Black cotton soil contains fine clay particles. This property induces a great affinity to water of such type of soil. Alternate swelling and shrinkage in extensive limit during wet and dry process respectively results cracks in soil without any warning. These cracks may sometimes extent to severe limit like ½” wide and 12” deep. So building to be founded on this soil may suffer severe damage with the change of atmospheric conditions.
Under reamed piles are the most safe and economical foundation in Black cotton soil. Under reamed piles are bored cast in situ concrete piles having bulb shaped enlargement near base. A Pile having one bulb is called single under reamed pile.
The bearing capacity of the pile is increased by increasing the number of bulbs at the base. Such type of foundations is ideally suitable in the areas where the black cotton soil or expansive soil is beyond 2.50 meter. The basic principle of under reamed pile is to anchor the structure at a depth where ground movement are negligible due to moisture variation or other reasons.
Simple tools are required for construction of under-reamed piles like spiral auger, under reaming tool, and boring guide. This is a well proven and established technology for construction of foundation in expansive soils. For speeding up the construction bore and under ream for large diameter and deeper pile a mechanical rig can be used. The construction and design of such foundation can be done in accordance with Indian Standard Code of Practice IS 2911-Part III.