Abstract: This paper has performed a group of undrained unidirectional cyclic loading tests on undisturbed clay samples near planned metro line of Hangzhou. To simulate subway load, this paper has considered the characteristic that the trains always leave at intervals, so cyclic load is designed to consider regular intervals. Under cyclic loading with different amplitudes, dynamic loading durations and intervals durations, excess pore water pressure accumulations are observed. In this paper, the critical dynamic amplitude under cyclic loading with intervals is obtained and is found to be similar with that under cyclic loading without intervals. After that, an explicit model to calculate excess pore water pressure is built with an independent variable that describes cumulative dynamic deviator stress level. It is a new stress state parameter that can describe the effect of long-term dynamic loading in a cumulative way. The influence regulation of cumulative dynamic deviator stress level, vibration number and interval duration are presented and the dynamic deviator stress level and vibration number are much more important to excess pore water pressure accumulation than intervals. This explicit model is in the form of hyperbolic function with one major variable and a few constants. When predicting excess pore water pressure under different vibration numbers, these constants are also different but has relation with vibration number. The method to calculate the constants is also presented. In the end of this paper, testing results of several samples are presented to verify this model. Comparing the results predicted by the new model with an existing model with the measured data, the new model performs well.

Key words: cumulative dynamic deviator stress level, cyclic loading, regular intervals, excess pore water pressure, undisturbed clay