In recent years, as the scale of structures has been enlarged and the construction works have been carried out in deeper sea areas, the loads due to constructions has become close or larger than the consolidation yield stress, Pc, of the Pleistocene clay layer beneath the Holocene layers. In this report, the compression and shear characteristics of aged Pleistocene clay in Osaka Bay, were reviewed with related to the case study on Kansai International Airport project.
1) The concept of standard compression curve (SCC) of marineclays was proposed. The standard compression curve consists of ultimate standard compression curve, USC, and the standard compression curves from an initial void ratio, SCC(eo). Both curves are determined mainly by the liquid limit of clay and the void ratio at the beginning of consolidation. Using the specific volume index, Isv, defined as ln (1 e) / ln (l eL), USC and SCC(eo) were given as a unique Isv - log p relationship.
2) By comparing in-situ specific volume of clay with value on of SCC, the degree of structure of marine deposits can be evaluated. It is shown that Osaka Bay Pleistocene clay seems to have well-developed structure due to aging, because the in-situ values of Isv are much larger than those of SCC-marine.
3) The consolidation characteristics of Osaka Pleistocene clay was examined with the purpose of investigating soil structural effect. At the pressure increment spanning the consolidation yield strees, effective stress increasing with elapsed time showed sudden decreasing tendency. The cause of this unique behavior seems to be attributed to the fact that radical increase of compressibility by the yielding of soil structure would surpasse the excess pore pressure dissipation. This behavior is particularly relevant to clays with a well-developed structure such as Osaka Pleistocene clay.
4) The undrained shear strengths and deformation characteristics of Pleistocene clay were studied by the recompression method with triaxial tests. The Pleistocene clay at large depths is brittle and shifts very rapidly from elastic deformation to plastic failure. Although the safety factor of slip circle which passes through the Pleistocene clay layer was larger than 1.20, the margin of the safety were not so much. The accurate estimation of the shear strength of Pleistocene clays and the field observations and construction control will be necessary to complete the projects safely.