TABLE OF CONTENTS In this chapter we will be discussing a variety of effects related to the different strain rates that occur throughout the range of rock mechanics processes and rock engineering applications. Following the introduction, highlighting the wide time ranges over which these effects are manifested, we discuss the basic theory of rock dynamics, obtaining dynamic rock properties and the relevance of the ideas in engineering.
In Chapter 6, it was noted that the compressive strength is the maximum stress that can be sustained by a specimen of rock. Let us now say that the compressive strength is reached at 0.1% strain, i.e. 0.001. If this strain is developed in 1 ? for example, during an explosion the strain rate is 1 10 3 s -1. If, on the other hand, this strain is developed over a period of 30 years, the average strain rate is of the order of 1 10 12 s -1. Between these two extremes, there are 15 orders of magnitude of strain rate, and so, if the rock exhibits any time-dependent behaviour, we would not expect to be able to use the same rock properties for an analysis of both cases. In Fig. 13.1, we illustrate two manifestations of these extremes of strain rate. Fig. 13.1(a) shows hackle marks that develop on rock fracture surfaces formed during high strain rate failure, in this case on the surface of a blast-induced fracture. Fig. 13.1(b) shows the effect of the gradual deterioration, and subsequent...
