抄録
It has been indicated that the sensitivity of the marine sediment in Ariake Bay area varies between 8 and 50, and rarely is 100. Recently, however, extra quick clays with sensitivity greater than 50 and sometimes near 1000 were found in this area. In order to find the cause of the formation of extra quick clays, the sediment samples of slightly-very quick and extra quick clay obtained from two sites in Ariake Bay area were analyzed with respect to their mineralogical, chemical, and geotechnical properties. There was no appreciable difference in caly mineralogy between the sediment samples from two sites, and they contained smectite as the main mineral. The principal difference between the sediment samples of slightly-very quick and extra quick clay was in the salt concentration in the pore water which was in the range of 1.02∼10.4 g/l and 0.05∼1.08 g/l, respectively.The experiments of salt addition to extra quick clays and salt leaching from medium quick clays demonstrated that extra quick clays could be developed by leaching out of the salt in the pore water. The fact that extra quick clays can be developed in the sediment in Ariake Bay area, in spite of the presence of smectite as the main mineral, was interpreted by considering smectite in the clays to be of low-swelling type.
