INFLUENCE OF STRESS RELEASE ON SAMPLE QUALITY OF PLEISTOCENE CLAY COLLECTED FROM LARGE DEPTH IN OSAKA BAY

 YOICHI WATABE and TAKASHI TSUCHIDA

 ABSTRACT:A data set of unconfined compression strengths of Osaka Bay Pleistocene clays collected from large depths tends to be much scattered. This is because of large stress release in the collected sample. In more detail, the two following factors are possibly considered. A residual effective stress larger than 98. 1 kPa is apparently impossible in a fully saturated soil because a negative pressure greater than a vacuum is non-existent, and some cracks are created in a sample when extruding it from the sampler and during trimming. In this study, a series of suction and unconfined compression tests for samples collected from 40-200 m depths in Osaka Bay was carried out to investigate the relationship between the residual effective stress and the undrained shear strength. Suction larger than 98.1 kPa was measured in undisturbed samples collected from large depths by applying a back air pressure. The disturbance ratio defined by Okumura (1974) ranges from I .5 to 3, implying that the samples show a small strength reduction of up to 150/0 . It is concluded that the quality of the undisturbed Pleistocene clay sample collected from a large depth is very good, if the effective stress is of concern and no crack is created. However, the unconfined compression strength of Osaka Bay Pleistocene clay tends to show a large variance due to the crack type disturbance that is created when trimming the specimen.

Key words: Pleistocene clay sample disturbance, sampling, suction, unconfined compression test (IGC: C6/D6)