1 INTRODUCTION
A new method for measuring the real area of contact between model joint walls is presented. The technique allows measurement of contact area changes at all stages of a compression or shear test. This provides an insight into the complex processes that contribute to the mechanical behaviour of rock joints. The experimental method and set-up are described and results are presented from normal closure tests conducted to validate the technique. Preliminary shear tests have also been conducted and their data are shown. The potential of the method for the investigation of fundamental problems in rock mechanics is discussed.
The mechanical behaviour of rock joints has received much attention over the last 40 years, but as yet there is an incomplete understanding of the processes that take place in response to changing stresses. In particular we have little knowledge of conditions at the actual points of contact between joint walls. Whilst it is clear that even for relatively flat, planar surfaces the real area of contact is very much smaller than the gross projected area of the joint, we still know little about how the real area of contact evolves, either during normal closure or shear. As the interaction between joint walls can only occur at these points, the nature of these contacts is of great significance. Observation of the relatively small areas of damage between joint walls is often all that is recorded following a shear test. Even then, this measured area is the result of cumulative contact over the total shear displacement throughout the test. Such measurements are clearly only imprecise indications of the factors that have contributed to shear behaviour throughout. The relatively limited research carried out to address this problem directly has included the use of thermodyes (Teufel & Logan, 1978, Logan & Teufel, 1986), contact sensitive plastics (Bandis et al., 1983) and optical interference methods (Dieterich & Kilgore, 1994).
The method used here to study the development of contacts in rock-like materials is known as the electrical contact resistance (ECR) technique. The technique was developed earlier this century to study the behaviour of metal electrical contacts (Holm, 1948). Bowden and Tabor (1950, 1964) used the technique to good effect in developing the 'adhesion theory of friction'. The theory behind the technique is illustrated in figure 1. A constant direct current is passed between the two samples, crossing the interface at a number of contact points. A separate circuit is connected to measure the drop in potential across the junction and resistance is calculated using Ohm's law. The contact resistance that is measured mainly results from the current flow being constricted as it passes through the small contact points between the samples, and spreading of the flow as it enters the second sample. The constriction of the current flow introduces a 'spreading resistance' the size of which depends on the electrical conductivity of the sample material and the size of the contact spot (Holm, 1946; Bowden & Tabor, 1950).
