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
Level One (Core FAQs)
Part I: Bored Piles and Mini-piles
1. What is the function of introducing cement plug before pouring
concrete in bored piles? (BP1)
It is a common practice in the construction of bored piles by adding a
cement plug before concreting of bored piles takes place. One of the
possible explanations is that the cement grout serves as a barrier and
protects fresh concrete from being washed away by water inside pile bore.
The cement grout helps to set aside water when the first batch of concrete
is poured down from the tremie pipe. As such, the quality of fresh concrete
is anticipated not to be seriously affected by water and this helps to
improve toe imperfection problem.
2. Why are bored piles usually cast higher than the required final
level?
It appears to be a common construction practice that bored piles are
normally cast up to a piling platform much higher than the formation level of
pile caps. Then, the excessive pile length has to be cropped and removed
to the correct level for the subsequent construction of pile cap. The
reasons of such practice are as follows:
(i) It is technically sound to employ such practice because laitance,
impurities and poorly-compacted concrete should have migrated to
the top of the piles. Therefore, additional length at the top of piles is
constructed to accommodate these sub-standard concrete, which
are subsequently removed and cropped to leave high quality
concrete over the entire length of piles.
(ii) In actual site practice, the details of construction sequence and
access arrangements have potential impact on the possibility of
constructing piles to correct level without the need of cropping.
3. What is the reason of adding water to bored piles? (BP2)
In water bearing ground, some water head (about 1 m) above the existing
ground water table is maintained to stabilize the bore during excavation
below casings by pumping water to the pile bore. This balanced head
condition is created to minimize the possible drawdown of surrounding
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