Abstract: Triaxial drainage shear tests are conducted on undisturbed and remolded samples of marshy and lacustrine clay to explore the mechanism of structure affecting shear strength and deformation. The undisturbed soil samples after the triaxial drainage shear test are photographed by scanning electron microscope (SEM) and the SEM photos are qualitatively and quantitatively analyzed by IPP program. In addition, the effects of soil structure on shear strength and deformation and the change of microstructure of marshy and lacustrine clay are examined under different consolidation pressures. Results demonstrate that before the yield failure of soil structure, the soil structure of the undisturbed soil could enhance shear strength and reduce shear deformation, while after the yield failure of soil structure, the soil structure of undisturbed soil reduces shear strength and instigates shear deformation which is similar to the remolded soil. Moreover, the peak structural strength of undisturbed soil increases linearly and the peak structural volumetric strain decreases exponentially with the growth of confining pressure. When consolidation pressure is smaller than structural yield stress, the soil structure damages slightly in consolidation stage, and the shear load is bore by the bonding and fabric of the undisturbed soil in shearing stage. Meanwhile, the pores are almost uncompressed and the shape and arrangement of particles and pores have changed slightly. However, when consolidation pressure exceeds the structural yield stress, the soil structure damages significantly in consolidation stage and the bonding and fabric gradually degrades in shearing stage. As a result, the pores are compressed, the shape of particles and pores gradually becomes circular, and the arrangement orientation of particles and pores adjust with the direction of load.

Key words: marshy and lacustrine clay, triaxial drained shear test, soil structure, microstructure, undisturbed soil, remoulded soil