Abstract:
Clay samples from Manurewa, South Auckland were tested to investigate the applicability of the Critical State Concept to the behaviour of undisturbed natural soil under conventional triaxial conditions. Special care was taken to ensure least amount of sample disturbance at all stages, from the sampling operation to the preparation of the triaxial specimens. The clay properties vary quite significantly, the textures vary from sandy clay to highly plastic clay. The in situ water content over the sampling area 2[m]x4[m], vary from 25 to 65%; within a cylindrical specimen 75[mm] dia., 170[mm] Iong the water content vary by as much as 20%. The LL varies from 46 to 103%; PI from 23 to 66%; Compression Index, Cc from 0.28 to 0.49. No relationship exists between natural water content and vane shear strength. It seems that, due to a complex geomorphological situation at the site, each specimen has a different preconsolidation pressure, although all were taken from the same depth. However, this diverse nature of the clay does not seem to affect its stress-strain and strength characteristics. When total volumetric strain, i.e. the strain induced during both the consolidation and shearing stages, is used instead of void ratio and the stresses are normalised using the apparent isotropic preconsolidation pressure, the resulting plots suggest that all the specimens can be regarded as one soil. The clay exhibits a stable post-failure behaviour, even for the overconsolidated specimens. It yields at about 3% of volumetric strain; the shape of the yield locus shares a common feature with the undrained stress paths and the the volumetric strain contours obtained from q/p' constant tests, i.e. it has a lIitle curvature,. The Critical State Concept is found to be inapplicable for the clay investigated. A Modified Critical State Concept is proposed and shown to be applicable to this particular clay. In this modified form, the critical state is a unique combination of the total volumetric strain (instead of void ratio) and the normalised effective stresses q/Pc and P’/pc (instead of q and p’), where pc is the isotropic preconsolidation pressure of the specimen.