Probabilistic Stability Analysis of Underground Mine Excavations (original) (raw)
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Estimation of the possible instability that may be encountered in the excavation slope(s) during the planning and application steps of the rock excavation processes is an important issue in geoengineering. In this paper, a modelling method is presented for assessing the probability of wedge failure involving new permanent or temporary slope(s) along the planned excavation direction. The geostructural rock slopes including wedge blocks are determined geometrically in the first step. Here, a structural data analysis system that includes a series of filterings, sortings, and linear equations used to reveal the necessary geometric conditions for the wedge form is developed and used. The second step involves the 3D visualization and Factor of Safety (FS) using the limit equilibrium analysis of wedges on both the actual and planned new excavation surfaces. The last step is the Monte Carlo simulation, which is used in assessing the instabilities on the actual and planned new excavation sur...
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Currently, most studies on stability of underground excavations include two separate analyses: the elastoplastic behavior of rock masses and/or kinematic analysis of possible wedges and blocks formed in the excavation walls. This paper presents a case study carried out at the Vazante Zinc Mine in Minas Gerais, Brazil, where studies on stability of underground excavations in discontinuous media included survey reports, laboratory tests and in-situ collected data. In this context, where galleries and mining stopes are excavated in discontinuous media, collapse events caused by the presence of discontinuities are common. First, the spatial orientation, geometric arrangement and mechanical characteristics of the discontinuities intercepted by the core samples were collected. The spatial orientation was based on guide layers, which are discontinuities with known dip direction and variable and dip. The geotechnical characteristics of the discontinuities were obtained by correlation with the roughness degree and the nature and weathering degree of the filling material. From there, the geological-geotechnical models were developed, which were the basis for the finite element analysis in discontinuous media of the designed excavations in the sections 13225 and 13300, between levels 210 and 345 of the mine. For comparison and complementation, wedge kinematic analysis and finite element analysis in equivalent continuous media were performed and, later, an arrangement for the reinforcement system was suggested. The results of these studies show that, in general, continuous models tend to be more conservative and have wider deformation zones, while discontinuous models are able to show in more detail where the displacements occur, and how the families of discontinuities affect the stability of excavations.
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The damage-failure criteria for numerical stability analysis of underground excavations: A review
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Failure of rock mass in deep underground excavations could be attributed to a broad range of performance malfunction, from plastic yielding of rock, generation of macro cracks on the boundary of the excavation, gravity driven rockfalls or even complete stress-induced collapse. The failure criteria determine the stress level (or strain level) at which the rock mass loses its load-carrying (or strain-carrying) capacity. Determination of the state of underground stability can be successfully achieved through implementation of appropriate failure criteria within the numerical analyses' tools. The choice of failure criteria in numerical stability analysis plays a key role in defining the behaviour of an underground excavation. A failure criterion will be useful only if selected based on the correct mechanism of failure. Plus, a right choice of failure criterion, significantly reduces the errors of quantifying an excavations behaviour. Therefore, this paper offers a critical review of the most common stress-based and strain-based failure criteria used in numerical stability analysis of underground excavations. Particular attention is paid to characterize different mechanisms of underground failure and recommendations are formulated for each failure mode. In addition, this paper addresses the theoretical considerations for the applicability of different failure criteria and highlights the practical limitations for their numerical implementation.