The International Information Center for Geotechnical Engineers

Prefabricated Vertical Drains

Benefits of PVDs

  • Decrease overall time required for completion of primary consolidation due to preloading
  • Decrease the amount of surcharge required to achieve the desired amount of precompression in the given time
  • Increase the rate of strength gain due to consolidation of soft soils when stability is of concern
  • Comparison to sand drains:
    • Economic competitiveness, less disturbance to the soil mass compared to displacement sand drains, and the speed and simplicity of installation. Also feasible to be installed in a nonvertical orientation. [12]

 

Rixner, Kraemer, and Smith (1986) have also outlined some technical advantages of PV drains compared to sand drains. They separate these advantages by sand drain type. There is displacement drains, which displace the soil as the drain is installed, and non-displacement, which does not displace the soil during drain installation. They found the following advantages of PVDs:

 

Displacement

  • considerably less disturbance of cohesive soils during installation due to: smaller physical displacement by mandrel and top, and typically static push rather than driving

  • installation equipment usually lighter, more maneuverable on site

  • do not require abundant source of water for jetting

 

Non-Displacement

  • do not require control, processing and disposal of jetted spoil materials; fewer environmental control problems

  • field control and inspection not as critical

  • definite potential for cost economy

  • eliminate cost of sand backfill of drains, quality control problems and related truck traffic

  • jon control and inspection requirements are reduced due to simplification of installation procedures

 

General

  • there is greater assurance of a permanent, continuous vertical drainage path; no discontinuities due to installation problems

  • PVDs can withstand considerable lateral displacement or buckling under vertical or horizontal soil movement

  • faster rate of installation possible

  • where very rapid consolidation is required, it is practical to install PVDs at close spacing

  • PVDs can be installed underwater and in a non-vertical orientation more conveniently

 

 

 

 

Disadvantages

 

  • If the compression layer is overlain by dense fills or sands, very stiff clay or other obstructions, drain installation could require predrilling, jetting, and/or use of a vibratory hammer, or may not even be feasible
    • Under such conditions, general pre-excavation can be performed if appropriate 
  • Where sensitive soils are present or where stability is of concern, disturbance of the soil due to drain installation may not be tolerable
    • In such cases, sand drains installed by non-displacement methods or an alternate soil improvement technique may be more practical
  • Winter Considerations
    • During cold winter conditions, the ground is subject to frost. A frost line, the depth at which the ground is subject to freezing, can typically be measured at 3 ft in the midwest United States, for example. The frost can reduce or prevent the drain discharge at the groundwater table or into the drainage blanket at the ground surface, building up a back pressure. The build-up of back pressure will temporarily retard the time development of the consolidation settlement; perceived as a flattening time-settlement curve. The flattening may lead to a false premise that primary consolidation has reached an end. However, once winter conditions pass, and the ground within the frost line thaws, settlement has potential to reinstate [5].

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