| Coal is the main energy source in China.The reserves of coal resources in the northwest region account for nearly 80% of the total reserves in China.The coal storage in the northwest region has characteristics including a large coal seam thickness and a large mining space.Under the mining conditions of thick coal seams with multi-layer hard roofs,super-strong mine pressure disasters are easily formed on working faces.To this end,the 1307 fully-mechanized caving faces of Zhaoxian Coal Mine in Shaanxi Province are taken as the engineering background in this paper,and comprehensive research methods including physical similarity simulation,theoretical analysis,numerical simulation calculation,laboratory test and field test are used to study the double thick sandstone roofs,the law of overlying rock movement in large-space stope,the mechanism of strong rock pressure appearance,the weakening mechanism of advanced deep hole pre-splitting blasting,and the engineering practice of deep hole pre-splitting blasting roof cutting.The main research results are as follows:(1)The structural characteristics and migration law of overlying strata in large-space stope with double thick sandstone roofs.In the mining of fully-mechanized caving faces,it is difficult to fill the goaf with the caving rock blocks of the immediate and main roof,forming a large space separation between the high thick sandstone layer and the lower rock layer,inducing rotary deformation fractures of the high thick sandstone layer,and forming ’large and small’ periodic fractures as well as the caving of the main roof of the overlying strata and the higher thick sandstone layer.When large periodic fractures occurs at the overlying strata in the stope,the stress of the overlying strata increases greatly and sharply,and when small periodic fractures occurs at the basic roof,the overall change range of the stress value of each rock layer is obviously smaller than that of large periodic fractures at the high thick sandstone layer.(2)The behavior mechanism of strong strata in the double thick sandstone roof largespace mining field.The evolution characteristics of the overlying strata structure in largespace stope are obtained through physical similarity simulation,and the mechanical model of thick sandstone rock beams in large-space stope is constructed.The fracture mechanical characteristics of the basic roof(low thick sandstone layer)and the high thick sandstone layer of overlying strata in stope are analyzed.Based on the key stratum theory,using the mechanical model of ’step-masonry’ rock beam structures,the calculation expression of the working resistance of the broken support in the high and low thick sandstone layer of the large-space stope with double thick sandstone roofs is analyzed and obtained.It is predicted in the example analysis that the 1307 fully-mechanized caving faces of Zhaoxian Coal Mine have an excessive resistance on the hydraulic support working face during the ’large and small’ periodic weighting period,which is easy to cause accidents such as the crushing of working faces.(3)The propagation law of dynamic shock waves during ’coal-sandstone’assemblage.The stress waves propagate from coal to sandstone.The peak strain of coal and sandstone samples increases with the increase of impact pressure,and that of coal samples is between 25με-100με,which is only 10%-20% of the strain of sandstone samples.When the stress waves continue propagating to the rock samples,the peak stress waves on them decrease sharply.On the contrary,the stress waves propagate from’sandstone to coal’,and their peak attenuation is less.With the increase of impact pressure,the peak value of stress waves of rock specimens under impact load is smaller than that of coal specimens,and the peak value of stress waves of rock specimens under an impact pressure of 0.5MPa is gradually larger than that of coal specimens.(4)The weakening mechanism of blasting damage of sandstone body.Under blasting disturbance,the excitation current of the rock mass around the center of the blasting source decreases gradually from the ring to the outside.Along the vertical direction,the damage degree to the upper part of the test body is weaker than that to the middle part,and that to the lower part is the smallest due to the constraint of the base boundary.The tensile effect of detonation waves plays a leading role in the damage and failure of rock mass.The stress peak boundary is obvious among the three areas,namely the ’upper’,’middle’ and ’lower’ area.The peak value of residual stress in the middle area is the smallest,that in the upper area is the second,and that in the lower area is the largest.With the increase of distance from the explosion source,the peak value of residual stress gradually increases.The failure of sandstone is controlled by the ’X’ type stress concentration caused by the transformation of detonation energy waves from compression waves to reflection tensile waves.The spacing of sandstone blasting charges has direct influence on the development of cracks and the degree of damage.(5)Large-space stope structure control and mine pressure control technology for double thick sandstone roofs.The deep hole pre-splitting blasting technology is used to actively control the overburden structure of large-space stope with double thick sandstone roofs.The initial weighting step of 1307 working faces in Zhaoxian Coal Mine is 58.5-61.7m,and the dynamic load coefficient is 1.18.The step distance of small periodic weighting is 10.1 ～ 15.9m,and the average step distance of weighting is 2.7 ～ 5.9m.The average pressure step of large periods is 109 m,the continuous step can reach 14 m,and the pressure strength is 15,392 k N.Through the roof cutting of advanced deep hole blasting,the first and periodic breaking of the basic roof and high hard roof is effectively controlled,realizing the control of the large-space mining field pressure appearance of double thick sandstone roofs,and the problem of strong mine pressure disasters caused by the instability of double-layer hard roof structures can be successfully solved.The research results of this paper reveal the mechanism of strong mine pressure in the large-space mining of double-layer hard roofs and the weakening mechanism of deep hole pre-splitting blasting in hard roofs.The success of engineering application provides theoretical and technical reference for the control of strong mine pressure disasters caused by the instability of multi-layer thick hard roof structures,which has high popularization value.105 figures,26 tables and 172 references. |
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