EFFECT OF VIBRATION ON FAILURE AND ACOUSTIC EMISSION IN A FAULT ZONE MODEL

Abstract

Experiments were carried out on biaxial compression applied to blocks of concrete consisting of two outer layers of high strength and a middle layer of low strength including variously sized gravels. The model design and the type of load chosen for the experiments produced the classical type of fracture: subparallel en-echelon cracks developing in the middle layer and a subsequent shear rupture. The sources of ultrasonic pulses were located to monitor the geometry and successive phases of rupture development. The experiments were carried out at a mean rate of longitudinal strain equal to 10-6 s-1. The principal compression was simulated by a sinusoidal vibration with periods 2, 10, 30, or 100 s. This resulted in a more gradual transition of the model to the overcritical state and lengthened the duration of deformation from the peak load to major rupture. The spectra of the rate of acoustic emission and its highest relative energy showed a maximum at the period of the applied sinusoidal load. At the same time the distribution of these characteristics during one period was considerably different from a sine function. The bulk of acoustic emission and the associated energy release was concentrated in a 20-30% interval around the maximum of the cyclic load. It is shown that the parameters of the vibration allow one to discover approaching instability without knowledge of the absolute values of the load and deformation. This approach can be used to evaluate a change of a rock mass to an overcritical state in the crustal conditions.