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
Level Two (Advanced FAQs)
Part I: Design of Tunnels
1. What is the difference in two common approaches in tunnel
support, i.e. support approach and reinforcement approach? (DT1)
For support approach it involves the application of reaction force at the
face of excavation by using heavy structures, primarily ribs and lagging.
For reinforcement approach, it involves the overall improvement of rock
mass performance by techniques such as rock dowels, rock bolts and
ground anchors. The target of reinforcement approach is to keep the rock
and blocks from moving and loosening so that a large dead load of rock
would not be exerted onto the support system. In fact, it holds the rock
together and causes the ground around the opening to form a
self-supporting ground arch around the opening.
There is a trend of tunneling industry to move from support approach to
reinforcement approach because it requires less amount of structural steel
support.
2. What are the major factors affecting ground-support interaction in
tunnels? (DT2)
In general, the in-situ ground stress reduces with an increase in inward
radial displacement in an unloaded ground/tunnel. The major factors
affecting ground-support interaction in tunnels are as follows:
(i) The relative stiffness of ground and linings plays an important role in
ground-support interaction. For instance, a stiff support could support
the unloaded ground at lower deformation than a flexible support.
(ii) Delay of support to tunnels results in the ground to be deformed by a
certain degree before the installation of linings. Consequently, the
linings take up less loads subsequently owing to less deformation of
ground generated after the action of delay of support.
(iii) As the ground is unloaded, stress is redistributed during excavation. In
case the ground is delayed in support and the ground respond from
elastic region to yielding region, it results in larger displacement and
supporting load.
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