What are vertical or wick drains, and vacuum consolidation

WHAT ARE VERTICAL OR WICK DRAINS?

Soil stabilization using Prefabricated Vertical Drains (PVDs) or wick drains are applied in areas with loose, compressible and water-saturated soils such as clay and silty clay. These soils are characterized by a very weak soil skeleton and a large pore space, usually filled with water (pore water). 

When a load such as a road embankment, a hydraulic fill or a dike, is placed on soft compressible soils, significant settlements may occur. These settlements can create serious problems. Any increase in load can also result in an increase of pore water pressure. In soils with low permeability, this water dissipates very slowly, gradually flowing from the stressed zone. Increased pore pressure may also cause soil instability and

slip plane failures may result. 

A vertical drainage system – drains are generally placed in a square or triangular pattern, spaced at about 1 to 3 meters – allows for faster removal of excess pore water, thus decreasing the risk of slip plane failure. The consolidation of soft cohesive soils using vertical drains can reduce settlement time from years to months ensuring that bearing capacity is adequate and construction can commence rapidly.

WHAT IS VACUUM CONSOLIDATION?

Vacuum consolidation is a process whereby vacuum pressure is applied to an area where PVDs have already been installed. The purpose is to potentially increase the drains’ effectiveness. 

Generally, this technique requires the application of a surcharge loading to squeeze water out of the soft clay soils. Such loading must be equal to or in excess of the service loading to which the developed land will be subjected. In vacuum consolidation, the vacuum pressure applied contributes to the surcharge loading, and therefore actual surcharge heights are reduced. 

An additional important advantage of the vacuum is the isotropic nature of the vacuum pressure and the correlated improvement of the stability under Pre-loading, reducing considerably the risk of slope failure resulting from the surcharge.

Note:this articles are excerpted from the iadc dredging, facts about soil improvement, and here we thank the authors give us the knowledge about dredging and soil improvement.