Engineering Rock Mechanics

The influence of geological factors on rocks and rock masses 19

Figure 2.10 Complete stress-strain curve for basalt-high stiffness, high strength,

very brittle.

&

Figure 2.11 Complete stress-strain curve for limestone-medium stiffness,

medium strength, medium brittleness.

conjunction with the applied loading conditions and loading rates. The

intact rock will also have such characteristics as inhomogeneity and

anisotropy: these factors are discussed in Chapter 10.

2.3.2 Discontinuities and rock structure

In the previous section we have indicated one major characteristic of the
intact rock, i.e. the stiffness, defined as Young’s modulus, E. In the pre-peak
portion of the curve in Fig. 2.8, the rock is behaving more or less elastically.
When materials are truly elastic they do not absorb energy; they react to
the loading instantaneously and they can sustain any stress levels. If rock
behaved in this way, and assuming that one were able to excavate it, there
would be no problem with either excavation, support or rock failure.
However, as we noted in the previous section, the rock does break and it
does have post-peak mechanical characteristics. The consequence of this is
two-fold:

(a) through natural processes, the in situ rock may have already failed and

formed faults and joints;

(b) these faults and joints may be the ’weak links’ in the rock structure.