Geology Reference
In-Depth Information
who was appointed as part of the team investigating the collapse by the
Health and Safety Executive (HSE, 2000). The monetary cost of
the failure was at least £400 million whereas the original contract
value was £60 million. The reputation of civil engineering in the UK
was also badly tarnished overseas.
The project was to be conducted by the New Austrian Tunnelling
Method (NATM), which was a novel approach for tunnelling in
London Clay, although it had been used successfully for trial tunnels
at Terminal 4. As described in
Chapter 6,
the traditional concept of
NATM relies on the rock mass locking up as joints and interlocking
blocks of rock interact and dilate during convergence towards the
excavation. The mass often forms a natural arch within the surrounding
rock mass as it relaxes, and no or little support is needed. Optimising
support requirements requires prediction of likely convergence rates,
making measurements as excavation proceeds and then applying any
support necessary to prevent collapse. Generally, shotcrete is used to
prevent loss of loose blocks and wedges, which would destabilise the
arch and maybe lead to ravelling failure. Further active support
options include the use of bolts, lattice girders and steel arches, as
appropriate. The principle is that the rock mass carries most of the
stress and by waiting until the appropriate time, the engineered works
are kept to a minimum. After the rock mass has stabilised, an inner
liner can be constructed, with or without waterproo
ng.
There is some debate over whether the method adopted for HEX
really was or should be termed NATM. Muir Wood prefers the term
informal support to describe the use of a primary shotcrete liner for the
Heathrow Express project; ICE (1996) uses the term sprayed concrete
linings (SCL). The common feature between NATM in rock and SCL
in weaker ground is the use of shotcrete as a preliminary support, but
in weaker ground the shotcrete is used to minimise the deformation
and settlement of the ground, which
effectively reverses the original
NATM principle of encouraging controlled ground deformation
'
'
(ICE, 1996). It is a subtle but important difference. Even before the
Heathrow tunnel project, considerable question marks had been raised
over whether the London Clay (an extremely weak,
ssured rockmass)
would behave as a jointed rock mass, appropriate to the use of a
NATM approach. In fact, the design concept was to use sprayed
concrete (with steel mesh and lattice girders) to form a load-bearing
closed ring (without any bolts or dowels as might be used for rock) to
limit settlement
a quite different concept to NATM in rock. One of
the features of the failure at HEX was cracking and perhaps repair
works of that damage prior to a complete folding in of the shotcrete
liner, and this has been blamed as a contributory factor although it was
argued, post collapse, by experts, that even with part of the invert
completely removed,
-
'
the tunnel, as designed, should still have stood
up for up to 80 days
'
(Wallis, 1999).
