Microseismic characteristics and rockburst risk of deep tunnel constructed by different excavation methods

Based on the stress state analysis and microseismic monitoring data of the surrounding rock of JinpingⅡ Hydropower Station, microseismic characteristics and rockburst risk of deep tunnel constructed by TBM-drilling and blasting method are compared and studied. The results show that: (1) The stress concentration of the surrounding rock caused by the drilling- blasting method is far more away from the tunnel wall than that by TBM, and the stress gradient is small, while the stress gradient caused by TBM excavation is large. (2) The strain energy of the surrounding rock mainly releases in several hours after blasting, especially in the first hour. Under the condition of TBM excavation, the stress of the surrounding rock unloads with the continuous mode and the energy violently releases in whole construction process. (3) The Microseismic magnitude and seismic source fracture scale caused by TBM excavation are larger than those by drilling-blasting method. (4) By the drilling-blasting method, the strain energy of the surrounding rock rarely dissipates in the form of rockburst but mostly in the form of rock mass cracking. By TBM excavation, the strain energy of the surrounding rock successively dissipates, resulting in frequent microseismic events. Part of strain energy can dissipate in the form of rockburst. Generally, mild rockbursts often occur in the same range. The inoculation process of high-grade rockburst accompanies with the occurrence of low-grade rockburst. For instance, the occurrence of medium rockburst accompanies with mild rockburst and strong rockburst accompanies with medium rockburst and mild rockburst, and so on. In conclusion, the drilling-blasting method is superior to TBM for rockburst prevention and control of deep tunnel with strong rockburst risk