Study On The Stress Deviator Invariant Distribution Of Gob Floor And Its Application

In the process of closely coal seams mining,The roadway location of the lower coal seam is related to the stability of the roadway itself,the layout of the mining area and the utilization rate of resources.For the reasonable arrangement of the lower coal roadway in the downward mining process,the traditional research mostly follows the methods of experience,stress propagation angle or vertical stress distribution.However,these methods failed to take into account the horizontal and shear stresses of the gob floor,and did not conduct in-depth research on the surrounding rock distortion energy,shear stress and strain state.However,these conditions are also important reasons for the deformation and damage of the surrounding rock and the instability of the roadway.herefore,the method of determining the location of the roadway by simply relying on the vertical stress distribution has a certain one-sidedness,and a new standard is urgently needed to measure the stability of the surrounding rock of the gob floor,so as to provide a basis for the layout design of the lower coal roadway in the process of closely spaced coal seams mining.In elastic-plastic mechanics,it is mentioned that the stress deviator is the main reason for the plastic failure of the material.The second invariant of the stress deviator can represent the distortion energy and shear stress of the material,and the third invariant of the stress deviator can represent the strain state of the material,so it is very suitable as a standard to measure the stability of the surrounding rock of the gob floor.Based on the distribution characteristics of typical closely spaced coal seams,this thesis deduces the mechanic formula of the second and third invariants of the stress deviator of the single gob floor by means of field testing,theoretical derivation,numerical simulation,similarity simulation and engineering monitoring.and revealed the mechanical response of the second and third invariants of floor stress deviator under different influencing factors through theoretical and numerical methods,and analyzed the evolution characteristics of second and third stress invariants during the mining process of upper and lower coal seams.Finally guide the practical engineering application.The main conclusions are:（1）Research on the engineering background,original rock stress and petrophysical and mechanical properties of closely spaced coal seams.The distance between the 02#and 2#coal seams being mined in Malan mine is about 7m,and the inclination angle is close to horizontal,which is a typical closely spaced coal seams.In view of this background,this thesis carried out on-site in-situ stress test and measurement of surrounding rock physical and mechanical parameters in the South No.6 mining area.The relevant data obtained from the field and laboratory provided a reliable basis for the study of the stress deviator invariant of the gob floor.（2）The derivation of various stresses and stress deviator invariant formulas of the floor under a single gob.The lateral support pressure of the gob is simplified as a linear load,and the mechanical model of the stress increment of the floor is established.Using the assumption of a plane semi-infinite elastic body,the vertical stressσ_z,the horizontal stressσ_x,the shear stressτ_xz,maximum and minimum principal stressesσ₁,σ₃ and three principal values s₁,s₂,s₃ of stress deviator under the gob floor are obtained by integral calculation.And according to the relationship between the stress deviator invariant and s₁,s₂,s₃,the formulas of the second and third invariants J₂and J₃ of the gob floor stress deviator are derived.According to the formula,the main factors affecting the stress deviator distribution of the gob floor are determined.（3）Numerical research and analysis on influencing factors on the distribution of stress deviator invariant in the floor under single goaf.According to the analysis of theoretical and numerical calculation results,it is found that the depth H where the coal seam is mined has no obvious influence on the distribution characteristics and transfer laws of J₂and J₃,but will cause the values of J₂ and J₃ to change;The horizontal pressure coefficientλwill change the distribution range of J₂,J₃ concentration area and the J₂,J₃ value,Whenλ<1.0,the J₂ concentration area is mainly distributed under the solid coal pillar,and the floor J₃ is mostly positive（compressive strain state）.Whenλ>1.0,the J₂ concentration area transfers to the gob floor,and the floor J₃ is mostly negative（tensile strain state）.With the increase of∣1-λ∣,the value of J₂,J₃ and the range of the concentration area will also change accordingly.The change of the inclination angleθof the coal seam will cause the distribution characteristics of the stress deviator invariants J₂,J₃ to rotate in the same direction,and the rotation angle is smaller thanθ.（4）Research on the distribution characteristics of the plastic zone of the gob floor.Through the theoretical results about the combination of the maximum and minimum principal stress formulas and the Mohr Coulomb failure criterion,and the post processing results of FLAC^3Dnumerical calculation,it is found that under the same stress environment,the larger C andφof the surrounding rock,the higher the strength of the rock mass and the smaller the plastic zone;under the same rock mass strength,the greater the mining depth H of the coal seam,the larger the floor plastic zone;The lateral pressure coefficientλgradually increases from less than 1.0 to greater than 1.0,and the plastic zone of the floor will gradually shift from below the solid coal pillar to the bottom of the gob floor,and the larger the deviation betweenλand 1.0,the larger the plastic zone.With the change ofθ,the area of the plastic zone does not change much,but the distribution range will rotate in the same direction withθ.It can be seen that the floor plastic zone presents a distribution characteristic that is basically consistent with the stress deviator second invariant J₂.（5）Research on the evolution law of the stress deviator invariant in the surrounding rock during the mining process of the lower coal seam.The post-processing results of numerical simulation show that during the mining process of the upper coal seam,the narrower the width of the coal pillar,the more serious the concentration and superposition of the J₂,J₃ in the coal pillar area,and the lower the stability of the surrounding rock.During the mining process of the lower coal seam,the variation characteristics of J₂,J₃ are not obvious when the upper and lower mining areas are staggered,which means that the overall stability of the surrounding rock is not damaged.When the upper and lower mining areas overlap or are staggered,J₂ on one side of the coal pillar will suddenly drop,but J₂ at the boundary of the lower coal mining area will rise sharply due to stress concentration,and the negative value area（tensile strain area）of J₃ will also increase,which is extremely unsafe for mining construction.（6）Research on similar simulation experimental of closely spaced coal seams mining.Similar simulation experiments of closely spaced coal seams mining show that under the condition of high horizontal stress,the stress distribution of the gob floor is uneven,the displacement is large,the bottom sag range is large,the floor stability is low.When the boundary of the lower coal mining area crosses the edge of the coal pillar,the stress in the coal pillar area rises sharply,the displacement of the surrounding rock increases asymmetrically,and the surrounding rock risk of instability and damage increases.In addition,the acoustic damage results of the floor surrounding rock also show the same distribution as the stress deviator invariant.（7）Engineering practice and application effect research.Combined with the actual geological parameters of the project,the study on the stress deviator invariant guides the layout of the 10607 gas drainage roadway in Malan Mine.The results of on-site drilling peep experiments and mine pressure monitoring show that the surrounding rock fissures at the 10607 gas drainage roadway are less developed and there is no obvious deformation and damage,which verifies the feasibility and rationality of the research in this thesis.