An investigation into coal burst process based on the stiffness variation of coal-rock system

The monitoring, early warning, and prevention of coal burst in underground coal mines have remained a worldwide challenge perplexing both academia and the mining industry, due to its inherent characteristics of process complexity, sudden occurrence, and diverse manifestation patterns. The underlying mechanisms governing the entire process of coal burst evolution (preparation-initiation-manifestation-termination) have not yet been effectively elucidated. Based on summarizing the mining-induced stress environment and the dynamic-static load superposition theory for coal burst initiation, a roof-coal seam-floor burst system model is established, and the full coal burst process is discussed based on the stiffness variation of coal-rock system, including two criteria of displacement mutation and energy release based on stiffness variation, as well as a generalized stiffness initiation condition and two dynamic load-induced coal burst effects (plastic deformation and load increment). True triaxial quasi-static loading, cyclic loading-unloading, and combined dynamic-static loading experiments on composite coal-rock specimens were carried out, which respectively validate the static load-induced coal burst condition of stiffness initiation and the dynamic load-induced coal burst effects of plastic deformation and load increment. The results indicate that coal burst will be induced by static loading while the yield stiffness of the coal is greater than the loading stiffness of surrounding rock, including roof and floor. The generalized stiffness initiation condition under superimposed dynamic loading is equivalent to the stiffness reduction of surrounding rock. As a result, the displacement mutation and energy release criteria can predict the spatiotemporal positions of coal burst occurrence. The essence of the plastic deformation-type induced coal burst effect is that the pre-peak accumulated permanent plastic deformation exceeds the critical strain value for the coal burst initiation under quasi-static loading condition. The essence of the load increment-type induced coal burst effect is that the input dynamic loading energy is greater than the dissipation energy required for the stress-strain development from the pre-peak state to the post-peak coal burst initiation. The scientific connotation of the coal burst definition includes three characteristics with initiation object, physical parameters, and macroscopic phenomena. Amongst, stiffness variation response, characterization and matching are the keys to guiding coal burst monitoring and prevention. The stiffness variation of coal-rock systems explains the influence of relative stiffness variations between the roof-floor surrounding rock loading system and the loaded coal on coal burst process, as well as the consequent dynamic energy equilibrium issues. It has integrated the core concepts of coal burst theories of dynamic-static load superposition, strength, stiffness, energy, and bursting liability, which could provide references for revealing the full-process mechanisms and energy release patterns of coal bursts.