Application Of Strain Softening Model In Numerical Simulation Of Shallow-buried Soil Tunnel

For the advancing of shallow-buried tunnel in soil, while the soil deformation reachesto a certain value, the soil strength rapidly reduces due to the strain softening behaviour ofsoil, the tunnel is apt to lose stability and collapse during tunnelling. Therefore, the strainsoftening behaviour of soil is adverse to the stability of shallow-buried soil tunnel. However,most of the existing literature on shallow-buried soil tunnels pays little attention to thestrain softening behaviour of soil.In this thesis, a3D numerical model was established by FLAC3D, and the strainsoftening behaviour of soil was considered in the tunnelling simulation, so as to investigatethe stability of the shallow-buried soil tunnel affected by the strain softening behaviour ofsoil.Firstly, the theory concerning the strain softening model in FLAC3Dwas introduced.Combined this with the direct shear test, the strain softening parameters were determined.Subsequently, based on the field test data for tunnelling, in the3D simulation model, whenthe soil was excavated, the preliminary support was installed, the second lining wasinstalled, the stress relaxation factors were identified.Secondly, the tunnelling was simulated by use of the Mohr-Coulomb strain-softeningmodel and the Mohr-Coulomb elastic-perfectly-plastic model inserted in FLAC3D,respectively. Results indicate that: compared with the elastic-perfectly-plastic model, thesoil deformation, the axial displacement at the tunnel face and the range of plastic zone ofthe soil are remarkably larger, as a result, with no consideration of the strain softeningbehaviour of the soil, the tunnel design for shallow-buried soil tunnels tends to bedangerous.Finally, the influence of related parameters of the strain-softening model in FLAC3D(the strength reduction factor, the critical plastic softening parameter) on the mechanicalresponse of the tunnelling was discussed. Results indicate that: the soil deformation, theaxial displacement at the tunnel face and the range of plastic zone of the soil increases with the decrease in the strength reduction factor, the critical plastic softening parameter, and theinfluence of the strength reduction factor on them is greater.The achievements in this thesis can be valuable for the design and construction ofshallow-buried soil tunnel.