How to Install Prefabricated Vertical Drains

Prefabricated vertical drains, usually referred to as “PVDs” but also called “wick drains” and “band drains” in industry-speak, serve to drain soil. Usually row upon row of PVDs are topped off with a series of horizontal strip drains that catch precipitation before it enters the ground; this speeds the wicking of water out of the soil into the PVD stacks themselves. Such a system is particularly useful in fine-grained soils -- peat, clay and silty clay-based soils -- that are extremely compressible, resulting in unpredictable settlement patterns. Providing an easy route for the water to exit the area decreases the difficulties involved in building on such soils.

Instructions

- 1
  Locate each drain site, all of which the project engineer should have staked out and numbered. This installation grid is typically determined by geotechnical analyses. Industry practice expects the eventual location of each drain to be within 6 inches of each stake installed by the engineer.
- 2
  Augur stiff upper soils to loosen them prior to bringing the mandrel equipment to each drain site. Industry best practice determines that such auguring extend less than 2 feet beyond the surface soil into the compressible soils beneath.
- 3
  Use a dedicated hydraulic installation rig to drive a mandrel that advances down through the soil to the depth determined by the architects and site management team. Specialist equipment that applies at least 12 tons of vertical force is designed to advance the mandrel at a constant rate; this minimizes disturbance of the subsoil. A penetration rate of 6 inches per second or greater is desirable.
- 4
  Install the drains to the precise depth mandated by the engineer’s plans. If soil resistance prevents this, the on-site engineer has the option of varying the depths; revisions to the number of drains and their spacing typically accompanies changes in desired depth.
- 5
  Ensure the drilling equipment is plumb before advancing each drain. Industry best practice expects a deviation from vertical of less than 1 inch per foot.
- 6
  Consult the on-site engineer if impassible below-ground obstructions are encountered. In such events, usual practice is to install a PVD to the depth reached by the mandrel, then attempt a second full-depth penetration within 18 inches of the obstructed drain.
- 7
  Splice connections within each PVD in such a way that continuity of the drain material is uninterrupted. Splicing is achieved through stitching; the process requires further dedicated equipment.
- 8
  Leave a neatly cut above-ground protrusion of between 4 and 8 inches at the top of each drain location. Some engineers require that an anchor plate be used, while others mandate only that the drain material be neatly at the upper end.

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