Effect of water intrusion on mechanical behaviors and failure characteristics of backfill body and coal pillar composite specimens under uniaxial compression

Water intrusion significantly impact the mechanical behaviors of the composite structure formed by the backfill body and coal pillars (BC), which is a crucial load-bearing structure for underground water storage reservoirs. In this study, Uniaxial compression tests, supplemented by acoustic emission (AE) experiments, were conducted to investigate the effect of water intrusion on the mechanical behaviors of BC composite specimens with varying coal volume fractions (0, 0.25, 0.36, 0.49, 1). The results indicated that the BC composite specimens’ behaviors are significantly affected by the volume fraction of coal and the water immersion pressure. The strength of BC composite specimens show a linear decreasing trend with the increase in water immersion pressure and the coal volume fraction. The variation in coal volume fraction significantly affects the stress-strain characteristics of BC composite specimens. At a coal volume fraction of 0.25, the composite specimen exhibits single-peak behavior, with stress gradually decreasing to 0 MPa after reaching peak strength as strain increases. Conversely, at coal volume fractions of 0.36 or 0.49, the composite specimens exhibit a double-peak characteristic. The AE evolution of BC composite specimens with varying coal volume fractions exhibit a high correlation with their deformation and failure processes. In addition, a strength model was introduced to quantitatively evaluate the load-bearing mechanism of BC composite specimens with varying coal volume fractions under different water immersion pressures. This research offers crucial insights for the design of load-bearing structures comprising backfill bodies and coal pillars.