Stability assessment of the Three-Gorges Dam foundation, China, using physical and numerical modeling—Part II: numerical modeling

Sliding in a dam foundation along potential sliding paths is generally caused by two kinds of external factors: one is the overloading of the designed upstream hydrostatic load due to flooding; and the other is the gradual degradation of the shear strength of joints due to seepage, deformation, damage, and geochemical reactions between water and joint surface minerals. Based on the conceptualized geomechanical model of the Three-Gorges Dam, described in the Part I paper, in this Part II paper the limit equilibrium method and finite element method are used to study the effects of gradual degradation of the shear strength of joints on the stability of the dam foundation. The numerical modeling focuses on the stability conditions of the no. 3 powerhouse-dam section which are estimated to be the most critical. The constraint influences from the adjacent no. 2 and no. 4 powerhouse-dam sections are also included. The failure mechanisms, factors of safety and critical displacements of these dam sections are derived numerically as the measures for stability evaluation. The factor of safety is defined as the ratio between the combined shear strength of joints and intact rock bridges, and the mean shear stress along potential sliding path required for limit equilibrium under the designed external loads. All the results obtained from these different numerical models, together with the results of physical model tests as presented in Part I, are compared in this paper. The comparisons show that both the numerical modeling and physical modeling results support each other and demonstrate the stability of the Three-Gorges Dam foundation as designed. Nevertheless, considering the overall engineering and social–economical importance of the Three-Gorges Dam complex, some additional treatment and reinforcement measures are recommended in this paper.