In situ observation of spalling process of intact rock mass at large cavern excavation

This paper presents the results of in situ observation of rock spalling process of a large underground powerhouse, with 34 m in span and 88.7 m in height, during excavation layer by layer over two years. A method for in situ observation of rock spalling process has been established in pre-installed boreholes by using digital borehole camera. Observation results clearly show the whole process of rock fracturing and the associated spalling failure. Not only could the development of rock fracturing and spalling affected by the step-by-step excavations along the cavern's axis be captured, but also could the effect of the subsequent excavation layer by layer on the previous rock fracturing and spalling area be observed. The failure mechanism of rock mass surrounding the large cavern, which involves complex excavation sequences, could be better understood through the observation results. The difference of rock spalling behavior between large cavern and small tunnel is discussed. Influence of geostress, geological structures, rock mechanical properties, and layered excavation process on rock spalling process has been analyzed. The results in this study is instructive for rethinking the engineering design including the excavation and support at similar large underground caverns, as well as for the dynamic adjustment of excavation schemes and support optimization during the construction of a large cavern subjected to high geostress.