Engineering Rock Mechanics

278 Stabilization principles

between the two (i.e. the pressure remaining to be equilibriated) versus
displacement. Equilibrium is reached when the pressure difference is zero.
Accepting that the pressure difference is a function of the displacement of
the lining, and that the displacement of the tunnel wall is a function of time
(because the displacement results from advancing the tunnel face), then a
convergencetime curve can be drawn. This final curve is the one that is
measureable in practice.
Convergence occurs rapidly as excavation proceeds; subsequently the
convergence rate decreases as equilibrium is approached. This leads direct-
ly to the observational method (sometimes referred to as the New Austrian
Tunnelling Method), in which sufficient support is installed, after the
period of initial high displacement rate. The disadvantage of this
approach is that the rock-support mechanics is not precisely known; the
advantage of the approach is that it is based on sound engineering prin-
ciples and can be tailored on site to the idiosyncrasies of the project. The
technique was used in the Bochum metro (Fig. 16.9) and the Channel
Tunnel sub-sea running tunnel crossover chambers.
An extensive review of excavation support techniques with tunnel
linings in the United Kingdom has been conducted by Craig and Muir
Wood (1978).

76.4.2 Rock support in discontinuous rock

A directly analogous ground response curve approach can be considered

for the use of rock support in discontinuous rock. As the rock becomes more

and more fractured with the attendant loss of strength, the ground

response curve would be expected to become progressively flatter, as

illustrated in Fig. 16.10. This effect is similar to the reduction in rock

mass modulus with increasing discontinuity frequency, as illustrated

in Fig. 8.2.

Figure 16.9 Construction of the Bochum metro in Germany.