Abstract: A finite element upper bound solution with unstructured mesh adaption to refinement criterion is presented to evaluate stability of shallow tunnel face and obtain accurate failure mechanism. Weight indexes of element energy dissipation are regarded as mesh adaption refinement evaluation criterion considering element scales and plastic strain. A two-dimensional finite element upper bound solution model with six-nodal triangular element and linear programming is compiled. It can perform mesh adaption with repeated mesh refinement during a series of computational process. Using strip foundation ultimate bearing capacity of foundation, the solution is verified to perform effectively in the aspects of accuracy of upper bound values and mesh features. Numerical calculation analysis is done using the finite element upper bound solution with mesh adaption. Different parameters are adopted to evaluate stability of shallow tunnel face. The relationship between element quantity and accuracy of upper bound value is analyzed during process of mesh refinement. The upper bound solutions of stability critical values of tunnel face at different depths and internal angles are demonstrated. Failure mechanism and variation law of stability of tunnel face are revealed.

Key words: shallow tunnel, face stability, finite element upper bound solution, six-nodal triangular element, failure mechanism, mesh adaptation