Mechanical properties and fracture evolution law of rock-coal-rock combination

According to the characteristics of thin coal seam mining and surrounding rock deformation of coal and rock roadway, this paper explores the deformation law of coal and rock mass, and reveals the uneven deformation characteristics of rock and coal combinations based on the mechanical properties of different rock formations. Through the uniaxial loading test of “rock-coal-rock” combination with different height ratios, the loading failure law of coal rock combination is analyzed. The results show that the combination uniaxial compressive strength is mainly affected by the height of coal body, and increases with the increase of coal body height. With the increase of coal height, the failure mode of the specimen changes from tensile failure to inclined plane shear failure, finally displays as coal extrusion failure. Moreover, the sandstones at both specimen ends improve the strength of the coal body by constraining the coal in the middle section. This constraint in the middle section of the coal body is the smallest, and the influence on the middle section decreases rapidly with the increase of the height of the coal body. Finally, the method of uniaxial loading test and particle flow analysis are used to determinate whole process of fracture development. The results show that the fracture development process of the combination can be divided into four stages: fracture pore compaction stage, fracture generation and stable development stage, accelerated fracture development and penetration stage, and post failure stage. The micro cracks are formed in the area of coal defects at first(only when the height of the coal body is very small can the cracks come from the sandstone). With the increase of the load, the cracks randomly distributed in the coal body connect with each other and extend to the sandstone part, resulting in the ultimate failure. The initial cracks are mainly shear cracks caused by axial compression and end effect. When loading to about 0.8 of the uniaxial compressive strength of the specimen, the internal tensile cracks begin to develop rapidly, indicating that the strength of the composite begins to lose.