In this article we will discuss about:- 1. Need for Using Sand Drains 2. Advantages of Sand Drains 3. Construction.
Structures constructed over highly compressible soils, soft marine clay deposits suffer very large consolidation settlements, which have the potential to cause serious damage or failure of such structures. Such soils, located in the coastal regions such as Mumbai, Calcutta, Visakhapatnam, and others, extend to large depths of up to 15 to 25 m or more. Pile foundations may not always be an effective solution for such conditions. The surface of such soil deposits is so soft that it is difficult to use heavy construction equipment for construction of structures over such soils. The progress of construction is often delayed or halted for several weeks, shooting up the project costs if conventional foundation techniques are used in such soils.
Compacted embankments are usually constructed over such soils to provide a hard surface for construction and also to minimize the differential settlements. An ideal solution for this type of problems is to accelerate the rate of consolidation such that the potential settlements are completed before the construction of the structure.
Vertical sand drains are used as an effective technique for construction of structures, such as oil or gas storage tanks or airports, sea ports, harbor works or highway or railway embankments or dams, over highly compressible soils. The principle of sand drains is to provide additional vertical drainage faces in the form of cylindrical holes filled with clean sand.
The following are the advantages of using sand drains:
1. Provision of sand drains allows drainage of pore water in radial direction in addition to the drainage in vertical direction.
2. Since the permeability of soil in horizontal direction is usually several times larger than that in vertical direction, the rate of consolidation becomes considerably faster compared to conventional soil system.
3. With faster consolidation, the soil gains shear strength rapidly, allowing faster pace of construction and thus reducing the project cost.
4. The long-term stability of the structure is also increased significantly as the potential settlements are completed mostly before or during the construction.
5. Sand drains avoid potential problems during construction in soft compressible soils.
Figure 11.30 shows the sand drains installed below an embankment. A sand layer of 50-60 cm thickness is initially laid over the soft soil that not only serves as a drainage layer for the soft soil at top but also provides a working platform for construction of both sand drains as well as embankment. The sand layer also serves as a graded filter between the soft soil and the embankment and prevents loss of embankment material in to the soft soil.
The embankment is built up to some height above the sand layer to facilitate partial consolidation of the soft soil. This facilitates the soft soil to gain some shear strength to support and sustain the sand drains. However, full height of the embankment is not constructed in single stage because the soft soil will not have sufficient shear strength to support the embankment, in which case the embankment would sink into the soft soil. After the embankment is constructed up to some height, the sand drains are installed.
A circular casing or hollow steel mandrel, of about 25-cm internal diameter and 6-m height, is driven vertically into the soft soil up to the required depth through the embankment and sand layer. The soil in the mandrel is removed and the hole is back filled with clean sand. The mandrel is then removed. The embankment is then constructed up to the full height in stages. After allowing sufficient time for consolidation to complete, the required structure, such as pavement, airport, or oil storage tank, is constructed.
The sand drains are arranged in square or triangular pattern. The spacing of sand drains is designed using Eqs. (11.76), (11.77), and (11.78) depending on the time required to complete consolidation, radius of sand drains, thickness of compressible layer, coefficients of consolidation in vertical and horizontal direction, etc. For sand drains in square pattern, the radius of circle of influence r is equal to 0.564l, where I is the c/c spacing of sand drains. For sand drains arranged in triangular pattern, the radius of circle of influence r is equal to 0.525l.
Tr = [Cvr/(2r)2] x t …(11.76)
Ur = 1 – eα …(11.77)
(1 – U) = (1 – Uz) (1 – Ut) …(11.78)