FRACTURE-MECHANICS OF ROCKS

This paper reviews recent studies on (1) AE (acoustic emission) and cr acking models, (2) failure processes and (3) frictional sliding proces ses, mainly based on work carried out in Japan. Techniques for AE data acquisition and hypocenter location have been greatly improved; one s ystem can record twenty-one channels of waveforms and can locate the A E hypocenter automatically. Another system can also record the occurre nce time and the maximum amplitude of the AE event without dead time. On the basis of these data, we are able to discuss the relation betwee n the distribution of the hypocenters, the occurrence intervals, and t he experimentally controlled physical parameters. For this purpose, ma ny studies have tried to develop quantitative expression for the stati stical characters of these distributions. Techniques for evaluating AE source parameters are still being developed, and there has been a gre at deal of improvement in our knowledge about cracking mode of AE. The focal mechanisms have been systematically studied based on the space distributions of the initial motion directions. The studies showed tha t shear type cracking becomes dominant with increasing axial stress. T hese mechanism solutions agree well with the local stress field sugges ted by the fracture plane. Increasing of the failure strength of rocks with increasing stress and strain rates under relatively low confinin g pressure has been studied experimentally. The failure process and th e rate dependency of the fracture strength in the low pressure regime are discussed on the basis of a stress corrosion cracking model. The f ailure mechanism under higher confining pressures of up to 3 GPa is al so examined. Some behavior including the variation of AE activity with axial stress differs between low and high confining pressures althoug h the stress-strain relations clearly show brittle deformation in both regimes. On the basis of these differences, the researchers proposed that 'high-pressure' brittle deformation was different from ordinarily observed brittle behavior at low pressure, and examined the failure m icromechanisms through an optical and an electron microscopes. Frictio nal sliding has also been intensively examined in the past decade. Exp eriments using large samples have demonstrated that the slip propagati on process is well described by a slip-weakening model. The relations between the dynamic parameters of slip propagation process and the phy sical parameters of slip surfaces is becoming clearer.