Microseismic characteristic and development mechanism of fault-slip rockburst in a deep-buried TBM excavated tunnel: A case study.

• The temporal and spatial evolution process of microseismicity in a fault‑slip rockburst were studied. • The development process and evolution mechanism of the fault‑slip rockburst were revealed. • The mechanism of intermittent occurrence of the rockburst and mud gushing near the fault was discussed. An intense fault-slip rockburst occurred in a deep-buried tunnel excavated by tunnel boring machine (TBM). After the intense rockburst occurrence, moderate rockburst and mud gushing intermittently occurred several times, which resulted in the main beam of the TBM being buried and delaying construction schedule for several months. In the present study, microseismic (MS) monitoring and geological survey were carried out to investigate the evolution process and development mechanism of the rockbursts. The temporal and spatial evolution of MS activity, dynamic stress drop, and fracture mechanism were analyzed. The results indicated that prior to 2–4 days of the fault-slip rockburst, the number of MS event gradually decreased, while the energy level and dynamic stress drop gradually increased. Prior to one day of the fault-slip rockburst, the number of MS event with high energy sharply increased. During the development stage of the fault-slip rockburst, most of the MS events were failure in tensile and a few events were in mixed and shear failure. The main MS event of the fault-slip rockburst is in mixed failure, result from the dislocation of the fault. The energy accumulated near the fault was not released fully through the intense rockburst. After that, the energy intermittently released, i.e. moderate rockburst and mud gushing intermittently occurring several times. It was because that some hard rock layers were within the fault fracture zone. As the mud gushing occurred, the stress applied on the hard rocks was unloading and a moderate rockburst was prone to occur. As the moderate rockburst occurred, the path of mud gushing was opening, and a mud gushing tended to occur. The research results can be helpful for further research on fault-slip rockbursts and planning appropriate warning and mitigation methods. [ABSTRACT FROM AUTHOR]