The International Information Center for Geotechnical Engineers

Prefabricated Vertical Drains

 

Installation


[I] PVD Installation

PVDs are installed by a hollow steel mandrel encasing the wick drain material. The mandrel is driven into the ground by a stitcher attached to an excavator carrier, as seen in figure I. This is a vibrating force, but static options are also available for areas near underground utilities. At the base of the mandrel, the wick is looped through a steel anchor to secure the drain in place. Once the desired depth is reached, the drain is anchored and the mandrel is extracted.  The mandrel is withdrawn 15 to 20 cm above the surface for the wick drain to be cut. If the soil the mandrel is driving into is exceedingly stiff, and the mandrel cannot be vibrated or hammered into the ground, predrilling may become necessary.

Depth of Installation

Drains are not likely to accelerate consolidation if induced effective stress is not greater than the preconsolidation stress. The optimum depth of the wick drains lies within the preconsolidation stress margin as the stress from the surcharge diminishes with depth. However, if there is a pervious soil layer below the preconsolidation margin, the wick drain should be extended into that soil layer. This will aid in assuring the discharge of the water [5].

Width of Installation

Soil strata are not defined as entirely uniform layers, therefore there may not be equal volumes of water to be drained. If some portions of a layer have a greater amount of drainage, the soil will settle to fill those voids. This leads to differential settlements and could prolong the consolidation time. To help avoid this issue, wick drains should be distributed across the entire footprint of an embankment and a small distance beyond. It is advised to place the outermost rows of drains between one third and one half of the proposed embankment's height beyond the embannkment. However when designing the wick drain’s layout, homogeneous soil can be assumed for simplicity [5].

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