Acoustic emission induced by progressive excavation of an underground powerhouse

We monitored acoustic emission (AE) events associated with the progressive excavation of an underground chamber for a powerhouse at a depth of 280 m below the surface in a porphyritic rock mass of the Mesozoic era. Large AE events rarely occur under such conditions; specifically, low-stress environments due to the shallow depth, careful excavation, and sufficient reinforcement. However, upon employing sensitive, high-frequency monitoring (15 to 40 kHz) in a relatively small region, some AE events were located and, by using fault-plane solutions, their fracture mechanisms were identified. Strike and dip angles of fracture planes and the directions of principal stresses, all derived from fault-plane solutions, were consistent with the directions of dominant joint surfaces, the measured initial stress conditions, and the shape of nearby excavated openings. This suggests that by employing sensitive, high-frequency AE monitoring, fault-plane solutions can be effectively utilized as a tool to assess the stability of a chamber excavated at shallow depth, as well as in deep mines where fault-plane solutions have already been used in practice to assess and control rockbursts.