Upper Bound Analysis of Suction Effect on Tunnel Stability

This paper reports numerical studies into mechanical characteristics of tunnel stability in unsaturated soils using a modified upper bound discretization scheme, the discontinuity layout optimization (DLO) method. A modified approach, unsaturated DLO (UNSAT-DLO) was utilized to take into consideration the effect of suction stress on strength. A parametric study was carried out using the UNSAT-DLO approach for two simulated unsaturated soils. This was to examine the influence of soil capillarity on strength and failure mechanisms for five different scenarios of a circular shallow tunnel. The scenarios comprised various combinations of suction profile, water table height (H-w), effective internal friction angle, cohesion (considering soil fabric changes), shotcrete strength, and air entry suction, representing different soil types or soil hysteresis. The numerical results revealed a nonlinear relationship for the tunnel stability with suction. The tunnel stability for both simulated soils was seen first to rise with increasing suction and then to fall, owing to desaturation, slightly before and beyond the residual suction. The shear strength parameters, as well as the increase in shotcrete strength, owing to hydration, showed significant increase in the tunnel stability. Finally, an increase of about 6.63% in tunnel stability was obtained at phi ' = 30 degrees, c ' = 0 kPa and H-w = -2 m when the air entry suction increased from 25 to 50 kPa. The modified UNSAT-DLO approach is, therefore, a promising tool that can be utilized to determine stability for various geotechnical applications at unsaturated conditions, such as tunnels, retaining walls, and slope stability problems.