Microcrack evolution and local stress inversion at the bottom of a high arch dam during operations across a major earthquake based on microseismic monitoring

High arch dam is susceptible to microcracking at its bottom due to stress concentration. Therefore, it is crucial to investigate microcracking at the dam bottom for evaluating the integrity and safety of the high arch dam during its operation. In this study, based on the microseismic (MS) data monitored at the bottom of the Dagangshan high arch dam during its normal operation across the Luding Ms6.8 earthquake, the moment tensor inversion and iterative joint inversion methods were implemented to analyze the focal mechanism of microcracks. This analysis revealed the evolution characteristics and fracture types of microcracks as well as explored the stress characteristics at the dam bottom. The results indicated that the MS events at the dam bottom were mainly distributed from the dam heel to the middle of the dam bottom. The fracture types were primarily compressive and tensile, which occupied approximately 38 % and 32 %, respectively, in average, and the microcracks form fracture channels in the NW-SE and NE-SW directions. The stress inversion analysis clarified that the occurrence of the microcracks at the dam bottom were primarily controlled by compressive stress, with the maximum principal tensile stress being dominant in the SW-NE directions. The primary stress axis of the stress field at the dam bottom exhibited minimal variations before and after the earthquake, suggesting that the bottom of the Dagangshan high arch dam remained stable after the earthquake. However, the earthquake and its aftershocks altered the opening and closing states of the microcracks, which was the main reason for the sudden change in seepage pressure at the dam bottom. Therefore, this study helps to reveal the damage mechanism and local stress characteristics of the high arch dam, and provides a reference for seepage control and removal, as well as safety and stability evaluation of the high arch dam.