Vertical drains are constructed in order to improve the quality of soil specially cohesive soil. Vertical drains are mainly of 3 types: Sand Drains, Band Drains, Sand wicks. Sand Drain is the most commonly used vertical drain. Let us explain sand drains in detail as under –
What is Sand Drains?
Sand drain is a system in which radial consolidation rate of drainage is increased making vertical bore holes. These holes is back filled with suitable grade of sand. All the holes are connected at above with a layer of sand known as Sand Blanket. Further a surcharge load is placed over it to increase in consolidation rate or to accelerate the drainage. Surcharge is generally in the form of dumped soil. Due to surcharge load, the pore water pressure increases in the embankment. The drainage occurs in the vertical and horizontal directions. The horizontal drainage occurs because of dissipation of excess pore water created by surcharge.
Pattern of Sand Drains
The drains are laid either in a Square Pattern or Triangular Pattern. The spacing of drains is kept smaller than the thickness of the embankment in order to reduce the length of the radial drainage path. The zone of influence of each drain in a triangular pattern is hexagonal in plan. Similarly, the zone of influence of each drain in a square pattern is circular in plan.
The zone of influence is approximately equivalent to a circle of radius R, where R is given by :
For triangular pattern; R = 0.525 S
For square pattern; R = 0.564 S
The radius of sand drain is represented by rw
The spacing between sand drain is represented by S
Theory of Sand Drains
The theory of Sand Drains was given by Rendulic in 1935 and Barron in 1948. Later Richart in 1959 summarized the theories. Depending upon the type of strain there are 2 cases – (i) Free strain case (ii) Equal strain case.
(i) Free strain case
If the surcharge load placed over the sand blanket is flexible, free strain case occurs. In this case, there is uniform distribution of surface loads, but the settlements at the surface are uneven.
(ii) Equal strain case
This case occurs when the surcharge applied is rigid, such as heavy steel plates. In this case settlements are uniform, but the distribution of pressure is non uniform.