The pile toe is usually cleared by airlifting prior to concreting. However, once the air-lifting operation ceases and concreting operation are not carried out simultaneously, suspended sediments would tend to settle and form the soil sediments at pile toe of bored piles.
The layer of soil sediments is considered to be unacceptable by most engineers as they claim that it would impair the structural performance of bored piles. For bored files with bell-out, the rock socket provided during the formation of bell-out tends to provide confinement to soil sediments. The horizontal stress will increase with the applied stress. Hence the settlement of soil sediments is very small and would not affect the performance of bored piles.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
If you have a query, you can ask a question here.