Long-term settlement of the reclaimed quasi-overconsolidated Pleistocene clay deposits in Osaka Bay

In Osaka Bay of Japan, a number of large-scale reclamations have been executed. The settlement due to those reclamations has been serious enough to provoke investigation. The compression of the stiff Pleistocene clay deposits has caused the half of this settlement, and it makes the quality control of the reclaimed lands very difficult. According to the geological history, the Pleistocene clays in Osaka Bay are thought to be normally consolidated aged clays that exhibit seemingly overconsolidated behavior due to the nature of quasi-overconsolidation. This makes the problem of settlement more complex. In this paper, characteristics of the long-term settlement of the reclaimed islands in Osaka Port are discussed based on the in-situ measured data. Long-term compression is found to occur even in the deeper Pleistocene clay layers, stress conditions of which remain close to or less than p(c). Discussion is extended to the applicability of conventional procedure in terms of elasto-viscoplastic finite element method to the long-term settlement measured in the Maishima Reclaimed Island in Osaka Port. The conventional procedure to assess the settlement is found to function well for normally consolidated clays, however, serious limitation exists for describing the time-dependent behavior of the Pleistocene clays, in which stress states remain close to or less than p(c). A series of long-term consolidation tests is done to assess the property of the quasi-overconsolidated Pleistocene clays. Laboratory experimental results also provide remarkable time-dependent behavior even in the region less than p(c). It is found that the rational modeling for compression in the region of close to and less than p(c) is indispensable to describe the actual time-dependent behavior of the quasi-overconsolidated Pleistocene clays in Osaka Bay.