Mesoscopic study on creep of sandy mudstone under loading axial stresses and unloading confining pressures based on Burgers hybrid contact

This study investigates the creep behavior and instability mechanisms of deep soft rock under varying ratios of axial pressure increment to confining pressure decrement(AITCD). The two-dimensional particle flow code(PFC2D) was further developed to incorporate a hybrid contact model, referred to as the Burgers-parallel bond(BPB) model, to establish a meso-creep model of sandy mudstone under different mining conditions. The model's reliability was validated using results from conventional triaxial compression tests and creep tests involving axial stress loading and confining pressure unloading of sandy mudstone. Based on this model, the mesoscopic creep characteristics and energy evolution of sandy mudstone were studied under various axial stress loading and confining pressures unloading stress paths. The results show that, under the three distinct AITCD ratios, the stress-strain curves of sandy mudstone exhibit strain-hardening behavior. Both the peak deviator stress and total crack number increase as the AITCD ratio rises. Under the same confining pressure and unloading rate, an increase in the AITCD ratio significantly reduces the duration of the accelerated creep instability phase, widens the difference between peak axial and radial strain, and raises the axial steady-state creep rate. As creep time progresses, the proportion of Burgers contacts of top coal caving and non-pillar mining initially increases before decreasing, while for protective seam mining, it first decreases gradually and then drops sharply. Additionally, the proportions of Burgers and linear contacts increase with higher AITCD ratios, whereas the proportion of parallel-bond contacts decreases. Energy analysis reveals that, prior to unloading, the total energy of sandy mudstone is dominated by elastic energy, while dissipative energy becomes the primary component after unloading. The elastic energy consumption ratio exhibits nonlinear, rapid growth at the initial stage of creep. It then declines in a "sawtooth" pattern during the first three creep stages until the onset of the accelerated creep stage, where it increases sharply. This growth rate diminishes progressively with higher AITCD ratios.