3 Stress
Stress is a concept which is fundamental to rock mechanics principles and
applications. For those encountering stress for the first time, it is not a
straightforward concept to grasp-unless explained very clearly. For this
reason, and at this stage in the book, we have adopted a key point approach
to explaining the concept of stress. This is a direct precursor to Chapter 4
on in situ stress and provides a link with strain in Chapter 5. Further
explanation of stress is given in Appendix A.3.1 Why study stress in rock mechanics and
rock engineering?There are three basic reasons for an engmeer to understand stress in the
context of rock mechanics. These are:
- There is a pre-existing stress state in the ground and we need to
understand it, both directly and as the stress state applies to analysis and
design. This has been discussed in Chapter 2 in the context of the
geological setting. It is emphasized again here that there can be
circumstances when, during the engineering, no new loading is
applied, e.g. when driving an unsupported tunnel in rock. In this latter
case, the pre-existing stresses are redistributed-which leads us to the
next reason. - When engineering occurs, the stress state can be changed dramatically.
This is because rock, which previously contained stresses, has been
removed and the loads have to be taken up elsewhere. In line with this
fact, it is also noted in Section 3.9 that all unsupported excavation
surfaces are principal stress planes, a concept we will explain. Further-
more, most engineering criteria are related to either the deformability
or the strength of the rock or rock mass and the analysis of these subjects
involves stresses. For example, almost all failure criteria are expressed
as a function of certain stress quantities. - Stress is not familiar: it is a tensor quantity and tensors are not
encountered in everyday life. The second-order tensor which we will be
discussing has, for example: