Many ground improvement techniques have been developed for the safe support of structures constructed on weak ground. The sand drain method is an easy‐to‐design, highly economical method of ground improvement using the principle of accelerated consolidation, which has been used successfully many times for structures of various sizes and types. Kansai International Airport was constructed on a man‐made island that was formed by land reclamation in deep water, 33 m thick, on a soft seabed. The sand drain method was carried out in a 20‐m layer of alluvial clay, and using the results of soil investigation and the monitoring of behavior of the ground, analyses were performed on ground settlement and strength improvement These methods were effective in reducing residual and uneven settlement and preventing the sliding of the seawall