Degradation Of Soil Structure And Anisotropic Plasticity Evolution Of Natural Clay

Soft clays are widely distributed near the littoral in southeast China. They usually have high water content, extensive compressibility and low strength just as other soft clays all around the world. Building on soft clay is always difficult and costive. Geotechnical designers commonly use test data of disturbed clays in their calculation and designing with the real behaviour of natual clay being ignored.Structure and anisotropy are the most important factors of natural clays. A constitutive model is developed based on critical state soil mechanics in this paper. Cam-clay model and modified Cam-clay model are reviewed and their limitations for natural clay are presented. Considering the anisotropic behaviour of natural clay the new constitutive model employs an inclined ellipse as the yield surface, no longer the aclinic ellipse used in traditional modified Cam-clay. The influence of over consolidation and structure are considered in this model based on the theory of super/subloading yield surface concept. Finally the undrained strength of soft clay is analysed according to the new model and the average undrained strength in practice is suggested. Some developments for soft clay are involved in this paper as below.(1) The yield surface of modified Cam-clay is revised. The new model employs an inclined ellipse as the yield surface instead of aclinic ellipse used in traditional modified Cam-clay according to the results of many laboratory tests for natural soft clays from different areas. The initial inclination of NCL for K₀ consolidated clay is analysed and compared with the test results. A correlation between the initial inclination of NCL and the parameters of M and K₀ is constituted on p'^q plane.(2) The structure and anisotropy of Whenzhou clay are studied. A series of stress probe tests are carried out by the GDS triaxial test system compared with the test procedure of Bothkennar clay. Whenzhou clay is a typical structured soft clay as well as its anisotropic behaviour. Whenzhou clay appears obvious destructuration along the stress path in loading, together with its distinctive isotropization behaviour.(3) Hardening rule based on thermodynamics is adopted. In this paper an...