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Owing to the relatively low presumed bearing value for founding rock (i.e. 5MPa for Grade II rock with total core recovery of 85% and unconfined compressive strength more than 25Mpa), it leads to the necessity to form bell-out in order to spread the pile loads in larger area, thus reducing the bearing stress at pile toes.
Firstly, in Hong Kong most of rock belongs to igneous rocks whose unconfined compressive strength is generally higher than normal concrete (no reinforcement in concrete in bell-out region). Therefore, someone may query the reason of replacement of strong rock by weaker concrete during the formation of bell-out.
When piles are concreted by tremie concrete method, soil sediments at the pile toe inside the bell-out are pushed aside and trapped in the tip of bell-out. Consequently, this would impair the functionality of the 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.