A NUMERICAL METHOD FOR SEISMIC SAFETY EVALUATION OF CONCRETE ARCH DAMS AGAINST FAILURE IN JOINTED ROCK MASS AT ABUTMENT

Seismic safety of concrete dams is a grave concern in view of the possible disastrous consequences of a rapid release of the reservoir when the dam fails under a large earthquake. In the case of concrete arch dams, which are often placed on jointed rock foundations, the earthquake-induced failure of rock joints at abutment is one of the dominant mechanisms that endanger the stability of dam. In this paper a numerical method is presented for the seismic safety assessment of concrete arch dams against rock joints failure. In this method, the results of dynamic analysis of a three-dimensional FE model of the dam and foundation are used for the next two-dimensional local analyses on critical regions of abutment. In the local analysis, which contains a 2-D horizontal domain of abutment at a critical elevation around its interface with dam body, the rock joints are modeled by joint elements. The time history of sliding and opening of rock joints can be obtained as a result of 2-D dynamic analysis. Then two so-called ‘Damage Indices’ are introduced based on the magnitude of failure on joints to indicate the failure status of abutment at each time of earthquake excitation. After several dynamic analysis of different amplification, which applied on a design earthquake motion, the maximum value of each Damage Index during the time history can be plotted against its peak ground acceleration. From this variation it is possible to find the PGA of critical earthquake, which causes the instability in abutment. For the behavior of rock joints in sliding, a shear softening constitutive law has been implemented to the FE analysis.