In order to improve a dredged clayey ground, the vacuum consolidation method (VCM) using horizontal drains has been developed and already effectively applied at 10 construction sites. In this consolidation method, the drains are horizontally installed in the very soft clayey ground using a drain installation ship, and a negative pressure was applied to the clay through the horizontal drains. In case of the VCM using vertical drains, the drains with 0.1 m in width and 0.003 m in thickness were usually used. On the other hand, the special prefabricated drains were used in case of the VCM using horizontal drains, where its dimensions were 0.15 m in width, 0.01 m in thickness and 250 m in length. Wide and thick drains bring an advantage to prevent the delay of consolidation when long drains are used, because the well-resistance of drains is small. In this paper, the design, the execution procedure and the four case histories regarding the VCM using horizontal drains are generally described from outcomes of the former study.
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