 m³/year (straight) and 250 m³/year (buckled); the project value must be confirmed against the actual confining stress. For surface drainage that complements vertical drains, see our corrugated drainage pipe.

What the wick drain is made of

A prefabricated vertical drain is a two-part band drain. The drainage core is a formed polypropylene profile, typically about 3–5 mm thick across the 100 mm band, whose open channels carry pore water to the surface. The core is wrapped in a nonwoven or spunbond PP geotextile filter that lets water in but holds back the soil fines that would otherwise clog the channels. Both parts are polypropylene, so the wick drain is chemically inert and durable in the soft, often saline soils where it is installed.

How PVDs are installed

Vertical drains are installed with a rig that pushes a hollow steel mandrel — with the band drain threaded through it and pinned at the bottom by an anchor plate — down to the design depth, then withdraws the mandrel leaving the drain in the ground. The drains are installed on a triangular or square grid; closer spacing speeds consolidation. A horizontal drainage blanket or collector connects the drain heads, and a preload or surcharge fill is placed on top so the squeezed-out pore water can escape upward through the drains. This is the standard PVD ground improvement and soil-improvement method for soft ground.

Where vertical drains are used

PVDs are used wherever a soft, compressible clay or silt deposit must be made to settle and gain strength before construction: highway and railway embankments over soft soils, reclamation and soft-soil land development, port and industrial fills, tank farms, and any preloading or surcharge program. Combined with a surcharge they turn ground that would settle for decades into ground ready to build on in months — an efficient, cost-effective alternative to deep removal-and-replacement for soil improvement.