Study On Morphological Characteristics And Shear Mechanics Characteristics Of Coal Structural Plane

Through obtain the three-dimensional morphology characteristics of the coal structure and shear mechanical parameters,we can provide necessary data support for the analysis and calculation of the coal deformation and stability.At present,most of the related tests all focus on the discontinuous structural plane in hard rock,there are little researchs on the structural plane of coal strucure.In order to obtain the morphological characteristics and shear mechanical properties of the coal structure,this paper study the coal body structure by laboratory tests,numerical simulation and theoretical analysis.The main research contents and conclusions are as follows:1)coal were taken from SiHe coal mine in JinCheng mining area,the coal containing structure was extracted from underground,the coal was processed into standard structure sample,then the surface morphology of the coal structure was scanned optically,and the direct shear test under different normal stress was conducted.The results show that:(1)maximum height of coal structure sample is 26.9 mm,maximum depth is 13.4 mm,the biggest ups and downs difference of structure surface is 26.9 mm,the minimum difference is 14.1 mm,inclination of the coal structure surface mainly concentrated in the 15 ° to 30 °,all tilt direction of coal structure showed obvious anisotropy,and there is an obvious advantage tilt direction.(2)the maximum and minimum value of JRC on coal structure sample are 19.2 and 9.3 respectively,and the JRC values are mainly between 14 and 18.(3)under low normal stress level,the normal stiffness values of coal structure range from 13 to 39.2 MPa/cm,shear stiffness value range from 3.1 to 26.6 MPa/cm,under secondary normal stress level,the normal stiffness values of coal structure range from 13.8 to 62 MPa/cm,shear stiffness value range from 4.3 to 18.2 MPa/cm,the average coal structure cohesion is 0.67 MPa,the average peak friction Angle is 37 °.According to the test results,with the increase of normal stress,the normal stiffness and shear strength of the coal structure both increase,and with the increase of shear deformation,the shear strength of the coal structural plane gradually increases and finally reaches a constant value.2)similar materials were used to make regular dip structure samples close to the strength of the coal body,and the dip angles of the bump were 30°,45° and 60°,respectively.direct shear test and cyclic direct shear test were carried out on the samples under different normal stresses.The results show that:(1)when the normal stress is 1~3MPa,the shear stiffness of the samples range from 9.4 to 44.6MPa/cm.When the normal stress is 1~5MPa,the shear stiffness of the structure range from 9.4 to 52MPa/cm.(2)under the same normal stress,the peak shear strength of the structure first increase and then decrease with the increase of the sawtooth fluctuation,the structure with dip Angle of 45° has the strongest shear resistance.Compared with monotone shear stage,under the cyclic shear test,peak shear strength is significantly reduced,between 42% to 71%.Meanwhile,the shear stiffness of the structure in the cyclic shear mode is also much lower than that in the monotone shear mode.(3)under the monotonic shear,the normal deformation of the structure will show the state of first shear contraction and then dilatation,while under the cyclic shear,the normal deformation of the structure will remain in the state of shear contraction all the time.The failure pattern can be divided into cutting-edge disruption of model structure,along the central convex body cut,cut along the convex body root three conditions,when the normal stress is small,the structure of the climbing effect more apparent,normal stress is larger,the structure of the climbing effect is constrained convex body damage form gradually from the tip damage to the central cut and cut the roots of the transition.3)based on the optical scanning results,PFC software was used to create the numerical structure model of the coal and the numerical model including the shear.After the completion of the mesoscopic parameter calibration of the model,direct shear tests were carried out on the model under different normal stresses.The results show that:(1)the peak shear stress,shear crack number and total crack number of the model structure all increase with the increase of normal stress.The change of shear direction will lead to the obvious anisotropy of the peak shear strength of the structural plane.When the shear direction is opposite to the dominant inclination direction of the structure,the shear strength of numerical model is the largest,which is significantly higher than that of the structural plane in other shear directions.(2)the presence of cleavage in the coal will significantly reduce the peak shear stress value of the structure,with the maximum reduction range of 1.9Mpa.Moreover,the number of shear cracks and the total number of cracks generated in the model will increase due to the presence of cleavage.The cracks are mostly distributed at the outcrop where the cleavage intersects with the model and at the intersection of the cleavage and the structure.(3)when the length of the structure and sawtooth height are constant,the dilatancy deformation value,peak shear stress,shear crack number of model structure presents trend of first increase and then decrease with the increase of bump dip,when the bump dip is 30 °,the dilatancy deformation value is maximum,when the bump dip is 50 °,the peak shear stress and cracks number of structure reached the maximum.