Civil Engineering Reference
In-Depth Information
5.8 Cut-and-cover tunnels
5.8.1 Introduction
The cut-and-cover method for constructing tunnels offers an alternative
approach to underground construction techniques. This method involves
constructing the tunnel structure in a braced or anchored, trench-type
excavation ('cut') and this is subsequently backfilled ('covered'). Pipelines,
such as sewers, vehicular tunnels and metro tunnels are often constructed
using this technique. In locations with no important constraints on, or close
to, the ground surface, using the cut-and-cover method is often cheaper
and more practical for shallow tunnels (10-15 m) compared to underground
tunnelling methods. However, depths of 30 m are quite common for metro
tunnels, which have been helped by advances in construction techniques,
such as the use of diaphragm walls (section 5.8.4). (Kuesel and King 1996)
Using the cut-and-cover method can also reduce the risk compared to
underground construction, particularly with respect to the health and safety
aspects of working underground. The approaches to immersed or bored
tunnel sections will often be constructed as cut-and-cover tunnels. It is also
common for metro stations to be constructed as cut-and-cover tunnels if
there is suitable access from the ground surface. However, in urban areas
cut-and-cover tunnel construction can be very disruptive as access to the
ground surface over extended areas and for long periods of time is difficult.
Even finding suitable sites for access shafts for bored tunnels can be
extremely problematic and this can dictate the alignment of a tunnel.
5.8.2 Construction methods
Two basic forms of cut-and-cover tunnel construction are available:
BOTTOM-UP METHOD
An excavation is made from the gorund surface and the sides are supported.
The tunnel is then constructed within this excavation. The tunnel may be
of
in situ
concrete, precast concrete, precast arches and corrugated steel
arches. The excavation is then backfilled, and the surface reinstated. This
method has the benefit of allowing good access to the construction area,
but means that the surface reinstatement happens last, which in congested
urban areas may be unacceptable.
TOP-DOWN METHOD
From the ground surface level, the support walls and capping beams (beams
constructed on top of the side walls) are constructed. These walls can be
constructed using diaphragm walls, contiguous or secant piled walls or
another method (these techniques are described later in this section). The
