Under reamed piles are bored cast-in-situ concrete piles having one or more bulbs formed by enlarging the bore hole.

The grade design are designed for a maximum bending moment of (wL2)/50 if the beam are supported during the construction, and for a bending moment of (wL2)/30 if the beams are not supported.

Design of grade beam, depth of beam and reinforcement required for various loading is provided by Central Building Research Institute, Roorkee.

Generally the diameter of the under-reamed pile bulb is kept equal to 2.5 times the diameter of pile steam. However, it may vary from 2 to 3 times the stem diameter of pile steam.

When the pile has only one bulb, it is known as single-under reamed pile, while the pile with more than one bulb is known as multi-under reamed pile.

The under-reamed pile foundation find application in widely varying situations in a type of soils where foundation are required to laid on undesirable effect of seasonal moisture change as in expansive soils.

The maximum spacing of the under-reamed pile should not normally exceed 2½ m as to avoid heavy capping beams.

The spacing of the piles shall be considered in relation to the nature of the ground, the types of pile and the manner in which the pile transfers the load to the ground.

The load carrying capacity of an under-reamed pile may be determined from safe load test, in the absence of actual test.

The under-reamed piles are connected by a reinforced concrete beam, known as capping beam or grade beam.