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
For a long pipeline, the frictional forces established between the jacking
pipes and soil is high. Sometimes, such resisting forces may be so high
that they can hardly be overcome by the jacks in jacking pits. Moreover,
even if the jacks can overcome the high frictional forces induced during
jacking, high loads are experienced in jacking pipes during driving.
Jacking pipe’s material e.g. concrete may not have sufficient strength to
resist these stresses and hence pipe strength is another factor that
governs the need for consideration of using intermediate jacks.
8. Why does the problem ground settlement occur when pipe-jacking
machine enters mixed ground with soils and boulders? (PP1)
The rate of cutting through soils is faster than that of cutting through
boulders for pipe-jacking machine. As such, when pipe-jacking machine
enters a region of mixed ground with soils and boulders, the machine has
the tendency to move towards the direction of soft soils because of the
difference of rate of advancement of pipe-jacking machine for soils and
boulders. Consequently, migration of soft soils occur which contributes to
ground settlement. The degree of settlement is dependent of the depth of
soil cover, soil property and the size of boulders.
9. Why does pipejacking machine usually get stuck when the ground
condition change from soil to very hard rock? (PP2)
When the pipejacking machine moves from a region of soil to very hard
rock, it will be subject to damage of cutting disc. To break and loosen the
rock, the pipejacking machine applies a large torque on cutting wheels.
However, with the change of soft region to hard region, the pipejacking
machine is still under the same jacking load. As such, this results in
insufficient or little space for the movement of the machine against the rock
face, leading to damage and exhaustion of the pipejacking machine.
10. Would ground settlement occur ahead or behind the jacking face
for pipe-jacking? (PP1)
It is reported by Lake (1992) that settlements are expected at the ground
surface at a distance of 1-2 times of tunnel depth ahead of the tunnel face
and 80% to 90% of settlement to be completed at a similar distance behind
the face. However, in the paper “Monitoring of ground response associated
with pipe jacking works – recent experience in Hong Kong”, the author
pointed out that based on their experience, development of longitudinal
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