Study On Coal Pillar Effect Of Multi-seam Mining And Stability Control Countermeasure Of Gob-side Entry

Close-range multi-seam mining is unavoidable in coal mine production,and the stability of surrounding rock of gob-side entry under upper coal seam goaf is the key problem needed to be solved for the safety production of lower coal seam.Based on the research background of track transportation tunnel of 9306 working face of Nantun Coal Mine,aiming at the difficult problem of large deformation control of surrounding rock,this paper systematically studies the coal pillar effect of multi-seam mining and the stability control countermeasures of gob-side entry under goaf by means of field engineering investigation,laboratory experiment,theoretical analysis,numerical simulation and engineering application.The main conclusions are as follows:(1)Through detailed analysis of geological conditions and mining conditions of track transportation tunnel of 9306 working face,it demonstrates that the roadway belongs to the gob-side entry under goaf of close-range multi-seam.This study summarizes the characteristics of deformation and failure of the roadway and main influencing factors,and points out that the existing support can not meet the need of stability control of roadway.It is essential to develop more reasonable and effective stability control countermeasures of the gob-side entry under goaf.(2)The load distribution law of surrounding rock under the condition of insufficient and sufficient collapse of overlying strata after mining in the upper coal seam is analyzed theoretically,and the mechanical models of floor strata under coal pillars under the two different overburden strata collapse conditions are established.The stress calculation formulas of the floor strata are derived when the floor is a single homogeneous stratum and a transversely isotropic stratum respectively.The stress distribution law of the floor under the coal pillar of the two different overburden caving states is studied by numerical simulation.Simulation results show that the stress concentration is too large when the overburden rock collapses insufficiently,which is not in accordance with the engineering practice,while the model when the overburden rock is fully collapsed is more practical.The influence law of goaf height,horizontal lateral pressure coefficient and pillar width on the stress distribution of floor strata are also analyzed by numerical simulations.It demonstrates that the goaf height and pillar width have a greater influence on the stress distribution in the floor rock,while the lateral pressure coefficient has a greater influence on the horizontal stress distribution,and the influence of these three factors on the shear stress is relatively small.The stability of surrounding rock of the lower coal seam is greatly affected by the residual coal pillar effect in the upper goaf.(3)Two-dimensional physical model experiments of deformation and failure process of the gob-side entry under goaf and multi-seam mining were carried out.The evolution laws of displacement,stress and temperature field of the roadway surrounding rock during the excavation of upper and lower coal seams and gob-side entry were obtained.Comprehensive analysis results showed that the roof and floor strata of 3 lower coal seam were damaged seriously before the excavation because of the multiple loading and unloading processes such as stress concentration and release during the mining process of 3 upper coal seam.In addition,the mining of working faces and the excavation of roadway of 3 lower coal seam penetrate the previously stable goaf of 3 upper coal seam,which aggravates the deformation and destruction of the roadway surrounding rock.The surface infrared temperature changes of surrounding rock can correspond to its displacement and stress changes.Thus,temperature changes can reflect the deformation mechanics characteristics of the surrounding rock.The stress concentration of surrounding rock caused by multiple mining in upper and lower coal seams leads to the development and expansion of cracks in surrounding rock,which is the main reason for the decrease of surrounding rock temperature near the roadway,and the range of temperature reduction can represent the range of displacement and deformation of the roadway surrounding rock.(4)The three-dimensional numerical calculation model of multi-seam and multi-face mining is established.The stress evolution law of the roadway surrounding rock during the mining of working faces in the 3 upper coal seam and the stress distribution and deformation law of the roadway surrounding rock during the mining of working faces and the excavation of roadway in the 3 upper coal seam are obtained.The stress distribution of the upper and lower coal seam working face and roadway excavation shadow are determined,based on the comparison and analysis of physical model test results,the influence of the excavation of upper and lower coal seam working face and roadway on the stress distribution of surrounding rock of gob-side entry is determined.The key positions of deformation and failure of gob-side entry are gob-side and roof.The mechanism of deformation and failure of gob-side roadway under goaf is revealed,that is,high gravity stress,horizontal tectonic stress and multiple mining stress overlap each other to produce intense stress concentration,coupled with uncoordinated stress and deformation between traditional supporting materials and supporting structure and surrounding rock,resulting in large deformation and failure of surrounding rock.(5)The main measures to control the surrounding rock stability of the gob-side entry under goaf are the reasonable layout of the gob-side entry,the reasonable width of coal pillar,the reasonable roadway support mode and the effective control of key positions.By analyzing the interaction process between the support and surrounding rock of roadway,the mechanism of coupling support with common-constant resistance and large deformation is revealed and the formulas for calculating the ultimate deformation and constant resistance of constant resistance anchor(cable)are derived,Two design schemes of common-constant resistance coupling support are put forward.The stress and deformation evolution law of the roadway surrounding rock during roadway driving and working face mining are compared and analyzed by numerical simulation,and the optimal support scheme and support parameters of the gob-side entry under goaf are determined.(6)Based on the above research results,the common-constant resistance bolt/cable coupling control strategy suitable for gob-side entry of 9306 working face in Nantun Coal Mine is put forward and the field engineering application is carried out.The monitoring data of rock pressure manifestation such as deformation of the roadway surrounding rock,the force of bolt/cable and advance support are comprehensively analyzed.Combined with the engineering application results,the successful application of the coupling control strategy of common-constant resistance and large deformation in the gob-side entry under goaf is verified.The research results can provide reference for the stability control of the surrounding rock of gob-side entries under multi-seam mining goaf under similar conditions.