Abstract:The large-scale finite element model for dam site zone of Xiaowan Hydropower Station is established,in which the influences of topography and river turning are taken into consideration. The method combining genetic algorithm with finite element method is applied to the first-stage back analysis of initial geostress field at dam site zone. Then,the small-scale finite element model of dam foundation excavation and relaxation is established,considering the main soft geological structures in the vicinity of the dam site. Based on the sub-model method,the displacement boundary conditions of the small-scale model can be achieved by interpolating from the large-scale model of the first-stage back analysis,and calculation for the second-stage analysis of geostress field is conducted. Through comparative analysis,it is shown that the main soft geological structures have much effect on the results of initial geostress field,especially in the vicinity of discontinuity during the first-stage back analysis. And the precision of the initial geostress field in the first-stage back analysis has been greatly improved in the second-stage calculation. The geostress of most measuring points are much closer to measured values and the relative error has also been reduced gradually. So the initial geostress field after two-stage analysis can be used in the later analysis of relaxation effect during dam foundation excavation. And the results also indicate that the method is feasible and of great reference to similar projects.