Under reamed piles have mechanically formed enlarged bases that have been as much as 6 m in diameter.
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. In its closed position, the under reamer fits inside the straight section of a pile shaft, and can be expanded at the base of the pile to produce the enlarged base.
The cost advantages of under-reamed piles are due to the reduced pile shaft diameter, resulting in less concrete needed to replace the excavated material. Under reamed piles are of two types: