Identification of inelastic deformation mechanisms around deep level mining stopes and their application to improvements of mining techniques.

[摘要] Mining induced fracturing and associated deformations can commonly be observed arounddeep gold mining excavations. As the rockmass behaviour and the stability of theexcavations are directly influenced by these processes, a proper understanding of thisinfluence would certainly improve current mining practices with respect to blasting, rockbreaking, support design and mining lay-outs.The main subject of this thesis is the physics of failure and post failure behaviour of rockand similar materials. Failure is denned here as a state at which the material has beensubjected to fracture and/or damage processes. The applicability of commonly usedconstitutive models in representing such failure and post failure processes has beeninvestigated mainly by means of numerical simulations. Mechanisms which controlfundamental fracture and damage processes have been analysed by comparing the resultsfrom relevant laboratory experiments with numerical models.Linear elastic fracture mechanics has been applied to explain and simulate the formation oflarge scale extension fractures which form in response to excessive tensile stresses. Usingthe flaw concept it is demonstrated that these fractures not only initiate and propagate fromthe surface of an opening in compressed rock, but that so called secondary fracturing canbe initiated from within the solid rock as well. The effect of geological discontinuities suchas bedding planes, faults and joints on the formation of (extension) fractures has also beeninvestigated and it has been shown how the presence of such discontinuities can cause theformation o f additional fractures.Micro mechanical models have been, used to investigate the interaction and coalescenceprocesses of micro fractures. It was found that the formation of large scale extensionfracturing can be explained from such processes, but so called shear fractures could notdirectly be reproduced, although such a possibility has been claimed by previousresearchers. The formation of shear fractures is of particular- interest as violent failure ofrock, which is subjected to compressive stresses only, is often associated with suchfractures. In an all compressive stress environment, only shear deformations would allowfor the relief of excess stress and thus energy.The formation of shear fractures is associated with complex mechanisms and shearfractures can therefore not directly be represented by tingle cracks. In contrast to thepropagation of tensile fractures, which can readily be explained by traditional fracturemechanics in terms of stress concentrations around the crack tip, the propagation of shearfractures requires a different explanation. In this thesis an attempt has nevertheless beenmade to reproduce shear fractures by direct application of fracture mechanics. This hisbeen done by representing a shear fracture as a single crack and by assuming fracturegrowth criteria which are either based on critical excess shear stresses, or on a maximumenergy release. Both criteria are completely empirical and require a value for the criticalshear resistance in the same way as a critical tensile resistance is required to represent theformation of tensile fracture; , The determination of a critical tensile resistance ( Kk ) isrelatively straight forward, as the formation of tensile fractures from a pre-existing flawcan be reproduced and observed in standard laboratory tests. The determination of a criticalshear resistance is, however, not a common practice, as the formation of a shear fracturefrom a pre-existing flaw is very infrequently observed.The application of shear fracture growth criteria nevertheless resulted in plausible fracturepatterns, which suggests that such criteria are realistic. It is argued here however that theformation of shear fractures cannot be associated with primary fracture growth, but ratherwith the localisation of failure and damage in an area which is subjected to plasticdeformation. The application of fracture mechanics is therefore not correct from afundamental point of view as these processes are not represented. For this reason plasticitytheory has also been applied in order to simulate failure in general, and shear failurelocalisation in particular. It was in principle possible to reproduce the shear fractures withthe use of this theory, but numerical restraints affected the results to such an extent thatmost of the simulations were not realistic. Plasticity theory can also be extended to includebrittle behaviour by the use of so called strain softening models. The physical processeswhich lead to brittle failure are however not directly represented by such models and theymay therefore not result in realistic failure patterns. It was in fact found that strainsoftening models could only produce realistic results if localisation of failure could beprevented. The effect of numerical restraints becomes even more obvious with a strainsoftening model in the case of failure localisation.While the plasticity models appear inappropriate in representing brittle failure, theydemonstrated that plastic deformations can be associated with stress changes which maylead to subsequent brittle fracturing. Although only indirect attempts have been made toreproduce this effect, as appropriate numerical tools are not available, it is clear that manyobservations of extension fracturing could be explained by plastic deformations precedingthe brittle fracturing processes. Many rocks are classified as brittle, but plastic deformationprocesses often occur during the damage processes as well. The sliding crack for instance,which is thought to represent many micro mechanical deformation processes in rock,directly induces plastic deformations when activated. A pure brittle rock, which may bedefined as a rock in which absolutely no plastic deformation processes take place, maytherefore only be of academic interest as it is inconceivable that such a rock materiel exists.Only in such an academic case would (linear) elastic fracture mechanics be directlyapplicable. As plastic deformation processes do play a role in real rock materials it isimportant to investigate their influence on subsequent brittle failure processes. The elasticstress distribution, which is often used to explain the onset of brittle fracturing, may bemisleading as plastic deformations can substantially affect the stress distribution . -recedinyfracture initiation.In an attempt to combine both plastic and brittle failure, use has been made of tessellationmodels, which in effect define potential fracture paths in a random mesh. The advantage ofthese models is that various failure criteria, with or without strain softening potential, canbe used without the numerical restraints which are normally associated with theconventional continuum models. The results of these models are also not free fromnumerical artefacts, but they appear to be more realistic in general. One o f the m;ij, rconclusions based on these results is that shear failure does not occur in a localisedfashion, but is associated with the uniform distribution and extension of damage. Shearfailure, which can be related directly to plastic failure, can however induce brittle, tensile,failure due to stress redistribution.While the theories of fracture mechanics and plasticity are well established, theirapplication to rock mechanical problems often leads to unrealistic results. Commonlyobserved firacture patterns in rock, loaded in compression, are most often not properlyreproduced by numerical models for a combination of reasons. Either a model concentrateson the discrete fracturing processes, in which case the plastic deformation processes areignored, or plasticity is represented, but brittle failure is pooxiy catered for. Whiletheoretically a combination of these models might lead to better representations andsimulations, numerical problems do affect all models to a certain extent and a practicalsolution is not immediately available. The results of numerical models can therefore onlybe analysed with caution and the underlying assumptions and numerical problemsassociated with a particular technique need to be appreciated before such results can beinterpreted with any sense. Many of the problems are identified here and this may assistresearchers in the interpretation of results from numerical simulations.Laboratory experiments, which have been chosen for analyses, involve specimens whichhave been subjected to compressive stresses and which contain openings from whichfailure and fracturing is initiated. Such specimens are less subjective to boundaryinfluences and are far more representative of conditions around mining excavations thantypical uni- and tri-axial tests. The uniform stress conditions in these latter tests allowboundary effects to dominate the stress concentrations, and thus failure initiation, in thespecimens. The large stress gradients, which can be expected to occur around undergroundexcavations, are not reproduced in such specimens. As a consequence failure is notu atained within a particular area, but spreads throughout the complete specimen in theuni- and tri-axial tests. Specimens containing openings are therefore far more likely toreproduce the fracture patterns which can be observed around deep level miningexcavations.Numerical simulations of brittle, tensile fracturing around mining excavations resulted inconsistent fracture patterns. Fracture patterns could however be strongly influenced by thepresence of geological (pre-existing) discontinuities such as bedding planes. Althoughtensile stresses are often assumed to be absent around deej:

[发布机构] University of the Witwatersrand