Current Seismic Safety Requirements for Large Dams and Their Implication on Existing Dams

Large dams, especially concrete dams, have been designed against earthquakes since the construction of Hoover dam in the 1930s. The design criteria and methods of analysis used in those times are outdated today and the actual seismic safety of these structures is not known in view of today’s requirements. Earthquake action was usually taken into account pseudostatically, through an inertia force characterized by a seismic coefficient. Since 1989 the ICOLD guidelines consider two levels of seismic loading, namely the operating basis earthquake (OBE) and the safety evaluation earthquake (SEE). Various methods of dynamic analysis are now available to predict the dynamic response of dams to strong ground shaking. Rehabilitation planning and design must take into account these developments, even in regions of low seismicity and assess whether seismic upgrading of a particular dam is necessary from a seismic safety point of view. As an example the seismic safety assessment of the Mattmark fill dam is discussed, which is located in the area of highest seismicity in Switzerland with a peak ground acceleration of the SEE of 0.43 g. INTRODUCTION Large concrete dams have been among the first structures that have been designed against earthquakes since the 1930s, however, the knowledge about their seismic safety is actually poor, because most of them were designed using seismic design criteria and methods of dynamic analysis that are considered obsolete today. Earthquake loading was represented by a pseudo-static force expressed as the product of a seismic coefficient and the weight of the structure, acting in upstream-downstream direction plus a hydrodynamic force acting on the upstream face of the dam. This type of analysis did not consider the dynamic characteristics of the dam, although both, inertia forces and the hydrodynamic pressure were taken into account. For embankment dams, the design practice was similar, but great efforts were made in understanding the dynamic behaviour of these dams after several failures had occurred due to liquefaction during major earthquakes in the 1960s and especially after the 1971 San Fernando earthquake in California. Earth dam design prior to the 1960s was mainly empirical using judgment guided by past experience. At those times, sites for dams were generally unproblematic and engineers were confident that they could build completely safe structures. Little attention was given to the consequences of a possible failure. The confidence in the ability to build safe dams was derived from the satisfactory performance of a large number of