In order to study the development height of the water-conducting fracture zone in a fully mechanized top coal caving face. The E2311 working face of Gaohe Coal Mine was chosen as the research object, and the combined-strata structure and the rock layer synergistic movement mechanism were determined by combining engineering geological investigation and theoretical analysis. The height of the water-conducting fracture zone at the working face was calculated based on the combined-strata structure, and then the theoretical results were verified by numerical simulation and field measurement. The results show that after the coal seam is extracted from the working face, the movement of the overlying rock layers is in the form of bending and sinking movement of the rock layer group as a unit. Each rock layer group is controlled by a supportive lower layer with greater thickness and strength, driving the upper layers of weaker rock layers to synchronize and coordinate the movement; the sinking curvature is the same, after the lowermost support layer is bent and broken, its overlying weaker rock layers will move and break at the same time. The height of the water-conducting fracture zone of the working face were obtained by theoretical calculation, numerical simulation, and field measurements, which are 83.82 m, 84.3 m, and 86.6 m, respectively. The results are nearly consistent, thus the prediction of the height of the water-conducting fracture zone under the combined-strata structure is more accurate.

In this study, a wide strain measurement range optical frequency domain reflectometry (OFDR) system based on self-correcting two-dimensional (2D) cross-correlation is proposed and experimentally demonstrated. Large strain events can lead to spatial mismatch caused by overstretching of the fiber, resulting in inaccurate demodulation of the cross-correlation spectrum during conventional OFDR data processing. The degradation of signal similarity due to spatial mismatch can be mitigated by creating a 2D array of reference signals in frequency domain with different positional deviations and performing a 2D cross-correlation operation with the measurement signal in frequency domain. In addition, based on the obvious trend difference between the main peak and the rest of the 2D cross-correlation result with positional deviation points, a self-correcting dictionary is constructed based on the standard deviation, which further improves the demodulation accuracy of the spectrum shift. In experiments, the strain gradient information from 1000με to 8000με is reconstructed free of outliers with a spatial resolution of 3mm located at the end of 50-m-long sensing fiber. The mean absolute error (MAE) is reduced from 14.12pm to 6.12pm, and the standard deviation is reduced from 13.09pm to 4.56pm compared to the conventional method. This novel and simple strain measurement range extension technique will provide new ideas for the development of economical, reliable, and practical large strain measurement systems.

In order to study the local deformation of an anchor bolt and the improvement in the shear strength of a structural surface under the misalignment of an anchorage structure surface, FLAC^3D software was used to simulate granite, sandstone, and coal specimens with anchorage angles of 90° to analyze the damage of the anchoring agent and the changes in the local axial and shear forces of the anchor bolts with the misalignment of the structural surface. The results show that the anchor bolt near the structural surface had significant local characteristics with the misalignment of the structural surface; that is, the length of the local deformation area of the bolt was approximately equal to the length of the damaged area of the anchoring agent, and the stress on the anchor bolt was in a coupled tensile–shear stress state when the bolt reached the yield state. For the fully grouted bolts, it was this significant local feature that made the shear strength of the structural surface increase rapidly under a small shear displacement so that the structural surface reached a stable state. The improvement in the shear strength of the anchoring structural surface was caused by the misalignment of the structural surface. This is referred to as the passive improvement of the shear strength of the anchoring structural surface, which is the mechanism of the bonding section anchor to control the shear displacement of the structural surface and realize the stability of the rock mass.

In this study, a fully distributed carbon dioxide (CO₂) sensor based on optical frequency domain reflectometry (OFDR) system is proposed and experimentally demonstrated. Commercially available polyimide (PI)-coated single mode fibers (fibercore, SM1500(9/125)P) are used as sensing fibers without the additional step of a complex and expensive coating process. Changes of CO₂ concentration can induce a volume expansion effect in the PI coating layer to transfer strain to the fiber core, and the distributed measurement of CO₂ concentration can be indirectly realized by demodulating the spectrum shift induced by the strain at each position of the sensing fiber through OFDR. In experiments, the individual sensitivity of PI-coated fiber to CO₂ was verified with methane (CH₄) and nitrogen (N₂) as references. Distributed CO₂ concentration measurement was successfully verified with 5 cm spatial resolution over 12 m sensing fiber. Moreover, the system was proven to have good stability and reliability. This simple, novel, robust, and inexpensive distributed CO₂ measurement technique has potential applications in downhole fire monitoring, air quality monitoring, and greenhouse gas monitoring.

Drilling unloading, and bolt support are widely used in the practice of coal mine roadway engineering as the means of impact prevention and support. However, the evaluation index of intact coal body is still used in bursting liability evaluation, and the evaluation results obtained do not match with the actual dynamic phenomena in the field, resulting in inaccurate evaluation results and even bringing serious impact accidents. In this paper, uniaxial compression and uniaxial loading/unloading tests are conducted on specimens in different states, and common evaluation indexes are used to evaluate the bursting liability of specimens in different states, and the impact kinetic energy of crushed blocks during uniaxial compression is calculated. Based on this, the bursting liability criterion based on the impact kinetic energy of the crushed block is established and the common bursting liability evaluation index is modified. The bursting liability obtained by the bursting liability discrimination criterion based on the impact kinetic energy of the crushed block is more consistent with the laboratory dynamic phenomena. Therefore, the bursting liability evaluation results based on the impact kinetic energy of the crushed block are more consistent with the actual engineering. And the numerical simulation results verify the correctness of the bursting liability criterion based on the impact kinetic energy of the crushed block.