Mechanical Properties Of Cracked Coal-rock Mass And Its Macro And Meso Mechnism Of Rockburst

Rockburst is a sudden destruction of coal-rock induced by crack initiation,expansion and penetration.In this paper,the mechanical characteristics of cracked coal-rock and its macro-and meso-mechnism of rockburst were discussed.The mechanical response of crack slip,the meso damage mechnism and dynamic fracture characteristics of cracked coal-rock,the macro mechanical mechnism of rockburst,the burst failure of roadway surrounding rock and the case analysis and application of the project were systematically researched by the means of theoretical analysis,numerical test,physical and mechanical test and engineering practice.The innovative results are as follows:(1)The direct shear numerical test of cracked coal-rock indicates that four factors including roughness,size effect,shear history and normal stress have influence on normal and shear mechanical properties,while shear rate has a weak influence on them.The results of direct shear test are fitted and analyzed,and the revised Barton's criterions are obtained as(?)(2)Constructing the discriminant index of coal-rock damage,heterogeneous and cracked coal samples based on UDEC development,the meso damage mechnism and dynamic fracture characteristics of cracked coal samples were analyzed.The meso damage of intact coal samples is shear cracks and induces shear failures.With the increasing ratio of height to width and heterogeneity,the fracture characteristics are changed from shear and splitting to global shear,the tensile and damage cracks decrease,the coal sample is transformed from ductility to brittleness and the impact tendency is enhanced.The hole-defects makes the fracture characteristics of coal samples change from global shear to multiple shears and splits,the proportion of damage cracks decreases,and the compressive strength decreases rapidly.With crack dip increasing,the tensile strength and compressive strength of coal samples show a"U"shape of reducing-increasing trend.The primary inclined and through cracks can obviously reduce the impact tendency of coal samples,promote the initiation,expansion and penetration of secondary cracks;the rock bridges between non-through cracks have played a role in reducing the crack growth,only resulting in the local slip of coal samples so that the reduction of impact tendency is limited.In crack network models,the more discrete the values of crack dip are,the stronger the impact tendency of coal samples is.(3)In the static and oscillatory stress combined tests,the stress drops of coal-rock samples with macro-crack defects and the high value points of the Acoustic Emission(AE)counts have good coupling.The crack expansion of sample is dominated by tensile,tensile-shear composite and shear in the stage of compaction,plasticity and failure.The loading of sinusoidal oscillatory stress provides the inertial force for meso-crack expansion,accelerating its initiation,expansion and penetration,which increase the strain softening characteristic and the impact tendency of coal samples.The increasing dips of macro-crack defects and length of rock bridge make the strength of samples show a"V"trend of reducing firstly then increasing.The sinusoidal oscillation stress makes the peak stress and strain of samples fluctuate,which affects the mechanical properties,such as strength and elasticity modulus.The coupling analysis of stress and AE signal indicates that the rapid reduction of b value(crack scale)and the statistical value Z>2(crack penetration probability)can be used as the precursor information for samples failure.The coupling degree of multi information,like micro fracture,displacement and stress,indicates that the fracture of samples with inclined cracks is mainly composed of shear and splitting,and that with horizontal or vertical cracks is mainly split.The AE CT inversion results show that the micro fracture and the high wave velocity zone are strip coupled before peak stress,while the micro fracture is evenly distributed and the high wave velocity zone disappears after peak stress,which reflects the characteristics of shear failure of the sample.(4)The models of coal thickness change,floor layer and Maxwell were established to carry out mechanical analysis.The vertical stress distribution rule indicates that the shear stress formed by the difference value is the main control factor for the failure of coal-rock with horizontal cracks.And the shear stress index is established to identify the rockburst risk of coal thickness change area.The monitoring results of stress and energy show that the horizontal stress is the main control factor leading to the stratiform splitting instability of the coal-rock with vertical cracks.The failure of coal-rock with inclined crack is stick-slip,forming the chain reaction of stress release–accumulation–rerelease and structural surface unlocking–locking–relocking,which causes stress wave and make cracks further extend.(5)The numerical models of roadway roof,floor and ribs were established to obtain the multi-information,such as stress,displacement and crack in the process of burst failure,and the coupling analysis is carried out.It is concluded that the impact stress wave increases the number of meso-cracks in the surrounding rock,and accelerates its initiation,expansion and penetration,which causes roof subsidence,floor heave and ribs deformation instantaneously.In comparison with the single prop,the bolts and lining supports can control the expansion and penetration of meso-cracks and control the burst failure of surrounding rock to a certain extent.The multi-information coupling of the roof,floor and ribs in the process of burst failure indicate that the crack and maximum principal stress in surrounding rock show"four arches"distribution.(6)Using the microseismic(MS)system to monitor the tremors in Zhaolou Coal Mine,the coupling characteristics between the source distribution in the 1305 working face and the coal seam bifurcation zone were analyzed.It is concluded that the shear stress is the main control factor leading to the slip instability of the coal-rock mass with horizontal crack.The wave velocity,stress and wave velocity gradient are analyzed by the vibration wave CT inversion in the 1306 working face.According to the distribution characteristics,the horizontal stress is the main control factor for roadway floor heave with vertical crack.The multi-parameter early warning indices of rockburst are proposed based on crack expansion.According to the"four arches"distribution characteristics of crack and stress in the surrounding rock,a comprehensive control system for controlling the crack expansion in roadway surrounding rock is proposed.