Deformation and failure characteristics of composite coal mass

This study analyzed stress characteristics and mechanical properties and compared the similarities and differences of composite coal mass with soft and hard coal mass in deformation and failure characteristics. Moreover, the mechanical properties and deformation and failure laws of the composite coal mass under unidirectional loading were investigated. The research results showed that for the composite coal mass, stress was distributed on each layer according to elastic modulus before reaching the peak pressure. However, after reaching the peak pressure, the soft layer first reached the strength limit, thus undergoing deformation and failure, and had softening effects on the overall bearing capacity of the composite coal mass. The degradation of the overall strength decreased and gradually increased with the rise of the height ratio of the soft layer. Under the same load, the degree of axial deformation of the composite coal sample reached the maximum. Deformation and failure of the composite coal mass could be divided into a stable stage, an accelerated stage and a failure stage. The failure of the soft layer induced damages to the upper and lower layers, while hard surrounding rock in the upper and lower layers constrained deformation and failure of the soft layer in the middle. Repeated adjustment of the stress field characterized the progressive failure of the composite coal mass.