Also known as Wick Drains, Prefabricated Vertical Drains (PVDs) are composed of a durable plastic core (usually polypropylene) wrapped around a synthetic geotextile (i.e. a filter jacket) to facilitate the movement of water through slow-draining soils.
In doing this, it prevents intrusion and clogging of soil particles. Prefabricated vertical drains have remarkable flow discharge capacity and are usually coupled with surcharging to accelerate preconstruction soil consolidation.
They are installed in soft clay layers which cater to the accelerated consolidation and gain in shear strength.
While penetrating soft clays, the pore water pressure is increased more rapidly thereby reducing the preloading time, increasing water dissipation, shortening pore water travel distance, and compressing soil voids.
The pressure exerted in the pore water can influence the drain flow. In this case, pore water will flow sideways to the nearest drain, as opposed to vertical flow to an overlying or underlying drainage layer.
PVDs are meant to serve 2 basic functions:
Prefabricated Vertical Drains installation can be successfully applied in various projects typically used as ground improvement system, including:
Generally, PVDs are supplied in rolls with specific roll length and width. There are a few things to watch out for before PVD installation.
In this guide, the prefabricated vertical drains installation process will be broken down into steps.
Prefabricated vertical drains are installed by a hollow steel mandrel encasing the PVD material. The first step is to drive the mandrel into the ground by a stitcher.
The stitcher is a vibrating force that is attached to an excavator carrier.
Mount the PVD roll on the side of the leader and channel the PVD through the steel mandrel.
The PVD is looped through a steel anchor plate at the bottom layer of the mandrel.
The anchor plate will be installed together into the compressible soft soil at a constant speed to firmly hold and retain the installed PVD at the required depth.
Once the required depth is reached, the mandrel is extracted back up into the ground.
Retrieve the mandrel from the ground and cut off the PVD with approximately 300mm or 500mm or 600mm, as per the Engineer’s specification.
Repeat all the steps for the entire installation process. Attempt at most two times when installing the drain within the stated radius.
If, after two attempts, the drain still could not be installed, then you may consider changing the drain location to a closer radius.
If the mandrel hits an obstruction and cannot be vibrated or hammered into the ground, then the pre-drilling or pre-auger method can be introduced to loosen any obstruction before re-attempting PVD installation.
If induced effective stress is less than the pre-consolidation stress, drains are not likely to accelerate consolidation.
Thus, the optimum depth of the PVDs hugely depends on the pre-consolidation stress margin. This is because the stress from the surcharge reduces with depth.
In cases where a previous soil layer is below the pre-consolidation margin, the PVDs should be extended into that soil layer to ensure the discharge of the water.
PVDs should be evenly distributed across the entire footprint of an embankment at a shorter distance to prevent soil from filling the drained layers.
This could prolong the consolidation time. For an ideal width, place the outermost rows of drains between 1/3 and ½ of the proposed embankment’s height beyond the embankment.
N.B: Assume homogeneous soil when designing the PVD’s layout; it aids simplicity.
As geosynthetics continue to improve, installation monitoring techniques become more sophisticated.
In fact, quality control of PVDs has also improved with the use of electronics to the installation equipment.
Will other aspects such as surcharging loading with PVDs improve in the future? You bet.