Undrained face stability analysis of dual circular tunnels using three-dimensional adaptive lower bound finite element limit analysis method

This paper studies the undrained face stability of dual circular tunnels with the use of three-dimensional lower bound finite element limit analysis (3D LB-FELA) method based on a node-stress-based mesh adaptive refinement strategy. The effectiveness and correctness of the proposed 3D adaptive LB-FELA are proven through extensive comparison. In this paper, the non-homogeneous feature of the shear strength increases linearly with depth for the undrained clay is considered, and the dimensionless coefficient Ns is adopted to assess the face stability. Through extensive calculations, the influences of depth C/D, the distance between dual tunnels S/D, soil gravity parameter gamma D/c0, and the strength coefficient rho D/c0 on the face stability coefficient Ns are deeply discussed. The research results show that the proposed 3D adaptive LB-FELA method can effectively evaluate the undrained face stability, and the evolution characteristics of failure mechanisms of dual circular tunnels can be studied according to the proposed method. The research results can also provide data support for evaluation of the face stability and formulating the reinforcement scheme for the dual circular tunnels.