Abstract: Stability and failure mechanism of shallow tunnel under surcharge loading is studied. The program is compiled based on finite element upper bound theory with rigid blocks translatory moving elements to obtain upper bound solution of critical surcharge coefficients and rigid block movements failure mechanism. Comparing with current results of rigid blocks upper bound solution and finite element upper and lower bound solutions of limit analysis, it is proved that the proposed upper bound solution is reliable. The results show that: critical surcharge coefficients increases with increment of soil internal frictional angle and ratio of depth to diameter of tunnel C/D; and decreases with increment of soil weight and cohesion factor ; C/D and have great influence on tunnel failure mechanism. Failure area expanded when C/D increases; as internal frictional angle increases, failure mechanism composed by rigid moving blocks mutually split up significantly. In contrast, influence of on failure mechanism is comparatively less potent. The proposed solution obtained for finite element upper bound method with rigid blocks translatory moving elements is accurate. Obtained failure mechanism composed by rigid moving blocks is presented as slip-lined shape, which precisely indicates characteristics of failure mechanism for unstable tunnel.

Key words: surcharge loading, shallow tunnel, stability, finite element upper bound method, rigid translatory moving elements, failure mechanism