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During the last ICOLD Congress in Brasilia, a common meeting between the technical committee on concrete and the technical committee on numerical methods for dams took place, during which the attention was called upon consequences of AAR on the structural behaviour of hyperstatic structures, and the question was raised on how to evaluate the different existing methods and software which have the ability to consider concrete swelling. The question was again put on the table at a workshop held in Gennevilliers in September 2009, just before the 10th ICOLD Benchmark Workshop on numerical methods of analysis of dams.
The numerical problem now proposed in the frame of Theme A of the 11th Benchmark Workshop is very representative of those raised to engineers by swelling due to AAR. The example selected is that of Kariba dam, a large arch dam built on the Zambezi River between Zambia and Zimbabwe in the late fifties, which has shown evident signs of swelling soon after the starting of its operation. Thanks to the careful and efficient surveillance made along years by the Zambezi River Authority who operates the dam, thanks also to the progressive implementation of additional instruments, the specific behaviour of the structure could be monitored, modelled and interpreted, which allows today to conclude that in spite of this swelling process, the dam behaves well as a whole and remains safe.
A very specific feature to Kariba dam case is the apparently strong anisotropy which exists between vertical and horizontal swelling rates, well reported by measurements of horizontal (topographic and pendulums) movements on one side, levelling measurements on the other. This has been interpreted as being due to the stress dependence of the swelling process, since major stresses are mainly developed in the direction of horizontal arches, while vertical stresses are only due to the dead weight and therefore much smaller.
The exercise proposed to participants to the 11th Benchmark Workshop consists in determining the adequate swelling law and parameters which allow the best identification with both horizontal and vertical movements of the dam vs time. Water level and monitoring results will be provided during the 1963‐1995 period to allow this identification, then only water level will be given to allow the prediction of further movements during the 1995‐2010 period of time.
Participants are free to select the more or less sophisticated swelling law they will use for the exercise.
In order to keep the exercise within reasonable limits of difficulty, some strong simplifications have been adopted, especially in
- the construction, joint grouting and impounding sequence,
- the thermal effects, made negligible by adequate selection of monitoring readings.
Additionally participants are strongly encouraged to adopt a simplified reservoir level curve with several successive periods during which the water level will be supposed constant.