Influence of spatial variability of soil Young's modulus on tunnel convergence in soft soils

Field data have shown the fact that soil spatial variability could aggravate the uncertainty of tunnel convergence Delta D (a key indicator for serviceability and safety of tunnels). This paper presents a detailed numerical analysis to investigate the probabilistic response of tunnel convergence in spatially varied soft soils. The soil Young's modulus E-s is highlighted and modeled with isotropic and horizontally stratified anisotropic random fields, respectively. The influence of scale of fluctuation (SOF) delta of the E-s on convergence Delta D is discussed in detail with respect to different directions, i.e., the vertical and horizontal directions both for delta and Delta D. It is observed that ignoring the spatial variability of E-s i.e., disregarding the possibility of unfavorable soft soil (low stiffness soil) locally around tunnel, can underestimate the mean value of Delta D. The horizontally stratified anisotropic random field is more appropriate than isotropic random field in the sense of an accurate prediction, especially when extreme tunnel convergence occurs. In horizontally stratified anisotropic random fields, the influence of horizontal and vertical SOF is different on tunnel convergence. The surrounding soils near tunnel crown and invert or across tunnel horizontal diameter are very critical to the tunnel convergence. In addition, the effect of horizontal SOF delta(x) on failure probability of the calculated Delta D exceeding the specified allowable Delta D-lim is limited when the delta(x) is larger than 4.84 times of tunnel outer diameter.