The role of dilatancy in shallow overburden tunneling

Abstract

The mechanical behavior of sandy ground during shallow circular tunneling is explored for various overburden heights H (=0.5D, 1.0D, 1.5D and 2.0D; D is the diameter of the tunnel) and various dilatancy coefficients (ψ/ϕ = 0, 1/3, 1/2, and 1; ϕ and ψ are the internal friction angle and dilation angle, respectively) through finite difference analyses. The ground is modeled as a linear elastic-perfectly plastic material that employs the Mohr–Coulomb yield criterion and obeys the non-associated flow rule. The ground reaction curve is applied in conjunction with the stress path as a conceptual tool for interpreting the mechanical response of the ground to tunneling. It is revealed that, at a certain relaxation value, a yield zone develops during tunneling and extends to the surface. This relaxation value increases with increases in the overburden and ψ/ϕ values for the cases of less shallow tunnels (i.e., H = 1.0D, 1.5D and 2.0D), while for the shallowest case (H = 0.5D), the extent of the yield zone to the ground surface is not sensitive to the ψ/ϕ value. The shear strain due to tunneling also increases with an increase in the ψ/ϕ value. Moreover, the ψ/ϕ value affects the radial displacement and the surface settlement due to tunneling. The magnitudes of the surface settlement and the radial displacement at the tunnel crown both decrease with an increase in the ψ/ϕ value. The relative difference in the displacement at the tunnel crown between the upper bound and lower bound values, ψ/ϕ (at the last computed stage), increases with an increase in the overburden height. It is recommended, therefore, that careful consideration be given to the dilatancy angle in the case of relatively less shallow tunnels.