During the excavations of the Qinling extra-long tunnels, severe rockburst occurred in several sections, where the initial subhorizontal stress is about 20MPa–30MPa and the overburden is no less than 1000 m. The magnitude of the in situ stresses are related to the rock mass's capacity of storing initial geostatic stress. Large in situ horizontal stresses are kept in intact gneiss, with unaxial strength more than 45MPa and overburden more than 200 m. The large subhorizontal in situ stress mainly plays the role of σ2 during tunnel excavations in the section of gneiss with severe rockbursts occurred. The initial stress in the rock mass at Qinling tunnels is not such large that severe rockbursts will be induced during tunnel excavations. The severe rockbursts should be the combination effect of in situ stress, gneiss fabric and excavations.
The original properties of rock mass around a tunnel are changed due to excavation and stress redistribution in the wall rocks of a tunnel or underground structure. The characteristics of disturbed zone around an excavation vary with geological conditions, excavation method, and opening geometry. The stability of tunnels and underground structures is related to the onset of wall rock yield due to boundary condition alteration (Martin et al. 2003). For a tunnel or underground structure excavated in brittle rock mass, as stress levels beyond this point of hard rock at depth, the bursting of wall rock can increase cost and safety concern (Diederichs et al. 2004). As a result, rockburst seems one of the major concerns for the stability deep underground structures in hard rocks or difficult conditions. The determination of the in situ stress level is one of the primary design issues (Heok & and Brown 1980, Zang & Stephansson 2009).