Microstructure change of structured clay under different stress paths is the key in understanding the intrinsic mechanism of structure variation of structured clay in K0 consolidated tests. Based on the triaxial shear tests under different stress paths for the natural clay in Hengshui area

the micro-structure of soil under different stress paths were compared and the mechanism of micro-structure variation of structured clay was studied from the microscopic perspective. The results show that variation of pore size

orientation of particles and pores is large

while the variation of particle and pore shape is small under different stress paths. The increase of spherical stress leads to the consolidation of soil particles

and the soil pores are compressed

resulting in the decrease of soil volume. The soil particles loose

the pore size and soil volume increase when the spherical stress decreases. The increase or decrease in deviatoric stress has similar impact on soil microstructures

which results in the compression of soil skeleton before the failure of soil structure. After the failure of soil structure

the soil particles are misaligned. They rolled and connected to each other

which leading to the expansion of inter-grain pores and dilatancy of soil occurs. The spherical stress and deviated stress caused by irregular shaped soil particle form mutual effects on volumetric strain and shear strain. The results reveal the microscopic mechanism of stress-strain characteristics of structured clay under different stress paths.