Stability Analysis Of Flat Large-span Tunnel Excavation Based On Strain Softening Model

With the trend of expressway construction, more and more large-span flat highway tunnels were built. As the flat ratio decreases, the engineering quantity and the sectional area of tunneling is reduced. At the same time, surrounding rocks go more unstable and foundation bearing capacity is highly required because of stress redistribution after excavation. Geological conditions, surrounding rock parameters and excavation methods have ineluctable influence to the stability of surrounding rock in the design and construction process of large-span flat highway tunnels. To improve the reliability of numerical simulation, parameter sensitivity analysis of the surrounding rock is quite necessary, the primary sensitive factors and secondary factors should be revealed.In this paper, several numerical simulation analysis methods widely used in current geotechnical engineering are summarized. The finite difference software, FLAC3D, is systematically introduced. The excavation simulated computation of tunnel model by using FLAC3D strain softening model is completed.Firstly, the two side-wall pilot tunnel method is used for excavation of tunnel model which length is 30meter and high-span ratio is 0.56. All the surrounding rock parameters of this tunnel model are from base parameter sets. The mechanical behavior of the displacement and stress of key observation points is analyzed with the process of excavation. The research also reflects the stress redistribution in rock mass.Secondly, based on the single-factor sensitivity analysis, the excavation model for tunnels with high-span ratio of 0.56 is simulated. The sensitivity of vertical crown displacement to different rock parameters, as well as horizontal haunch displacement and skewback principal stress, is researched. The results show that the uppermost sensitive parameter is elastic modulus and poisson’s ratio takes second place. Meanwhile, the maximum principal stress under the situation of different flat ratio is calculated. It shows that stress concentration strengthens rapidly as the flat ratio decreases. The research provides a reference for engineering applicaions.