The changes of in-situ stress magnitude and directions caused by the excavation disturbance effect of pilot tunnel are usually neglected. Firstly, the surrrouding stress field of pilot tunnel was inversed finely by multiple linear regression inversion anlaysis in the local mesh model. In order to obtain the entire stress distribution of damsite region, the inversion analysis method of second stress field for the whole damsite region was presented, which combined the ground abrasion process of strata and support vector machine model. Considering the abrasion process of the strata, the second stress value of measuring points is calculated in the entire mesh model. Numerial results of real project indicate that the in-situ stress field obtained by second fine inversion method fits for the distribution law of V-shaped vally generally.
Back-analysis of in-situ stresses is of top priority to the stability analysis in the rock engineering. Most river valleys in southwest of China are mountain valleys and are the symmetrical V-shaped or the nearly symmetricalV- shaped (Fig. 1). There are extensive crustal movement, abundant active faults, extensively complicated geological environment, and the physiognomy of alpine gorges in the southwest. Its in-situ stress field is complicated due to many tectonic movements such as tectonic stress, overburden and exhumation, in addition to the crash and squeezing between Indian plate and Eurasian plate which is the primary cause to give rise to the horizonal deformation of rock mass in the depth of Chinese Mainland (Liu 2006). The measurement of in-situ stress is timeconsuming, expensive and onerous, and furthermore, it is very demanding on the skill, so it is impossible to carry out in engineering field. The measuring results are limited and discrete to some extent due to the in-situ stress and the measurement errors (Ljunggren 2003).