Prediction of rockburst in tunnels at the Jinping II hydropower station using microseismic monitoring technique

Rockburst hazards are frequently encountered in tunnels at the Jinping II hydropower station, which pose serious threats to the safety of personnel and equipment. Abnormal stress concentration caused by the rock mass interfaces and geological defects along the #3 headrace tunnel makes the rockburst difficult to predict using routine estimate and monitoring methods. A two-body interaction model was used to interpret the mechanism of the rockburst, which divided the evolution of a rockburst into three stages, i.e., the stress buildup period, early warning period, and rockburst occurrence period. The microseismic (MS) monitoring technique, which can capture the associated seismicity in the stress buildup period and early warning period, was employed to study and predict the rockburst hazards. An intense rockburst occurred at the right sidewall of the #3 headrace tunnel on September 21, 2011. The rockburst location was recognized by analyzing the spatial-temporal distribution and density contour plot of the recorded MS events from September 12, 2011 to September 21, 2011. Some seismic precursors to the rockburst were obtained based on quantitative interpretation of the source and statistical parameters of the 367 MS events, and they are a sharp increase in the daily event count and average energy level, the energy index remains at extremely high levels accompanied with a rapid increase in the cumulative apparent volume, a sharp decrease in the b value to an extremely low level, and a rapid increase in the seismicity index to an abnormally high level. The multi-parameter precursors to the rockburst should be comprehensively used to improve the prediction accuracy. The validity of using the MS method to predict the rockburst was verified by the preliminary achievement at the #3 headrace tunnel. This study may be valuable to improve the accuracy of rockburst prediction, and helpful for the safety assessment during construction of deepburied tunnels.