The cross-river subway in the Hangzhou Bay area often passes through deep, thick, soft soil at the bottom of the river. At the same time, overlying erosion, siltation, and changes in water levels adversely affect the deformation of the subway, thereby causing hidden dangers to its safe operation. Using two-way dynamic triaxial testing, the effects of cyclic loading of the cross-river subway on the soft clay foundation were investigated for the first time, using simulation methodology as the prime objective of the present study. A strain development curve for the soft clay was obtained as a result. Considering the effects of effective confining pressure (p′) and radial cyclic stress ratio (τ_r), an explicit model of accumulative strain on soft clay under cyclic loading of the cross-river subway was established. The results showed that the accumulative axial strain (ε_d) was closely related to p′ and τ_r. Under certain conditions, as p′ and τ_r increased, the ε_d produced by the soil tended to decrease. Furthermore, through non-destructive testing based on nuclear magnetic resonance (NMR), pore distribution and pore size changes in soft clay during cyclic loading were analyzed. It was observed that under the action of the cross-river loading, the large internal soft clay pores were transformed into small pores, which manifested as a significant decrease in the number of large pores and an increase in the proportion of small pores. Lastly, the macroscopic dynamic soil characteristics observed during triaxial testing closely correlated with the microscopic pore size of the soil obtained in the NMR test, which indicated that using pore distribution and pore size changes to describe microscopic changes was a valid method.

The hydraulic conductivity k is one of the most important engineering properties of soft clay, and it plays an important role during the whole life cycle of underwater tunnel. Therefore, it is necessary to systematically study the response of k to dynamic load. In this study, a series of seepage tests were conducted on remolded Hangzhou soft clay using dynamic triaxial seepage apparatus after cyclic loading. The results show that the hydraulic conductivitys of all samples decrease after cyclic loading, and the maximum drop of the hydraulic conductivity can reach 29.1%. Also, the hydraulic conductivity of soil samples is significantly affected by cyclic stress ratio (CSR). With the increase of CSR, the decrease range of hydraulic conductivity under cyclic loading is between 22.3% and 24.3%. The dynamic load reconstructed the microstructure of clay and made the number of large pores getting decreased and the small pores increased. This resulted in a decrease in the smaller hydraulic conductivity (k) of the specimens. Futhermore, based on the NMR scanning on the microstructure of soil samples under cyclic loading and the microscopic pore channel theory, a prediction model for the k of soft clay is proposed. The validity of the model is verified based on test data. It is believed that the proposed model could provide some theoretical support for the design and long-term performance prediction in practical engineering issues.

The stress‐strain characteristics of soil depend primarily on the previous stress history and stress path, thus related to both the stress magnitude and direction. To have a better understanding of the response of soft clay under heart‐shaped stress paths, 18 cyclic hollow cylinders tests were performed on undisturbed Hangzhou soft clay under different cyclic stress ratios, deviatoric consolidation ratios (K₀), and loading frequencies. The result shows that as the vertical dynamic stress amplitude, K₀ value, and loading frequency increase, the degradation index gradually decreases. Moreover, the degradation index of the soil under the cyclic torsion shear (CTS) test is always higher than that under the cyclic triaxial (CT) test. The increase in the amplitude of the cyclic stress ratio (CSR), K₀ value, and the decrease in the loading frequency will promote the initial accumulative plastic strain and accelerate the failure rate of the soil sample; it shows that the effects of cyclic stress ratio amplitude, deviatoric consolidation ratios K₀, and loading frequency on the accumulative plastic deformation of soil cannot be ignored. On the basis of the test results, a logarithmic relationship between the degradation index and the loading frequency is determined. A new empirical formula of accumulative plastic strain degradation of soft clay has been established, and its accuracy has been further verified by test data. The research results can provide theories for predicting and calculating the long‐term settlement and deformation of clay foundation.