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 underlying principle of the EPB method is that the excavated soil itself
is used to provide continuous support to the tunnel face by balancing earth
pressure against the forward pressure of the machine. The thrust forces
generated from rear section of TBM is transferred to the earth in the
cutterhead chamber so as to prevent uncontrolled intrusion of excavated
materials into the chamber. When the shield advances at the face of
excavation, the excavated soil is then mixed together with a special foam
material which changes its viscosity or thickness and transforms it into a
flowing material. This muck is then stored and is used to provide support
and to balance pressure at the tunnel face during the excavation process.
14. Why do Slurry Shield TBMs have difficulty when tunneling in clay?
(TP2)
In Slurry Shield TBMs, the slurry forms a filter cake on the face of
excavation which has the following purposes:
(i) It provides the surface for slurry pressure to act on.
(ii) It acts as a seal against the y of groundwater into the tunnel.
(iii) In case the TBM breaks down, this filter cake serves as a sealing
membrane at the tunnel face which allows man-entry into the
excavation and working chamber upon provision go compressed air.
Slurry Shield TBMs are widely used for non-cohesive soils ranging from
fine-grained sand to coarse-grained gravel. It is less suitable when
operating in clay because:
(i) Most slurry separation plant could not separate clay from slurry. As
such, the cost of frequent replacement of bentonite slurry is
substantially increased.
(ii) Clayey materials to clog and cause blockages in slurry system
leading to sudden pressure surges.
15. Why are aqueous lubricants considered inappropriate for
cohesive soils in pipe jacking?
Lubrication performs effectively in pipe jacking by maintaining a layer of
lubricant between the outside surface of jacking pipes and the adjacent soil.
However, once the ground has collapsed onto the jacking pipe, the effect of
lubrication would be greatly reduced. Hence, it is important to maintain
sufficient pressure to avoid these occurrences.
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