Engineering Rock Mechanics

356 Underground excavation instability mechanisms

14

12

• '..


• '.,

Figure 19.17 Optimal aspect ratio for an elliptical excavation.

7 9.2.3 Analysis of instarbility around underground

openings

There are three main modes which will be addressed here:

(a) fracture zones around the excavations caused by stress induced failure

(b) the possibility of slip on pre-existing discontinuities because of the

(c) the special case of slip within a stratified rock.

of the intact rock;

induced stress field; and

Development of fracture zones. The discussion of fracture zones is

illustrated with reference to circular excavations in plane strain, but

the ideas apply to all excavations. In Fig. 19.19, there is a zone shown

around the opening where the Mohr-Coulomb criterion for the intact

rock has been satisfied. For the conditions of a hydrostatic field stress,

as shown, this zone is circular and concentric with the centre of the

opening.

Closed form solutions for the radial extent of the fracture zone, the

stresses within it, and the stresses within the remaining elastic zone

can be derived from first principles for this geometry and stress state-

with the usual CHILE assumptions. The solutions are given in

Fig. 19.19.

Although these equations apply for an idealized case, they can provide

guidance to intact rock failure potential and to what extent the rock might

be damaged. The expressions for stresses within the fractured zone and the

radius of the fractured zone both contain the parameter pi, the internal

pressure. This pressure may be a fluid pressure (water or drilling mud, for

example) or may be produced by the installation of mechanical support.

In the latter case, the equations enable one to examine the effect of support

on the stability of an excavation. This theme will be continued in

connection with the ground response curve in Chapter 20.