Abstract: A high-precision microseismic(MS) monitoring system was installed to evaluate the stability of the #5, #6 bus-bar tunnels at Lianghekou hydropower station during excavation. The dangerous areas of underground powerhouse during excavation were delineated through real-time monitoring of MS accumulation and analyzing frequency of microseismic signal in gathering area. Based on previous exploration data and site excavation progress, and with the aid of UDEC discrete element program, the deformation distribution and failure mechanism of bus-bar tunnels after excavation were revealed. The MS activities were closely related to excavation of underground powerhouse. The excavation of #5, #6 bus-bar tunnels resulted in the accumulation of MS events and the increasing of stress gauge reading. The free surfaces formed by excavation were prone to instability. The high frequency components of MS signal decreased at first and then increased before surrounding rock fractured. Displacements at the top arch of bus-bar and the upstream side wall of main transformer chamber were remarkable and presented tensile failure after excavation. The research results provide important references for excavation and reinforcement of underground powerhouse at Lianghekou hydropower station.

Key words: microseismic monitoring, underground powerhouse, frequency characteristics, distinct element, surrounding rock mass stability, bus-bar tunnels