Numerical analysis on the bearing mechanism of branch tunnel in upper softer and lower hard ground

Abstract

It is of great interest to investigate the excavation disturbance mechanism of branch tunnel in the upper soft and lower hard ground. In this paper, a fine 3D numerical simulation, considering the longitudinal gradient of tunnel, was performed to investigate the bearing mechanism of branch tunnel. By comparison of the characterises of stress redistribution, two different bearing mechanisms were obtained: the beam bearing mechanism dominates the capacity of super-large section, and the beam bearing zone and stress concentration zone are identified in the arch zone; In the small interval section, the arch bearing mechanism works, and the stress loosening zone and arch bearing zone appears above the arch. The attributed reasons are related to the thickness of hard layer above the arch and the span of tunnel. Correspondingly, the arch settlement in the super-large section is much larger than that of the small interval section, while the deformation at arch foot of both sections moves outside. Furthermore, through the analysis of the maximum principal stress direction, the determination method of the outer boundary of the pressure arch was discussed.