Seite - 1131 - in Book of Full Papers - Symposium Hydro Engineering

In ICOLD bulletin No. 93 [4] the first five years after EoC are not counted for ageing. Therefore, ageing starts five years after EoC according to [4] since all damage that occurs before is considered to be originated in construction shortcomings. The monitoring and detection of ageing processes are evident in order to assess the dam behaviour correctly. For large embankment dams with permanent reservoirs the measurement of the seepage conditions is of first priority and has to be usually done by the Owner (see [13]). For embankment dams and levees which are only impounded temporarily a continuous surveillance is frequently considered to be not required. For small embankments and small levees frequently no monitoring is conducted [11] [14]. Under favourable conditions the ageing process will not reveal considerable deterioration effects during the life span of the structure. But, e.g., if the embankment dam was designed without safety margin so that with proceeding ageing the limit equilibrium conditions are reached and exceeded. This results in an overload of the system and potential failure. The safety margin should be assessed so that the design requirements are also fulfilled at the end of the service life span of the dam. The dam will subsequently suffer from damages, acceleration of ageing, and finally from failure. Therefore, a safety margin should be considered to cover ageing effects. This safety margin should be also integrated in the specific technical guideline and codes. Not only material degradation should be integrated but also foreseeable processes, such as sedimentation, an increase of hydraulic loads, etc. The safety margin should be established by the selection of adequate design parameters such as permeability, shear strength, etc. Generally, for all high risk large embankment dams it is recommended to consider a safety margin and not to design it at “the limit equilibrium border”. In Fig. 3 the effect of ageing on the design approach is shown. Depending on the starting design approach and the influence of ageing the dam may reveal an under or over design during the operation period until the life span is reached. The under design shows an unacceptable risk; the over design reuqires the utilization of more resources but are welcome in consideration of safety reasons. A conservative design should be applied if the dam’s behaviour during the life span shows uncertainties which cannot be assessed reliable. 1131