A three-dimensional fast Lagrangian program FLAC3D is adopted to conducted the detailed study on the dynamic responses of encloure structure in a 500 kV underground substation of the Shanghai World Expo.The Mohr-Coulomb model is used and static analysis

dynamic analysis and artesian water influence are included in this research.The unified strength theory is introduced to the model by secondary development in FLAC3D to determine the stress state of the envelope′s reinforced concrete for truly reflecting the responses of the envelope under the action of the dynamic force.The results show that the ring stiffness of the cylindrical enclosure structure is large.When it is forced

most of the lateral loads will be transferred to the arch structure in the ring direction

which controls the deformation of the pit and the surrounding soil in both static and dynamic conditions.The excavation as a whole is in a safe state in a variety of operating conditions.