Research on Mechanical Properties and Damage Evolution of Lignite Under Uniaxial Cyclic Loading and Unloading: Insights into Crack Propagation and Energy Dissipation

In lignite open-pit mines, the blasting mining method and large-scale mechanical shovelling processes induce substantial cyclic disturbances in coal seams at the terminal slope during lignite extraction, significantly increasing the risk of slope destabilisation and damage. Consequently, uniaxial cyclic loading and unloading experiments were conducted to evaluate the mechanical properties and energy evolution of lignite. Acoustic emission (AE) characteristics and macroscopic crack evolution of lignite under cyclic loading and unloading conditions were analysed using AE counts and b-values. The energy evolution of lignite was further examined to elucidate the mechanisms of crack propagation and instability failure. The results indicate that initial damage exists within the lignite, and cyclic loading weakens its mechanical properties. Specifically, the irrecoverable damage resulting from the continuous development of internal cracks leads to the continuous deterioration of the mechanical properties of lignite. During the process of damage accumulation, the energy evolution characteristics of the lignite shift from being dominated by plastic energy dissipation to being dominated by elastic energy storage, which triggers higher energy dissipation and release at the cumulative damage stage. Furthermore, as the stress level increases, the cracks in the lignite transition from tensile–shear composite cracks to predominantly tensile cracks. These findings provide critical insights into the mechanisms of instability and failure in open-pit slopes subjected to cyclic loading and unloading, contributing to the advancement of slope stability management in lignite mining operations.