Underwater Soil Improvement for Port Facilities by Compaction Piles of Oyster Shell-Sand Mixture

In Japan many of port facilities such as breakwaters and quay walls are constructed on the soft sea bottom, and it is necessary to improve these soft grounds before construction of upper structures, in order to maintain the stability of facilities. One of most popular methods of soil improvement in Japan is sand compaction pile (SCP) Method, in which many compacted columns of good quality sand are performed inside the soft ground. In Miyagi Prefecture, 4 thousand tons of oyster are harvested annually (2^nd largest amount in Japan) and 30 thousand tons of oyster shell are produced, 75 % of which has been either disposed of or stocked without any eventual use in mind. Oyster shell is composed mainly of calcium carbonate. It is rather thin, but well graded when crushed. Crushed oyster shell is comparatively light and pervious, and mixability with soil is good. Making use of such advantageous characteristics, large amount of oyster shell can be used as material of SCP. After carrying out a series of geotechnical investigations such as laboratory soil tests and model ground SPT tests on crushed oyster shells and shell-sand mixtures, trial usage of oyster shell-sand mixture to 81 SCPs under the breakwater in Ishinomaki Port were performed in 1992 with some relevant laboratory and underwater field tests. In 1993, practical usage to 420 SCPs and some relevant investigations were performed. Since then, about 10 thousand tons of crushed oyster shell have been used annually together with 20 thousand tons of sand as material of SCP for improving soft ground under the breakwaters and quay walls in Ishinomaki Port for more than 15 years. This utilization contributed to improving the environment and the economy.