Steel sheet piles are indispensable material for construction of harbor structures and have been used with cathodic protection in submerged zone.
　It is reported that in the case of large scale earthquake, liquefaction of backfill materials behind quay walls damaged some steel sheet piles at the portions where aluminum alloy anode were underwater-welded.
　One of the major causes of that damage may be the brittleness of sheet piles ascribed to underwater welding.
　Under this background, the following items are studied in this paper.
(1) Review of the damages of steel sheet piles found in past large scale earthquakes
(2)Full-scale test execution of underwater welding
(3)Experimental confirmation of fractural mechanism of underwater-welded steel sheet piles
(4)Experiment to find chemical composition of steel effective to improve the fractural mode of underwater-welded steel sheet piles
(5)Proposition of new standard of chemical composition suitable for steel sheet piles