Study On Strong Rock Pressure Behavior Mechanism And Pressure Relief Control Of Gob-side Entry In Caojiatan Mine With Extra Thick Coal Seam

Shenyu mining area in China is rich in coal resources,and most of the coal seams are extra thick coal seams.The extra thick coal seams are disturbed greatly in the process of fully mechanized top coal caving mining,the ground pressure appears violently,and the surrounding rock deformation of roadway is serious.In particular,the deformation mechanism of roadway surrounding rock under the superposition of ground pressure is more complex due to the influence of two mining movements in the mining process of goaf side roadway in double face stope.Therefore,aiming at the bottom heave deformation of return air roadway under strong mining in extra thick coal seam of caojiatan mine,this paper systematically studies the overburden movement law of the whole stope and the deformation control of roadway surrounding rock through the research means of field investigation,indoor test,theoretical analysis,numerical simulation and engineering practice,and finally forms a set of key technologies suitable for roof cutting and pressure relief roadway protection in extra thick coal seam.The main research results are as follows:(1)Based on the key layer theory,the fracture characteristics of the double key layer in the overburden of the stope are analyzed,and the mechanical models of the transverse and longitudinal pressure bearing structure under the influence of double key layer are established.The instability of the transverse pressure bearing structure is divided into three kinds: friction squeeze force double control,friction force control and squeeze force control,and the instability of the longitudinal pressure bearing structure is divided into friction slip instability,squeeze rotation The analytical expressions of system instability in each state are solved,and the effects of fracture angle,key block degree,friction angle and other parameters on system stability are analyzed.At the same time,combined with the energy dissipation theory,the behavior law of impact dynamic pressure under the condition of simultaneous and non simultaneous breaking of double key layer pressure bearing structure is analyzed.The results show that the breaking of near-field single key layer is the main reason for small cycle pressure in stope,while the simultaneous breaking of near-field and far-field double key layers is the main reason for large cycle pressure in stope Strong ground pressure behavior mechanism of small periodic pressure.(2)Based on the elastic theory,the mechanical model of floor stress distribution under asymmetric load is established,the variation laws of floor vertical stress,horizontal stress and shear stress with depth and horizontal position are analyzed,and the mechanical mechanism of roadway asymmetric floor heave is revealed.According to the stress characteristics of roadway composite floor,the deformation mechanical models of soft and hard strata of composite floor are established by using slip line theory and catastrophe theory.The soft rock flow floor heave is divided into three types: angular asymmetric floor heave,local asymmetric floor heave and global asymmetric floor heave.Then,combined with energy dissipation theory,the total energy of floor system is divided into floor compression deformation accumulated energy The sufficient and necessary conditions for sudden instability of hard rock are solved,and the cooperative asymmetric deformation mechanism of composite floor under asymmetric load is revealed.(3)Based on the pull rod damage theory and considering the Weibull distribution of rock micro elements,the damage mechanical model of double hole shaped charge blasting is established,and the correlation between damage degree and shaped charge coefficient and blasting center distance is analyzed.The results show that the double hole spacing and damage degree are positively correlated with shaped charge coefficient and explosive diameter,while the damage degree is negatively correlated with blasting center distance.At the same time,LS-DYNA is used to numerically analyze the crack propagation characteristics under the conditions of non concentrated energy blasting and concentrated energy blasting,and the attenuation laws of effective stress and particle vibration velocity are analyzed.The results show that directional concentrated energy blasting can effectively block the transmission of stress in the non concentrated energy direction,and then increase the energy accumulation in the non concentrated energy direction,so that the effective stress The particle vibration velocity attenuates more slowly in the energy accumulation direction and promotes the crack propagation in the energy accumulation direction,which reveals the mechanical mechanism of directional crack propagation in energy accumulation blasting.(4)Based on the pressure distribution characteristics of the whole stope,the horizontal and vertical pressure relief mechanism of the roof cutting stope by shaped charge blasting is analyzed.After the roof cutting,the transverse large cantilever structure of the stope overburden is interrupted,which makes it collapse and expand,enriches the goaf,supports the overlying strata,and then reduces the concentration of coal pillars laterally;The roof of the longitudinal stope changes from fixed support to simple support,which makes the roof easier to collapse,shortens the collapse step,and then reduces the lateral bearing pressure of the longitudinal working face.At the same time,the numerical simulation analysis of roof cutting is carried out by using3 DEC software.The research shows that the small cycle pressure step distance of the double key layer stope is 20 m and the large cycle pressure step distance is 40 m.When the stope is pressed with large cycle,the stress concentration at the end of the return air roadway in 122108 working face is the most obvious.Then,the pressure relief effect under different roof cutting directions,different roof cutting angles and different roof cutting heights is analyzed,The results show that the two-way pressure relief effect of roadway side can be achieved by cutting the roof in the direction of working face;The cutting angle mainly affects the bearing capacity of the short cantilever structure formed after cutting.A larger cutting angle is more conducive to the rapid collapse of the roof and better pressure relief effect;The key stratum is the main rock stratum for the transmission of ground pressure.When the cutting length does not interrupt the key stratum,the stress in the end area decreases more with the increase of the cutting length,but the positive correlation effect becomes less obvious after cutting the key stratum.At the same time,the cutting length affects the crushing and swelling filling rate of gangue.The rational use of the crushing and swelling characteristics of gangue is the key to the control of stope ground pressure.(5)Based on the theoretical analysis and numerical calculation results,the key parameters such as roof cutting direction,seam cutting angle,roof cutting height,blast hole spacing and charge quantity are optimized,four schemes are designed,and the engineering application of roof cutting and pressure relief is carried out on site.The monitoring results show that there are large and small periodic pressure in the stope,which verifies the rationality of the theoretical and numerical analysis.The results of ground pressure change after roof cutting show that the roof cutting has little impact on the ground pressure at the far end of the working face,but has an obvious pressure relief effect on the end of the near end return air roadway.The abutment pressure,pressure step and coal stress concentration factor after pressure relief are significantly reduced,while increasing and strengthening the blasting hole and reducing the NPR anchor cable support density are conducive to the roof collapse of the goaf after roof cutting,It is more favorable for pressure relief effect and bottom drum treatment.Under the optimal roof cutting test scheme,the coal stress is reduced by 50 ? 55%,and the floor heave is reduced by about 66.7 ? 90% on average.The implementation of this technology achieves the purpose of pressure relief and floor heave control,forming a set of roof cutting pressure relief roadway protection technology system for extra thick coal seam.