Strength hardening and damage mechanism of semiwetted coal under cyclic loads with different initial stress levels

When a coal pillar serves as part of the dam structure in an underground coal mine reservoir, it is subjected to the combined effects of water immersion and dynamic loading. Therefore, understanding the mechanical properties of coal under such conditions is crucial for assessing the stability of underground coal mine reservoirs. While previous studies have investigated the mechanical behavior of coal under cyclic loading conditions, systematic research remains limited regarding structural evolution and mechanical response characteristics under varying initial stress levels in partially saturated environments, particularly for underground reservoir applications. This study investigates the mechanical properties of semi-wetting coal under cyclic loading and unloading at varying initial stress levels, with a focus on stress-strain characteristics, modulus changes, deformation, failure, and energy evolution. The results show that both the strength and the loading/unloading modulus of coal decrease as the initial stress level increases. However, the coal's strength remains higher than its conventional uniaxial compressive strength, indicating a hardening effect due to cyclic loading. This hardening diminishes with higher initial stress levels. Additionally, larger particle sizes and fewer small fragments are observed as initial stress levels rise, suggesting that at lower stress levels, new cracks form but tend to reclose, improving strength and deformation resistance. The cumulative dissipated energy decreases with lower stress levels, highlighting that more energy is required to induce failure at these conditions, further confirming the hardening effect. These findings offer a theoretical foundation for assessing the impact of dynamic loading on coal structure stability under varying initial stress levels.