Three-dimensional continuous field of initial stresses in Ertan site is determined with stress function approaching the date of a number of measurement points. Then a 3-d FEM analysis of a set of powerhouses with three large chambers is made

and the distribution of failure region

low and high stress areas in rock masses are obtained

which is compared with the results of 2-d FEM analysis. The results from 2-d analysis are not always more conservative than that from 3-d analysis

because the tension failure region from 3-d analysis is larger than that from 2-d analysis. A method to determine approximately failure area of jointed rock masses is also proposed in this paper. In addition

deformation monitoring of a testing chamber in site and its back analysis are presented. This back analysis demonstrates well the measurement results of mechanics parameters and initial stresses.