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
settlement was observed at a distance of 3-4 times of tunnel depth behind
the tunnel face and little settlements were reported immediately above the
tunneling face.
11. Why does ground heaving sometimes occur during pipe-jacking?
(PP1)
It is commonly recognized that ground settlement is one of the major
concern in pipe jacking operation. However, engineer should also pay
attention to the problem of ground heaving during grouting work of
pipe-jacking. For instance, if excessive slurry or grout pressure is applied
so as to exceed the overburden pressure, ground heaving would result.
Alternatively, if the ground contains loose soils with high porosity, the same
phenomenon also occurs. Proper control on the applied pressure and
viscosity of grout/slurry is necessary to prevent such occurrence.
12. What are the potential advantages of segmental tunneling when
compared with hand-shield pipe-jacking?
In segmental tunneling, the jacks are installed at the shield so that it is not
necessary to install thrust wall at the jacking pit. This provides the
opportunity for smaller size of the pit because of the absence of thrust wall.
Moreover, as the jacking operation involves the jacking of small length of
segmental liner plates and hence smaller force would be required for pipe
jacking when compared with traditional pipe jacking (jacks at jacking pits)
where the jacking force is needed to overcome long lengths of pipe drives.
On the other hand, the use of segmental tunneling offers better control in
alignment because the steering operation could be performed at the shield.
13. How can Earth Pressure Balance TBM maintain stability of tunnel
face? (TP2)
Earth Pressure Balance (EPB) TBMs are used in excavating and
advancing tunnels through any type of soft ground or soil condition,
particularly below the water table. The EPB method consists of a cutting
chamber located behind the cutterhead. This chamber is used to mix the
soil with water foam. It is maintained under pressure by the mucking
system. The ground at the cutting face is supported by earth pressure by
balancing the advancement of the tunnel with the discharge rate of the
excavated soil.
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