Steel pipe sheet pile cutoff walls built up by installing steel pipe sheet piles (SPSPs) in coastal waste landfill sites are important elements for preventing leakage of leaching from the reclaimed waste to the open sea. Adhesion with the peripheral ground has to be maintained in the installation of SPSPs for manifestation of the cutoff function of the SPSPs with joint sections. In this study, attention is paid to the disturbed area generated in the peripheral ground around the steel pipe sheet pile cutoff walls. Moreover, by using seepage and advection/dispersion analysis, we evaluated the impact of the disturbed area on the environmental preservation function of coastal waste landfill sites as a whole, the effect of the installation method which suppresses the formation of the disturbed area and the influence of soil improvement by the sand compaction pile method. One of the results of this study has shown that the disturbed area generated in the lower deposition clay layer greatly influences the quantity of leakage of toxic substances in a specific route. On the other hand, the installation of SPSPs by using soil cement, in which the soil improvement of the periphery ground is undertaken, is effective in suppressing the leakage of toxic substances
