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Coal mass is often affected by consecutive loading-unloading cycles in coal mining, so the study of energy conversion and failure mechanism of coal mass under repeated loads has a guiding significance for understanding the mine dynamic disasters. By using MTS815. 03 servo experimental system, the uniaxial loading cycle tests were performed for the mechanism of energy accumulation, dissipation, and release in the failure process of coal sample under different loading rates, and the fragmentation distribution was also analyzed based on the theoretical analysis of energy and fractal. It provides a basis for the study of burst response and failure mechanism of coal and rock under cyclic loading. The experimental results show that the phasic characteristics of energy conversion are obvious, which can be divided into three stages:initial energy accumulation stage, accelerated energy accumulation stage and fast energy dissipation stage. The proportion of energy dissipation of coal samples encounters the course from high to low again to high. The elastic energy has an opposite trend of change, the proportion of elastic energy drops or the proportion dissipated energy increases at the later stage of loading, the coal samples enter the accelerated failure stage. The energy dissipation and the energy release are closely related with the loading rate. With the increase of loading rate, the proportion of elastic energy increases before the peak stress, sample is damaged severely after the peak stress and its macro failure mode transits from shear tension and splitting failure to ejection failure. Because more energy is stored in the coal sample body in the form of elastic energy before the coal sample is destroyed, and more energy is released after the rock is destroyed, which makes the coal sample destroyed more severely. The phasic characteristics of fractal features of coal samples are obvious under cycling loading and unloading and the self-similarity characteristics are obvious within the range of threshold, the fractal dimension is between 2-3, which increases linearly with the loading rate. The fractal dimension of fragments increases with the increase of loading rate, and the higher the broken degree of coal samples, the smaller the proportion of large fragments. In this case, the higher of the broken level of more fragments and the smaller the mass of single fragments is, the greater the risk of dynamic disasters of coal samples. [ABSTRACT FROM AUTHOR]

The increase in the cover depth and length of longwall face leads to some drastic changes in mine pressure occurrence and spatial distribution. Such changes result in a great challenge in surrounding rock control. In order to realize an effective control of the abnormal mine pressure occurrences, happening frequently and difficult to be predicted in deep longwall face with ultra-large length,the roof rupture characteristics and support resistance distribution in such a longwall face are investigated. Pre-existing fracture development significantly increases due to the increase of cover depth. The fracture size and orientations obey lognormal and normal distributions, respectively. The main roof with randomly distributed fractures is reconstructed based on such statistical distributions and DFN method. The large length of longwall face increases the probability of the existence of fractures in exposed main roof. Sub-regional rupture model is established for the main roof with one and two fractures, whose limit load-bearing capacities are achieved. The mechanical condition for the occurrence of the sub-regional rupture of main roof and its subsequent migration is achieved using upper bound theory. Spatial distribution of the broken blocks after the main roof fracturing is non-uniform. The block volume in the middle section of the longwall face is small while that of the blocks on two sides of the longwall face is relatively large. Such distribution reasonably explains the non-uniform distribution of the support resistance along the dip direction of the longwall face. The theoretical conclusions are in good accordance with the experimental results and field measurement. [ABSTRACT FROM AUTHOR]

The mesozoic medium particle sandstone slices of northeastern and western coal mines were observed under microscope，and the mineral particles，cementing composition and crystallization degree were analyzed for the following triaxial cell loading and cyclic loading acoustic emission (AE) experiments. The variation laws of strength criterion，dissipated energy，Kaiser effect and load/unload response ratio (LURR) were discussed. The results show that the northeastern calcareous cemented sandstones show a higher strength than that of the western calcareous argillaceous cemented specimens by the Mohr-Coulomb criterion. At the equal stress level the northeastern sandstone elastic energy is higher and the dissipated energy is lower comparing with western sandstone. During the cyclic loading，the Kaiser effect usually occurs at the elastic stage within low stress level，and the northeastern specimens contains more Kaiser points than that of western specimens. As the stress increases，the LURR decreases sharply at low stress stage，then declines steadily，and reaches minimum at the peak strength，moreover the northeastern sandstone LURR is lower than that of the western specimens.The differences of the mechanics and energy dissipation can be ascribed to the diversity of cementing composition and crystallization lithogenesis degree. [ABSTRACT FROM AUTHOR]

In the western mines，the deformation，movement and breakage of overlying strata induced by high intensity mining are the root causes of geological hazards (e.g.large area roof falling，water and sand inrush) and environment damage (e.g.water and soil loss). Incorporated with numerical simulation，in-site micro-seismic monitoring and theoretical analysis，the paper study on influence factors of overlying coal strata falling and mine pressure behaviours. Taking Xiaojihan Coal Mine as an example，the deformation，breakage，falling and “three zones” formation are well discussed，the ground pressure behaviours at different mining speed are also investigated in detail，and the influence rules of different mining height and excavation speed on failure of coal strata are finally clarified. Results of in-site micro seismic monitoring show that，different mining rates of working face causes different range and completion degree of ground stress adjustment. Through comprehensive analysis，the caving characteristics of overlying coal strata and the law of “three zones” origination and evolution are systematically revealed，which will provide scientific basis for establishing failure law and stability criterion of overlying strata in high intensity mining. [ABSTRACT FROM AUTHOR]