2D stability analysis of concrete and masonry gravity dams. Guideline comparisons related to deterministic and semi-probabilistic approaches. Application for the determination of mechanical properties
31 May 2011
In France, hydro power companies need to periodically reassess the stability of their dams due to the revaluation of loadings (hydrology, seismicity, sedimentation) or ageing of structures. In 2001 the French utility company EDF, which operates more than 350 concrete and masonry gravity dams, published internal guidelines, called the CTH guide, for the 2D stability analysis of gravity dams based on a classical determinist approach. In 2004 the French Committee of Dams & Reservoirs (CFBR) initiated a working group in order to unify French practices concerned with the stability evaluations of gravity dams. Provisional guidelines, based on a semi-probabilistic approach, were published in January 2006. Both methods followed similar steps, i.e. the identification of project conditions and related load combinations, the determination of dam and foundation mechanical properties, and the evaluation of safety criteria (against sliding, shear failure and cracking). Due to the differences between these two approaches, global safety factors have been considered in the CTH guide and partial safety factors in the CFBR guide, and the determination of mechanical properties is also influenced by these two approaches. The CFBR guide contains rather high values concerning materials, for which mechanical data is available (conventional vibrated concrete or roller compacted concrete), whereas the CTH guide provides conservative values including safety provisions. In addition, the CFBR guide provides additional project conditions and methodology for the mechanical properties determination as follows: 1) Extreme hydrostatic condition (more than 5000-10,000 years of flood), in addition to conditions usually taken into account, e.g. normal top water level, design flood condition (1000 years for large dams), and the maximum credible earthquake condition, and 2) Tools to determine foundation properties (cohesion and the coefficient of internal friction); the Hoek & Brown failure criterion for the homogeneous rock or the Barton criterion for rock joints.
