Excavation induced stress change is a significant and considerable factor to drive the brittle failure in the underground opening. Rockburst, as a type of brittle failure, has became a great threat to the construction of mining, traffic tunnels, hydropower station etc. With Hoek-Brown brittle parameters m=0 and s=0.11, failure of the auxiliary tunnel in Jinping II Hydropower Station was analyzed using a BEM program examine2D, and the calculated depth of brittle failure accords with the field observation very well. This paper transformed the in situ stress at the embedded depth of about 1182m from the principal stress coordinate system to the tunnel coordinate system. With these stress values and brittle parameters, the brittle failure of B-auxiliary tunnel was analyzed with examine2D program, and the depth of failure was estimated as about 2.2∼2.3 m. This paper also supplied an analysis on the spalling limit because of excavation based on elastic theory.
The stability of underground openings can be drastically influenced by excavation-induced stress change (Kaiser et al. 2001). Martin et al. (1999) pointed that as in situ stress magnitudes increase, the fractures growing parallel to the excavation surface due to the induced stress will dominate the process of brittle failure, and the failure regions are localized near the opening perimeter at intermediate depths while at great depths the whole boundary of the excavation may be enveloped by the brittle fractures. Rockburst, as a kind of brittle failure, always results in the damage of equipment, delay of construction and even wounds and deaths of workers. Several failure phenomena observed in B-auxiliary tunnel are presented in Figure 1 (photographed by China Railway Shisi Group). On Nov. 28, a very strong rockburst happened in the drainage tunnel of Jinping II and caused 7 deaths and 1 wounded (Liu 2010).
