Abstract
The mechanical and hydraulic properties of joints included in rock masses are affected significantlyby geometrical characteristics of the two surfaces making up each joint. It is recognized that the amplitude of seismic waves transmitted across a rock joint is more sensitive to changes in the joint properties than the velocity of transmitted waves. In this study, the real area of contact between the two surfaces of a joint under normal stresses is examined by means of measurig the amplitude of seismic waves transmitted across the joint. Seismic experiments are conducted using a joint model which consists of a single aluminum plate (2mm in thickness) ranging from 10mm to 50mm in diameter inserted between two cylindrical aluminum blocks (50mm by 50mm) or consists of multiple plates with an identical diameter of 10mm. It is shown from the experiments that seismic waves transmitted through the joint model attenuates monotonously with decreasig the total contact area of plate (s). Then the relationship between the amplitudes of seismic waves and the contact areas is given by A/A0=α0.75, where A0 is the amplitude of seismic waves transmitted through the no-joint model, A is that of seismic waves through the joint model, and α is the ratio of contact area to the cross- sectional area of the cylindrical blocks. The reason for the equation being non-linear is that the energy of seismic waves passing through the central part of the circular section is greater than that of seismic waves passing through the outer part, which is caused by the difference in diameters between the joint model and the transducers. Thus the above relationship is valid only for the present measuring system. With the seismic measurements, three series of uniaxial loading are conducted on an artificial rock joint under mated and unmated contact conditions. The real contact areas estimated using the equation are 90% and 11% for mated and unmated conditions at the normal force of 12kN, respectively. The difference in contact areas is also reflected in the normal force-closure behaviors.
