TABLE OF CONTENTS A hypothetical material capable of sustaining only compressive stresses and straining without resistance in tension is in many respects similar to an ideal plastic material. While in practice such an ideal material does not exist, it gives a reasonable approximation of the behaviour of randomly jointed rock and other granular materials. While an explicit stress-strain relation cannot be generally written, it suffices to carry out the analysis elastically and wherever tensile stresses develop to reduce these to zero. The initial stress (modified Newton) process here is natural and indeed was developed in this context.103 The steps of calculation are obvious but it is important to remember that the principal tensile stresses have to be eliminated.
The 'constitutive' law as stated above can at best approximate to the true situation, no account being taken of the closure of fissures on reapplication of compressive stresses. However, these results certainly give a clear insight into the behaviour of real rock structures.
Figure 4.19(a) and (b) shows an application of this model to a practical problem.103 In Fig. 4.19(a) an elastic solution is shown for stresses in the vicinity of an underground power station with 'rock bolt' prestressing applied in the vicinity of the opening. The zones in which tension exists are indicated. In Fig. 4.19(b) a no-tension solution is given for the same problem, indicating the rather small general redistribution and the zones where 'cracking' has...
