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COMMISSION INTERNATIONALE DES GRANDS BARRAGES ------- VINGT-SIXIÈME CONGRÈS DES GRANDS BARRAGES Autriche, juillet 2018 ------- COLLAPSE OF AGRICULTURAL EARTH-DAMS BY A RECENT EARTHQUAKE AND ITS RECONSTRUCTION Fumio TATSUOKA Professor Emeritus, UNIVERSITY OF TOKYO and TOKYO UNIVERSITY OF SCIENCE JAPAN 1. COLLAPSE AND RECONSTRUCTION OF IRRIGATION DAMS A pair of old irrigation dams in Fukushima Prefecture collapsed by the 2011 Great East Japan Earthquake (Fig. 1a). The collapse was due to generally poor soil compaction and the use of inadequate soil type (sandy soil) in the top layer despite homogeneous type of the dam in addition to a prolonged strong seismic motion. The new dams (Fig. 1b) were constructed to be much more stable by the use of adequate type soils, good compaction and an adequate structure. a) b) Fig. 1. Fujinuma dam, Fukushima Pref. Japan: a) collapsed; & b) reconstructed. Settlement 0 10 20 30 40 50 (m) Slide 4 Slide 2 Sediment Slide 3 Reservoir Downstream Masonry wall Slide 1 PM15:11 after the E. Q. (starting PM14:46:18, 11 March 2011 (by the courtesy of Fukushima Prefecture) Main dam Vol.= 232,000 m3 Auxiliary dam Vol.= 48,000 m3 CoreShell Shell Filter Rock Drain Rock Crest length= 149.2 m width= 8 m Crest elevation= EL 417.4 m 31.4 m EL 401.99 m Crest elevation= EL 417.4 m Filter Rock Core Crest length= 86.8 m; width= 7 m EL 405.00 m Shell Shell Backfill Backfill Backfill 18 m Drain Backfill 9