One of the more interesting applications of vibratory hammers is the installation of wick drains, a type of vertical drain. But vertical drains are of interest far beyond vibratory technology.
Soils are a composite of solid soil particles, water and air. When soils reside below the water table (phreatic surface,) there is no air, and the soils are referred to as saturated. The soils dealt with in vertical drainage are generally saturated.
Soil particle sizes vary, and with that variation come many of the variations in soil properties. Soils with large particles (sand and gravels) are referred to as cohesionless soils. Soils with smaller particles are usually silt or clay soils and are referred to as cohesive soils.
In either case, water not only fills the space (voids) between the soil particles, but it is capable of flowing through the soils as well. Flow in rivers and streams is due to the fact that the water is flowing “downhill” due to gravity, and the same phenomenon can take place in the soil voids. The property of soils relating to their allowance of water flow in the voids is referred to as permeability. As a general rule, the smaller the particle size, the lower the permeability of the soils.
One action that can result in water flow in a soil is the placement of a new load on top of the soil, which in turn exerts downward pressure on the soil. Unless the soil particles are in their most compact arrangement (which is unlikely,) water will be forced out of the soil voids under the new load. If this water is forced out, the structure on top will settle, sometimes significantly.
In the case of cohesionless soils, the large particle size enables relatively rapid water flow out from under the load, and the settlement can be very rapid. But if the soil is cohesive with small particles, the water movement (and thus the settlement of the structure) can be very slow, sometimes months or years. Structures built on top of this kind of soil can be fine to start with but over time settle significantly, creating serious structural damage and requiring expensive repair or demolition of the structure.
Although there are several ways of dealing with the problem, one of them is to drain water out of the cohesive soils before placing a structure on top of them, thus getting the settlement out of the way and enabling a stable structure to be built. The method used to do this is referred to as vertical drainage, and specifically two types of vertical drains and their installation will be described here: sand drains and wick drains.
A sand drain is basically a hole drilled in a cohesive soil and filled with sand. Since the sand has larger particle size, its permeability is much higher, thus water will flow through it much more easily. As shown above, an array (it’s actually a two-dimensional array) of sand drains is installed, and a load is applied on top of the drains. The load shown above is an embankment, such as is used on a highway, and an additional, or surcharge, load is used to speed up the drainage process. The excess water is collected at the top and directed away from the jobsite.
The tricky part comes in getting the sand drains in the ground. The obvious solution is to simply drill the holes and fill them with sand, but if the soil is soft (which is frequently the case,) the holes will collapse. Although sand drains were first used in California in the 1930’s, the sand drain projects that were of special interest to pile hammer equipment manufacturers took place in the late 1950’s and early 1960’s during the development of the Meadowlands area of northern New Jersey.
Both Vulcan and McKiernan-Terry (MKT) developed equipment to install sand drains. Vulcan developed a special series of differential-acting hammers referred to as sand drain hammers. The major difference between the sand drain and conventional hammers was in the cylinder head at the top of the hammer. Instead of using sheaves for the hammer lifting cables, a bar which interfaced with a retractable hook was used. Vulcan developed a special hook block which was patented (U.S. Patent 3,171,552.)
Basically, the hammer would first drive a mandrel (a piece of pipe) into the ground. After this, the sand drain door (the large piece just below the hammer) would be opened, and sand would be dumped into the mandrel. Compressed air is then applied to the sand, the hammer is hooked to the crane with the hook block, and the mandrel is pulled out of the ground, leaving the sand column in the earth to do its job.
A detailed description of the process is given in MKT Bulletin 71 (why Vulcan didn’t develop a piece of literature like this is beyond me.) Below is another view of a Vulcan hammer over a sand drain dump tube.
A cursory examination of the procedure for sand drains shows that the procedure is fairly involved. It invites simplification, at least for some applications. A popular simplification is that of wick drains.
A wick drain is just what the name implies: a geosynthetic “rope,” usually about 100 mm wide and 5mm thick, which acts as a high-permeability conduit for water to flow out of the soil and to the surface, in the same manner as takes places with sand drains. As is the case with sand drains, they are installed as an array, generally in 3 metre spacings.
Candle makers have the luxury of melting the medium into which their wicks are places. Since things aren’t so simple for the contractor, he or she has to use a mandrel to insert the wicks. The simplest way to do this is to push the mandrel/wick combination into the ground, but some soils are too stiff for this, so the mandrel is frequently vibrated.
Vulcan vibratory hammers have been used in some cases to install wick drains. Since many drains are installed, this is a fairly demanding application for a vibratory hammer, but it is another example of the versatility of vibratory pile drivers.
A much more detailed explanation of the theory and installation of vertical drains–and especially of wick drains–can be found in the FHWA document RD/86/186, Prefabricated Vertical Drains, August 1986, which can be found by clicking here.