Abstract: Based on the understanding of geological conditions, tunnel excavation response and the physico-mechanical properties from laboratory tests of rock, deformation gradation and safety factor of the soft rock tunnel are proposed. Considering that the root cause of tunnel instability after excavation is the excessive energy of the tunnel native rock; the energy, which is combined with the ratio of the total deformation and elastic deformation, is used to analyze the stability of the tunnel; and on the basis, the safety factor is obtained by increasing-decreasing energy of the original rock. Through the practical application and compared with traditional evaluation methods, the method metioned above is proved to be reasonable and practical. The geological conditions of surrounding rock, the cross section shape, size, excavation and other factors, which are rarely considered by traditional evaluation methods are taken into account, so as to supply important references for the underground engineering.

Key words: soft rock tunnel, large deformation, energy, safety factor, deformation gradation