Abstract: The mechanical characteristics of the shallow buried large span and unsymmetrical tunnel construction with imbalanced pressures could be very complicated since its surrounding rock could be very easily to cause the stability issue and even lead to tunnel collapse under the different construction activities. In order to analyze the excavation process of the ultrashallow buried, largespan & bias tunnel with the double side drift method, a threedimensional model of the tunnel was established using the commercial finite element software ABAQUS. The stress and displacement fields of the surrounding rock mass as well as the stress features of the supporting structures were studied during the excavation. The results show that: (1) the settlement ratio between the ground surface and the vault at the same section is close to 1, and the settlement of monitoring points in deep buried side of the tunnel is larger than those of in the shallow depths; (2) during the excavation, the stress of the surrounding rock mass changes constantly;  the surrounding rock mass at the vault of tunnel, the middle and bottom of the arch of the deep buried side have greater tensile stress, whilst those at the both side walls, and the middle of the arch of shallow buried side have a greater compressive stress; (3) when the benches of the both headings are excavated, the plastic area comes into being in the middle core soil of the tunnel; (4) the force of first lining changes continuously in process of the excavation; when the whole section of the tunnel is closed, the tensile stress concentration occurs mainly at the arch bottom and vault, whilst the smaller tensile stress occurs at the arch bottom of the second lining.

Key words: Ultra-Shallow Buried Tunnel, LargeSpan Biased Tunnel, Numerical Simulation, Double Side Drift Method