The Wudongde hydropower plant under construction is located where there exists strong regional tectonic movement. Faults

folds and joints are well developed

and most of the river valleys are steep and narrow in this area. Therefore

the distribution characteristic of geostress is quite complex

which exhibits a significant nonlinearity. To accurately obtain the distribution rule of initial geostress field in the dam area

a new two-stage back analysis method is presented

which considers the optimized distribution characteristics of geostress. Firstly

the geostress is decomposed into three components including gravitational stress

tectonic stress and nonlinear stress. By considering the influence of topography

geological structure

and inhomogeneity of the rock mass

a model in numerical software FLAC3D is developed. The superposition method is applied to the first-stage back analysis of initial geostress field. Then

by considering the small scale geological structures and local excavation in the vicinity of the dam area

a refined three-dimensional discrete element model using 3DEC is established. The preliminary lateral pressure coefficient and gravitational stress modified coefficient of the refined model are obtained from the developed model. Moreover

the calculation for the second-stage back analysis of geostress field is conducted by optimizing the coefficients based on a uniform design. During the first-stage back analysis

the calculated results agree well with the measured data on the whole

but the difference between them is large in the vicinity of the local geological structures. The calculated results are in agreement with measured data in the second-stage back analysis by considering the small scale geological structures

local excavation and the results in the first-stage back analysis. The commercial FLAC3D and 3DEC software can be combined to obtain the initial geostress field under complex conditions. The distribution characteristics of geostress field shows continuous in a large scale and discontinuous within a small range due to local geological structures.