Microseismic monitoring and deformation early warning of the underground caverns of Lianghekou hydropower station, Southwest China

The underground powerhouse caverns of the Lianghekou hydropower station are characterized by their depth, high geo-stress, and complicated surrounding geological conditions. To identify the excavation-induced damage zones and realize the deformation early warning of surrounding rock masses of the underground powerhouse, a high-resolution microseismic (MS) monitoring system was adopted to conduct real-time monitoring of the inner microfracture activity of the surrounding rock mass. The spatiotemporal evolution of the MS events recorded in the aggregation areas prior to the deformation of the surrounding rock mass was analyzed. The analysis results show that two typical MS event clustering zones were closely related to the blasting excavation disturbances of the omnibus bar caves and the working surfaces. The MS activity shows an obvious spatiotemporal migration evolution pattern controlled by excavation, which can be analyzed to provide some reference for future support measures. The Es/Ep value of approximately 75% of the MS events in one typical clustering zone was below 10, which indicates that the rock microfractures were dominated by non-shear failures. Furthermore, prior to the deformation relaxation of the surrounding rock mass, the MS events in the selected clustering zone increased and became rapidly concentrated, the apparent stress of the corresponding MS events suddenly decreased, and the cumulative apparent volume increased dramatically. This phenomenon of MS parameter variations can be regarded as an evaluation index of the deformation early warning of surrounding rock masses. The analysis results prove that MS monitoring is essential for the assessment and mitigation of rib spalling and collapse risks in deep underground caverns, and can be used to minimize catastrophic events involving equipment and personnel. The results of the current study may be valuable for future excavation management and safety evaluations for similar underground caverns with high in situ stresses.