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
9. What is the consideration in selecting the number of segments for
segmental linings in tunneling? (SLT3)
For the construction of tunnels by segmental linings, the choice of the
number of segments affects the cost and durability of tunnels. With an
increase in the number of segments, the number of joints also increases
accordingly. This raises the potential for water ingress into the tunnels.
However, if the number of segments is kept to a minimum, the speed of the
erection of segments can be increased. However, it is expected that higher
bending moment would be induced in the tunnel rings for smaller number
of segments and extra cost is incurred in the provision of additional
reinforcement.
10. What is the purpose of subdivision of tunneling faces in Sprayed
Concrete Linings (SCL) method? (SLT4)
In employing sprayed concrete lining methods for lining a large tunnel in
soft ground, the tunneling face is normally divided into several parts
because of the following reasons [62]:
(i) It enables early closure of part of the invert of the tunnel.
(ii) With the excavation in each part taking place at different times, it helps
to reduce the area of exposure of the tunnel face so that there is better
control on tunnel stability.
(iii) For unit advancement in any part of the tunnel excavation, the amount
of excavation and sprayed concrete is reduced. As such, this allows for
early provision of primary support.
11. What are the functions of lattice girder in tunneling?
Lattice girders are supporting elements in tunnels and they normally
consist of steel bars laced together in triangular pattern. They are made to
suit the shape of the tunnel. Owing to their small steel reinforcing area,
they are not expected to contribute much to the overall support of tunnel.
Instead, they are designed and provided for the following reasons:
(i) They have similar spacing with rock bolts and they are intended to
provide temporary support to rock which is readily to loosen and fall.
(ii) Their presence provide an indication if sufficient thickness of
shotcrete is applied.
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