TABLE OF CONTENTS When a piece of rock is subject to sufficiently large stresses, a failure of some kind will occur. This implies that the rock changes its shape permanently, and possibly also falls apart. The condition is accompanied with a reduced ability to carry loads. Rock failure is an important phenomenon also for petroleum related rock mechanics, as it is the origin of severe problems such as borehole instability and solids production. It is therefore useful to be able to predict under which conditions a rock is likely to fail.
This chapter discusses the most elementary and well known models for rock failure. One should keep in mind however, that these are only simplified descriptions of real rock behaviour. Rock failure is a complex process which is still not fully understood. Much of the framework used to handle rock failure is therefore based on convenient mathematical descriptions of observed behaviour, rather than derivations from basic laws of physics. Some of the concepts used here, including the concept of failure, may thus be poorly defined and sometimes not very relevant, as will be seen below.
For most of the chapter we shall assume that rocks are homogeneous and isotropic. Anisotropy is discussed in Section 2.9, while some consequences of inhomogeneities are briefly discussed in Sections 2.5 and 2.8.
The stress level at which a rock typically fails is commonly called the strength of the rock. Obviously, as stress level is not a uniquely defined...
