Progressive failure characteristics and damage constitutive model of rock samples under immersion conditions

To investigate the effect of immersion time on the stability of water-rich pillars in deep goaf areas and rock burst mechanism, water immersion treatment and uniaxial compression acoustic emission experiments were conducted on mine pillar rock samples. The impact of immersion time on the softening characteristics, failure characteristics, energy dissipation characteristics and acoustic emission evolution characteristics of pillar rock samples was explored. The results showed that the mechanical properties of rock samples softened significantly under water immersion condition. The compressive strength, elastic modulus and deformation modulus decreased exponentially with immersion time. Under the high stress condition, rock samples immersed in water exhibited static failure characteristics with no obvious rock burst features. In contrast, natural rock samples showed intense dynamic rock burst failure characteristics. Immersion time significantly influenced the energy storage characteristics of the rock samples. Water increased the dissipation strain energy by reducing the stored elastic strain energy of rock samples, preventing the elastic strain energy concentration and weakening the energy storage characteristics of rock samples. Compared with water-immersed rock samples, the cumulative acoustic emission energy of natural rock samples was greater under peak stress. The lubricating effect of water reduced inner particle friction and hindering the acoustic emission signal propagation. This reduced the ringing count of water-immersed rock samples. During the initial loading stage, an increase of the r-value indicated low-energy events caused by closure of primary cracks. A sudden decrease of the r-value indicated high-energy events resulting from secondary crack propagation and breakthrough. Under uniaxial loading, all different water-immersed rock samples experienced shear failure. However, the failure mode showed no obvious relationship with the water immersion state. For each rock sample, the damage variables (Db and W) showed a trend of slow growth—steady development—rapid increase with strain. Based on acoustic emission ringing counts, the improved model was superior to the original statistical damage model. It better reflected the hydraulic damage process of rock samples under uniaxial loading. The research results can provide theoretical basis for evaluating the stability of water-rich pillars in deep goaf areas and rock burst prevention.