A group of earthquakes typically consists of a mainshock followed by multiple aftershocks. Exploration of the dynamic behaviors of soil subjected to sequential earthquake loading is crucial. In this paper, a series of cyclic simple shear tests were performed on the undisturbed soft clay under different cyclic stress amplitudes and reconsolidation degrees. The equivalent seismic shear stress was calculated based on the seismic intensity and soil buried depth. Furthermore, reconsolidation was conducted at the loading interval to investigate the influence of seismic history. An empirical model for predicting the variation of the accumulative dissipated energy with the number of cycles was established. The energy dissipation principle was employed to investigate the evolution of cyclic shear strain and equivalent pore pressure. The findings suggested that as the cyclic stress amplitude increased, incremental damage caused by the aftershock loading to the soil skeleton structure became more severe. This was manifested as the progressive increase in deformation and the rapid accumulation of dissipated energy. Concurrently, the reconsolidation process reduced the extent of the energy dissipation by inhibiting misalignment and slippage among soil particles, thereby enhancing the resistance of the soft clay to subsequent dynamic loading.

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.

Pile penetration in soft ground involves complex mechanisms, including significant alterations in the soil state surrounding the pile, which influence the pile negative skin friction (NSF) over time. However, the pile penetration process is often excluded from finite element analysis. This paper investigates the impact of pile penetration on the generation of NSF and dragload. A stable node-based smoothed particle finite element method (SNS-PFEM) framework is introduced for two-dimensional axisymmetric conditions and coupled consolidation, incorporating the ANICREEP model of soft soil with a modified cutting-plane algorithm. A field case study with penetration process is simulated to verify the numerical model's performance, followed by a parametric analysis on the effect of penetration rate on NSF during consolidation. Results indicate that without the pile penetration process in NSF analysis can result in an unsafely low estimation of NSF and dragload magnitudes. The penetration rate affects dragload only at the initial consolidation stage. As consolidation progresses, dragload converges to nearly the same magnitude across different rates. Additionally, current design methods inadequately predict the beta value (where beta is an empirical factor correlating vertical effective stress of soil with the pile skin friction) and its time dependency, for which a new empirical formula for the time-dependent beta value is proposed and successfully applied to other field cases.

The long-term settlement of subsea pipelines on a clayey seabed is crucial for the on-bottom stability of the pipelines, especially in deep waters. In this study, a poro-elasto-viscoplastic finite element analysis is performed for predicting long-term settlement of subsea pipelines by incorporating a rheological constitutive model. A method for identifying the creep-settlement (Sc) from the total-embedment (Sk) is proposed on the basis of the obtained linear relationship between the secondary consolidation coefficient (C alpha e) of the clayey soil and the total-embedment (Sk) of the pipe. The identifying method is validated with the existing theoretical solutions and experimental data. Parametric study is then performed to investigate the key influential parameters for long-term settlement of subsea pipeline. A non-dimensional parameter Gc is introduced to quantitatively characterize the soil rheology effect on pipeline settlement. The relationship between the proportion of creep-settlement in the total-embedment (Sc/Sk) and Gc is eventually established for identifying whether the proportion of creep-settlement in the total-embedment is remarkable.

The under-consolidated state affects the deformation behavior of deep excavation in soft soil and poses potential risk to the safety of adjacent facilities. However, the deformation model of deep excavation in under-consolidated ground has not been well investigated yet. This study presents a series of numerical analyses on the deformation characteristics of deep excavations in under-consolidated and normally-consolidated ground, each of which is retained by diaphragm walls with rigid struts and a bottom improvement layer. Under-consolidated cases without excavation activities (i.e., simplified as Consolidate cases) were also included for comparison. The modelling results showed that, with the lateral constraint of inner rigid support system, the under-consolidated ground resulted in only a slight increase of lateral wall deformation but a significant increase in ground settlement as compared to normally-consolidated ground. The under-consolidated ground with lower initial average consolidation ratio, thicker surface fill, higher permeability, and longer construction period produced greater wall deformation and ground settlement during excavation. Besides, this study proposed an empirical method to estimate the settlement envelope for deep excavation in under-consolidated ground as the superposition of two parts: settlement induced by excavation activities, and settlement induced by residual consolidation with consideration of average consolidation ratios before and after excavation.

Steel slag is an environmentally friendly material with significant potential as an alternative to gravel for encased columns in soft ground improvement. However, the performance of composite foundations improved by geosynthetic-encased steel slag columns (GESSC) remains somewhat unclear. This study compares the working performances of GESSC and geosynthetic-encased stone column (GESC) composite foundations, as well as untreated foundations, through a series of large-scale experiments. Additionally, cone penetration tests were conducted on both the untreated and GESSC foundations to assess changes in soil strength before and after loading. The results show that both GESSC and GESC significantly increase the bearing capacity of soft clay, demonstrating an approximate 10-fold increase compared to the untreated foundation. The GESSC composite foundation marginally outperforms the GESC in bearing capacity during the elastoplastic stage. Furthermore, upon reaching the ultimate bearing capacity, the GESSC exhibits greater radial strain and less settlement than the GESC, owing to the unique redistribution of steel slag and gravel. Both types of foundations effectively transmit vertical pressure to deeper soil layers, with GESSC demonstrating superior load transmission capabilities and a more uniform distribution of soil stress along the depth. The excess pore-water pressure and its accumulation rate within the GESSC foundation are typically lower than those in the GESC composite foundations, underscoring the superior drainage capabilities of GESSC. This enhanced drainage capacity leads to a higher consolidation ratio within the soil, resulting in a significant improvement in soil strength after loading compared to the untreated foundation.

Soft clay is the primary soil type encountered in engineering construction in the eastern coastal regions of China. The deformation characteristics of soft clay are closely related to its inherent stiffness. Under the action of long-term geostatic stress and external load, the dynamic behavior and characteristics of soil in vertical and horizontal directions are different, i.e., anisotropy. In this study, the dynamic parameters of saturated soft clay samples were investigated through bidirectional dynamic step-amplitude cyclic triaxial experiments. The anisotropic stiffness evolution of soft clay over a wide strain range was analyzed, and the effects of different consolidation states on the development of dynamic shear modulus and damping ratio were also examined. Under the same confining pressure, the soft clay samples subjected to axial step-amplitude cyclic loading exhibited higher ultimate dynamic stress values in backbone curves compared to those under radial step-amplitude cyclic loading, while the obtained shear modulus showed the opposite trend. The anisotropic stiffness ratio of soft clay samples tended to increase with increasing confining pressure, with an average value of 1.25 in the range of 100-300 kPa. The shear modulus of the samples increased with increasing confining pressure and consolidation stress ratio but decreased with increasing overconsolidation ratio (OCR).

A series of cyclic triaxial tests were conducted on marine soft clay deposits to establish and validate a predictive model for cumulative plastic strain. Additionally, a numerical model of particle flow code in cyclic triaxial tests was developed. The effects of confining pressure, moisture content, and dynamic stress ratio on the dynamic properties of marine soft clay were examined, considering factors such as volume deformation and microscopic failure patterns. The results indicated that both the predictive model and numerical model showed strong consistency with the experimental data. The plastic strain of marine soft clay was influenced by moisture content, stress ratio, and confining pressure in a consistent manner, with moisture content being the primary factor. A predictive model for the cumulative plastic strain of marine soft clay was successfully established, allowing for the evaluation of dynamic properties from the perspective of cumulative plastic strain. During the loading process in the numerical model, microcracks within the soil structure gradually compacted, and the main displacement of the specimen extended from the vertical center axis to the sides, ultimately resulting in shear damage.

Accurately predicting pile penetration in marine soft clays is crucial for effective construction, load-bearing design, and maintenance of offshore pile foundations. A semi-analytical solution employing the combined expansion-shearing method (CESM) is introduced to model pile penetration in soft clays. This method innovatively simplifies the Pile penetration into undrained cavity expansion and vertical shearing. Using the S-CLAY1S model, which incorporates the anisotropy and structure of natural soft clays, an exact semi-analytical solution was developed to describe soil behavior around the pile under undrained vertical shearing, expanding upon existing undrained cavity expansion solutions. The accuracy and innovation of the CESM were validated through the results of field tests and finite element simulations. Additionally, a comprehensive parametric study highlighted the significant impact of soil's initial structure and stress state on pile penetration response. The study findings strongly align with theoretical calculations, field Measurements, and numerical simulations. Compared to the conventional cavity expansion method, CESM excels in resolving soil stresses at the pile shaft, albeit with a slight limitation in evaluating excess pore water pressure of soils at the pile shaft. The proposed solution considers the fundamental properties of soft clays, including their anisotropy and structural behavior, while incorporating the vertical shearing experienced by the soil during pile installation, thereby providing a simplified yet precise theoretical framework for addressing pile penetration challenges.

Stiffness degradation of soft clay around offshore monopile is caused by the long-term effect of lateral complex cyclic loading such as wave and wind. Offshore wind turbine structure is a dynamic sensitive structure. It is urgent that the effect of complex cyclic loading on stiffness degradation of soft clay around pile and natural frequency of offshore wind turbine. A series of variable cyclic dynamic shear tests were conducted. The effect of initial shear stress and cyclic shear stress on soften characteristics of soft clay was investigated. It was found that as the initial shear stress is less than the cyclic shear stress, softening index decreases with the increase of cyclic stress ratio. Based on the test results, a soften model of soften clay with considering the effect of initial stress and cyclic shear stress was then built. By combining dynamic motion equation and this soften model of soft clay, a calculation method of natural frequency for offshore wind turbine structure was established to consider the effect of initial shear stress and cyclic shear stress. This method is verified by combining with the results of practical engineering and numerical data. Some parameters influence analysis were performed to explore the effect of amplitude and number of shear stress on the natural frequency of offshore wind turbine structure. The results showed that natural frequency of offshore wind turbine structure decreases with the increase of initial shear stress. As the amplitude and number is increased, the natural frequency decreases.