Three-Dimensional Probabilistic Analysis of the Surface Settlement Based on Spatial Variability of Soil Properties: Case Study Zarbalizadeh NATM Tunnel

The design and construction of a tunnel in urban areas need special considerations. Usually, the excessive settlement caused by tunnelling during the construction process damages the adjacent infrastructures and utilities, especially if the tunnel is excavated by the new Austrian tunnelling method (NATM). Thus, it is important to make accurate predictions and effective control on tunnelling-induced settlement. In this study, the soil’s Young’s modulus was modelled using a three-dimensional random field and coupled with a finite difference method (FDM) analysis to reveal the scale of fluctuations (SOF) effects on the maximum surface settlement (S_max). The Fourier series method is employed to generate the field of Young’s modulus for soil, and sensitivity studies are further performed via Monte-Carlo simulations (MCS). The results demonstrate that the mean value of S_max increased from 28 to 31 mm and its coefficient of variation (COV) increased from 0.02 to 0.35, with an increasing horizontal SOF but both stabilize at higher values of SOF. Furthermore, the probability of failure increases as the COV increases for each allowable limit greater than the verification FDM of S_max. It is observed that ignoring the spatial variability of the soil properties leads to an underestimate of excessive surface settlement risk.