A Method for Determining Nonlinear Foundation Reaction Coefficient of Shield Tunnels Based on UH Model

To accurately capture the nonlinear characteristics of subgrade reaction surrounding shield tunnel under surcharge, a determination approach for the subgrade reaction of shield tunnel was implemented on the basis of the soil strength design method (MSD method) and the unified hardening model(UH model). The practical simplified version was proposed, by conducting the description of the mechanical properties of the entire soil zone with soil constitutive curves at the height of tunnel center. Furthermore, using the statistically derived UH constitutive parameters for soil layers ④ and ⑤1 in Shanghai, the subgrade reactions for both soil layers were predicted and fitted by using a hyperbolic curve function. After a secondary fitting process was conducted to obtain the initial stiffness and ultimate strength in relation to the constitutive model parameters, the functions of subgrade reactions for Shanghai's shield tunnels was established. Eventually, multifactorial evaluation of Shanghai's shield tunnels under surcharge conditions was performed, confirming the capacity of the developed formulas for predicting the large deformation of tunnel under surcharge. The convergence deformation of the tunnel under surcharge is more sensitive to the initial stiffness of subgrade reactions, governed by various soil parameters such as κ/λ and confining pressure, than to its ultimate strength.