Characterization of microseismicity induced by TBM excavation at the –280 m experimental level in Beishan underground research laboratory, China

With the rapid development of nuclear energy, the safe disposal of high-level radioactive waste (HLW) has emerged as a global challenge. Beishan underground research laboratory (URL) is the first underground facility for the development of China’s HLW repository. The ramp of Beishan URL is a spiral structure excavated using a tunnel boring machine (TBM). A quantitative assessment of the disturbance induced by TBM excavation on the surrounding rock is essential for understanding the mechanical behavior of rock masses at depth and will support China's future implementation of TBMs in constructing the HLW repository. In this work, microseismic (MS) technology was used to monitor the disturbance characteristics of the surrounding rock during TBM excavation at the –280 m experimental level at Beishan URL. A monitoring tunnel was first excavated, and a series of MS sensors were installed prior to TBM excavation to capture the complete MS response of the surrounding rock. The results indicate a positive correlation between the number of MS events and TBM advancement. During TBM excavation, the rock mass experiences increased disturbance, characterized by reduced apparent stress, elevated energy index, and increased apparent volume. Using the moment tensor inversion method, the primary failure mechanism of the rock mass was identified as shear failure. A relationship was observed between the center frequency of MS events and the scale of failure, with lower center frequencies indicating larger failure scales. Compared with shear failure, tensile failure generally occurs on a larger scale. TBM excavation resulted in the formation of four disturbance zones, with an approximately 8 m range from the tunnel periphery. MS events within these zones presented low magnitudes and low energy release, demonstrating limited disturbance to the surrounding rock mass. Field investigations revealed that MS events were predominantly concentrated in areas characterized by fracture development. Comparative analyses of typical stress and displacement measurement results validate the accuracy of MS monitoring in evaluating surrounding rock disturbances.