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: Pipejacking
1. What is the difference between close mode, open mode and mixed
mode of trenchless methods? (TP1)
(i) Close mode
It refers to mechanically operated TBM using bulkhead or slurry to
balance earth pressure and groundwater. There is no manual access
to the face of excavation.
(ii) Open mode
It refers free air hand-dug tunnel or compressed air handing tunneling
with manual access to face of excavation.
(iii) Mixed mode
It is similar to close mode except that it allows access to the face of
TBM for manual removal of obstructions.
2. Which method of pipe jacking is better, pressure balance method
or compressed air method? (TP2)
Pressure balance method normally requires the use of mechanically
operated tunnel-boring machine at its cutting head in pipe jacking. Slurry or
steel bulkhead is commonly adopted to provide the balance of earth
pressure and groundwater in front of the boring machine. Slurry used in
balancing earth pressure and ground water pressure is constantly supplied
to the face of the cutting wheel through slurry pipes. The excavated
materials drop into a crusher for reduction in material size. Later, the debris
and spoils will enter the spoil removal chamber near the invert of the shield
and will be transported to ground level through slurry discharge pipes. This
method of construction is normally adopted in sand and gravel. However,
it suffers from the demerit that it is quite difficult to remove large rock
boulders during the advancement of the machine. It is quite
time-consuming for workers to go inside the relatively small airlock
chamber and remove large bounders by hand tools.
The other type of pressure balance technique is called earth pressure
balance method which is commonly used in clay and silty soils. It makes
use of the principle of maintaining the pressure of excavation chamber the
same as the pressure in ground. The excavated materials are transported
through screw conveyor to the jacking pit.
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