Portals are frequently an exceedingly difficult area in terms of ground control due to the near-surface, weathered, and highly discontinuous rock mass conditions. Surface and subsurface failures involving portals were analyzed using over 500 case histories which were organized into a database. Critical factors contributing to both stability and instability were isolated, and failures were classified according to location. Correlations between rock mass classes and types of portal failure were made and a four step stability analysis methodology defined. To determine critical sections of portal approach cuts for stability analysis, the Geomechanics Classification System was appended with discontinuity orientation adjustments. The most common type of failure for active portals, that of 'Crown Face Overbreak' failure, was investigated and modelled for design and support purposes. Results are confirmed using case study data. Excavation and support guidelines, based on database information the predicted failure zone from the 'Crown Face Overbreak' model are provided. / Ph. D.

A review of behavioral rock properties used for input to the finite element method are summarized. Rock properties presented in the literature were primarily obtained from laboratory specimens. Methods to determine applicable field properties via testing, calculations and empirical correlations are included. Suggested behavioral properties of the structural concrete-to-rock interface are proposed. Specific property values, resulting from the literature review, are presented as input for a finite element parametric evaluation of navigation structures. / Master of Science

No description available.

published_or_final_version / Civil and Structural Engineering / Master / Master of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy

Geotechnical characterization of crystalline rock is often dependent upon the influence of the rock's fracture system. To test ensemble fracture behavior in situ, a series of cross-hole and single-hole electrical conductivity measurements were made within saturated Oracle granite. The tests were conducted with a point source and a point reference electrode and employed electrode separations ranging from 8 inches to over 100 feet. A volume of rock approximately 50 x 50 x 150 feet was tested (as bounded by the vertical test borings). Analysis of the data in terms of an equivalent homogeneous material showed that the effective electrical conductivity increased with electrode separation. The cross-hole data indicate that the rock can be treated as a non-homogeneous, isotropic material. Further, the spatial variation of measured conductivities along a line can be fit to a fractal model (fractional Brownian motion), implying that the scale-dependence is a result of a fractal process supported by the fracture system. Scale-dependence exists at the upper scale limit of the measurements, hence a classical representative elemental volume was not attained. Two directions were taken to understand the scale-dependence. The rock mass is treated in terms of a disordered material, a continuum with spatially varying conductivity. First, a percolation-based model of a disordered material was examined that relates the conductivity pathways within the rock to the backbone of a critical percolation cluster. Using the field data, a fractal dimension of 2.40 was derived for the dimensionality of the subvolume within the rock that supports current flow. The second approach considers an analytic solution for a non-homogeneous, isotropic material known as the alpha center model (Stefanescu, 1950). This model, an analytic solution for a continuously varying conductivity in three dimensions, is a non-linear transform to Laplace's equation. It is employed over a regular grid of support points as an alternative to spatially discretized (piece-wise continuous) numerical methods. The model is shown to be capable of approximating the scale-dependent behavior of the field tests. Scaling arises as a natural consequence of the disordered electrical structure caused by the fracture system.

Thesis (Ph.D.)--University of the Witwatersrand, Faculty of Engineering, 1988. / Mining induced fracturing and associated deformations can commonly be observed around deep gold mining excavations. As the rockmass behaviour and the stability of the excavations are directly influenced by these processes, a proper understanding of this influence would certainly improve current mining practices with respect to blasting, rock breaking, support design and mining lay-outs. The main subject of this thesis is the physics of failure and post failure behaviour of rock and similar materials. Failure is denned here as a state at which the material has been subjected to fracture and/or damage processes. The applicability of commonly used constitutive models in representing such failure and post failure processes has been investigated mainly by means of numerical simulations. Mechanisms which control fundamental fracture and damage processes have been analysed by comparing the results from relevant laboratory experiments with numerical models. Linear elastic fracture mechanics has been applied to explain and simulate the formation of large scale extension fractures which form in response to excessive tensile stresses. Using the flaw concept it is demonstrated that these fractures not only initiate and propagate from the surface of an opening in compressed rock, but that so called secondary fracturing can be initiated from within the solid rock as well. The effect of geological discontinuities such as bedding planes, faults and joints on the formation of (extension) fractures has also been investigated and it has been shown how the presence of such discontinuities can cause the formation o f additional fractures. Micro mechanical models have been, used to investigate the interaction and coalescence processes of micro fractures. It was found that the formation of large scale extension fracturing can be explained from such processes, but so called shear fractures could not directly be reproduced, although such a possibility has been claimed by previous researchers. The formation of shear fractures is of particular- interest as violent failure of rock, which is subjected to compressive stresses only, is often associated with such fractures. In an all compressive stress environment, only shear deformations would allow for the relief of excess stress and thus energy. The formation of shear fractures is associated with complex mechanisms and shear fractures can therefore not directly be represented by tingle cracks. In contrast to the propagation of tensile fractures, which can readily be explained by traditional fracture mechanics in terms of stress concentrations around the crack tip, the propagation of shear fractures requires a different explanation. In this thesis an attempt has nevertheless been made to reproduce shear fractures by direct application of fracture mechanics. This his been done by representing a shear fracture as a single crack and by assuming fracture growth criteria which are either based on critical excess shear stresses, or on a maximum energy release. Both criteria are completely empirical and require a value for the critical shear resistance in the same way as a critical tensile resistance is required to represent the formation of tensile fracture; , The determination of a critical tensile resistance ( Kk ) is relatively straight forward, as the formation of tensile fractures from a pre-existing flaw can be reproduced and observed in standard laboratory tests. The determination of a critical shear resistance is, however, not a common practice, as the formation of a shear fracture from a pre-existing flaw is very infrequently observed. The application of shear fracture growth criteria nevertheless resulted in plausible fracture patterns, which suggests that such criteria are realistic. It is argued here however that the formation of shear fractures cannot be associated with primary fracture growth, but rather with the localisation of failure and damage in an area which is subjected to plastic deformation. The application of fracture mechanics is therefore not correct from a fundamental point of view as these processes are not represented. For this reason plasticity theory has also been applied in order to simulate failure in general, and shear failure localisation in particular. It was in principle possible to reproduce the shear fractures with the use of this theory, but numerical restraints affected the results to such an extent that most of the simulations were not realistic. Plasticity theory can also be extended to include brittle behaviour by the use of so called strain softening models. The physical processes which lead to brittle failure are however not directly represented by such models and they may therefore not result in realistic failure patterns. It was in fact found that strain softening models could only produce realistic results if localisation of failure could be prevented. The effect of numerical restraints becomes even more obvious with a strain softening model in the case of failure localisation. While the plasticity models appear inappropriate in representing brittle failure, they demonstrated that plastic deformations can be associated with stress changes which may lead to subsequent brittle fracturing. Although only indirect attempts have been made to reproduce this effect, as appropriate numerical tools are not available, it is clear that many observations of extension fracturing could be explained by plastic deformations preceding the brittle fracturing processes. Many rocks are classified as brittle, but plastic deformation processes 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 be defined as a rock in which absolutely no plastic deformation processes take place, may therefore 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 directly applicable. As plastic deformation processes do play a role in real rock materials it is important to investigate their influence on subsequent brittle failure processes. The elastic stress distribution, which is often used to explain the onset of brittle fracturing, may be misleading as plastic deformations can substantially affect the stress distribution . -recediny fracture initiation. In an attempt to combine both plastic and brittle failure, use has been made of tessellation models, which in effect define potential fracture paths in a random mesh. The advantage of these models is that various failure criteria, with or without strain softening potential, can be used without the numerical restraints which are normally associated with the conventional continuum models. The results of these models are also not free from numerical artefacts, but they appear to be more realistic in general. One o f the m;ij, r conclusions based on these results is that shear failure does not occur in a localised fashion, but is associated with the uniform distribution and extension of damage. Shear failure, 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, their application to rock mechanical problems often leads to unrealistic results. Commonly observed firacture patterns in rock, loaded in compression, are most often not properly reproduced by numerical models for a combination of reasons. Either a model concentrates on the discrete fracturing processes, in which case the plastic deformation processes are ignored, or plasticity is represented, but brittle failure is pooxiy catered for. While theoretically a combination of these models might lead to better representations and simulations, numerical problems do affect all models to a certain extent and a practical solution is not immediately available. The results of numerical models can therefore only be analysed with caution and the underlying assumptions and numerical problems associated with a particular technique need to be appreciated before such results can be interpreted with any sense. Many of the problems are identified here and this may assist researchers in the interpretation of results from numerical simulations. Laboratory experiments, which have been chosen for analyses, involve specimens which have been subjected to compressive stresses and which contain openings from which failure and fracturing is initiated. Such specimens are less subjective to boundary influences and are far more representative of conditions around mining excavations than typical uni- and tri-axial tests. The uniform stress conditions in these latter tests allow boundary effects to dominate the stress concentrations, and thus failure initiation, in the specimens. The large stress gradients, which can be expected to occur around underground excavations, are not reproduced in such specimens. As a consequence failure is not u atained within a particular area, but spreads throughout the complete specimen in the uni- and tri-axial tests. Specimens containing openings are therefore far more likely to reproduce the fracture patterns which can be observed around deep level mining excavations. Numerical simulations of brittle, tensile fracturing around mining excavations resulted in consistent fracture patterns. Fracture patterns could however be strongly influenced by the presence of geological (pre-existing) discontinuities such as bedding planes. Although tensile stresses are often assumed to be absent around deej: <y vel excavations because typical hanging- and foot-walls are subjected to compressive horizontal strain and thus stress, the numerical models identified alternative locations o f Ix 'sile stress and also mechanisms which could induce secondary tensile stresses, A failure criterion has therefore been identified as the most likely cause of large scale fracturing while shear fracturing may only occur in the absence of such tensile stresses .and only as a consequence of failure localisation in damaged rock rather than fracture propagation (in solid rock). Geological discontinuities can easily induce tensile stresses vVher mobilised and may even replace the mining induced fractures by offering a more efficient meat s for energy release. The latter possibility is a true three dimensional issue which has not be en addressed any further in this study, but may be very relevant to jointed rock. Although dynamic failure has not directly been addressed, one of the micliamsms lor brittle, and thus stress relieving, failure under compressive strass conditi ons has been investigated in detail, namely shear fracturing. Shear fractures are effect vely the only discontinuities which allow for stress relief under such conditi ons', in the ibaence of preexisting, geological discontinuities, and are therefore quite rele vant to dynamic rock failure, such as rock bursts, in deep level mining conditions. Potential mechanisms for shear fracture formation and the numerical simulation of these features have been investigated and this may especially assist further research into rock bursts.

[pt] No ano 1999 as quedas de blocos rochosos já representaram cerca de 8% dos diferentes tipos de escorregamentos registrados no Rio de Janeiro. A atividade antrópica gera um aumento das áreas de risco devido às construções próximas da base de escarpas rochosas e uma aceleração destes fenômenos. Desde 1993 o número de quedas de lascas e blocos rochosos a partir de faces de pedreiras desativadas tem aumentado. As condições geológicas e estruturais da região favorecem este fenômeno ao discretizarem blocos nos taludes rochosos. Muitas destas quedas tem sido reportadas em condições climáticas particulares, em períodos relativamente secos correspondentes aos meses de junho, julho e agosto. O presente trabalho discute que, dentro dos possíveis mecanismos para a ocorrência destes fenômenos, está a variação térmica diária, a qual pode criar tensões que favorecem a propagação de fraturas existentes dentro dos maciços rochosos. Por tal motivo, simulou-se em laboratório as condições de um maciço rochoso fraturado e obtiveram-se dados das variações diárias de temperatura, mediante a disposição de blocos rochosos graníticos simulando a forma da fratura e com o auxilio de sensores térmicos em diferentes posições, como na superfície, no interior e na fresta. A partir disto elaborou-se um modelo de bloco com auxílio do software ABAQUS para se determinar a variação dos valores de concentração de tensões sob a influência térmica. / [en] In 1999, the falls of rock blocks had represented about 8% of the different types of slides registered in Rio de Janeiro. The anthropic activity generates an increase of the risk areas due the building of vulnerable houses near to foot rock scarps, and an acceleration of these phenomena. Since 1993 the number of falls of rock blocks from slopes of disactivated quarries has increased. The geologic and structural conditions of the region favor this phenomenon forming blocks in rock slopes. Many of these falls have been reported in particular climatic conditions, in relatively dry periods correspondents to the months of June, July and August. This work argues that, the daily thermal variation could be one of the possible mechanisms for the occurrence of these phenomena, which can create stresses to propagate cracks already existing on the rock mass. For such reason, conditions of a broken rock mass was simulated in laboratory to obtain daily temperature variations, it was made by the disposal of granítics rock blocks simulating a fracture form, where was placed thermal sensors. From this, a model of rock block in the ABAQUS sofware was elaborated to determine the variation of stress concentration factor values under the thermal influence

A caracterização geotécnica de maciços rochosos é um dos aspectos mais importantes para o sucesso de um empreendimento mineiro. Desta forma é preciso ter um investimento significativo para a aquisição de dados durante a fase de pesquisa mineral, bem como um grande esforço para manipular os dados obtidos. Para se determinar as propriedades de um maciço, é necessário estabelecer, primeiramente, as diferenças entre rochas intacta e maciço rochoso. O comportamento mecânico destas duas classes pode apresentar grandes diferenças quando analisadas em laboratório. Um dos métodos mais utilizados no ambiente mineiro para classificar o maciço rochoso é o Rock Mass Rating (RMR), sistema que consiste em ranquear os seguintes parâmetros: Rock Quality Designation (RQD), espaçamento entre fraturas, resistência a compressão simples (UCS), qualidade das descontinuidades e presença de água no maciço. Este trabalho tem por utilizar a geoestatística para primeiro se conhecer algumas das particularidades destas variáveis, tais como a não linearidade, o comportamento em diferentes direções e a união de diferentes populações e posteriormente usar as técnicas de krigagem para criar um modelo geotécnico tridimensional. A geoestatística nos permite obter uma estimativa mais precisa e ter um maior conhecimento da incerteza do fenômeno, entretanto o RMR, por não ser uma variável aditiva, não se pode aplicar diretamente o uso das técnicas de krigagem. Assim uma metodologia é proposta para se estimar cada um dos parâmetros, ranqueá-los de acordo com seus valores e por fim obter a classificação RMR através do somatório destes ranques. Os resultados são comparados com a geologia estrutural da região de estudo, para se analisar a relação desta com a qualidade do maciço. A metodologia proposta apresentou uma melhora significante na qualidade de informações no local estimado e mostrou ser uma tentativa consistente para se criar um modelo geotécnico 3D. / The geotechnical characterization of rock masses is one of the most important aspects for the success of a mining enterprise. In this way, it is necessary to have a significant investment to acquire data during the mineral research phase and to make a great effort to manipulate and interpret the obtained data. In order to know a rock, it is necessary to establish the differences of properties between rock intact and rocky mass. The mechanical behavior of these two classes can present great differences when analyzed in the laboratory. One of the methods most used in the mining environment to classify the rock mass is the Rock Mass Rating (RMR) system, which consists of ranking the following parameters: Rock Quality Designation (RQD), fracture spacing, simple compression strength (UCS) quality of the discontinuities and presence of water in the massif. This work has to use geostatistics to first know some of the peculiarities of these variables, such as nonlinearity, behavior in different directions and the union of different populations and later to use kriging techniques to create a three - dimensional geotechnical model. The geostatistics allows us to obtain a more accurate estimate and to have a better knowledge of the uncertainty of the phenomenon, however, the RMR is not an additive variable, so cannot be directly applied the use of kriging techniques. Thus a methodology is proposed to estimate each of the parameters, rank them according to their values and finally obtain the RMR classification through the sum of these ranks. The results are compared with the structural geology of the study region, in order to analyze its relation with the quality of the rock mass. The proposed methodology presented a significant improvement in the quality of information at the estimated site and proved to be a consistent attempt to create a 3D geotechnical model.

This Master´s Thesis studies the applicability of existing numerical model to predict the conditions in the drifts at the mine. Damage mapping, covering the existing production levels, has been carried out to study the condition of the surface support, i.e. shotcrete, mesh and rock bolts to quantify the amount of deformation visible in the tunnels. Irregularities in the floor levels were also mapped. The rock support applied varies between different areas, from only a layer of shotcrete to areas where shotcrete, mesh and cable bolts are applied sequentially. The large amount of support in some regions is required because of high in situ rock stresses in Kemi Mine. The geological conditions are challenging, with large local variation making efficient supporting and damage prediction difficult. Access drifts at the mine have varying service times, on average 6 years, which is a long time in a difficult environment. The results from this work are a mine specific damage classification, used in the mapping to capture the range of damages seen on site. A reoccurrence of same areas showing damages on several production levels is noted. Digitized versions of the damage maps were made and these have been compared to simulation results. The comparisons were made to plots of deviatoric strain, deviatoric stress and total displacement. Based on the work done during this project it can be concluded that the studied parameters have varying levels of agreement with the drifts, both when comparing the levels with each other and the results in the same level between spring and autumn. The best agreement is found with the deviatoric strain increment and the yielded elements. Rock mechanics and geological reasons for the variations seen between the mapping results and the simulations results can be further studied in the future, as can the alternatives for increasing the accuracy through changes in the numerical model or model type used for comparison.