Abstract: The underground powerhouse of Houziyan hydropower station is a typical deep-buried powerhouse with characteristics such as large scale excavation span, complex structures and stress condition of overlying rock mass. The stability of rock mass in the process of excavation unloading has always been one of the most important and difficult problems during the project construction. So far conventional monitoring methods such as GPS, multiple position extensometers cannot effectively measure and reveal the microfractures inside the surrounding rock mass of underground powerhouse. A high resolution microseismic monitoring system was installed in the underground powerhouse on April 2013, aiming to real-time analyze the deformation and stability of surrounding rock mass subjected with continuous excavation. The real time positioning and investigation of microfractures in deep rock mass is achieved. The monitoring results show that the temporal-spatial evolution distribution regularity of microseismicity can demonstrate the failure mechanism of initiation, propagation, coalescence, interaction and breakthrough till macroscopic deformation of microcracks in deep rock mass of underground powerhouse. Meanwhile, microseismic monitoring can dynamically identify and delineate micro-fracture clustering areas of surrounding rockmass and the potential instability risk areas in the underground powerhouse along with continuous excavation. Finally, assisted with acoustic detection results, a relationship is established between microseismic clusters and rock mass quality. The results could provide guidelines for later excavations and supports in the underground powerhouse of Houziyan hydropower station. Moreover, it’s also a new idea for the stability evaluation of surround rock mass in similar deep-buried underground engineering.

Key words: microseismic monitoring, underground cavern group, excavation unloading, damage zone, wave velocity