Earthquake sequences frequently occur in seasonally frozen regions, where soils are simultaneously subjected to freeze–thaw cycles and repeated seismic loading, leading to complex liquefaction behavior. However, the mechanisms controlling sand liquefaction resistance under the influence of coupled freeze–thaw and foreshock sequences remain insufficiently understood. In this study, a series of undrained cyclic triaxial tests were conducted on saturated soil to investigate the effects of seismic loading sequences and freeze–thaw history on liquefaction resistance. Cyclic loads with varying stress ratios were applied to simulate foreshock sequences prior to a mainshock, and both unfrozen specimens and those subjected to one freeze–thaw cycle were examined. The evolution of excess pore water pressure (EPWP) and axial strain during foreshocks was analyzed, and liquefaction resistance was evaluated during the mainshock. The results indicate that the loading sequence of foreshocks significantly affects the cyclic response and liquefaction resistance of sand, with freeze–thaw cycles leading to greater EPWP generation and increased strain accumulation. When the maximum EPWP ratio during foreshocks does not exceed 0.8, liquefaction resistance exhibits a clear relationship with the accumulated EPWP based on the observed data trends. Conversely, in cases where large EPWP develops during foreshocks, the residual axial strain becomes the dominant factor influencing subsequent liquefaction resistance. These findings underscore the combined effects of seismic sequences and freeze–thaw processes on sand liquefaction resistance in seasonally frozen regions. ... Read more

Deep excavations in urban areas underlain by structured soft clay are critical for transportation infrastructure development, yet their deformation behavior is strongly influenced by natural soil structure and stress-path-dependent degradation. Conventional constitutive models (e.g., MCC and HS/HSS), which neglect soil structure and intermediate principal stress, often lead to unconservative deformation predictions. A modified constitutive model for structured soft clay is developed within the critical state framework, incorporating the superloading surface concept to represent soil structure. The model considers three key mechanisms governing excavation-induced deformation: (i) the intermediate principal stress effect through the SMP yield criterion, (ii) the initial soil structure described by a structural parameter R* , and (iii) strain-path-dependent structural degradation. A modified evolution law is introduced, in which a parameter A is used to distinguish the relative contributions of plastic volumetric and deviatoric strains to structural degradation. The model is implemented in finite element analyses and validated against centrifuge model tests. Parametric analyses are conducted to investigate the effects of intermediate principal stress, initial soil structure, and the plastic strain ratio parameter. The results show that neglecting intermediate principal stress underestimates wall deflection and ground surface settlement, while accounting for soil structure increases predicted deformation. This effect is moderated when strain-path-dependent structural degradation is considered, leading to improved agreement with experimental observations. Overall, the proposed approach provides a more realistic framework for deformation prediction of deep excavations in structured soft clay, with direct relevance to underground transportation infrastructure design. ... Read more

Laboratory tests on undisturbed and remolded Shanghai clay were conducted with a novel mixed boundary true triaxial apparatus. The strength and deformation behaviors of the structured soft clay at different Lode angles were carefully investigated. The relationship between the shear stress ratio and the dilatancy ratio of the clay in three different stress spaces, that is p–q space, transformed stress (TS) space, and tij modified stress space, is discussed in detail. In particular, the influences of the stress path and the structure of nondisturbed clay on the strength, deformation, and failure criterion of Shanghai clay were quantitatively evaluated. The shear strength and dilatancy of undisturbed/remolded clay decreases with the increase of the Lode angle. It is also found that the shear strength and dilatancy of the undisturbed clay are larger than those of the remolded clay, while the influence of the structure on the dilatancy decreases with an increase in the Lode angle. In the π-plane, the maximum strength of Shanghai clay under different stress paths generally obeys the spatial mobilized plane failure criterion. The coincidence of the stress–dilatancy relationship given by the TS and tij stress spaces with the test results is much better than that given by p–q stress space. Furthermore, the linearity of the plastic potential curves of the undisturbed/remolded clay is more evident in tij stress space. ... Read more

Shanghai soft clay is a typical marine clay with specific structural characteristics. However, the effects of surface surcharge loading on structured soft clay and existing tunnels remain unclear. In this study, the effect of repeated surface surcharge loading on tunnel displacement was numerically investigated, considering the structural characteristics of Shanghai Layer 4 soft clay. An elastoplastic constitutive model (Shanghai model) that describes the mechanical properties and structural characteristics of natural clay was used to simulate the soil response. The results indicate that the first cycle of repeated surface surcharge loading had the greatest effect on the displacement of the tunnel and overlying clay; subsequent loading cycles further increased the displacement. With repeated surface surcharge loading, the displacement of the overlying clay exhibited a significant decreasing trend with increasing depth. The maximum excess pore pressure of the clay exhibited a decreasing trend with increasing loading time. The displacements of the tunnel and overlying clay were greater when the effects of the soil structure were considered. However, the initial degree of the structure had no significant effect on the accumulation of excess pore pressure when the surface surcharge loading did not reach the ultimate bearing capacity of the clay. ... Read more

Shanghai soft clay is a typical marine clay with specific structural characteristics. The tunnel and overlying soft clay may undergo repeated surface surcharge loading, such as the temporary soil stacking. Assessing the extent of structured soft clay deformation and tunnel displacement caused by repeated surface surcharge loading is of great significance for evaluating of the safety of ground structures and underground tunnels. In this study, the effects of repeated surface surcharge loading on the soil and tunnel displacement were numerically investigated. The finite element code DBLEAVES with an elasto-plastic constitutive model (Shanghai model) that describes the mechanical properties and structural characteristics of natural clay was used to simulate the soil response. The parameters of the constitutive model were obtained through geotechnical testing. The effects of the soil structural characteristics, seepage conditions, and loading conditions on the soil response and tunnel displacement were analyzed. The numerical results show that the maximum excess pore pressure of clay decreased as the number of loading cycles increased. The effects of the structural characteristics cause greater displacement, whereas the effects of the degradation parameters of the structure are more significant than the initial degree of the structure. The differences in the vertical displacement of the tunnel and overlying soils owing to the structural characteristics become apparent with an increase in surface surcharge loading. However, the effects of structural characteristics become less significant as the depth increases. The seepage conditions and loading method primarily affect the build-up of excess pore pressure and the development of effective stress paths. For soils inside the surcharge area, a flexible surcharge produces a greater vertical displacement than a rigid surcharge. As the burial depth increased, the effects of the seepage conditions and loading method showed a declining tendency. ... Read more

The pile leg-mat foundation is a novel composite foundation designed for jack-up offshore platforms. The seismic response of such foundations in sandy seabeds is an issue that requires significant attention. In this study, shaking table tests were conducted under various input seismic waves to investigate the dynamic response of a seabed platform system subjected to seismic loads. The effects of the peak ground acceleration (PGA), seismic wave frequency, wave type, pile leg insertion depth, and pile length on the response of the seabed platform system were investigated. The results indicated that as the PGA increased, the acceleration response of the soil and platform model increased, increasing the accumulation of excess pore water pressure and the horizontal displacement of the platform model. The acceleration and horizontal displacement responses of the seismic wave with lower frequencies were higher than those with higher frequencies. The peak acceleration amplification factors under the measured and regular seismic waves showed significant differences, with the dynamic response of regular waves to the platform model being pronounced. Under seismic waves with low PGAs, the horizontal displacement of the platform model decreased with increasing pile leg insertion depth or pile length. ... Read more

An integrated numerical model was developed to investigate the interaction between a tsunami-like solitary wave and a monopile on a sloping sandy seabed in this study. The solitary wave motion is governed by the RANS equations with the k-ε turbulence model. The porous sloping sandy seabed is governed by Biot’s equation (u-p approximation). The solitary wave is validated with previous experimental data. Meanwhile, a further comparison of solitary wave scattering by the monopile is carried out to verify the numerical model. Then, the effects of different monopile locations were examined in investigating the solitary wave–monopile interaction problem. The velocity magnitudes and the free-surface elevation changes in the solitary wave around the monopile are investigated at various monopile locations. In addition, the response of the sloping sandy seabed and monopile under the solitary wave are examined. The numerical results demonstrate the accuracy of the current method in simulating solitary waves and wave height variation around monopiles. Wave run-up is observed in front of the monopile, with a high-velocity forward-moving water jet forming behind it. The maximum fluid velocity, wave run-up height in front of the monopile, excess pore water pressure (EPWP), and bending moment of the monopile increase as the monopile approaches the shoreline. However, at the closest location to the shoreline, due to the strong dynamic interaction between the solitary wave and the monopile, significant wave shoaling and breaking are observed, resulting in a slight decrease in the wave force acting on the monopile. ... Read more

The mechanical behaviors of structured soft clay in consolidation tests and undrained triaxial shear tests were investigated, revealing evident structural properties of undisturbed Ningbo clay. Based on the experimental results, the limitation of the original constitutive model in simulating the structural properties of clay were discussed, and the evolution rule for the structure in the original constitutive model was revised. The parameter A that represented the ratio of plastic deviatoric strain to plastic volumetric strain was proposed to account for the distinct differences in the plastic deviatoric and volumetric strains on structural degradation. This modification enhances the ability of the constitutive model to accurately simulate the mechanical behaviors of structured soft clay. The performance of the modified constitutive model was verified by the consolidation and triaxial shear tests. Furthermore, the structure degradation of undisturbed Ningbo clay was investigated. For undisturbed Ningbo clay, the plastic volumetric strain plays a major role in causing progressive structural degradation compared to the plastic deviatoric strain. ... Read more

The mat foundation is a new type of jack-up wind power installation platform suitable for clay seabed. Because of the large size of the mat foundation, a large suction force must be overcome during the uplift process. In this study, the eccentric uplift of a mat foundation was experimentally and numerically investigated. The effects of the eccentric distance, eccentric position, and foundation shape on the suction force of the mat foundation were discussed. The results revealed that the breakout force of the mat foundation mainly comprised the foundation weight and suction force. Both the uplift position and eccentric distance affected the suction force; however, the effect of the uplift position was considerably smaller than that of the eccentric distance. The dimensionless suction force for the eccentric uplift tended to decrease linearly as the eccentric distance increased. When the dimensionless eccentric distance was 0.5, the suction force was 0.35–0.5 times that in the case of central uplift. ... Read more

This paper presents a new true triaxial apparatus with mixed boundaries for soft soils. A novel rigid–flexible loading device with four sliding rigid plates is designed. In the new loading device, the sliding block and the loading plate are separated, which allows the plates to be enclosed by a rubber membrane. This unique design can not only avoid the interference between adjacent plates and the corner effect, but also achieve a full π plane stress path. The friction between the sliding plates and specimen is reduced to a negligible level by using Teflon films together with lubricant. Paired sliding plates are connected precisely by a special transmission device and driven by a servo motor, which makes it possible to keep the specimen central, in either displacement control or load control modes. The new apparatus was used to investigate the mechanical properties of Shanghai Layer-4 clay under the three-dimensional loading condition. Typical test results are presented and discussed comprehensively. The influence of Lode angle on the stress–strain relation of the clay in finite deformation is investigated and the applicability of the new apparatus to soft soil testing is confirmed. ... Read more