How do wick drains pull water from the ground?

Wick drains do not pull water from the ground, any more than a well without a pump does. When a load (like a highway embankment) is applied to saturated soils, the only way the soils can compress is by squeezing the water out. The wick drains simply make it easier and faster for that water to be forced out of the soils.

How do you determine the spacing and depths of the wick drains?

The first thing that needs to be done is a geotechnical investigation to determine the soil types and the compressibility properties of the soil. The wick drains generally, but not always, extend through the compressible layers. The amount of settlement under the design load, as well as the time required, can then be determined after the geotechnical properties are known. Once this information is determined, the wick drain spacing can be calculated using established analytical techniques, such as discussed in the Technical Topics section of this website.

What do I do with the wicks after the surcharge is removed? Won't they keep pulling water out?

The wick drains remain in place. After the consolidation process is complete, i.e., the excess pore pressure in the compressible layer is zero, the wick drains serve no function whatsoever. They do not make the soil settle.

Can I install Wick Drains myself?

Originally, sand drains were used to accelerate consolidation of soft clays. Sand drains can be installed using conventional drilling equipment, and can certainly be installed by anyone with a drill. However, wick drains require expensive specialized installation equipment.

How do you account for smear effects?

Smear is just what it sounds like, a zone of disturbed soils resulting from the friction of the steel mandrel as it is forced through the soft clays. In laboratory tests, this smeared zone has a lower permeability than the original soil, which results in a slower rate of settlement, i.e., a longer settlement period. In real life, however, even the most conscientious test programs indicate consolidation rates which may vary by factors of 3 or more. The smear effect is simply not very significant, given this level of uncertainly in the design values.

Should we require a vibrator in addition to a static crowd stitcher?

There are certain instances where a vibrator, in addition to a static crowd, is beneficial. However, it has been our experience, as well as others, that a static-vibratory stitcher can not produce a static crowd as high as a static crowd stitcher alone. The primary reason for this is that most static-vibratory stitchers do not use counterweights. If the hydraulic gear drive on a stitcher has the capacity to create 15 tons of pushing force, you need 15 tons of resisting force, which balanced static-vibratory stitchers do not have, hence the driving force is much less than the theoretical maximum.

Do I have to use a drainage blanket?

The tops of the wick drains need to have a backpressure of zero. Stated otherwise, the wick drains need to be vented to the atmosphere. This can be done naturally if the upper soils are clean sands or gravels. Otherwise a drainage blanket of clean sand or gravel is required. Alternatively the wick drains can be connected to horizontal strip drains, with the strip drains being vented to the atmosphere.