Abstract: Macroscopic and microscopic triaxial shearing experiments were carried out on Shanghai soft clay to study the evolution characteristics of pores under loading. The results show that local deformation occurs at the beginning of loading. With the increase of the loading, number of small aggregates increase, pores continually enlarge, interconnected pores occur, the pore orientation is obvious, and a shear band is formed. Microstructure parameters are nonlinearly positive to the shear stress ratio. The maximum pore area, void ratio, anisotropy ratio and fractal dimension of pores increase slowly at an initial loading stage, and then increase rapidly at the later stage. The maximum anisotropy ratio reaches 0.68 and void ratio is up to 1.96, when the axial stain is 8%. The microstructure parameters of shear band are larger than those outside the band. During the triaxial loading, soil microstructure deteriorates and the whole deformation process of soft clay is divided into three stages: damage initiation, damage developing and shear band formation, as well as soil failure. Microstructure degradation is closely related to macroscopic mechanical properties of soil.

Key words: soft clay, field emission scanning electron microscope (FESEM), shear band, pore, fractal dimension