Due to inappropriate mining practices, water-conducting fracture zones can develop in an aquifer, not only destroying the surface-water environment but also causing water inrush, even hurting or killing workers. To avoid such disasters, investigating and simulating the evolution mechanism of water-conducting fractures are becoming a research focus in mining engineering, especially regarding the organisation and development of fractures. Our work mainly involved the design of low-strength analogous materials and the simulation of fracture evolution for weak-roof problems in shallow seam mining based on a self-built experimental hydromechanical coupling system. The experimental results show that the vertical stress in the roof increases first as the working face approaches and finally decreases to near its initial value as the working face passes. The relationship between fracture depth and coal-seam excavation distance is obviously nonlinear. The leakage velocity of surface water remains stable in the early stage of excavation and increases when the fracture develops through the main aquifuge. The maximum fracture depth is 76.18 m for the Yili coal mine with weak roofs and shallow coal seams. In addition, we numerically simulated and verified the evolution patterns with the FLAC3D platform. The simulated fracture depth of the Yili coal mine agreed with the in situ borehole observation very well and was more accurate than the output of the empirical formula. Our work provides new methods and relevant data for research on the evolution of water-conducting fractures in weak roofs during shallow seam mining.

The mechanical response characteristics of rocks under cyclic loading conditions are crucial factors for evaluating and analyzing the stability of rock mass during underground excavation. In this study, based on fractal theory and a series of tests using the MTS815.02 rock mechanics test system, the classification and fractal characteristics of limestone specimen fragments are investigated. The results show that limestone specimens subjected to cyclic loading can generate more small-sized fragments than conventional compression, but the large-fragment-producing abilities of the two tests exhibit small difference. The mass fraction of the fragments in the cyclic loading test is obviously greater than that in the conventional test when the fragment size is less than 4.75 mm; however, only a small difference is observed between the cyclic loading tests with frequencies of 0.25 and 0.5 Hz. In the same type of test, a confining pressure is helpful in reducing the fragmentation of limestone specimen. As the size interval decreases, the shapes of limestone fragment transition from rectangular to long slice and then to square. The results also indicate that the confining pressure has a significant influence on the size-quantity and size-mass fractal dimensions of limestone fragments. The former has a positive correlation with the confining pressure, whereas the latter decreases with confining pressure. The conclusions obtained in this investigation can enrich the theoretical research on the failure response and mechanism of rock under cyclic loading conditions.

This paper mainly involved the testing of self-designed hydrophilicity-related geomaterials and application of overlying strata monitoring in a mining area in Northwest China. We employed the orthogonal testing technique to select the ingredient ratio of hydrophilicity-related geomaterials and optimized the mechanical properties of materials, such as density, elastic modulus, Poisson ratio, compressive strength, tensile strength, and water absorption rate on the basis of regression analysis. It can be seen that the proportion of the mixture clearly determines the mechanical properties of similar materials. Among them, the content of silicone oil and the cement-Vaseline ratio have the most obvious effects on the mechanical properties of the material. By using the hydrophilicity-related geomaterials with ingredient optimization, we built a physical model to simulate the failure progress of the workface in one of the coal mines in Northwest China. It was shown that three remarkable characteristic parts, including collapse zone, fissure zone, and layer-separating space, appear in the overlying strata. Furthermore, the fractured zone above the separating band is just located under the main aquifer. Finally, compared with in-field data, it was verified that the height of the collapse zone and water-guiding fissure zone measured at three boreholes are in good agreement with the experiment. Thus, the failure mode of overburden and the vertical stress and displacement changes are consistent with the actual engineering. Self-developed hydrophilicity-related geomaterials can be applied to laboratory physical simulation experiments of overlying strata. It provides the basis for future research on large-scale physical water-containing similar simulation experiments.