STRUCTURAL DUCTILITY IN DAM ENGINEERING

This paper describes the importance of reduction of hydrological and hydraulic risks during construction of dams in view of uncertainties of design parameters. In practice the risks during the short period of construction may be much larger than the risk during operation of the dam. To achieve balanced safety in different phases of the project (construction, first impoundment and operation), for most projects increased safety during construction would be required. The Guhuo dam failure in China in 1993 indicated that failure of a 3 MCM reservoir could cause 350 deaths. Adopting more conservative criteria for design of the diversion system would be not only uneconomical but also may in practice would increase the risk to the downstream if a higher cofferdam is required. One of the most important strategies proposed by Holistic Design of adaptive hydraulic Structures for coping with uncertainties is enhancing structural ductility in dam engineering. Design hydraulic structures to adapt to extreme events far larger than design parameters and remain inherently safe (structural ductility). In this context, overtopping resistant concrete and earthfill cofferdams may offer great potentials in increasing safety and reducing costs. In 1990’s construction of many dams was initiated in Iran. In all, 5 upstream cofferdams (along with 6 downstream cofferdams) were overtopped. Except for Delvari arch gravity cofferdam, which resisted two overtopping (one of them in a 80-year flood), all the other cofferdams failed. In addition the damage to the construction equipments was minimized by nonstructural risk reduction measures. It is concluded that if the failed cofferdams had been protected by an RCC overlay or other options, they would have overtopped safely. It is concluded that cost effective and expeditious risk reduction during construction and operation may be achieved through integration of structural ductility and nonstructural risk reduction measures. The historical Kurit dam near Tabas, Iran, would serve as an illustrating example of structural ductility. The 60 m high dam, which was the highest dam for 550 years in world, has resisted more than 1000 overtopping ever since it has been constructed. The excellent overtopping resistance of the dam has resulted from the combined effect of arch configuration, suitable geotechnical conditions of abutments and foundation and erosion resistance of masonry.