Abstract: Hollow cylinder samples tests were performed with high-density saturated silt from the sea entrance of the Yangtze River. The stress paths consisted of undrained cyclic principal stress rotation at the range of 180° with different shear stress ratio and frequency. It is found that isotropic consolidated samples would liquefy after cyclic principal stress axis rotation, which is featured by the excess pore water pressure reached initial effective confining pressure. Before the liquefaction, due to the different shear stress levels the development of pore water pressure was divided into three stages by graded phase transformation point and cataclysmic phase transformation point or two stages by cataclysmic phase transformation point. As for the development of strain components, it was divided into two stages by collapse point. Meanwhile the cataclysmic phase transformation point of pore water pressure and the collapse point of strain occurred simultaneously, which reflected the state of structure collapse. Besides the deviator strains when the collapse state occured were limited within a narrow range of 0.2%-0.4%. The stress state of collapse can be normalized in - space with a line when samples underwent cyclic principal stress axis rotation after isotropic consolidation. Furthermore the development of pore water pressure can be fitted well by modified Seed model in which the collapse state was used as the normalized denominator.

Key words: cyclic principal stress axis rotation, silt, high relative density, phase transformation point, collapse state, pore water pressure