Excavation characteristic analysis of columnar jointed rock mass considering unloading damage

The quantitative expressions of unloading damage effect of columnar jointed rock mass are obtained via establishing a mathematical relationship between radial stress ratio and mechanical damage factor of columnar jointed rock mass. Then combined with FLAC3D, a new calculation method for excavation of columnar jointed rock mass is proposed, which takes unloading damage into account. By the proposed method, mechanical parameters of each element are dynamically adjusted according to its stress state in the process of excavation calculation. On this basis, the excavation characteristics of the experimental tunnel in columnar jointed rock mass are analyzed. The calculation shows that the results that take into account of unloading damage match the measured data well. The displacement, volume strain increment and plastic zone of the experimental tunnel are gradually increasing with the increase of excavation section, presenting a distribution law that these variables are maximum at sidewall, medium at floor, and minimum at vault. Finally, volume strain increment larger than 0.10% and plastic zone are both taken as discriminant index of relaxation of the columnar joints rock mass, and the corresponding analysis results are verified by the borehole acoustic test data.