: Equivalent properties models are used when joints, faults, fractures and similar structural discontinuities are too numerous to be individually taken into account. Such models are essentially methods for averaging behavior at the joint level of detail up to an aggregate level that can be reasonably accommodated in numerical analysis. Influence functions allow for the recovery of important but “averaged out ” details, such as failure mechanisms. While equivalent properties models are occurring with increasing frequency in the rock mechanics literature, none deal with influence functions with the exception of the NRVE (non-representative volume element) approach. Examples of influence function use, first at the element level and then at the engineering level of analysis, using the popular finite element technique, illustrate the recovery of joint failure mechanisms for assessing stability of jointed rock masses. The main conclusion is that the usefulness of equivalent properties in jointed rock mechanics is greatly enhanced when linked to influence functions that allow failure mechanisms to be taken into account.
1 INTRODUCTION
Equivalent properties models are used when joints, faults, fractures and similar discontinuities are too numerous to be individually taken into account (e.g., Duncan & Goodman 1968, Morland 1973, Gerrard 1982, Chen 1986, Cho et al, 1991). Such models are essentially methods for averaging behavior at the joint level of detail up to an aggregate element level that can be reasonably accommodated in engineering analysis. Averaging methods range from empirical scaling rules to more rigorous theoretical treatments that consider rock masses as composite materials composed of joints and intact rock between. Averaging thus homogenizes the considered element volume. Regardless of the approach, identity of individual joints is lost with the consequence that mechanisms of yielding are lost as well. An open pit mine slope or underground mine slope may be appear safe “on average ” just as a tall engineer who has not learned to swim may feel safe attempting to cross a stream that is, on average, one meter deep. Unfortunately, the consequences of information lost through averaging may be severe. There may be a three meter hole in the stream bed. Potential sliding and toppling failures associated with joints may go undetected. Raveling and caving underground may occur because of joint failure. Thus, design against failure in jointed rock masses using equivalent properties, although often necessary, may be unreliable.
