Engineering Rock Mechanics

378 Design and analysis of underground excavations

complex set of caverns and tunnels, so that these considerations become
increasingly important. Operational requirements may dictate other, non-
optimal (from the point of view of rock mechanics design) sequencing
arrangements and the consequences of adopting these can be assessed
using the concept of zone of influence.
In Figure 20.20, there are two cases of multiple excavation schemes, one
referring to a common mining layout, and the other to a three-tunnel civil
engineering railway scheme. In the mining layout, the footwall access drives
have to be excavated before the process of stoping can begin. From the
figure, it is directly evident that whilst the zones of influence of the footwall
access drives are unlikely to significantly affect the stress field applied to
the stope, the excavated stope will definitely significantly affect the stresses
applied to the footwall access drives, which may have to be protected for
this eventuality. In the case of the railway scheme, however, the primary
criterion may be to accurately establish the ground conditions by excavating
a small-diameter service tunnel before excavating the main large-diameter
running tunnels. Such a procedure may be thought necessary if there were
any doubt about the suitability of the rock for specific tunnel boring
machines which may be used to excavate the running tunnels. Although
the stresses applied to the service tunnel will change as the running tunnels
are excavated, the known situation may be preferable to excavating the
running tunnels in an unknown geomechanical environment.

Elliptical openings. Apart from a circle, the only other excavation shape
for which a closed form stress solution is available is an ellipse. In a similar
way to that illustrated in Fig. 20.17, contours of stress perturbation can be
derived around excavations and hence the extent of the zone of influence
can be determined. The calculation of the zone of influence on this basis is
time consuming, and in a similar fashion to that illustrated in Fig. 20.17,
we can adopt an elliptical approximation to the zone of influence for an
elliptical opening.
In Fig. 20.21 are the equations for the circumscribing elliptical
approximation for a zone of influence. The similarity between the diagram

Mining engineering Civil engineering

I\ r.

Up-dip I/

advance I/ II n

access

I/ Yves

0

Running

00

I

Service

tunnel

0

Figure 20.20 Illustration of zone of influence and excavation sequencing in different

circumstances of multiple excavations.