Study On Pressure Relief Effect Of Protective Layer Considering Residual Strength Of Coal

Protective layer mining is the preferred technology to prevent coal and gas outburst.It is of great significance to ensure safe production of coal mines to achieve the purpose of unloading damage and increasing permeability of coal and rock mass through protective layer mining.The mechanical properties of coal and rock mass,especially the mechanical properties after peak,are important factors affecting the pressure relief range of protective layer.In-depth study on the mechanical properties after peak can accurately judge the evolution of physical fields such as stress and deformation after mining of protective layer,and provide a basis for the analysis of pressure relief effect of protective layer.In this paper,the upper protective layer mining in 84 mining area of Qinan Coal Mine is taken as the research background,and the basic mechanical properties and post-peak strain softening behavior characteristics of tectonic coal and primary coal are obtained through laboratory test,theoretical analysis and field investigation.In order to solve the problem that the residual strength is difficult to measure,the residual strength estimation method is proposed based on the post-peak modulus and strain ratio.The experimental data in published literature are summarized and analyzed to determine the value range,and the numerical simulation is used to verify.The strain softening model considering residual strength is applied to the numerical simulation analysis of protective layer mining,and the pressure relief effect of protective layer mining is investigated combined with engineering practice.The main conclusions of this paper are as follows:(1)Uniaxial and triaxial conventional loading tests were carried out on raw coal and tectonic coal.The results show that the mechanical strength of raw coal is significantly higher than that of tectonic coal,and the uniaxial compressive strength,elastic modulus and cohesion of tectonic coal are 25.45%,11.95%and 42.86%of raw coal,respectively.Macroscopic failure modes are also significantly different.The primary coal generates macroscopic shear failure surface,and the tectonic coal shows obvious dilatancy characteristics.Both of the two coals showed strain softening characteristics after peak.Under the same confining pressure,the residual strength of primary coal was lower,and the post-peak softening was more obvious.The post-peak strength degradation parameters are used to calculate the conventional loading and pre-peak confining pressure unloading of tectonic coal.The results show that the confining pressure unloading significantly reduces the post-peak strength of coal.(2)The stress-strain curves of intectonic coal,tectonic coal,mudstone and sandstone are compared and analyzed.The results show that residual strength is the main influencing factor of post-peak stress-strain relationship.Aiming at the difficult problem of residual strength test,the residual strength estimation method is proposed based on the post-peak descent modulus and strain ratio,and the range of loading test data of coal and rock mass in more than 170 published literatures is summarized and analyzed.The strain-softening constitutive relation considering the influence of residual strength is established and applied to the numerical simulation verification of triaxial loading.The results are consistent with the measured results,which proves that the method is reasonable and reliable.(3)The simplified hydraulic punching condition was analyzed by FLAC3D with strain softening constitutive model and Mohr-Coulomb elastic-plastic model considering residual strength respectively.The results show that there is a deviation in the evaluation of pressure relief effect calculated by elastic-plastic model due to different residual strength values,which is not in line with the actual situation.The strain softening model considering residual strength should be used to reasonably analyze the evolution of coal seam physical field.The numerical simulation of the protected layer was carried out by using the strain softening model considering the residual strength,and the analysis showed that the vertical stress of the coal seam decreased by 45%in the range of 62°direction pressure relief angle,and the fully pressure relief was obtained.(4)The gas extraction amount of each drilling field of the protected floor gas drainage roadway during the mining period of the working face in the engineering field is counted.The analysis data show that the pressure relief angle along the strike is61.2°.It is comprehensively considered that the pressure relief angle along the strike is 61.2°?62°,which is slightly larger than the specified value of the pressure relief angle of 56°?60°in Prevention rules.Field test results and empirical calculation determine that the floor crack development depth of 6₁ coal seam mining is 20 m,and the pressure relief effect of protective layer mining is remarkable.The paper has 65 Figures,15 tables and 89 references.