A physical and numerical model study of the state of stress and deformation associated with large scale fracture roughness

Butt, Stephen Douglas (1994) A physical and numerical model study of the state of stress and deformation associated with large scale fracture roughness. Masters thesis, Memorial University of Newfoundland.

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Abstract

The focus of this research study was to quantify the influence of a single large scale fracture roughness asperity on the fracture stiffness and displacement, the local stress field in adjacent wall rock and on the mode and patterns of any induced secondary fractures under various states of applied stress. This was accomplished through the use of a discrete fracture numerical modelling code and experiments on an instrumented high strength concrete model of a fracture plane in a stiff biaxial loading frame. -- Initial numerical modelling was completed to determine a suitable asperity morphology for the physical model. Laboratory experiments were conducted on test specimens to determine the material characteristics of the intact concrete and the simulated fracture for input into the numerical model and to measure the 0 to 40 kHz acoustic emission signature of induced fracturing. Nonlinear numerical modelling was completed to predict internal stresses in the concrete model during two normal and two shear loading cycles and to predict modes and patterns of secondary fracturing. The concrete sample was subsequently tested under three normal and three shear loading cycles. Strain gauges cast into the concrete model measured the internal strain field. Displacement transducers mounted on the model measured average fracture displacements and an accelerometer monitored acoustic emissions. The sample macroscopically failed at the peak of the final shear loading cycle and was impregnated with a low viscosity resin to enable "post mortem" analysis of secondary fracturing. -- Results of the study confirmed that the large scale asperity had a significant influence on the local stress field and that several forms of enhanced fracture porosity were associated with plastic and brittle deformation near the asperity. Comparisons between the numerically predicted and experimentally measured stress fields showed that the uniformity of fracture mating about the asperity and secondary fracturing significantly influenced the measured strain field and the accuracy of the numerical predictions. Finally, measured patterns of induced secondary fracturing were very similar to those predicted from results of the numerical modelling.

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