Morphological features and fractography analysis for in situ spalling in the China Jinping underground laboratory with a 2400 m burial depth

Abstract Brittle spalling of hard rock is a typical failure in deep underground engineering that induces challenges for construction safety. Seventy marble spalling samples were collected from the China Jinping Underground Laboratory Phase-II (CJPL-II) to analyse their mesoscopic morphology and failure mechanism. The 3D scanning and corresponding statistical analysis indicated that the spalling surfaces were not smooth but with low roughness and evident anisotropy, and that the morphological transition (i.e. from the mirror zone to the mist zone then to the hackle zone) on the spalling surface was observed. The fractography of the CJPL-II spalling samples showed typical features of brittle fracture dominated by the granular morphology of intergranular fracture based on the Scanning Electron Microscope technology. What’s more, the simulated spalling by the laboratorial true triaxial unloading tests showed similar morphology with that of CJPL-II spalling. Moreover, by comparing the marble spalling samples with other granite spalling and basalt sapling samples, the effect of the mineral particle size of the surrounding rock on the roughness of the spalling slice was discussed. This study deepens the understanding of the in-situ spalling mechanism of hard rock under high geostress conditions.