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As mines become deeper, the potential for coal and gas outbursts in deep rock cross-cut coal uncovering is enhanced. The outburst precursors are unclear, which restricts the effectiveness and reliability of warning systems. To reveal the evolution characteristics of coal and gas outburst precursor information in deep rock cross-cut coal uncovering, briquette specimens are constructed and experiments are conducted using a self-developed true triaxial outburst test system. Using acoustic emission monitoring technology, the dynamic failure of coal is monitored, and variations in the root mean square (RMS) of the acoustic emissions allow the effective cracking time and effective cracking gas pressure to be defined. These characteristics are obviously different in deep and shallow coal. The characteristic parameters of gas outburst exhibit stepwise variations at different depths. The RMS and cumulative RMS have stepped failure characteristics with respect to changes in gas pressure. The characteristic parameters of coal failure are negatively correlated with the average in-situ stress and effective stress, but positively correlated with the lateral pressure coefficient of in-situ stress and the critical gas pressure. The transition characteristics are highly sensitive in all cases. The critical depth between deep and shallow coal and gas outbursts is 1700 m. The expansion multiple of acoustic emission intensity from the microfracture stage to the sharp-fracture stage of coal is defined as the outburst risk index, N1. For depths of 1100–1700 m, N1 ≥ 7 denotes a higher risk of outburst, whereas at depths of 1700–2500 m, N1 ≥ 3 indicates enhanced risk.

In order to obtain the weakening influence of coal seam water injection on coal and gas outburst, taking the 1100 m outburst coal seam in Sunjiawan Coal Mine in Fuxin of China as the research object, coal and gas outburst energy conversion model was established. The influence of water content on coal and gas outburst test was studied by using a self-developed simulation experimental system. The relationships between water content and critical gas pressure, outburst strength and energy conversion rate were analyzed. The results show that gas pressure is the main power source of coal and gas outburst, and the relationship between water content and critical gas pressure increases exponentially. When the water content increases from 0% to 3% and 6%, the relative strength of outburst decreases by 9.6% and 1.2%, respectively. With the increase in water content, the relative outburst strength and absolute outburst strength will gradually decrease. Gas internal energy is the key factor affecting coal and gas outburst energy which is closely related to the outburstforming and the outburst-triggering process. As the water content increases, the outburstforming energy and the outburst-triggering energy increase linearly. However, the energy conversion ratio decreases linearly and the less prone to coal and gas outburst. KeywordsCoal and gas outburst • Water content • Energy conversion rate • Critical gas pressure • Outburst strength

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To investigate the effects of the different conditions of water cooling at high temperature on the tensile strength and split surface roughness characteristics of hot dry rock in the Songliao Basin, the physical characteristics, tensile strength, and split surface roughness of granite under different conditions of water cooling at high temperature were studied. In addition, the relationship between tensile strength and split surface roughness under different conditions of water cooling at high temperature was established. The results showed the following: (1) as the rock temperature increased, the number of water injection cycles increased or the water injection temperature decreased, the mechanical properties of the specimen weakened, and the roughness of the split surface increased. The threshold for the effect of the rock temperature on the split surface roughness of granite was 300°C. At 400°C, the tensile strength greatly decreased. At 600°C, the tensile strength, height mean square error (MSE), fluctuation difference, roughness coefficient, and roughness profile index of the specimen were 0.21, 2.51, 2.57, 8.92, and 1.06 times those at 100°C, respectively. After five heating-cooling cycles, the tensile strength, height MSE, fluctuation difference, roughness coefficient, and roughness profile index of the specimen were 0.57, 1.33, 1.49, 1.29, and 1.01 times those after one cycle, respectively. (2) The roughness angle calculated using the root mean square of the first derivative of the profile was always greater than that derived using the roughness profile index. In addition, the higher the temperature, the lower the water temperature, the more high-temperature-water cooling cycles, the greater the difference between the above two calculations. (3) When the tensile strength varies, the factors affecting the variation in the height MSE and surface roughness were in the following descending order: rock temperature, number of heating-cooling cycles, and water temperature. In addition, the higher the tensile strength, the lower the roughness coefficient. This study is expected to provide a reference for the selection of different conditions of water cooling at high temperature for thermal recovery in the Songliao Basin.