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
the quality of already-placed concrete. The fresh concrete may be placed
under the portion of concrete which have set already so that it would raise
the mass of already-placed concrete and induce cracks on it.
11. In piling works, normally founding levels of bored piles are
defined by using total core recovery or rock quality designation
(RQD). Are there any problems with such specification?
The use of total core recovery to determine the founding level may not be
able to indicate the quality of rock foundation for piles because it depends
on the drilling technique and drilling equipment (GEO (1996)). For instance,
if standard core barrels are used to collect samples, it may indicate
sufficient core recovery for samples full of rock joints and weathered rock.
On the other hand, if triple tube barrels are used for obtaining soil samples,
samples with joints and weathered rock can also achieve the requirements
of total core recovery.
In case RQD is adopted for determining founding levels, it may also result
in incorrect results. For instance RQD does not indicate the joints and
infilling materials. Moreover, as it only measures rock segments exceeding
100mm, rock segments exceeding 100mm is considered to be of good
quality rock without due consideration of its strength and joint spacing.
12. What is the principle of airlifting for cleaning pile bores? (BP7)
Airlifting is normally carried out prior to concreting to remove debris and
clean the base of pile bores. It essentially acts as an airlift pump by using
compressed air. The setup of a typical airlifting operation is as follows: a
hollow tube is placed centrally inside the pile bore and a side tube is
connected to the end of the tube near pile bottom for the passage of
compressed air inside the tube. The upper end of the tube is linked to a
discharge tank for the circulation of pumped fluid from pile base.
The efficiency of airlifting operation is dependent on the performance of air
compressor. During airlifting, compressed air is piped down the tube and it
returns up to the discharge tank carrying it with the fluid. It functions by
imparting energy to the fluid and forces the fluid to move vertically upwards.
The injected air mixes with the fluid, resulting in the formation of lower unit
weight of the combined mixture when compared with surrounding fluid.
This hydrostatic pressure forces the fluid/air mixture up to the discharge
tanks.
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