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
Compressed air method in pipe jacking is commonly adopted in locations
where groundwater table is high. An air pressure of less than 1 bar is
usually maintained to provide the face support and to avoid water ingress.
Pressurization and depressurization has to be conducted for workers
entering and leaving the pipe-jacked tunnels. In case of porous ground,
certain ground treatment like grouting has to be carried out. The removal of
boulders by this method is convenient but it has the disadvantages of slow
progress and significant noise problem generated by generators and
compressors.
3. What is the function of packing materials in the joint of concrete
pipes in pipe jacking? (EP1)
Packing materials are about 10mm to 20mm thick and are normally made
of plywood, fibreboard or other materials. In case packing materials are
absent in pipe joints for pipe jacking, then any deflection in the joints
reduces the contact area of the concrete and it leads to spalling of joints
due to high stresses induced. With the insertion of packing material inside
the pipe joints, the allowable deflection without damaging the joint during
the pipe jacking process can be increased.
4. Is thrust wall an essential element in pipe jacking? Can it be
omitted if there is insufficient depth for constructing normal thrust
wall? (EP2)
Thrust wall is an essential element in pipe jacking and it provides the
reaction against the pipe jacking operation. In poor ground, consideration
may be given to using piling or other methods to increase the stiffness of
thrust wall. When there is insufficient depth to construct thrust wall (e.g.
jacking through an embankment), a structure has still be constructed to
provide the reaction to pipe jacking. In this case, the resistance to
horizontal jacking loads is resisted by piles, ground anchors or other
methods to reinforce the structure.
5. Packers are normally introduced in pipe joints in pipe jacking. Why
should packers be kept 20mm back from the edge of concrete? (EP1)
Joint stress is induced in pipe joints during pipe jacking. Packers are
normally installed in pipe joints to avoid localized stressing of joints leading
to concrete crushing. In essence, packers should be elastic enough to take
the reloading jacking force. Moreover, it should be thick enough to take the
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