(1. State Key Laboratory of Geomechanics and Geotechnical Engineering，Institute of Rock and Soil Mechanics，Chinese Academy of Sciences，Wuhan，Hubei 430071，China；2. Ertan Hydropower Development Co.，Ltd.，Chengdu，Sichuan 610051，China)
Abstract：The time delayed rockburst(TDR) is named based on mechanism and characteristic of rockburst occurring in the diversion tunnels of Jinping II hydropower station；and then it is analyzed and investigated systematically in terms of microseismic and rockburst data in situ. The conclusions are acquired as following：(1) Stress adjusting after rockburst zone is excavated and disturbing outside of stress adjusting range of rockburst zone work together results in the TDR bursting out. 80% of TDR occur from 6 to 30 d after the rockburst zone is excavated and inside of 80 m ranges from tunnel face. (2) All kinds of structural planes，such as joint，fracture，weak intercalation and so on，are main characteristic of surrounding rockmass in rockburst zone，the strike of structure plane is parallel or has a small angle with the axial of the tunnel. (3) As the rockburst zone is excavated，the magnitude and rate of stress adjusting is very large；fracture frequency of surrounding rockmass is quite high；and the microseismic events are located at the same zone in three-dimension space and its number increases continuously with time. Apparent volume of surrounding rock mass has a sudden increasing trend. Energy index has a sudden drop trend in the same time. Before the TDR burst out，microseismic activity has an obviously “sleep” period and the changes of apparent volume and energy index of surrounding rock mass are not very obvious. (4) The tensile，shear and mixed fracture all live during process of rockburst zone excavated. Then the fracture along the structure plane is main trend and tensile fracture is main fracture form. At last，the shear failure is main as the rockburst occurring. A union prevention scheme that firstly the concrete layer with steel fibre is built in time；secondly，the rock bolts are constructed；thirdly，the steel mesh is connected to the surrounding rockmass；lastly， the concrete with steel fibre is built again，which is proposed for TDR，based on the evolution characteristic，law and mechanism of TDR.