The conventional analysis methods for face stability of shield tunnels always assume that the soil is homogeneous

isotropic medium

and ignore its heterogeneity. Therefore

the influence of spatial variability of shear strength on face stability in sand is investigated in this paper. Based on random field theory

three dimensional random fields of friction angle are generated using the covariance matrix decomposition method. The performance of the coefficient of variation and the auto-correlation distance of friction angle is analyzed to explore the influence of that on the failure mechanism and the critical support pressure. Meantime

the selection of the characteristics value of critical support stress is discussed by probabilistic analysis approach. Results show that the spatial variability of friction angle does have significant influence on the stability of tunnel face. The critical support stress scatters as the coefficient of variation of friction angle increases. The failure mechanism is closely related to the auto-correlation distance. When the auto-correlation distance is close to the tunnel diameter

the local failure may happen. The concept of characteristics value of critical support stress is proposed and defined preliminary based on the probability of failure.