Required Lining Pressure for the Stability of Twin Circular Tunnels in Soils: Fid-Bau Portal

Required Lining Pressure for the Stability of Twin Circular Tunnels in Soils

Sahoo, Jagdish Prasad / Kumar, Jyant

In this study, the stability of twin circular tunnels in purely cohesive and cohesive-frictional soils was evaluated. It is assumed that the internal compressive normal pressure (σ_i) required to support the tunnels, by means of lining and the anchorage system, becomes uniform (isotropic) on the tunnels' periphery. The interference effect of twin tunnels on the magnitude of σ_i was examined by employing upper-bound limit analysis in conjunction with finite elements and linear optimization. The internal support pressure (σ_i) needed to maintain the stability of the tunnels is expressed in the form of a dimensionless parameter (σ_i/c), which becomes a function of the dimensionless variables S/D, H/D, γD/c, ϕ, and m, where c refers to soil cohesion, S is the clear spacing between the tunnels, H implies tunnel cover, D corresponds to diameter of each tunnel, ϕ is the internal friction angle of soil mass, and m accounts for the rate at which the cohesion increases linearly with depth. The effect of the changes in S/D on the magnitude of σ_i/c was examined for different combinations of H/D, γD/c, m, and ϕ. The results are presented in the form of nondimensional stability charts that can be readily employed by practicing engineers for the purpose of design.