Study On Instability And Time Dependence Of Pillar In Hard Rock Mass

The stability of stope is more important with the mining progress in room-pillar mining. Pillar is a pivotal stope unit and it supports stope, so, it not only ensures stability of roof beam and wall rock, but also stability pillar increases the mining rate of ore body obviously. At present, underground mining is developing toward high stage, high subsection, large scale, trackless in the world, consequently, the height of controlling roof increases. The holistic steady behavior of pillar debases markedly with the ratio of height to width increasing. Furthermore, the geometrical shape of pillar is irregular, and rock mass contains various flaws, which influence on the stability of pillar obviously, and the load-carrying capacity decreases of consequently. Usually, the pillar instability occurs when subcritical crack growth reaches to some extent, which results in time dependence of pillar instability. As mentioned above, the author carried out the research deeply and systematically on instability and time dependence of pillar in hard rock, combining with the item "study on the behavior and time dependence of deformation and failure for ultra high pillars in hard rock mass" , supported by National Natural Science Foundation of China. The main content of the dissertation is as follows:Firstly, simplified mechanical models of pillar were established under different mining conditions and instabilities of the systems were discussed by means of potential energy principle and cusp catastrophe theory. The necessary-sufficient condition of instability and the jump value of displacement of pillar and the released energy expressions were derived, which laid foundation for quantifying of the pillar instability. The critical behavior and the forming mechanism of pillar instability were discussed by external disturbance under critical conditions according to nonlinear theory. The essence of pillar instability is revealed consequently.Secondly, a simplified mechanical model of ultra high pillar with initial geometrical imperfection was established and the stability of the system was discussed by means of initial flexibility function, thestress-strain relation of rock, the strain-curvature relation and static balance at elastic stage and at elastic- plastic stage. The compliance relations between the stability of ultra high pillar and initial geometrical imperfections were discussed and limit loads of ultra high pillar with initial geometrical imperfections were obtained at elasticity- plasticity stage. On the basis of crack tensor, the behavior of pillar rock mass containing a random distribution cracks is discussed by regarding cracks as initial damage. The equivalent modulus and the equivalent Poisson's ratio were derived and the effect of initial geological flaws on the stability of pillar was discussed.Thirdly, subcritical crack growth of double torsion specimens made of ore, lherzolite, marble and granite was tested using Instronl342 type electro hydraulic servo test machine. The relations between the mode-I stress intensity factor Kj and the subcritical crack growth velocity V and the fracture toughness KIC were obtained by the double torsion constant displacement load relaxation method. The behavior of subcritical crack growth was analysed in different rocks. The testing results provide basis data for the study of time-dependency of pillar stability.Finally, based on testing results of subcritical crack growth, according to the relations of the mode-I stress intensity factor Ki versus the subcritical crack growth velocity V (Ki-V curve) and geometrical parameters of cracks, the time-dependency of pillar stability was discussed.In short, the research of this dissertation, which is based on the frontal of the subject, applying the newest mathematical and mechanical methods and advanced experiment means to study on instability and time dependence of pillar in hard rock, is of theoretical significance and engineering application values to the safe, economical, and reasonable mining of the underground mineral resources.