Abstract：The size of the north anchor pit of Runyang Bridge is about 69 m×50 m×50 m. The support for this pit is concrete diaphragm wall with internal support structure，and the diaphragm wall is embedded into rocks. In order to obtain the safety coefficient and the failure process of this pit，3D nonlinear finite element analysis is carried out with ANSYS. The excavation process is simulated with the “element dead/active” function of ANSYS. The self-weight of soil in finite element model is increased hypothetically until support structure fails，so that the failure process of support structure can be simulated. Comparing the assumed self-weight of soil with the true value，the safety coefficient of the pit can be obtained. The numerical results show that all the failures of the pit begin with the collapse of the internal support and the failure models are different in various excavation depths. This method can also be applied to the other pits if their support structure is relatively stiff.
