Abstract: According to the site measurements of geostress and engineering geological conditions in Ada dam area of west route of South-to-North water transfer project, the established 3D simplified geological model of the dam is calculated by FLAC3D combined with the linear multivariate regression analysis principle. Then, the optimum regression coefficients have been achieved. Comparing the computed results with the measured geostress data, it is found that the two are very close in values and directions, which suggests the accuracy and reasonability of the geostress field regression. Therefore, the distribution rule of the initial in-situ stress field can be obtained accurately. The further analysis is shown as follows: (1) The stress concentration at the bottom of the river valley is obvious; and the vertical depth of the zone is about 80 m with the peak value of 15 MPa. (2) There exists considerable difference between the maximum horizontal principal stress direction of two boreholes which are only 350 m apart, which reflects the control action of the direction of compressive principal stress by geographical and topographic features of the river valleys. (3) The initial geostress field in dam area is a medium stress field which is dominated by tectonic stress field at the shallow parts; but control action of gravity field increases gradually with the increment of buried depth. The results above provide important reference for design and construction of Ada dam and its diversion tunnel axis layout.

Key words: South-to-North water transfer, initial geostress field, back analysis, linear multivariate regression analysis, river valleys