Dr. K.M. Soni, Superintending Engineer, CPWD, Bhopal
Black cotton soil, a cohesive soil, is considered a problematic soil for civil engineers. It has characteristics of swelling during rains and shrinking during summer. In both the conditions, it poses problems. During swelling, structure has uplift pressure and produces heave in the foundations, plinth beams, bottom of floors of buildings and canals, roads surfaces etc. and on shrinkage, cracks develop in walls, slabs, plinth protection, floors etc. Though, local construction technology has been developed traditionally in most of the places where such type of soil is found, still cracks are observed in many structures as sufficient precautions are not taken during the construction and maintenance period of the buildings.
Index Properties
Black cotton soil of India has a liquid limit between 60-90%, plastic limit between 30-50% and shrinkage limit about 8-18% as given in Table 1.Black cotton soils having high plasticity index more than 23 described as critical and severe having free swell more than 100% are to be taken special care to avoid cracks in the structures.
In Situ Properties
Black cotton soil is very hard on drying but becomes slippery and slushy during rains. Its density varies along the depth. On the surface, density is about 1.6 g/cc and reduces to about 1.4 g/cc in dry state along the depth while on saturation, it is almost constant along the depth. When soil is dry, shear strength is quite high on the surface and it reduces marginally along the depth while during rains shear strength reduces and is very low near the surface. Thus, saturated state also has effect on its shear strength. However, properties change up to a certain depth only and then become constant.Black cotton soil consists of montmorillonite which has swelling characteristics. On its surface and internal layers, it has an electric charge. When the particles come into contact with water, a film is formed around clay particles and the water reaching into layers uplifts the layers and produces heave into the soil. If heave is prevented, say by floorings, it produces uplift pressure. Therefore, to avoid uplift pressure, important consideration is to avoid water coming into contact with black cotton soil.
Damages to Buildings
Various types of damages can occur in the buildings due to up lift forces and settlement caused by the expansive soils such as:- Diagonal and vertical cracks in smaller direction of slab.
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