Sliding stability of concrete dams : A study on the influence of rock joint characteristics

Sammanfattning

Sliding stability is a key aspect of dam safety, especially for concrete dams founded on jointed rock. It is influenced by factors such as joint inclination, length, frictional properties, and hydraulic conditions. While shear strength of rock joints have been widely studied, the specific influence of joint characteristics on sliding stability, particularly how analytical models respond to these variables, remains partially unclear. This thesis addresses these gaps by examining how variations in joint behavior affect sliding stability. Analytical calculations and numerical simulations using COMSOL Multiphysics were performed for both buttress and gravity dams. Two joint configurations were studied: (1) a vertical joint intersecting an inclined joint, and (2) two inclined joints forming a passive wedge downstream. Parametric studies explored the sensitivity of stability to changes in joint inclination, length, friction angle, aperture size, and hydraulic conductivity. The results showed that the influence zone for Model 1 was larger for buttress dams than for gravity dams. For all tested joint extension lengths (𝑙𝑥), buttress dams exhibited critical inclination angles (α) of approximately 20°–25° analytically and 15°–20° numerically, while gravity dams showed a narrower range, with critical angles around 10°–13° analytically and 7°–10° numerically. In contrast, Model 2 achieved the required stability only when the joint extension length 𝑙𝑥1=10 𝑚. It showed a higher critical angle of approximately 26° for both dam types when combined with a second joint inclination angle α2=15°. For buttress dams, longer and steeper joints significantly increased safety factors, whereas in gravity dams, the stabilizing self-weight reduced sensitivity to joint behavior. Numerical models offer a more nuanced assessment of stability by capturing uplift dissipation and joint interaction, but their sensitivity requires validation through site characterization. The ratio between joint and rock mass hydraulic conductivity proved critical for uplift response, and joint aperture had a notable impact on uplift and overall stability. In conclusion, a combined approach is recommended, analytical methods for initial screening and validated numerical models for detailed analysis, to ensure reliable safety assessments in jointed rock foundations.