Mechanical response of sandstone under variable loading rates and acoustic emission-based identification and early warning of fracture precursors

HUA Xinzhu; LI Chen; YANG Peng; LIU Xiao; YAN Jiyuan

doi:10.12438/cst.2025-1571

Abstract：Gob-side entry retaining with roof cutting is an important technique for achieving safe and efficient deep coal mining.However,under the combined effects of primary roadway excavation and secondary intensive mining-induced disturbances,severe surrounding rock deformation often occurs,with floor heave becoming a prominent problem.To clarify the stress transfer mechanism between the overlying strata and the floor during gob-side entry retaining,as well as the fracture evolution behavior of floor rock under variable loading rates,similarity simulation tests and uniaxial variable-rate loading experiments were conducted.The floor stress response and the mechanical be-havior,acoustic emission(AE)characteristics,and fractal features of sandstone under different loading rates were systematically analyzed.The similarity simulation results indicate that the floor surface experiences instantaneous unloading after roadway excavation.With the ad-vancement of the working face and periodic roof caving,the accumulation and compaction of caved gangue lead to stress recovery in the floor.After the overlying strata structure becomes stable,variations in floor stress gradually diminish,and the system enters a quasi-static equilibrium stage.Based on this stress evolution background,variable-rate loading experiments reveal that the loading rate has a signific-ant influence on the fracture mode and mechanical response of sandstone.Under rapid loading,high strain rates promote concentrated crack initiation and rapid coalescence,accompanied by intense energy release,resulting in typical brittle failure.When quasi-static loading is introduced at an earlier stage,cracks initiate at multiple locations and propagate slowly,the pre-peak nonlinear stage is prolonged,brit-tleness is reduced,and ductility is enhanced.Under fully quasi-static loading,damage accumulation is most sufficient,and the fracture pro-cess exhibits a progressive failure mode.AE results show that rapid loading induces sudden and highly concentrated AE activity,with sharp pre-peak increases in AE counts and energy,leading to an extremely short warning window.As the loading rate decreases,crack propagation is restrained,AE events become continuously active,and high-energy release persists even after peak stress.Multifractal ana-lysis demonstrates that the AE fractal spectra exhibit a bell-shaped distribution,which initially broadens and then converges with increas-ing stress.Rapid loading or late-stage rate reduction leads to left-skewed spectra with enhanced burst characteristics,whereas lower load-ing rates or early-stage rate reduction produce more symmetric spectra,indicating progressive crack development.Early-warning indicat-ors constructed from the variance of multiple AE parameters further show that under rapid loading,the variance rises sharply shortly be-fore peak stress,resulting in delayed and short-lived warning signals.With decreasing loading rate,the variance exhibits sustained growth with dense multi-peak features,triggering L1,L2,and L3 warning levels sequentially and significantly extending the warning window.Un-der fully quasi-static loading,variance fluctuations become more frequent,enabling earlier identification of accelerated crack coalescence.These results indicate that a lower loading rate promotes more complete fracture evolution and enhances the identifiability of failure pre-cursors.The findings provide deeper insight into rock fracture characteristics and offer a theoretical basis for stability monitoring and dis-aster early warning in deep rock engineering.