Civil Engineering Reference
In-Depth Information
'undrained' movements may be small compared to the time dependent
processes. The maximum settlement may reach between two to four times
the short-term value and the trough width parameter at the surface from
1 to 2.5 times the short-term value. Fang
et al.
(1993) proposed a hyperbolic
time settlement relationship to describe the increase in maximum settlement
with time for earth pressure balance tunnelling machines commonly used
in these soils. Alternatively, when tunnelling with traditional compressed
air methods, a number of authors have suggested that long-term settlements
in soft soils increase linearly per logarithm of time (Devriendt 2006). Work
in this area is continuing, for example Wongsaroj
et al.
(2007) investigated
the long-term ground movements in London resulting from the construction
of the Jubilee Line Extension, which opened in 1999.
7.1.4 Multiple tunnels
The previous work described in this chapter has related to single tunnels.
However, in many situations rather than constructing a single large tunnel,
twin tunnels are constructed. This has to be taken into account when
determining the ground movements generated by the tunnel constructions.
New and O'Reilly (1991) proposed equations for the prediction of cumula-
tive displacements for parallel tunnels with a given separation based on the
principle of superposition. However, Attewell
et al.
(1986) discuss the 'inter-
ference volume' effect where the volume loss is commonly greater when a
second tunnel is excavated adjacent to the completed tunnel (asymmetric
effect in the final settlement trough will occur for two side-by-side tunnels
of the same cross section and depth) (Devriendt 2006).
Addenbrooke and Potts (2001) considered this phenomenon numerically
and found it to be due to the accumulation of shear strain adjacent to the
first tunnel. This results in a lower stiffness and hence greater displacements
where subsequent tunnels are constructed close to the first tunnel (Devriendt
2006). This has also been investigated by Cooper
et al.
(2002), Hunt (2005)
and Chapman
et al.
(2007), with observations from case history data,
numerical analyses and small-scale physical modelling, respectively. These
authors propose a method for estimating the ground movements (both
surface and subsurface) above closely spaced twin side-by-side tunnel con-
structions in soft ground. This method should be considered if the clear
separation between the tunnels is approximately three diameters or less.
7.2 Effects of tunnelling on surface and subsurface
structures
One of the most important aspects of tunnelling in soft ground is assessing
the effect of any ground disturbance caused by the tunnelling operations
on surface or subsurface structures. This is particularly important in urban
areas. It is important to recognize that the ground movements, caused by
