Ageing effects on consolidation properties - Based on the site investigation of osaka pleistocene clays

Soil samples were recovered from the Osaka basin, where Holocene as well as Pleistocene marine clays are thickly deposited. It is found from X-ray diffraction analysis that the component of clay minerals for these clay layers is almost identical through all depths. This fact suggests that the origin of these sediments has not been drastically changed through at least more than 1,000,000 years. Using data obtained by oedometer tests, this paper examines ageing effects on consolidation properties. It is found that void ratio for the Pleistocene clays keeps large even under high in situ effective overburden pressure (p'(vo)). Salinity of pore fluid decreases with depth and becomes nearly zero at depths deeper than 70 m. However, the change in salinity does not give any influences on the index properties, suggesting that the relatively large void ratio for the Pleistocene clays is caused by other reasons than leaching. It is also found that the Pleistocene clays reveal no particular differences in consolidation properties, but fundamentally similar to those of Holocene clays. There is no distinguished difference in the overconsolidation ratio (OCR), non-linearity of the e - log p' relation after yield consolidation pressure including the maximum compression index (C-cmax) and the hydraulic conductivity (k). The only prominent difference in the Osaka Pleistocene clays, compared to the Holocene clay, is relatively small void ratio due to the large p'(vo).