Nonlocal regularized numerical analyses for passive failure of tunnel head in strain-softening soils

Abstract

This work looks into the passive failure of tunnel head in strain-softening soils modeled using a nonlocal constitutive model. The Drucker–Prager yield criterion and plastic potential are used, and an over-nonlocal approach is utilized to regularize the ill-posed boundary value problems emerging from material softening and strain localization inelasticity. The nonlocal variable is incorporated through a Helmholtz-type partial different equation (PDE), instead of nonlocal integration. A nonlocal implicit gradient enhancement is proposed, and its numerical implementation is discussed. The proposed methodology is verified to produce mesh-independent strain-softening responses accompanied by localized deformation. The passive failure analysis of tunnel head is conducted to study key parameters influencing plastic strain accumulations and ground surface deformation. It is revealed that the strain-softening mechanism can promote local concentrated passive deformation and substantial ground surface movement near the tunnel head. Increasing the friction angle increases face supporting force, but only affects the inelastic deformation in a minimal way. The increase in the dilation angle widens the extent of horizontal soil deformation and vertical ground deformation, while a larger cover depth suppresses the passive failure response of shallow tunneling.