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Abstract Because of the large size of the underground powerhouse of Baihetan hydropower station at left bank，the local failures of surrounding rock occurred frequently subjected to excavation. To study the mechanisms of rock deformation and failure subjected to unloading caused by excavation，an ESG(Engineering Seismology Group) monitoring system was applied to monitor the microfractures in real time during the excavation processes in the underground powerhouse. Microseismic(MS) activity was found to be in good agreement with the excavation. Three potential failure zones of the underground powerhouse under the first layer excavation were revealed，i.e.，the downstream spandrel between unit 4 and unit 5 of the main powerhouse，the upstream spandrel between unit 6 and unit 7 of the main powerhouse，and the crown above unit 8 of the main powerhouse to the crown of the transformer chamber. The moment magnitudes and Es/Ep of microseismic clusters controlled by the excavation strength and faults were very different，resulting in the different potential failure modes of surrounding rock. The targeted supports should be adopted in the MS clusters based on the evolutional MS characteristics. Several days before the local deformation of the surrounding rock，the area manifested with the accelerated cluster of MS events，quick increase of the apparent stress，but rare change of the cumulative apparent volume.
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2016, Vol. 35 
