The 117 m high Mattmark dam rests on a soil layer with a maximum depth of 88 m. The damis situated in the highest seismic zone of Switzerland. In connection with a plannedheightening of the reservoir level, a comprehensive seismic safety evaluation was carried outbased on the draft of the new Swiss earthquake guidelines for dams. The dynamic analysiswas performed using a two-dimensional finite element model of the dam-foundation system.The input motion consisted of three different sets of spectrum-compatible artificialaccelerograms with horizontal and vertical peak ground accelerations of 0.42 g and 0.28 grespectively, corresponding to the safety evaluation earthquake (SEE). The dynamic materialproperties were selected on the basis of the results of static and dynamic laboratory tests andinformation available in the geotechnical literature. The analysis showed that shallow slidescould move 2 m to 3 m during the SEE. For deeper slides, the maximum displacement wouldbe less than 80 cm. From the safety point of view, the deeper slides are more critical, as theycut through the whole core, whereas the shallow slides do not. The seismic settlements wereestimated based on the reduction of the shear stiffness of the various materials during theSEE. The freeboard remaining after the sliding displacements and the seismic settlementscaused by the SEE should be sufficient after the earthquake. The well-graded core materialtaken from a moraine and the wide filter layers will prevent any piping effects.
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