In situ experimental investigation of basalt spalling in a large underground powerhouse cavern

Rock spalling is one of the most frequent break modes and a serious stability challenge for the supporting design during engineering excavation under high geo-stress and hard rock conditions. The primary objective of this work was the field investigation and experimental testing of basalt spalling in more than 50 cases in a large underground cavern in China. The field characteristics of basalt spalling, including the spatial distribution of events, its micro-surface morphology, the joint effect, the time-dependent relation to the excavation, the spatial distance to the opening face, and the supporting condition, are first presented via detailed statistical investigation. An in situ study of the full spalling process, including the surface development of rock slabbing and inner cracking extension of the surrounding rock, was recorded in a time series via a borehole camera, displacement measurement, and in situ photos. These experimental results exposed the time-dependent development of surface spalling performance and the inner cracking evolution of rock mass ahead of spalling. All of these actual spalling cases in a large underground cavern and the corresponding statistical analysis provided meaningful evidence for the tensile failure mechanism and prevention measures for rock spalling.