Abstract: Referring to the original soft clay in the northern slope of South China Sea, the dynamic triaxial tests were carried out under different conditions of consolidation ratio and cyclic stress ratio CSR. The development rule of the residual dynamic strain, residual pore water pressure and the relationship between them were discussed. Considering the correlation between residual strain and residual pore pressure in the test process, the failure standard of dynamic triaxial test based on strain-pore pressure mode was proposed. The mechanism of mutual feed and its interaction mechanism were revealed by the scanning electron microscopy (SEM) technique. The research results indicate that under the condition of designated and different CSR, the development trends of the residual dynamic strain and cyclic vibration frequency curves show a significant difference, while the change laws of the residual dynamic pore pressure and cyclic vibration frequency curve are basically the same. With the increase of , the critical value of CSR gradually increases, while the final residual dynamic strain gradually decreases. The similar characteristics of changing law can also be found from the curve representing the relationship between residual dynamic strain and residual pore pressure. Furthermore, the conventional failure standard based on strain value is extended to the failure region controlled by the inflection point of the strain-pore pressure curve, which can effectively define the failure vibration frequency and completely describe the whole failure process of the specimen, even reveal the inherent mutual feedback mechanism among effective stress, strain and pore pressure. Research results from this study can provide reliable reference for the establishment of dynamic softening model in soft clay, the evaluation and prediction of marine geological hazards, and the foundation design of ocean engineering in the northern slope of South China Sea.

Key words: northern slope of South China Sea, dynamic triaxial test, residual dynamic strain, residual pore pressure, failure standard