Odedra, Anita; (1998) Laboratory studies on shear fracture of granite under simulated crustal conditions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Stresses in the earth cannot exceed the strength of rocks. I have conducted 60 conventional drained triaxial rock deformation experiments to address the effects of confining pressure, temperature and pore fluid pressure upon the shear failure parameters and post-failure behaviour of Tsukuba granite. The experiments were conducted on a recently constructed high pressure, high temperature, true triaxial rock deformation pressure vessel at the Earthquake Research Institute, University of Tokyo. The results presented in this study are the first suite of tests conducted on this apparatus. Confining pressures ranged from 70-480 MPa, pore fluid pressures ranged from 10-300 MPa, temperature ranged from 25°C to 480°C and strain rate ranged from 10-4s-1 to 10-7s-1. I present a failure law that is based on a series of wet shear fracture experiments that can predict the strength of Tsukuba granite under conditions prevalent in the brittle crust and upper brittle-plastic transition zone. The mechanical effect is incorporated in the effective stress law (which has been obeyed at 10-5s-1 for Tsukuba granite). The effects of effective normal stress and temperature on the shear resistance of intact wet granite in the brittle-plastic transition regime have been quantitatively evaluated. The chemical effects of water are enhanced at temperatures above 300°C, below which wet and dry fracture strengths are wholly within the brittle region. Above 400°C, semi-brittle effects and ductility are observed. The shear fracture data has subsequently been recast within the framework of a slip-dependent constitutive relation and the effects of simulated crustal conditions upon the parameters (peak shear stress, residual shear stress, fracture energy, breakdown stress drop and critical slip displacement) have been investigated. All the parameters are temperature dependent above 300°C. Shear fracture energy is found to be a function of the product of the breakdown stress drop and critical slip displacement.

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