Abstract:In order to demonstrate and evaluate the stability of rock masses for the arch dam shoulders of Xiaowan power station on Langcang River in China,a three-dimensional geomechanical model test was conducted to study the features and mechanism of deformation-failure of the rock masses of right dam shoulder under the condition of excavation,designed load and overload,and to evaluate its stability and overloading capacity. The test result proves that attentions should be paid to the partial stability of the slope at the upper reaches with excavation. Under designed load,there isn’t any cracking in the rock masses and the compressed deformation of the weak zones is restricted in the distribution range of accessional stress caused by designed load. When the load is exceeded up to three times,the minute fissures begin to appear at the upper reaches of the dam foundation. The stability of the dam shoulder is mainly controlled by the deformation of weak zones and the deformation-failure process of the dam shoulder can be described by 3 stages,namely,beginning crack of dam foundation,compressed crack of dam shoulder and large deformation of dam shoulder. There is sufficient safety storage in the dam shoulder’s rock masses because its safety degrees under the normal case and the instable deformation are 3.0 and 7.0,respectively.