The choice of an under-reamed pile in unstable or water-bearing ground is generally to be avoided. There is a danger of collapse of the under-ream, either when personnel are down the hole, or during concreting.
Important Notes: On the basis of limited experimental studies conducted on model under reamed piles in cohesion less soil the following conclusions are drawn.
1.
By providing under reamed bulbs the ultimate load capacities of piles increases significantly.
2.
The ultimate load bearing capacities of the under reamed piles with angle of under reamed bulbs of 45 0 and zero are almost same.
3.
Three or more under reamed bulbs are advantageous only when the spacing ratio (S / ) is two or less, and when (S / ) is greater than two, multi-under reamed piles do not have specific advantages.
4.
The ultimate load bearing capacities of piles are maximum when the spacing between two under reamed bulb is 2.5 times the diameter of the under reamed bulb. It appears that the spacing between two under reamed bulbs suggested in (1.25 to 1.5 times) IS 2911(1980) is not the optimum,
5.
The expression suggested in IS 2911(1980) can be used for predicting the ultimate load carrying capacity of under reamed piles with spacing ratio (S / ) less than 2.5.
) is two or less, and when (S / 
) is greater than two, multi-under reamed piles do not have specific advantages. 
) less than 2.5. 
