Under-reamed Pile Foundation - Black Cotton Soils.

An under-reamed pile is a pile of shallow depth (1 to 6 m) having one bulb a its lower end. If this bulb is taken or provided at a level lower than the critical depth of moisture movement in expansive solis, the foundation will be anchored to the ground and it would not move with the movement (i.e. swelling and shrinkage) of the soil. These piles may vary from 15 cm dia. to 50 cm dia., and are suitably spaced. Special under-reaming tools are available with the help of which these may be bored at site, and then concreted. They are nominally reinforced to take tensile stresses.

The piles spacing may vary between 2 to 4 m. The piles are connected by a rigid capping beam, suitably reinforced, over which the wall is constructed. The capping beam is kept 8 to 12 cm above the ground level, so as to provide air gap to accommodate the soil movements without adversely affecting the super-structure. Experience has shown that the buildings constructed on under reamed piles in expansive soils are free from distress normally caused in these soils.