Engineering Rock Mechanics

The nature of rock masses 1 2 1

n 3 5 P r E

a

__ Shearing failure

--

I

10 30 50 70 90

Inclination of discontinuity normal to major principal

stress axis

Figure 8.4 Effect of a fracture on the strength of a rock specimen.

and the shear stress on the fracture by

It1 = 2(o~^1 -q)sin2jJW.

It1 = c, + a, tan4,, where c, and 4, are the cohesion and angle of
friction of the fracture, gives for the 2-D case

((TI - o3) = 2 (c, + a3 tan 4,) / (1 - cos 8, tan 4,) sin 28,.

This results in the type of locus illustrated in Fig. 8.4.
Thus, when the rock mass initially reaches its peak strength due to the
application of stresses, either the intact rock or a fracture will begin to
fail3. Hence, the properties that govern the failure of rock masses are
the failure properties of the intact rock and the fractures. The orientation
of the applied stresses relative to the fractures governs how failure
occurs, bearing in mind that the failure of a rock mass is a complex
structural breakdown process following a similar curve to that in Fig. 6.2
for the intact rock. The Hoek-Brown empirical failure criterion applies to
rock masses through the appropriate choice of the governing parameters
(Hoek and Brown, 19974).
Finally, the behaviour of the rock mass after initial failure will be a
function of the loading conditions. Where the load is constant, as in a
rock block sliding down a slope, once failure initiates, it will usually
propagate. However, in underground circumstances, the loading may
reduce as failure occurs, e.g. when failure occurs in a weak, soft mine
pillar loaded by stronger, stiff roof and floor strata.
There are other factors influencing rock mass failure, notably rock de-
terioration due to exposure and other time-dependent effects, which will
not be included here. We will, however, discuss the general principles of
water flow through rock masses in the next chapter and note the effects
of excavation in Chapter 15.

Substituting these values in the Mohr-Coulomb criterion,

We have used the word 'failure' here to mean that some kind of limit state has been

Hoek E. and Brown E. T. (1997) Practical estimates of rock mass strength. Int. 1. Rock

reached.

Mech. Min. Sci, 34,8,1165-1186.