Quantitative analysis of the spalling mechanism of CJPL-II by step excavation under in situ stress using the flat–joint model and moment tensor

Spalling failure is a typical static failure phenomenon in deep surrounding rock. Resolving the spatiotemporal evolution of the spalling mechanism by step excavation under in situ stress is critical to understanding the spalling occurrence process. However, this evolution is not well understood. In this study, we introduce an alternative perspective for studying the spatiotemporal nature of the source and reanalyze the spalling mechanism of the CJPL-II by step excavation under in situ stress. The nature of source mechanics is quantitatively identified by the flat-joint model and moment tensor. The range and width of slabs are formed, and the tensile mechanisms (nonpure tensile) and implosive mechanisms (nonpure implosion) play dominant roles, while mixed shear mechanisms (shear–tensile/compressive) also contribute. As excavation progresses, the stress field tensor changes significantly, leading to microscopic damage and the formation of localized weakening zones around the tunnel. Our findings raise the question of the“effect of step excavation under in situ stress on the spalling evolution process?”: a mesoscale quantitative perspective using the flat-joint model and moment tensor is meaningful.