The foundation soil below the structure usually bears the combined action of initial static and cyclic shear loading. This experimental investigation focused on the cyclic properties of saturated soft clay in the initial static shear stress state. A range of constant volume cyclic simple shear tests were performed on Shanghai soft clay at different initial static shear stress ratios (SSR) and cyclic shear stress ratios (CSR). The cyclic behavior of soft clay with SSR was compared with that without SSR. An empirical model for predicting cyclic strength of soft clay under various SSR and CSR combinations was proposed and validated. Research results indicated that an increase of shear loading level, including SSR and CSR, results in a larger magnitude of shear strain. The response of pore water pressure is simultaneously dominated by the amplitude and the duration of shear loading. The maximum pore water pressure induced by smaller loading over a long duration may be greater than that under larger loading over a short duration. The initial static shear stress does not necessarily have a negative impact on cyclic strength. At least, compared to cases without SSR, the low-level SSR can improve the deformation resistance of soft clay under the cyclic loading. For the higher SSR level, the cyclic strength decreases with the increase of SSR.

A comprehensive series of tests, including dynamic triaxial, monotonic triaxial and unconfined compressive strength (UCS) tests, were carried out on reconstituted landfill waste material buried for over twenty years in a closed landfill site in Sydney, Australia. Waste materials collected from the landfill site were treated with varying percentages of cement, and both treated and untreated specimens were investigated to evaluate the influence of cement treatment. The study examined the dynamic properties of cement-treated landfill waste, including cumulative plastic deformation, resilient modulus, and damping ratio, and also analysed the impact of cyclic loading on post-cyclic shear strength in comparison to pre-cyclic shear strength. The UCS tests and monotonic triaxial tests demonstrated that untreated specimens subjected to monotonic loading exhibited a progressive increase in strength with rising axial strain, whereas cement-treated specimens reached a peak strength before experiencing a decline. During cyclic loading, with the inclusion of cement, a significant reduction in cumulative plastic deformation and damping ratio was observed, and this reduction was further enhanced with increasing cement content. Conversely, the resilient modulus showed substantial improvement with the addition of cement, and this enhancement was further amplified with increasing cement content. The formation of cementation bonds between particles curtails particle movement within the landfill waste material matrix and prevents interparticle sliding during cyclic loading, leading to lower plastic strains and damping ratio while increasing resilient modulus. Post-cyclic monotonic testing revealed that cyclic loading caused the partial breakage of the cementation bonds, resulting in reduced shear strength. This reduction was higher on samples treated with lower cement content. Overall, the findings of the research offer crucial insights into the possibility of cement-treated landfill waste as a railway subgrade, laying the groundwork for informed design decisions in developing transport infrastructure over closed landfill sites while using landfill waste materials available on site.

In marine environments, cyclic loads induced by earthquakes can lead to complex soil responses in marine coral sand. Waves and storms, often at different frequencies, can also contribute to these responses. These factors can finally contribute to instability or failure of offshore structures. To better understand the effect of loading frequency on the dynamic properties of marine coral sand, a series of cyclic triaxial tests on saturated coral sands were carried out. These tests were performed with different gradations at different loading frequencies and loading modes. A GDS dynamic triaxial instrument was used for the tests. The experimental results demonstrate that loading frequency has a significant effect on the cyclic response of coral sand. The maximum shear modulus of saturated coral sand rises with increasing loading frequency. The cyclic strength of saturated coral sand also increases with loading frequency. A strong linear relationship exists between the maximum shear modulus and cyclic strength. This suggests the existence of a cyclic yield strain that is relatively insensitive to loading frequency. Loading frequency significantly affects the axial strain development of saturated coral sand under diverse loading modes. Three stages of axial strain development were identified employing incremental strain analysis. Based on these findings, a new model for axial strain development is proposed, the accuracy of this model is verified by fitting it to the data from this study and existing literature.

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.

Evaluating the cyclic strength development using energy-based methods is a novel concept in studying the dynamic properties of sand-clay mixtures under cyclic loading. In this study, a series of undrained cyclic triaxial tests were conducted on sand-clay mixtures, and the performance of different fine-grain contents on the dynamic properties and the energy dissipation of sand-clay mixtures was investigated based on the energy-based methods. The results demonstrated a gradual increase in the cyclic strain amplitude and the residual axial strain with increasing fines content (FC) under cyclic loading with a controlled cyclic stress ratio; in contrast, the accumulation of pore-water pressure slowed down. An initial decrease in the cyclic strength of the mixtures was observed with an increase in their fines contents; however, further increasing the FC enlarged the cyclic strength of the sand-clay mixtures. This transition was observed when the threshold fines content reached about 30%. The viscous energy dissipation ratio (VEDR), which is a nondimensional energy ratio based on the relationship between cyclic stress and strain and reflects the characteristics of dynamic properties, was utilized to compare three critical phase transition points, namely, VEDRvalley, VEDRpeak, and VEDR5%strain, in the energy dissipation of the sand-clay mixtures. Based on the VEDR results, the cyclic strength development indexes were established. Furthermore, low-vacuum environmental scanning electron microscopy revealed that as the FC increased, the particle composition of the sand-clay mixtures transitioned from predominantly coarse-grained to fine-grained, resulting in a change in the cyclic behavior of the mixtures from sandlike to claylike. The cyclic strength development indices provided further insights into and quantified the effect of fines contents of the sand-clay mixtures on their cyclic strength development process.

An accurate understanding of the cyclic behavior of clays and plastic silts is important for system performance predictions during earthquake loading. This paper presents the results of a numerical investigation into the individual and combined influences of static shear stress and viscous strength gain on the cyclic resistance of clays and plastic silts. Using the viscoplastic constitutive model PM4SiltR implemented in the finite difference program FLAC 8.1, the cyclic behaviors of the plastic soils were simulated using single-element cyclic direct simple shear simulations. A parametric analysis was performed with different combinations of viscous strength gains and static shear stresses. The effects of static shear stress and viscous strength gain varied under monotonic and cyclic loading conditions. Numerical findings suggest empirical correlations developed using scant laboratory data may not accurately predict the reduction of cyclic strengths with increasing static shear stress. Furthermore, sizable magnitudes of monotonic viscous strength gains only produced a marginal increase in cyclic strengths. The findings from this study highlight the need for future experimental laboratory testing to validate the numerical findings, to improve the accuracy of performance predictions of geosystems constructed with clays and plastic silts during and following earthquake loading.