In the hierarchical approach of constitutive modelling, the damage accumulation and resulting strain-softening behavior of geologic materials can be modelled by introducing a concept of damage evaluation. The damage accumulation and softening behavior are affected by the size of specimens. A series of uniaxial tests on cylindrical specimens of a simulated rock were performed. The effect of specimen size on the parameters of the damage model were investigated by using the test results. The empirical results show that the damage parameters can be related to a characteristic dimension, which was expressed in terms of length and diameter of the specimen.

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

A model is proposed in which intraplate earthquakes are triggered by the tensile failure of asperities by subcritical crack growth. The principle subcritical crack growth mechanism is fatigue driven by cyclic tensile stresses within an asperity. Cyclic stresses result from cyclic pore pressure variations which propagate from the Earth's surface because of annual variations in the height of the water table. Asperities are postulated to be porous and permeable masses of saturated host rock hydrologically connected to fluids within an open fracture. Porosity and permeability within asperities are assumed to be due to microcracks within the rock matrix. Tensile stresses within an asperity are due partly to mechanical loads, but pore pressure is the primary Inechanism by which tension is developed and fatigue operates primarily in tension. Fatigue crack growth is enhanced by chemical subcritical crack growth mechanisms such as stress corrosion and these mechatusms are proposed to act in unison. Faulting may be initiated when a population of asperities is either driven to failure by these mechanisms alone or when it is weakened to the point at which it is vulnerable to small applied stresses. Numerical modeling of the interaction of pore pressures and stresses within asperities indicates that a small transient increase of pore pressure on the order of a fraction of a megapascal will increase the tensile stresses within an asperity by a fraction of a megapascal. Consequently, it may be possible for a small increase in pore pressure, due to elevated water table levels, to trigger asperity failure and result in seismicity. / Master of Science

A Thesis presented to the Department of Geophysics of the University of the Witwatersrand, Johannesburg / Violent failure of the rock surrounding under ground excavations forms a major hazard and obstacle in deep-level mining. (Abbreviation abstract) / AC 2018

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by David Thomas Barr. / M.S.