Civil Engineering Reference
In-Depth Information
A number of methods have been used for subdividing the face. In a stiff
ground, such as London Clay, full face excavation is possible up to 30 m
2
in cross section, although water-bearing ground or 'sensitive' structures
adjacent to the tunnel construction may dictate otherwise (ICE 1996). Full
face excavation would normally be advanced in a stepped profile of heading
and bench (and possibly invert) as shown previously in Figure 5.46.
For larger cross sectional areas, the face must be subdivided. The
objectives of this subdivision are (ICE 1996):
•
Reduction of the exposed face area to provide better control of face
stability, convergence and settlement.
•
Reduction of the quantities of excavation, reinforcement and sprayed
concrete per increment of advance thereby providing earlier support.
•
Early invert closure in each of the subdivisions.
•
Improved access for plant and operatives.
ICE (1996) recommends four basic excavation sequences as illustrated
in Figure 5.50. It should be noted that these divisions of the cross sectional
area can also be used with the traditional NATM approach.
The use of sprayed concrete has been developed far beyond the early
applications. There is nearly no limit to the ground conditions or tunnel
size and geometry, which cannot be safely excavated and supported with
the use of sprayed concrete (except high water table in soft ground). The
range of use for NATM/SCL covers:
•
short tunnels;
•
non-circular tunnels or tunnels of varying geometry;
•
caverns;
•
heterogeneous or varying geology;
•
high ground pressure.
The choice of NATM/SCL is, however, very project specific, for example
'short' can be as long as 5 km or more. The benefit of using sprayed concrete
is still its flexibility.
5.7.3 LaserShell ™ technique
The LaserShell
™
technique was developed to meet the requirement of the
Health and Safety Executive in the UK (HSE 1996, statement 310), which
states that 'no person should be allowed to approach the heading until all
exposed ground has been supported'. Although the technique was specific-
ally developed for tunnelling in London Clay, it is equally applicable to
other types of soft ground. The method is described in section 8.2.5 as part
of the case history on the PiccEx Junction tunnels at the London Heathrow
Terminal 5 project. A detailed description of this method is also given in
Eddie and Neumann (2003 and 2004) and Jones
et al.
(2008).
