Three-dimensional observations of faulting process in Westerly granite under uniaxial and triaxial conditions by X-ray CT scan

Observations of spatial fault development in granite undergoing compression provide new insights into the process of faulting. Dry intact Westerly gra nite samples were loaded under a confining pressure of 100 MPa (triaxial co nditions) and 5 MPa (similar to uniaxial conditions), and the progress of f aulting was controlled by maintaining the increment of circumferential disp lacement at a constant rate, which apparently stiffened the machine. The sa mples were unloaded after they experienced some degree of stress drop and w ere successfully recovered before faulting progressed further. A convention al medical X-ray CT scanning system was used to image the sample interiors. Three-dimensional fault systems were detected with sequential X-ray CT ima ges. It was found that three-dimensional reconstruction by X-ray CT images yields not only three-dimensional images of the fault system, but also prov ides fault cross-section images with much less artificial noise (artifacts) than does direct X-ray CT imaging. Three-dimensional images show that a fa ult system that developed under uniaxial conditions is much more complicate d than a fault system produced by triaxial conditions. In addition, the fau lt plane produced under uniaxial conditions is inclined at a lower angle to the maximum compressive axis than under triaxial conditions. Comparing X-r ay CT images, we show that a fault nucleates locally on the sample surface just after peak stress, then develops into the final fault plane in the res idual stress stage of the complete stress-strain relationship under triaxia l conditions. (C) 1999 Elsevier Science B.V. All rights reserved.