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La rupture brutale des piliers conditionne-t-elle les effondrements miniers ? : approches énergétiques par modélisation numérique

Abstract : At different locations around the world, the room-and-pillar mining method truns out to be associated with two major types of hazards affecting the surface: the continuous subsidence and the sudden collapse. The Lorraine iron basin (France) have known on many occasions both continuous subsidences and sudden collapses, notably since the decline of the French mining activity in the 1960’s. The collapses are more dangerous than the continuous subsidences. This is why researchers and engineers developed statistical methods capable of distinguishing between the sectors subjected to a collapse hazard and those subjected to a continuous subsidence hazard. The objective of that study if to develop indicators capable of making the same distinction but on the basis of mechanical criteria, for the purpose of improving the understanding of the collapse mechanism and for validating the statistical indicators. The main assumption of this work is that both the continuous subsidences and the collapses are caused by damage of the pillars within the mines. Chapter 1 presents a literature review concerning the actual knowledge about the pillars mechanical behaviour. We notice that pillars are traditionally characterized by two criteria: the factor of safety (pic compressive resistance/average vertical stress), which determines whether or not pillars are susceptible to fail, and a criterion of stability in case of a failure, which consists of comparing the flexural stiffness of the surrounding rock to the pillars axial stiffness in the post-pic domain of their behaviour. We identify two limits regarding the traditional approaches: i) The question of the average vertical stress has only been little studied in the case of pillars with irregular geometry, which is often the case in old mines; ii) The model allowing to estimate whether or not an instability is susceptible to occur comes from a 1D model proposed in the 1960’s and that has never been developed furthermore for being applied to real configurations in various geomechanical conditions. In Chapter 2, we develop a new method for taking the irregularity of the pillars dimensions into account in estimating their average vertical stress, for the purpose of having a better precision in the factor of safety calculation. We introduce the concept of relative extraction ratio and show, on the basis on numerical modelling, that a quadratic function of this ratio is more precise in estimating the average pillar stress than the classical tributary area method.Chapter 3 is devoted to the development and the application of an original explicit method for modelling pillar instability based on the kinetic energy. We show that the numerical results obtained in 2D configurations are compatible with the traditional 1D model of pillar instability. Moreover, we show how the modelling method we are using helps to locate and to quantify the kinetic energy in excess induced by the instability. The magnitudes of modelled kinetic energies are similar to Richter magnitudes of typical seismic energies recorded during real pillar bursts. The problem of the Lorraine iron basin collapses is treated in Chapter 4. Based on 3D modelling results, we show that pillar failure and instability criteria are capable of well distinguishing between the sectors affected by collapses and those affected by continuous subsidences in the iron basin. Both the sectors subjected to collapses and continuous subsidences are characterized by a factor of safety lower than one regarding the limit of elasticity. But the collapsed sectors are distinguished by a factor of safety also lower than one regarding the pic compressive resistance. This observation is confirmed by simplified 2D modelling, which suggests, furthermore, that the properties of the overburden have no effect on the occurrence of the collapses. Some studies provide contrary conclusions. So, we propose some points to be discussed
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Submitted on : Thursday, October 19, 2017 - 9:55:30 AM
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  • HAL Id : tel-01619122, version 1

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Thomas Hauquin. La rupture brutale des piliers conditionne-t-elle les effondrements miniers ? : approches énergétiques par modélisation numérique. Sciences de la Terre. Université de Lorraine, 2016. Français. ⟨NNT : 2016LORR0164⟩. ⟨tel-01619122⟩

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