Under Reamed Piles

Greetings!

The under reamed piles are the most common and appropriate type of foundation used in expansive soils. In this case the superstructure of the building is supported over beams, which are clear off the ground surface, and span over the piles anchored at the depth where the soil has nearly constant water content. This depth is usually found to be 3.5 m in India.

Length of Under reamed Pile:

The length of pile varies from 3.5 m to 4.0 in deep deposits of black cotton soils, and pile is carried to non expansive soil layer up to a depth of at least 0.6 m. 

Spacing of Piles:

Depending upon bearing capacity, spacing of piles varies from 1.5 m to 3.0 m. A pile is to be provided under every wall junction or where a point load is acting on the plinth beam. 

Diameter of pile shaft:

It varies from 200 to 500 mm, and the ratio of the diameter of the bulb to shaft i.e. Du/D varies from 2 to 3. In case of multi-reamed piles, the first bulb should be at minimum depth of 2.Du, below the ground level.  

Center to center distance between 2 bulbs may vary from 1.25 to 1.5.Du. The pile is reinforced throughout its length to take care of tensile stresses. Steel reinforcement should be sufficient to take care of shrinkage effects when the pile acts as the column, so 0.8 to 6% (IS: 456-2000) of reinforcement, which we provide for the columns should be provided at least to make it as a column.

As per IS:2911-1980(part 3), the ultimate bearing capacity of the soil can be calculated and a factor of Safety(FOS) of 2 to 2.3 is applied to calculate the safe load/allowable load.

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