Frozen soil is a common foundation material in cold region engineering. Therefore, the control and prediction of cumulative plastic strain for frozen soil materials are essential for the construction and long-term stability of actual foundation engineering under complex dynamic loadings. To investigate the influence of complex cyclic stress paths on frozen soil, a series of complex cyclic stress paths were conducted using the frozen hollow cylinder apparatus (FHCA-300).These cyclic stress paths included the triaxial cyclic stress path (TCSP), directional cyclic stress path (DCSP), circular-shaped cyclic stress path (CCSP), elliptical-shaped cyclic stress path (ECSP), and heart-shaped cyclic stress path (HCSP).The results indicated that the cumulative plastic strain under the five cyclic stress paths at three temperatures (-1.5,-6,and-15 degrees C) can be ranked as follows: DCSP>ECSP>HCSP>CCSP>TCSP. The cyclic stress paths are quantified based on the combined effects of the maximum shear stress (q(max)) and the principal stress axis angle (a). A developed model predicting cumulative plastic strain, considering complex cyclic stress paths, is introduced and demonstrates excellent predictive performance. The study's findings can offer insights into foundation engineering's deformation characteristics and settlement predictions under diverse complex dynamic loadings

Seismic events and wave action can induce volumetric strain (ev) accumulation in saturated sandy soils, leading to damage to the ground surface and structures. A quantifiable relationship exists between the generation of ev in sandy soils under drained conditions and the development of pore water pressures under undrained conditions. In this study, the impact of relative density (Dr), cyclic stress path, and stress level on the characteristics of volumetric strain (ev) generation in saturated coral sands (SCS) was evaluated through drained tests employing various cyclic stress paths. The test findings demonstrate that the rate of ev accumulation in SCS is notably affected by the cyclic stress path. The rise in peak volumetric strain (evp) in SCS, as a function of the number of cycles, conforms to the arctangent function model. The unit cyclic stress ratio (USR) was employed as an indicator of complex cyclic loading levels. It was determined that coefficient (evp)u is positively correlated with USR at a specific Dr. At the same Dr, coefficient CN1 exhibits a positive correlation with USR, while coefficient CN2 displays a negative correlation with USR, following a power-law relationship. Irrespective of cyclic loading conditions, evp rises with an increase in generalized shear strain amplitude (yga). A power function model was established to represent the relationship between evp and yga. The coefficient 41 decreases as Dr increases. Comparisons were drawn between evp and yga for Ottawa sand and SCS. The results indicate that, as Dr of Ottawa sand increases from 30 % to 70 %, the coefficient 41 decreases from 1.54 to 0.73, representing a reduction of approximately 53 %. In contrast, under identical conditions, the coefficient 41 of SCS exhibits a less pronounced decrease, from 1.16 to 0.79, corresponding to a reduction of roughly 32 %. These observations suggest that variations in Dr have a more substantial impact on generating evp in Ottawa sand compared to SCS.

Accurate prediction of excess pore water pressure (EPWP) generation in saturated sandy soils remains one of the most challenging issues in sandy site responses to strong earthquakes and extreme marine environments. This paper presents experimental results of undrained and drained multidirectional cyclic hollow cylinder (MCHC) tests on saturated coral sandy soils under various cyclic loadings. The results show that threshold generalized shear strain gamma ga,th, below which EPWP and volumetric strain can be neglected, is an inherent property depending only on the soil type and initial state. Furthermore, there exists a virtually unique form of relationships between the generalized shear strain amplitude (gamma ga) and the cumulative dissipated energy per unit volume of soil (Wc) at different relative density (Dr), irrespective of drainage conditions and cyclic loading conditions. These findings highlight the fundamental mechanism for cyclic deformation behavior and the uniqueness of correlations among rup (peak EPWP ratio), epsilon vp (peak volumetric strain), and gamma ga of saturated sandy soil at the similar Dr, regardless of cyclic loading conditions. Based on these findings, a novel unified model of gamma ga-based cyclic shear-volume coupling and EPWP generation is established, which is independent of cyclic loading conditions over a wide loading frequency range. Then the applicability of the proposed model is validated by the experimental data of the same tested coral sandy soil and siliceous Ottawa sand, as well as the data of siliceous fine sands in previous work. It is found that the proposed model surpasses the existing strain- and stress-based models in accurately predicting EPWP generation under complex cyclic loadings, which can offer new insights into the mechanisms of the EPWP generation in saturated sandy soils.

The railway foundation is subjected to periodic loads and time intervals during operation. Previous tests on permanent deformation of soft soil by applying the continuous cyclic loading under undrained or drained conditions cannot fully reflect the actual situation. To evaluate the cyclic triaxial behaviors of saturated soil under more realistic loading conditions, a series of bidirectional cyclic triaxial tests were designed. These tests considered the influence of drainage conditions and variable confining pressure throughout the intermittent and cyclic loading phases. The experimental results demonstrate that the progression of cumulative axial strain and excess pore water pressure under intermittent cyclic loading exhibits notable distinctions compared to previous tests conducted under continuous cyclic loading. Besides, the cumulative axial strain and excess pore water pressure of test samples under variable confining pressure conditions are affected by the cyclic stress ratio (CSR) and inclination of cyclic stress path (eta ampl). When CSR and eta ampl increase simultaneously, the cumulative axial strain of the soil tends to increase more significantly. It means that the increase in cyclic confining pressure leads to an increase in the generation of pore water pressure in the sample. This, in turn, reduces the effective stress and shear strength of the soil, resulting in a greater cumulative axial strain, regardless of the CSR. The present study highlights the importance of considering drainage conditions, variable confining pressure, and intermittent time when assessing the cyclic triaxial characteristics of soil subjected to cyclic train loads. The findings of this research emphasize importance of considering these factors when accurately evaluating the behavior of the soil under such loading conditions.