Geology Reference
In-Depth Information
cation systems introduced in
Chapter 4
are linked to
charts allowing decisions to be taken as to the immediate (temporary)
support measures required. These are reviewed by Hoek
et al
. (1995). In
practice, decisions may often be biased by other considerations such as
the materials and equipment at hand and the workers
Rock mass classi
perceptions of the
degree of risk and how well previous support measures have worked.
This may of course have cost implications and may also later become a
matter of dispute as to what was really necessary, as discussed and
analysed by Tarkoy (1991). The importance of good engineering geolo-
gical records during construction is emphasised. In severe situations such
as high stress or intense water in
'
ow, steel lining may be used but even
then this sometimes proves inadequate as happened during the construc-
tion of the Tai Po to Butter
y Valley water supply tunnel in Hong Kong,
where unexpectedly high water pressures buckled the liners (Robertshaw
& Tam, 1999; Buckingham, 2003).
There are two main areas for consideration:
firstly, the area around the
portal, especially for tunnels that are part of a road or rail system, and,
secondly, need for a permanent liner.
6.5.5.2.1 PORTAL DESIGN
The area above the entrance to a tunnel often requires careful engineering
to make it safe, both during construction and during operation. The
problems are essentially the same as for general slope stability design,
as discussed later in this chapter, but the need for long-term inspection
and maintenance, whilst maintaining tunnel usage, sets portal design in a
rather special category. A canopy is often constructed to protect the portal
area from falling rock and other debris, as illustrated in
Figure 6.17.
Catch nets, barriers (such as gabion walls) and
in situ
stabilisation can
Figure 6.17
Canopy extending
out from tunnel
liner (being
waterproofed), to
protect portal area.
A55, North Wales.
