Determination of coal cracking initiation strength under uniaxial compression and analysis of its key factors

In order to explore the key influencing factors of coal cracking strength and its response sensitivity to various factors，the crack volume strain method and acoustic emission method were used to determine the crack initiation strength of coal samples under uniaxial load. Based on the CT scanning results of coal samples，the distribution state of internal structural defects was obtained by binarization，and the DFN grid model was established. The numerical core was reconstructed by the synthetic rock mass method(SRM)，the failure process of coal sample was reproduced by un⁃ confined loading，and the fracture initiation strength，mode and location of numerical core were obtained. The influen⁃ cing factors of cracking strength were divided into three categories:matrix framework，matrix interaction and prima⁃ ry defects. Two indexes were selected from each of them，and single factor and interactive analysis were conducted by response surface method. The results show that the crack initiation strength of coal is 10.06 MPa and 9.26 MPa re⁃ spectively by crack strain method and acoustic emission method，and the numerical core crack initiation strength is 9.56 MPa，which is 47.2% of the peak strength. The results of single factor analysis show that the sensitivity of coal cracking strength to each factor is DFN grid density > matrix particle elastic modulus = PBM model cohesion > SJ model friction coefficient > matrix porosity，indicating that the primary defects have the most significant effect on the fracture initiation strength. The results of multi - factor interaction analysis show that the interaction re⁃ sponse curve of particle elastic modulus and PBM model cohesion has the highest distortion degree， which indicates that the interaction between them on coal sample cracking strength is more obvious. The interaction response surface between micromechanical factors and DFN grid density is steeper，and the projection isoline is more dense in the direction of DFN grid density，which proves that the internal structure characteristics of coal sample have great influence on coal sample cracking strength. The effect of mechanical properties is more significant，which is consistent with the results of single factor analysis. The field measurement shows that the coal body in the shallow part of the roadway has a high degree of fragmentation and low strength. After grouting and filling structural defects， the integrity，homogeneity and strength of coal body are improved.