Civil Engineering Reference
In-Depth Information
7
Ground movements and
monitoring
This chapter introduces the topics of how to estimate ground movements
caused by the construction of tunnels in soft ground, the importance of
these ground movements with respect to their effects on adjacent structures
and how these movements are monitored during the construction process.
In addition, the important topic of how assessment of the stability of the
tunnel during the construction for open face tunnels, such as NATM, via
in-tunnel monitoring is introduced.
7.1 Ground deformation in soft ground
When tunnelling in hard ground (rock), ground movements are not
normally a problem, except in squeezing ground conditions, and ground
movements propagating up to the ground surface as a result of the
excavation are unlikely unless the cover depth of the tunnel is relatively
small, i.e. in portal areas, or where the groundwater in the overlying soft
ground may be affected. In soft ground, however, displacements can occur
due to a number of reasons and these are shown for a shield tunnel on
Figure 7.1. These components are (after Mair and Taylor 1997):
1
deformation of the ground towards the face due to stress relief;
2
radial ground movements due to the passage of the shield, possibly due
to an overcutting edge (bead) used to help steering, or whilst trying to
maintain alignment of the shield (pitching and yawing angles);
3
tail void due to the difference in diameter of the tail of the shield and
the installed lining, and hence the tendency for ground to move into
this gap;
4
distortion of the tunnel lining as it starts to take the ground loading;
5
time dependent consolidation in fine grained soils.
Component 1
is particularly important with open face tunnelling
methods. However, if TBMs with pressurized faces, such as EPB and slurry
TBMs, are used this component can be negligible if good face control is
achieved. It should be noted that over-pressurization at the face can lead
