Helping soft soils settle
What are Wick Drains?
Long term settlement of soft silt and clay often creates serious problems in construction. Ten or more years may be required for consolidation of thick compressible layers of soil. This design problem can be overcome by creating closely-spaced artificial vertical drainage paths to which the pore water can flow, thus decreasing the consolidation time to a matter of months. These artificial drainage paths, called wick drains, consist of a central plastic core, which functions as a free-draining water channel, surrounded by a thin geosynthetic filter jacket. A typical wick drain is approximately 4 inches wide, 1/8 inch thick and comes in rolls up to 1,000 feet in length.
How are Wick Drains Installed?
Wick drains are installed with specialized equipment, called stitchers. The stitchers, which are mounted on either backhoes or cranes, consist of a vertical mast housing a special installation mandrel. The mandrel, containing the wick drain, is hydraulically pushed or vibrated into the ground to the desired treatment depth, typically to the bottom of the soft-soil stratum. As the mandrel is withdrawn back into the mast, the undamaged wick drain is left in place within the soil mass. The mandrels are generally less than 10 square inches in cross-section, thus causing minimal disturbance to the soil. Depending upon soil conditions, installation rates may be as high as 2,000 feet per hour.
What are Typical Applications?
Wick drain projects have included dams, large storage areas, highway embankments, sedimentation ponds, tanks, bridge abutments, buildings, and airport runways.
The most common application of wick drains is for accelerating the settlement rate of compressible soils. At some sites, the primary purpose of wick drains is to increase the shear strength of underlying soft soils. As soil consolidates, the shear strength increases due to a reduction in the water content. This application is particularly important when high area fills have to be placed over soft soils with low shear strength. Potential instability problems can be overcome by loading the site in stages and allowing the soft soils to gain in strength under each stage before placement of the next stage of fill.
Wick drains have also been used in conjunction with other ground improvement techniques, such as dynamic compaction, to allow rapid dissipation of excess pore pressures.
How are Wick Drains Designed?
The rate of soil consolidation or settlement is controlled by how rapidly the pore water can escape from the soil. The controlling variables are the spacing between the wick drains and the permeability of the soil. The amount of consolidation is independent of whether wick drains are present and is determined by the soil compressibility and the weight of the fill above the wicks. By developing a set of design curves of drain spacing, fill height, and consolidation time, the most economical drain spacing and height of fill can be selected to achieve a given degree of consolidation in a specified time period.