This chapter numerically investigates how the nucleation and propagation of plastic deformation zones, including those leading to localized banding, may promote the failure of shield tunnel faces excavated in sand. A material state-dependent model is employed to capture the mechanical responses of sand, and nonlocal enhancement is introduced through the increase in volumetric strain, which drives changes in the void ratio and associated material hardening/softening behavior, to regularize ill-posed boundary value problems caused by the activation of strain localization. The simulations of tunnel face failure indicate that lower initial void ratios result in more concentrated plastic deformation near the face and lead to a strain-softening trend in the global deformation response. In contrast, looser initial states cause the deformation zone to propagate spatially toward the ground surface, and the resulting global deformation response exhibits strain hardening. The second-order work is used to explain the relationship between material instability around tunnel faces and initial density, showing that material instability typically occupies a subset of the plastic deformation region and diminishes as the initial sand density increases.