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
3. In Slurry Shield TBMs, should high or low slurry pressure be
maintained to support excavated face? (TBM3)
In Slurry Shield TBMs, the slurry supports the wall of tunnel face in a
manner similar to diaphragm wall. The bentonite forms a filter cake on the
tunnel face on which the slurry exerts its pressure.
The face of a slurry shield tunnel boring machine is stabilized by bentonite
slurry, which is kept under pressure. If the slurry pressures provided are too
low, instability of the face results with occurrence of large settlements. On
the contrary, in case the slurry pressures are kept too high, it leads to an
excessive loss of bentonite and significant soil disturbance would occur.
4. How can shotcrete stabilize tunnels? (TBM4)
When shotcrete is sprayed on a rough ground surface, it fills small
openings and cracks. It serves as initial support and also immediate
support after excavation. It decreases the possibility of relative movement
of rock bodies or soil particles and, therefore, controls the loosening of the
exposed ground surrounding the tunnel.
Shotcrete lining could take up significant loads though it forms a flexible
support system. In fact, shotcrete lining is expected to undergo large
deformations which enable the intrinsic strength and self-supporting
properties of the ground to be mobilized as well to re-distribute stresses
between the lining and ground. During the deformation of lining, stresses
within the shotcrete lining are relocated to the surrounding ground. As such,
this mechanism of load transfer in turn generates subgrade reaction of the
ground which gives support to the shotcrete lining.
Friction between shotcrete and the ground also reduces the differential
movement of the ground. Even though shotcrete may not form a complete
ring, the frictional forces between shotcrete and the ground could provide
support to the ground.
5. Can shotcrete be adopted as permanent linings in tunnels? (TBM4)
Shotcrete can be employed as permanent linings in tunnels. However, it is
only expected that they would not carry loads in eth same way as structural
linings. The use of shotcrete is limited to temporary works in tunnels with
fair good ground conditions which is self-supporting. Its presence is used
for avoiding rock falls or erosion problems.
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