Abstract: Jinping II hydropower station is located in a high in-situ stress region in Southwest China. During the excavations of the transportation and drainage tunnels, more than 460 rockburst events were recorded in the transportation tunnel and 110 in the drainage tunnel, which has a serious and negative influence on the tunnels’ construction and the safety of staff and equipment. In the paper, the characters of rockburst patterns are analyzed for the transportation and drainage tunnels. The results are illustrated as follow: (1) Most of intensive rockbursts occur in the layer T2b, and continuous occurrences of rockbursts are more frequently observed than those in other layers. (2) The critical overburden depth of rockburst in the transportation tunnel is 600 m, and the length of the continuous occurrence section of rockburst is smaller than 25 m. The damaged depth of the rockburst has the tendency to increase with the increasing overburden depth, and the maximum damaged depth is over 3.5 m. (3) From east to west (west to east) in Jinping II hydropower station, the rockburst usually takes place in the right (left) side of tunnel working face, and then the left (right) or roof of the tunnel. The total length of the continuous occurrence section of rockburst is 57.4%–62.2% of the overall rockburst length, followed by the rockbursts of flake-splitting type and other types. (4) Compared with the transportation tunnel, the intensity of rockburst in the drainage tunnel is higher while the length of the continuous occurrence section of rockburst is smaller. The rockburst section with length less than 10 m and depth of 1 m mainly occurs in the layer at a depth of 1 800–2 000 m. The influences of opening geometry and excavation method on the characteristics of the adjacent zone are great, but the influence of the stress among the tunnel group induced by excavation is relatively low.
1 Introduction
Rockburst is a kind of geological hazard triggered by the brittle rupture of surrounding rocks during unloaded excavation in high in-situ stress environments. It is basically accompanied by a sudden release of elastic strain energy and some other phenomena, such as slabbing, spalling, ejecting or throwing [1]. Rockburst has tremendous and potential threats to both the safety of staff and equipment, and may cause a series of casualty and property loss as unexpected.Jinping II hydropower station is located in the boundary zone between Qinghai—Tibet Plateau and Sichuan Basin in Southwest China. The phenomenon of rock core disking frequently occurred during drilling in this field. The maximum principal stress measured is approximately 42.1 MPa. The four drainage tunnels, each about 16.7 km in length running parallel to and crossing the Jinping Mountain, connect the sluice gate and the plant units. The
cross-section of two transportation tunnels A and B is U-shaped with sizes of 5.5 m × 5.7 m and 6.0 m × 6.25 m (width × height), respectively. In addition, the round drainage tunnel has been constructed for water inrush or dewatering. The transportation tunnels are excavated by blasting, with more than 460 rockbursts recorded during excavation. The drainage tunnel is excavated by TBM, with more than 110 rockbursts recorded from the beginning of excavation.
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