Sand Drain vs. Wick Drain: What is the Difference?

Solid particles, water, and air are the main components of soil. When soils reside below the phreatic surface (also known as the water table), air is not present, and the soils are deemed saturated.

Vertical drainage mainly deals with saturated soils.

Soil properties are determined by the soil particles and their sizes. The term “cohesionless soils” refers to soils that include big particles, such as sand and gravel. Cohesive soil has smaller particles and is usually made of silt or clay.

In either case, water can move through the soil and fill the spaces between the soil particles. Gravity causes the water in rivers and streams to flow “downhill,” which is also a process that can occur in soil voids.

Permeability is the term used to describe the characteristic of soils that permits water to flow through the voids. The permeability of soils decreases generally as particle size decreases.

What Causes Water to Flow in Soil?

Placing a new load on top of the existing one causes the soil to experience downward pressure, which can cause water to flow through it.

Water will be driven out of the soil spaces under the extra weight unless the soil particles are in their most compact arrangement (rather unlikely to happen). The upper structure will settle, occasionally noticeably, if this water is driven out.

Cohesionless soils can settle quite quickly because the large particle size allows for relatively quick water flow out from under the load.

But if the soil is made up of microscopic particles that stick together, water flow and the structure’s settlement can be very slow, sometimes taking months or years.

Buildings constructed on top of this type of soil may initially be alright, but with time they may settle dramatically, causing serious structural damage and necessitating costly repairs or the need to demolish the structure.

Vertical Drainage

There are many solutions to the issue, and one of them involves draining water from the cohesive soils before building on top of them. By doing this, the problem of settling is solved, making it possible to build a strong structure. This process is known as vertical drainage.

In this article, we will discuss the installation of two different types of vertical drains—wick drains (a.k.a. prefabricated vertical drains) and sand drains.

The Functions of Drains

Drains are mostly used to consolidate vast regions of loading, like reservoirs, runways at airports, and big storage areas. Another prominent application has been in conjunction with driving piles in soft clayey soils.

The drain eliminates any risk to already-placed piles by dissipating the significant increase in interstitial pressure.

Sand Drains

A sand drain is a hole that has been dug in cohesive soil and filled with sand. Sand has bigger particles, which increases its permeability and makes it easier for water to pass through. A number of sand drains are set up, and then a load is placed on top of them.

The load is an embankment, similar to those found on highways. Water that is in excess is gathered at the top and directed away from the worksite.

The challenging element is installing the sand drains. Drilling the holes and filling them with sand is the apparent fix, but if the soil is soft, as is frequently the case, the holes will collapse.

Basically, a mandrel (a piece of pipe) would be driven into the ground first by the hammer. Sand would then be poured into the mandrel once the sand drain door (the sizable component directly beneath the hammer) was opened.

Then the sand is given compressed air, the hammer is attached to the crane with the hook block, and the mandrel is removed from the ground, leaving the sand column in the earth to do its job.

Sand drains typically have diameters between 25 and 40 cm and can go as deep as 40 to 45 m. Depending on the properties of the soil, the average production might range from 70 to 100 linear meters per hour.

Wick Drains

Wick drains are also known as Prefabricated Vertical Drains (PVD).

The process of sand drainage is fairly complex. Simplification is encouraged for certain applications. In this regard, wick drains provide a typical simplification.

Wick drains are exactly what the name suggests: a geosynthetic “rope” that is typically around 100 mm wide and 5 mm thick. It functions as a high-permeability conduit for water to flow out of the soil and to the surface, similar to how sand drains work.

They are set up in an array, often with 3 meter gaps, just like sand drains.

Due to the complexity of the situation, the contractor must use a mandrel to insert the wicks. The easiest method is to press the mandrel/wick combination into the ground. However, some soils are too rigid for this, so the mandrel is frequently vibrated.

In some instances, Vulcan vibratory hammers were utilised to install wick drains. This is a rather tough application for a vibratory hammer as numerous drains must be placed.

All in All

Wick drains, also known as prefabricated vertical drains, consolidate the soil. It’s a conduit for the removal of pore water from easily compacted soil.

Sand drains, on the other hand, necessitate digging a vertical shaft and filling it with filtering sand of the correct grade.

Companies looking for a prefabricated vertical drain contractor to carry out prefabricated vertical drain installation are encouraged to make an appointment with our PVD installation specialists for a full consultation.