我国煤系地层“顶板-煤-底板”结构中坚硬顶板约占三分之二，坚硬顶板弱化是保障煤矿安全高效开采的重要环节之一，常规顶板弱化技术存在一定的局限性。现有研究表明微波照射和液氮冻融可有效劣化岩石力学强度，但两者交互作用下砂岩致裂机理尚不清晰。本论文以陕北矿区煤系地层坚硬顶板中的致密砂岩作为研究对象，开展整体式与钻孔式微波-液氮交互致裂砂岩实验，研究微波-液氮交互作用下致密砂岩渐进破坏机理。所作主要研究工作如下：

       (1) 阐明了微波作用对含水砂岩弱化效果的影响。比较分析0.8 kW微波功率照射下不同含水率砂岩的升温速率，明晰了水在微波照射砂岩过程中的作用主要体现在快速升温、小幅升温和热平衡过渡这三个阶段。对比分析2.0 kW微波功率照射下不同含水饱和度砂岩的弱化效果，厘清了微波辐射下含水砂岩的爆裂原因，包括低孔隙率、微波整体性加热和蒸汽压急剧增大。

       (2) 揭示了整体式微波与液氮交互作用下砂岩裂隙时空展布特征。采用超声波测速仪，实时记录了微波与液氮交互致裂砂岩前后纵波波速数值，计算了微裂隙损伤因子，定量表征了砂岩内部损伤。获取刻画了不同交互次数下砂岩表面裂隙扩展情况，研究表明：微波加热与液氮冻结对砂岩裂隙发育具有显著促进作用，砂岩易形成复杂再生裂隙网络。

       (3) 明确了整体式微波与液氮交互作用下砂岩力学特性与能量演变规律。开展微波与液氮交互作用后砂岩单轴力学试验，精准测定加载过程声发射相关参数，分析砂岩受载破坏后的能量储存和耗散情况。研究表明：微波-液氮交互次数增多，砂岩峰值强度呈现线性降低，弹性模量显著减小，砂岩抵抗变形能力明显衰减，砂岩破坏形式由剪切破坏向拉剪混合破坏过渡，峰值强度对应的总应变能级和弹性应变能级大幅减小。

       (4) 探究了钻孔式微波与液氮交互作用下砂岩的潜在破裂特征。预制含孔洞砂岩试样，聚焦含水率差异性，开展微波照射和液氮压裂力学试验，结合声发射定位等手段，动态还原交互致裂下砂岩的二次破坏过程。研究表明：提高砂岩含水率，微波作用效果增强，损伤区域增大，有效缩短微波与液氮交互致裂时间，为陕北矿区煤系地层坚硬致密顶板微波与液氮交互压裂的现场应用提供理论基础支撑。

        In the "roof-coal-floor" structure of my country's coal-bearing strata, the hard roof accounts for about two-thirds. The weakening of the hard roof is one of the important links to ensure the safe and efficient mining of coal mines. Conventional roof weakening technology has certain limitations. Existing studies have shown that microwave irradiation and liquid nitrogen freeze-thaw can effectively deteriorate the mechanical strength of rocks, but the sandstone fracturing mechanism under the interaction of the two is still unclear. This paper takes the dense sandstone in the hard roof of the coal-bearing strata in the northern Shaanxi mining area as the research object, carries out integral and drilled microwave-liquid nitrogen interactive fracturing sandstone experiments, and studies the progressive destruction mechanism of dense sandstone under microwave-liquid nitrogen interaction. The main research work is as follows:

        (1) The effect of microwave on the weakening of water-bearing sandstone was clarified. The heating rates of sandstones with different water contents under microwave power of 0.8 kW were compared and analyzed, and it was clarified that the role of water in the microwave irradiation of sandstone was mainly reflected in the three stages of rapid heating, small heating and thermal equilibrium transition. The weakening effect of sandstones with different water saturations under microwave power of 2.0 kW was compared and analyzed, and the reasons for the explosion of water-bearing sandstone under microwave radiation were clarified, including low porosity, microwave overall heating and a sharp increase in vapor pressure.

        (2) The temporal and spatial distribution characteristics of sandstone cracks under the interaction of microwave and liquid nitrogen were revealed. An ultrasonic velocimeter was used to record the longitudinal wave velocity values before and after the microwave and liquid nitrogen interaction cracked the sandstone in real time, calculate the microcrack damage factor, and quantitatively characterize the internal damage of the sandstone. The expansion of cracks on the sandstone surface under different interaction times was obtained and characterized. The study shows that microwave heating and liquid nitrogen freezing have a significant promoting effect on the development of sandstone cracks, and sandstone is prone to form a complex regenerative crack network.

        (3) The mechanical properties and energy evolution of sandstone under the interaction of integrated microwave and liquid nitrogen were clarified. Uniaxial mechanical tests on sandstone after microwave and liquid nitrogen interaction were carried out, and the AE related parameters during the loading process were accurately measured to analyze the energy storage and dissipation of sandstone after load failure. The study showed that as the number of microwave-liquid nitrogen interactions increased, the peak strength of sandstone decreased linearly, the elastic modulus decreased significantly, the deformation resistance of sandstone was significantly attenuated, the failure form of sandstone transitioned from shear failure to tension-shear mixed failure, and the total strain energy level and elastic strain energy level corresponding to the peak strength were greatly reduced.

        (4) The potential fracture characteristics of sandstone under the interaction of drilling microwave and liquid nitrogen were explored. Sandstone samples with holes were prefabricated, and the differences in water content were focused on. Microwave irradiation and liquid nitrogen fracturing mechanical tests were carried out. Combined with AE positioning and other means, the secondary damage process of sandstone under interactive fracturing was dynamically restored. The study showed that increasing the water content of sandstone can enhance the effect of microwave action, increase the damage area, and effectively shorten the interactive fracturing time of microwave and liquid nitrogen. This provides a theoretical basis for the field application of microwave and liquid nitrogen interactive fracturing in the hard and dense roof of coal-bearing strata in the northern Shaanxi mining area.

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