Many engineering activities are conducted on marine soft soil foundations, with various types of soft soil influencing these projects differently. Studying the engineering characteristics and deformation mechanism of marine soft soil is crucial for the design and construction of marine structures. To reveal the dynamic mechanical characteristics of marine soft soil under wave loading, a set of soil samples under different confining pressure conditions were tested via a dynamic triaxial apparatus. Furthermore, a constitutive model was developed to predict the dynamic strength of marine soft soil subjected to wave loading. The experimental results demonstrate that the dynamic stress-strain behaviour of marine soft soil progresses through three stages: compaction, deformation, and failure. The dynamic strain-time history curve of the soil exhibited a cyclic trend characterized by a superposition of monotonic changes, which was attributed to the simultaneous occurrence of plastic deformation and cyclic deformation. The strain rebound gradually disappears with increasing number of loading cycles; the strain accumulation mainly occurs as compressive strain during the postvibration period. Within each stage, the dynamic shear modulus decreases with increasing shear strain, showing consistent curve characteristics across different dynamic stress amplitudes. During long-term cyclic loading, the damping ratio initially decreases and then stabilizes, with a negligible influence from the confining pressure. The Martin-Davidenkov constitutive model effectively characterizes the correlation between the dynamic shear modulus and shear strain, with fitting curves closely matching the measured data.

Marine structures are commonly situated near the mildly sloping sandy seabed characterized by the slope angles (alpha) not exceeding 10 degrees. The seabed liquefaction can be triggered due to the generation of the excess pore water pressure (EPWP), posing a threat to the stability of marine structures. This study focuses on the analysis of waveinduced liquefaction in the mildly sloping (MS) sandy seabed. A dynamic poro-elasto-plastic seabed model is developed to simulate the behavior of the MS sandy seabed under wave loading. The results indicates that the loading cycle required to trigger the initial liquefaction decreased as the position moved from the toe towards the crest of the MS sandy seabed. The amplitude of shear stress increases with the loading cycle and tends to increase with growing alpha before liquefaction, resulting in a slower accumulation of EPWP with larger alpha. Both the horizontal and vertical displacements induced by wave action reach the maximum at the crest of the sloping seabed. Notably, the horizontal displacement is much greater than the vertical displacement in the seabed under wave action. The displacement of the MS sandy seabed depends on not only the shear stress amplitude developed in the soils but also the accumulation of EPWP required to trigger the liquefaction in the seabed.

Calcareous sand is a favored unbound granular fill-in island project where complicated stresses are often applied. Traffic and ocean loadings are frequent in praxis, but poorly understood about the normalization of the pore pressure for both. This paper deals with an experimental simulation study on the pore pressure of calcareous sand subjected to ocean waves, traffic, and cyclic loading. Particular attention is devoted to the effect of the initial shear stress and the dynamic stress ratio (CSR). Results showed that, owing to the features of traffic loading and initial shear stress, the soil skeleton can still withstand partial external loading when reaching the failure criterion, hence the pore pressure at failure(ruf) is much smaller than liquefaction. In terms of the influence mechanism, unlike the initial shear stress, the increase in CSR accelerates the destruction of the soil skeleton, reducing the ruf, but having less effect on the critical pore pressure. Expressions for the number of cycles at failure (Nf) and ruf were obtained and the exponential model was simplified by changing N/Nf to epsilon 1/0.05 to reflect the characteristics of traffic loading. To normalize the pore pressure under different loadings, each stress component on Nf and ruf has been analyzed and the noteworthy finding is that torque has a very minor impact on the soil skeleton. Based on this finding, a new normalization method was proposed in which Nf needs to combine all loads including torque, while ruf only needs to consider axial forces. Hence, a pore pressure equation under three different loadings was established, taking into account the role of CSR and the initial shear stress.

The current design practice for offshore monopiles applies widely accepted simplifications of complex irregular loading scenarios. Highly cyclic loads from varying directions offshore are simplified by means of classification methods. This procedure involves reordering individual cycles in irregular loading scenarios by frequency, mean load, and amplitude. This approach is based on the assumption that the resulting accumulated deformation in the soil is independent of the ordering of the cycle packages, i.e. the validity of Miner's rule. Different contributions in literature investigate the validity of Miner's rule by evaluating the effect of the ordering of cycle packages on the resulting damage in the soil, i.e. the accumulated volumetric strain. This paper addresses the question of the validity of Miner's rule from a different perspective. It investigates the load-displacement behaviour of sand in direct simple shear tests due to dynamic load scenarios. Hence, the load sequences during testing vary while the energy spectrum being employed to generate the irregular load signals remains constant. This method enables capturing natural conditions offshore during testing and allows for comparing individual loading scenarios as the wave spectrum in the frequency domain defines the distribution of wave amplitudes. The test program is carried out on medium dense fine silica sand. The test results are evaluated regarding the accumulated volumetric strain and relate to previous studies on the validity of Miner's rule in sand.