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The coupling effect of freeze-thaw (F-T) temperature and dynamic load on the dynamic mechanical properties and fracture mechanism of saturated yellow sandstone was experimentally investigated in this research. The dynamic compression tests on the specimen after different F-T temperatures (i.e., −5°С, −10°С, −15°С, −20°С, −30°С, and 20°С) have been carried out with split-Hopkinson pressure bar (SHPB) setup under eight F-T cycle numbers. The density and P-wave velocity of the specimens were obtained before and after the F-T tests. After the F-T tests, the specimen microstructures were examined via the scanning electron microscope (SEM). The dynamic fracture process was visualized by the high-speed camera. The particle size distribution and fragment shapes of the specimens were analyzed using a classifying screen. In addition, the energy dissipation law of specimens during the impact test was also discussed. Experimental results show that the dynamic elastic modulus, strength of the specimen, and the average particle size decrease with decreasing F-T temperature. SEM results reveal that low F-T temperature leads to severer internal damage of the specimen by inducing freeze-swell holes, interconnected cracks, and pore clusters. In addition, the fragmentation shapes of the failed specimens exhibit double-cone failure, single-side slope failure, double-side slope failure, and split failure. The energy dissipation increases gradually with increasing F-T temperature. This study helps to prevent geological disasters and optimize engineering design in cold regions.

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Under the geological condition of soft and hard rock interaction stratum, the overburden damage height can provide a quantitative support for the design of the locations of gas drainage boreholes in the roof mining fracture zone and the determination of the hydraulic fracture zone in coal seam mining. The interbedded structure of overlying mud rock and sandstone in the Lu’an mining area in Shanxi is a typical soft and hard rock interaction stratum. In view of the lack of soft rock fracture mechanics analysis and the improper calculation of the damage height of overburden rock caused by constant rock residual bulking coefficient to be used regularly in the analysis, in this paper, we constructed a fracture model of soft and hard strata by giving a quantitative classification criterion of soft and hard rocks and introducing a fracture failure criterion of soft rock strata and the space constraint condition of broken-expansion rock formation. Aiming at improving the calculation precision of overburden damage height, we presented a calculation method based on fracture mechanics analysis of soft and hard strata, which could delineate the extent of intact rock in overlying strata from bottom to top to determine the damage height of overburden rock. This research took Yuwu coal mine in Lu’an mining area as an example. Results showed that (1) by the calculation method, the overburden damage height of the N1102 fully mechanized caving face in Yuwu coal mine was 51.44 m, which was less than the value obtained by an actual borehole TV method as well as the numerical simulation result of 53.46 m, with a calculation accuracy about 96.22%, which is quite high for both. The calculation accuracy of the proposed method was higher than that of the three conventional theoretical methods, and it effectively solved the limitation of the fracture analysis method without the inclusion of the soft rock layer in design and the distortion problem due to the residual bulking coefficient to be improperly used in simulation. (2) There was no noticeable fractures in the broken soft rock zone, and the whole fractures were mainly low-angle rupture; the fractures in hard rock layer had obvious ruptures and multiangle cracks, and the average fracture width of soft rock was 2.8 mm smaller than that of hard rock. The fracture modes of soft rock and hard rock were mainly tensile failure and tensile shear failure, which verified the correctness of the fracture mechanics model of soft and hard rock layers constructed in this paper. (3) It is noticed that the tensile strength of rock in this method needs to be obtained through rock mechanics experiment on overlying strata in the study area, and our proposed method was applicable to the mining conditions of near horizontal coal seam. The calculation accuracy of this method meets the engineering error requirements and can be applied to the prediction of overburden damage height in near horizontal coal seam mining.

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Hydraulic fracturing has been widely used in practical engineering as an essential means to prevent coal seam gas outburst, increase coal seam permeability and improve gas drainage efficiency. Accurate prediction of fracture propagation law is an important basis for optimizing fracturing parameters to achieve high-efficiency gas drainage in coal seams. In this paper, a new seepage–stress–damage coupling model considering permeability anisotropy is first established and then used to study the evolution laws of crack initiation pressure (σci), fracture pressure (σcd), AE behavior and pore water pressure with the lateral pressure coefficient (ξ) and permeability anisotropy coefficient (λ) in the process of hydraulic fracturing. Finally, the influence of initial pore water pressure on σci is discussed, and an efficient gas drainage method is proposed. Research results indicate that: the in situ stress still plays a leading role in the approach of crack propagation whether the permeability is isotropic or anisotropic; the non-uniform pressure condition is favorable for the crack growth compared with uniform pressure under the isotropic permeability condition; when the direction of maximum permeability is consistent with the direction of maximum principal stress (ξ = 0.5, λ < 0), the coal seams are easily fractured; AE behavior of fracturing holes can be divided into three stages: initiation stage, fracture smooth expansion stage and the breakdown stage for any λ or ξ; and the more complex the crack distribution, the more the area of the gas pressure release zone (GPRZ) increases, which is very beneficial to achieve high-efficiency gas drainage. This study can provide a basis for optimizing fracturing parameters and technology in improving the efficiency of coal seam gas drainage using the hydraulic fracturing method.