Volume changes in soil caused by freeze-thaw cycles can affect the shear performance of the saline soil-geotextile interface. To investigate this issue, the study examined changes in shear strength, deformation characteristics, and failure modes of the saline soil-geotextile interface under different numbers of freeze-thaw cycles. The experimental results indicate that with the increase in freeze-thaw cycles, the shear stiffness of the interface initially increases and then decreases, demonstrating the reduction in elasticity and resistance to deformation caused by freeze-thaw cycles. And the enhancement of normal stress can effectively increase the density of the soil and the adhesion at the interface, thereby improving shear stiffness. Meanwhile, the salt content in the soil also significantly impacts the mechanical properties, with notable changes in the dynamic characteristics of the interface as the salt content varies. Furthermore, after freeze-thaw actions, the soil becomes loose, reduces in integrity, features uneven surfaces, and sees increased internal porosity leading to slip surfaces. Trend analysis from this study provides new insights into the failure mechanisms at the saline soil-geotextile interface.

Calcareous sands provide the foundational support for various marine infrastructures. In the harsh marine environment, earthquake or wave loads apply multidirectional cyclic shear stresses to the foundation soil. To explore the undrained multidirectional cyclic response of sand, a series of simple shear tests were performed on reconstituted sand specimens considering the effect of phase difference (theta). By comparing the results with those of siliceous sand under similar conditions, the behavior of calcareous sand under multidirectional cyclic loading became clear. The results demonstrated that calcareous sand shows a lower degree of cyclic instability compared to siliceous sand, corresponding to the weaker strain-softening observed in calcareous sand during monotonic shear tests. The trend in normalized pore water pressure evolution in siliceous sand exceeds that in calcareous sand. Furthermore, under multidirectional cyclic shear conditions, the liquefaction resistance decreases by 30 % in extreme cases, irrespective of sand type. The liquefaction resistance of calcareous sand surpasses that of siliceous sand. However, as the cyclic stress ratio decreases, the reverse trend is observed, regardless of the impact of theta. Subsequently, the possible causes of the above experimental phenomena are explored from the perspectives of shear modulus and energy dissipation.

The soil fabric varies significantly depending on the deposition process that forms the grain skeleton. Each deposition method produces a specific type of soil fabric, which can be linked to a particular soil density. When represented as relative density, determined using limit densities from standard index tests, a wide range of relative densities can be observed for different sands produced by the same deposition method. The influence of this variation in relative density, resulting from a single deposition method, on the development of the excess pore water pressure (PWP) should be further investigated. A fast testing of the excess PWP accumulation in sandy soils during undrained cyclic shearing can be easily performed using the newly developed PWP Tester. In the PWP Tester, specimens are prepared through sedimentation in water, which yields a comparable fabric in different sands but significantly different relative densities. Despite these relative density differences, the rate of the excess PWP evolution during undrained shearing is remarkably similar among different sands. This indicates that relative density should not be regarded as a primary factor influencing the development of the excess PWP and that the soil fabric plays equal or even a greater role.

The stability of geotechnical structures after an earthquake is primarily determined by the residual strength of surrounding soils that have not fully liquefied. This research employs the discrete element method (DEM) to study the undrained post-cyclic shear behaviour of sand under triaxial conditions, focusing on the effect of varying degrees of liquefaction (LD) simulated by subjecting the samples to different lengths of cyclic loading. Different types of cyclic loading, i.e. symmetric (fully reversal), partially reversal, and non-reversal ones, as well as the effect of sample density, have been considered. The results indicate that the samples under fully or partially reversal cyclic loading eventually liquefied, displaying a cyclic mobility failure mode. In contrast, samples under non-reversal cyclic loading develop plastic strain accumulation (PSA) failure without liquefaction. The post-cyclic shear stiffness of the samples is affected by both LD and the type of cyclic loading. For samples under reversal cyclic loading, the post-cyclic shear stiffness decreases as LD increases. Notably, the liquefied samples (LD = 1) initially exhibit near-zero stiffness during post-liquefaction shear until highly anisotropic force chains are formed along the loading direction, with their buckling leading to stiffness recovery. The length of the low-stiffness stage is influenced by the static shear stress and the relative density of the sample, which determines the rate of anisotropy accumulation during cyclic loading. The onset and completion of stiffness recovery are marked by a peak in anisotropy and an abrupt increase in effective anisotropy, respectively. For samples under non-reversal cyclic loading, the post-cyclic shear stiffness initially decreases with the increase in LD but increases at higher LDs due to the significant anisotropy developed during the cyclic loading stage.

The liquefaction and weakening of saturated sands under cyclic stress loading is a major concern in earthquake engineering. This study proposes a model based on initial cyclic shear strain (gamma c,i) to predict the excess pore pressure generation in undrained saturated sands. Here, gamma c,i is defined as the average cyclic shear strain prior to the significant accumulation of excess pore pressure. To calibrate and validate the model, a series of undrained stress-controlled cyclic triaxial (CTX) tests were conducted on Fujian sand with 10 % Kaolin clay (FS-10) and Silica sand no.7 with 5 % Kaolin clay (SS7-5). The FS-10 and SS7-5 specimens displayed typical flow liquefaction and cyclic mobility as they approached initial liquefaction. A critical excess pore pressure ratio (ru,c) is introduced to characterize the effects of liquefaction failure modes on excess pore pressure generation. The model also incorporates reduction factors related to small-strain secant shear modulus and reference shear strain to account for variations in calculating gamma c,i. Ultimately, the initial cyclic shear strain-based model exhibited a strong correlation with experimental data under different confining pressures and loading cycles. In addition, it provides a critical initial cyclic shear strain for assessing soil liquefaction in engineering practices, particularly for improved ground with complex stress states.

In this research, an energy formulation is proposed for the evaluation of pore pressure generation, incorporating the influence of the initial state of static stresses, both normal and shear, prior to cyclic loading. The proposed model focuses on obtaining a law of evolution of pore pressures under cyclic loading in saturated soils regardless of their susceptibility or not to liquefaction. The energy approach developed in this research extends previous energy based models developed for granular soils (susceptible to liquefaction and without initial static shear stress) incorporating: a) the integration in the formulation and interpretation of both the work dissipated and consumed during the dynamic process; b) the normalization of the formulation considering initial static stresses both normal and shear; c) obtaining and validating the model parameters with conventional tests of cyclic shearing equipment. The proposed model was validated with 116 cyclic simple shear tests under different in situ vertical effective stresses and different combinations of static and cyclic shear stresses. However, the model can be easily calibrated for other soils with cyclic simple shear tests without static shear stress, widely used in laboratories with dynamic equipment.

Offshore wind turbines are subjected to more significant wave and wind environmental loads at extreme weather conditions, making subsoil experience various loading stages with different amplitudes. To investigate the coupling effect of both cyclic shear stress ratio (CSR) and stage amplitude ratio (Ar) between normal and extreme weather conditions, a series of bi-directional simple shear tests with five different Ar and three CSR values were conducted on marine sand using the variable-direction dynamic cyclic simple shear (VDDCSS) apparatus. In the tests, soil samples were compacted under vertical stress and then sheared in undrained conditions by applying two shear stresses acting in different horizontal directions. Test results indicated that the cyclic strain, pore water pressure ratio, and cyclic strength were significantly determined by the value of stage amplitude ratios and the CSRs: at the same CSR, cyclic strains, and pore water pressure increased while cyclic strength decreased with the Ar. Comparing the test data between various cyclic stress ratios found that the CSRs can accelerate shear strains, pore pressure accumulation, and cyclic strength attenuation.

In the waterway construction projects of the upper reaches of the Yangtze River, crushed mudstone particles are widely used to backfill the foundations of rock-socketed concrete-filled steel tube (RSCFST) piles, a structure widely adopted in port constructions. In these projects, the steel-mudstone interfaces experience complex loading conditions, and the surface profile tends to vary within certain ranges during construction and operation. The changes in boundary conditions and material profile significantly impact the bearing performance of these piles when subjected to cyclic loads, such as ship impacts, water level fluctuations, and wave-induced loads. Therefore, it is necessary to investigate the shear characteristics of the RSCFST pile-soil interface under cyclic vertical loading, particularly in relation to varying deformation levels in the steel casing's outer profile. In this study, a series of cyclic direct shear tests are carried out to investigate the influential mechanisms of roughness on the cyclic behavior of RSCFST pile-soil interfaces. The impacts of roughness on shear stress, shear stiffness, damping ratio, normal stress, and particle breakage ratio are discussed separately and can be summarized as follows: (1) During the initial phase of cyclic shearing, increased roughness correlates with higher interfacial shear strength and anisotropy, but also exacerbates interfacial particle breakage. Consequently, the sample undergoes more significant shear contraction, leading to reduced interfacial shear strength and anisotropy in the later stages. (2) The damping ratio of the rough interface exhibits an initial increase followed by a decrease, while the smooth interface demonstrates the exact opposite trend. The variation in damping ratio characteristics corresponds to the transition from soil-structure to soil-soil interfacial shearing. (3) Shear contraction is more pronounced in rough interface samples compared to the smooth interface, indicating that particle breakage has a greater impact on soil shear contraction compared to densification.

This study presents a comprehensive investigation into the cyclic shear behavior of sand-fly ash mixtures through experimental and data-driven modeling approaches. Cyclic direct shear tests were conducted on mixtures containing fly ash at 0%, 2.5%, 5%, 10%, 15%, and 20% by weight to examine the influence of fly ash content on the shear behavior under cyclic loading conditions. The tests were carried out under a constant stress of 100 kPa to simulate field-relevant stress conditions. Results revealed that the fly ash content initially reduces shear strength at lower additive contents, but shear strength increases and reaches a maximum at 20% fly ash content. The findings highlight the trade-offs in mechanical behavior associated with varying fly ash proportions. To enhance the understanding of cyclic shear behavior, a Nonlinear Autoregressive Model with External Input (NARX) model was employed. Using data from the loading cycles as input, the NARX model was trained to predict the final shear response under cyclic conditions. The model demonstrated exceptional predictive performance, achieving a coefficient of determination (R2) of 0.99, showcasing its robustness in forecasting the cyclic shear performance based on the composition of the mixtures. The insights derived from this research underscore the potential of incorporating fly ash in sand mixtures for soil stabilization in geotechnical engineering. Furthermore, the integration of advanced machine learning techniques such as NARX models offers a powerful tool for predicting the behavior of soil mixtures, facilitating more effective and data-driven decision-making in geotechnical applications. Evidently, this study not only advances the understanding of cyclic shear behavior in fly ash-sand mixtures but also provides a framework for employing data-driven methodologies to address complex geotechnical challenges.

Geosynthetics-soil interfaces are exposed to varying temperatures coupled with complex stress states. Quantifying the mechanical response of the interface considering this combined influence of temperature and complex stress is always a huge challenge. This study proposes a new displacement and stress-loading static and dynamic shear apparatus that is capable of testing the geosynthetics-soil interfaces with high and low-temperature controlling function. The apparatus satisfactorily simulates monotonic and cyclic direct shear tests, and creep shear tests on geosynthetics-soil interfaces at temperatures ranging from -30 degrees C to 200 degrees C. To validate the functionality of this device, a series of temperature-controlled experiments were conducted on different types of interfaces (sand-geogrid interfaces, sand-textured geomembrane interfaces, sand-smooth geomembrane interfaces). The experimental results indicate that the apparatus can simulate static, dynamic, and creep shear loading on geosynthetics-soil interfaces in high and low temperature environments, and these can be measured reliably. It also manifests that temperature has a non-negligible influence on all mechanical interface responses. These findings highlight the significance and potential of the proposed apparatus and its practical implications.