The mineralogy, geochemistry and geotechnical properties of clay samples in Ariake Bay, Japan are presented. The mineralogy and geochemistry are discussed in terms of depositional and postdepositional processes, and the correlations of the clay mineralogy and geochemistry with the geotechnical properties were examined by multiple regression analysis. The most predominant clay mineral in the deposits was found to be smectite. The vertical distribution of trace elements in the deposits was indicative of the periodical changes in the parent materials involved in the depositional processes. The chloride concentration in the pore water varied, according to a change in the sea level, from brackish to marine, and then brackish towards the surface. The higher iron oxide contents near the surface and in the deepest zones were the results of pyrite oxidation in the deposits due to weathering under subaerial conditions. Smectite content was the most predominant governing factor for the consistency limits and activity; the iron oxide content was the most predominant for the sensitivity, the cu/p value, and the overconsolidation ratio. Overconsolidation characteristics in the near-surface zone and in the deepest zones were attributed to interparticle cementation by the iron oxides produced by pyrite oxidation.
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