Abstract

The stress path experienced by a rock mass, during and immediately after excavation is complex and cannot be replicated by a simple loading of samples to induce microcracking. We have simulated excavation stresses in the laboratory by true-triaxial unloading. To do this we have utilised an existing polyaxial system at Imperial College and modified it to undertake acoustic emission (AE) monitoring. We have then used triaxial strain, measurements, three- dimensional ultrasonic surveys and Ali studies to investigate laboratory induced cracking. In particular we have developed and applied a moment tensor inversion procedure to located AE. This allows a direct analysis of the mechanics driving the cracking. We present and compare results from two tests (triaxial loading and triaxial unloading) using Springwell sandstone to show the capabilities of the laboratory and processing techniques. The AE source mechanisms in both tests are shown tu have a significant isotropic component. The mechanics of the cracking fits neither a Mode I tensile or shear source, but a combination of the two. The mechanism orientations agree with cracking parallel to.

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