Waste disposal sites along the coast in Japan are usually constructed on the thick clay ground, which has sufficient cutoff performance against the seepage from the bottom. Vertical seawalls of steel sheet piles or pipe sheet piles, as an impervious wall as well as a retaining structure, are considered one of the most useful methods for the landfill, because the walls can be embedded deeply into the impervious seabed.
　The present paper describes field tests at an actual coastal area, Kure port, Hiroshima, Japan. The tests investigated the cutoff performance about the seven types of steel seawalls newly developed for the purpose of improving the impermeability of the joints between steel piles so as to further enhance the credibility of the coastal waste disposal landfill. Test facilities for the seven types of cutoff walls were designed and constructed, then the cutoff performance was evaluated by leakage tests by filling water for about one year. After that, the facilities were actively deformed by hydraulic jacks or other methods in order to investigate the deformation-following performance of the joints. The results showed that the present seven types of walls had sufficient cutoff performance in accordance with the design standard prescribed in a ministerial ordinance, and that those were ready for practical use from the viewpoint of installation work as well.
　In addition, the applicability of the present cutoff walls is discussed and tabulated in the paper. Several seawall structures combined with the present cutoff joints proposed are studied from the viewpoint of the fail-safe design including backup functions, inspections, monitoring, and repair works, where advantages and
disadvantages are compared among the proposed structures.