| This thesis is intended to contribute to a better understanding of unstable violent failures around underground excavations, generally called strainbursts. With the increasing depths of mining and the growing need for larger openings due to mechanization, the problem of designing excavations against frequent unstable violent failures warrants serious attention. During strainbursting the damaged zone around the underground openings falls violently.; In laboratory testing, the dependence of unstable failure on the loading machine stiffness and on the rock's post-peak characteristics is well known. However, the problem of defining the loading system stiffness for an underground opening of an arbitrary shape, in non-hydrostatic stress field, is more complicated. The primary objective of this thesis is to highlight the importance of three parameters namely, the stress level, the system stiffness and the releasable energy. It is shown that these three parameters are interrelated and must be considered together when assessing violent failure. It is concluded that: (1) The stress level, i.e., the induced stresses relative to rock mass strength represents the failure potential of the rock mass. The mining-induced stresses must be high enough to exceed the in situ strength of the rock mass. At this level, the rock mass behavior changes from continuum to discontinuum. (2) The mine system stiffness alone is insufficient to assess the violence of the unstable failure in brittle rock mass. (3) The amount of released energy determines the violence of failure. This released energy is largely a function of the stress level and the system stiffness.; The transition from continuum to discontinuum behavior provides the second focus of this thesis. The knowledge of continuum to discontinuum transition is vital in understanding the change from gradual, stable failure to sudden, violent failure. The system response is examined for detached volume of rock (such as macro slabs, wedges, blocks, etc). However, it is necessary to define the failure zone before the system response can be assessed, as the violence depends on the volume of rock mass involved in the failure process.; For a single isolated opening, various processes during the failure initiation, the propagation and the arrest are examined. For a single slab, a “macro-slabbing” model is developed to capture the transition from continuum to discontinuum behavior. If the slabs do not come out sequentially, the failing rock block is assumed to take a triangular wedge shape.; The unstable failure process around multiple interacting openings is studied using 3D numerical models to demonstrate the spatial variation of the loading system response. It is shown that the system response varies along the stope length and that the violence of the failure process depends on the boundary conditions of the failing rock mass block.; Other conclusions of this thesis are: (1) The unstable failure is affected by the in situ stress ratio, the shape and the size of the opening, and the geometry of the damaged rock region around the excavation. (2) At larger depths, strainbursting problems can be reduced by reducing opening size, maintaining the integrity and controlling the dilation of rock mass in the damage zone. (Abstract shortened by UMI.)... |
