Failure and deformation of rocks in compression for underground design

Abstract

Rock contains discontinuities at all scales. These discontinuities make rock behave in a complex way. This dissertation discusses a new approach to underground design based on the theory of rock fracture mechanics. Due to the important role of coal for energy in the US, coal which is classified as a weak rock was selected as a test material. The mechanism of deformation and failure of coal obtained from the McKinley Mine and the Twenty Mile Coal Mine were studied by observing the distributions of length, orientation, and spacing of the pre-existing as well as stress-induced cracks. Different types of laboratory tests were employed to observe the different scales of cracks and to obtain different types of crack information. The crack information is dependent on the scale used. The cracks propagate along the intersections of the pre-existing cracks, and both extensile and shear crack growth occurs depending on the direction of the load relative to the bedding planes. An analytical model that takes into account both shear and extensile crack growth was developed to predict the nonlinear stress-strain behavior of coal including strain-hardening and strain-softening. In order to solve problems with complex boundary conditions, this model was implemented into two and three dimensional finite element programs. The implementation involved a series of modifications that took into account stress transformation, transverse isotropy, and the calculation of the effective elastic moduli due to cracks. Simple examples were taken to verify the results of the numerical analyses.