Abstract：The underground workshop of Xiaolangdi Hydrojunction is a large-scale underground opening complex consisting of the diversion tunnel，the main power house，the bus bar chamber，the main transformer chamber，the tailgate chamber，the draft tube and the tailrace tunnel. The three-dimensional continua fast Lagrangian analysis software FLAC3D is employed in the analysis. The viscoplastic model in FLAC3D for the rheological constitutive relation of surrounding rock masses is adopted. The three-dimensional meshes for numerical calculation have a range of 357.10 m×246.44 m×451.50 m，and have 44 600 elements and 48 705 nodes. The draft tube and tailrace tunnel’s concrete lining is simulated using the shell elements. The supporting structures of the main power house，such as prestressed cables，tension bolts and shotcrete lining，are simulated through improving rock mechanical parameters within the anchored scope. The whole excavation process is divided into seven excavation-supporting steps in the light of the practical excavation process. The time to exert excavation load for each step is set at the middle point of the corresponding step. For the application of the initial geostress，deadweight stress is selected as vertical component；and a lateral pressure coefficient of 1.0 is chosen for horizontal stress based on measured and theoretical analysis results. Mechanical parameters of rock masses are determined through back analysis of displacement measurement data of 5 years (1997－2002). Several analysis results and conclusions are given：(1) After the excavation，the horizontal displacements at the middle of the main powerhouse upstream and downstream sidewalls are considerably large with a maximum value of 1.66 cm. Only a small scope of plastic zones appears during the excavation. (2) Both the calculated and measured convergence rates are about 0.003 mm/day in the 4 years after completion of the project (1998－2002). (3) Rock creep will exist in a very long time，but the creep rate is very small and gradually reduces to an extremely small value，and no new plastic yielding occurs within 100 years considered in the calculation. So the surrounding rock masses are in good stability condition. But the creep displacement may reach to a considerably great extent and be harmful to the rock supporting structures and other concrete structures in the underground workshop.
