1563 5. CONCLUSIONS (1) Thick-hard roofs have high strength, great thickness, and strong integrity. After face extraction, such hard, thick, and difficult-to-cave roofs do not cave timely. Large-area roof hanging has the characteristic of instantaneous breakage, leading to strong face weighting accompanied by air blast phenomena, causing face equipment damage. Addressing the challenge of controlling strong strata pressure from thick-hard roofs, a technical strategy of underground regional fracturing for thick-hard roofs to prevent strong strata pressure was proposed. By modifying the roof breakage structure and reducing surrounding rock abutment pressure through fracturing and destressing, the manifestations of strong strata pressure and rib spalling/caving problems are controlled. (2) China's first set of underground regional fracturing technology and equipment with 5 m³/min capacity, underground large-displacement abrasive jet technology, and comprehensive evaluation technology for destressing effectiveness were innovatively developed. Based on the geological conditions of the 10m ultra-large mining height fully mechanized face, key strata for fracturing were determined, and the fracturing plan was dynamically adjusted. Kilometer-scale directional drilling rigs were used to construct 400–800 m horizontal boreholes. Utilizing precise directional horizontal drilling and large-displacement segmented retreat fracturing technology, complex hydraulic fracture networks were created within the target hard roof strata. (3) After applying roof hydraulic fracturing, the issues of large-area roof hanging and minor air blasts during the initial mining stage of the ultra-large mining height face were resolved. The first weighting span was 49.35 m, with the proportion of support leg safety valves opening being 24.81%. Periodic weighting spans were controlled below 25 m, and the intensity of strata pressure behavior during periodic weighting was significantly alleviated. The average and maximum retractions of hydraulic support legs decreased by 33.3% and 23.4%, respectively. The proportion of periods with an average dynamic load coefficient greater than 1.5 decreased by 25.6%. Rib spalling was concentrated within 0.2–0.5 m, within a controllable range. ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Grant No. 2023YFC2907502), the National Natural Science Foundation of China (Grant No. 52404129), and the Science and Technology Innovation Fund of CCTEG Coal Mining Research Institute (Grants No. KCYJY2026-ZD-01, KCYJY-2025-MS-04). The authors thank Dr. HUANG Zhizeng, Dr. ZHANG Zhen, Dr. LIU Xiaogang, Dr. LIU Qianjin, and Mr. LI Shaopeng for their assistance in strata pressure monitoring and field data collection. REFERENCES [1] AMADEI B, STEPHANSSON O. Rock stress and its measurement[M]. London: Chapman and Hau, 1997. [2] CHEN Dongdong, SUN Siqing, ZHANG Jian, et al. Technical system and engineering practice of coal seam permeability improvement through underground directional long borehole hydraulic fracturing[J]. Coal Science and Technology, 2020, 48(10): 84−89. [3] CHEN Zuo, XU Guoqing, JIANG Manqi. The current status and development recommendations for dry hot rock fracturing technologies at home and abroad [J]. Petroleum Drilling Techniques, 2019, 47(6): 1-8. [4] ECONOMIDES M J, NOLTE K G. Reservoir stimulation[M]. 3rd ed. New York: Wiley, 2000. [5] KANG Hongpu, FENG Yanjun, ZHANG Zhen, et al. Hydraulic fracturing technology with directional boreholes for strata control in underground coal mines and its application[J]. Coal Science and Technology, 2023,51(1): 31-44.
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