In situ monitoring of rockburst nucleation and evolution in the deeply buried tunnels of Jinping II hydropower station

Rockbursts occurred frequently during the excavation of several parallel tunnels in the Jinping II hydropower station under a maximum overburden of 2525 m over an average length of 17.5 km. In order to investigate the nucleation and evolution mechanism of rockbursts, a comprehensive monitoring campaign consisting of a digital borehole camera, cross-hole acoustic apparatus, and sliding micrometer was undertaken for in situ measurements in two specially excavated test tunnels B and F. This paper presents the comprehensive monitoring methods applied, and results of numerical analysis applied to a typical rockburst that fortuitously occurred during the testing period. Precursory characteristics preceding rockbursts are: (a) abundant crack initiation, propagation and coalescence, (b) deformation of surrounding rock mass involving an accelerated deformation stage, quiescence stage and reaccelerated deformation stage, and (c) decrease of the characteristic elastic wave velocity of the rock mass. The nucleation and evolution of rockbursts discussed consist of four stages: a) stress adjustment, b) energy accumulation, c) crack initiation, propagation and coalescence, and d) fractured rock collapse and ejection. The results provide a direct case history to assist the prediction and support of rockburst disasters, and contribute to field excavation of deeply buried tunnels.