A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
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.
To maintain the stability of hanging side slope of bell-out, it requires the
bell-out to be formed wholly inside sound rock. However, the rock above
bell-out may be weather rock so that the hanging rock is liable to fall down
and soil may collapse.
7. There is a general trend that toe defects in bored pile construction
have become more serious. Why? (BP5)
Direct coring method is commonly employed to check the quality and
workmanship of bored piles. It is further divided into two main types,
namely interface coring (pile/rock interface) and full coring (entire concrete
pile length). In the past, the implementation of full-depth coring usually
takes place near the pile centre to prevent physical conflicts with
reinforcement cage. When placing concrete by tremie method, concrete
pouring by tremie pipe is carried out near pile centre and soft and weak
materials are usually displaced to the sides of bored piles. As a result, the
pile/rock interface at pile centre should indicate the best results. However,
with the recent trend of using interface coring through reservation tube
attached to the reinforcement cage, there is a high chance that interface
coring gives poor results as the reinforcement cage may trap soft and weak
materials and they are located far from the pile centre.
8. Is it totally unacceptable that soil sediments are found at pile toe of
bored piles? (BP5)
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.
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