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 realistic prediction of excess pore water pressure generation and the onset of liquefaction during earthquakes are crucial when performing effective seismic site response analysis. In the present research, the validation of two pore water pressure (PWP) models, namely energy-based GMP and strain-based VD models implemented in a one-dimensional site response analysis code, was conducted by comparing numerical predictions with highquality seismic centrifuge test measurements. A careful discussion on the selection of input soil parameters for numerical simulations was made with particular emphasis on the PWP model parameter calibration which was based on undrained stress-controlled/strain-controlled cyclic simple shear (CSS) tests carried out on the same sand used in the centrifuge test. The results of the study reveal that the energy-based model predicts at all depths peak pore water pressures and dissipation behaviour in a satisfactory way with respect to experimental measurements, whereas the strain-based model underestimates the PWP measurements at low depths. Further comparisons of the acceleration response spectra illustrate that both the strain- and energy-based models provide higher computed spectral accelerations near the ground surface compared with the recorded ones, whereas the agreement is reasonable at middle depth.

Submarine landslides are a geological hazard that may cause significant damage, and are among the most serious problems in offshore geotechnics. Understanding the mechanism of submarine landslide/offshore structure interaction is essential for risk assessment, but it is challenging due to its complexities. In this study, ten centrifuge tests were conducted to determine how offshore wind turbines founded on four piles respond to consecutive submarine landslides. The tests highlighted two mechanisms of soil deformation and foundation settlement associated with the landslide cycle: (1) deformations of the clay were associated with induced excess pore water pressure, and increased with the number of landslides; and (2) by contrast, foundation settlements largely depended on the dynamic impact of the first cycle and remained unchanged for the remaining events. The settlements were 0.5 m for the 10 m pile foundation and about 0.1 m for the 20 m pile foundation, both in clay and in sand. It was also found that increasing pile length reduces the excess pore water pressure, soil deformation and foundation settlement.

Evaluating the stability of coral islands and reefs in dynamic marine environments, such as waves, tsunamis, storm surges, and earthquakes, is a critical scientific issue in the field of marine geotechnical engineering. Nansha coral sand was used as the study object, and stress-controlled drained and undrained cyclic-loading tests were conducted. The undrained excess pore-water pressure and the drained cumulative volumetric strain of saturated coral sand were determined at various non-plastic fine contents (FC), relative density (D-r), and cyclic stress ratio (CSR). The results indicated that cumulative volumetric strain (epsilon(vp)) developed in coral sand via two modes: cyclic stabilisation and cyclic creep. Analyses revealed that when the potential damage coefficient (DP) x CSR 0.05, epsilon(vp) transitioned into the cyclic creep mode. Utilising cumulative dissipation energy as a linking factor showed an arctangent function relationship between the excess pore water pressure ratio (R-u) and epsilon(vp) values of saturated coral sand with different FC, D-r, and CSR. This relationship was applicable to both stress- and strain-controlled cyclic-loading tests. Parameters m and n of the R-u-epsilon(vp) function model increased with an increasing CSR. Additionally, an increase in the D-r or FC resulted in a decrease in m and an increase in n. Multiple regression analysis further revealed that model parameters corrected for compactness and cyclic stress levels exhibited distinct trends as the void ratio (e) increased. Specifically, CSR alpha x m(D)(R) decreased, and CSR1-alpha x n(D)(R) increased. Both parameters displayed a single power function relationship with e. Based on these findings, a coupled incremental model for the cyclic pore pressure and volumetric strain of saturated coral sand, based on energy conversion, was developed.

Considering the occurrence of an earthquake, the bearing capacity of a strip footing placed on a saturated cohesive-frictional soil mass has been computed by performing a pseudo-static rigorous analysis incorporating the existence of (i) excess pore water pressures, and (ii) additional seismic-tractions and body forces. The analysis has been carried out by using lower and upper bounds finite elements limit analysis (FELA) in conjunction with the second order cone programming (SOCP) using the Mohr-Coulomb (MC) yield criterion. The generation of the excess pore water pressure in the event of an earthquake has been incorporated by defining a pore pressure coefficient ru-a ratio of the excess pore water pressure to the total vertical overburden stress at any point. The analysis has revealed that the bearing capacity reduces considerably with an increase in the magnitude of horizontal earthquake acceleration. For a given magnitude of earthquake acceleration, the bearing capacity reduces extensively further with an increase in the value of ru. All the computational results have been presented in a non-dimensional manner, and for the validation purpose, necessary comparisons have also been made. The study will be useful for designing foundations in a seismically active zone.

This article introduces a novel system identification technique for determining the bulk modulus of cohesionless soils in the post-liquefaction dissipation stage following seismic excitation. The proposed method employs a discretization of Biot's theory for porous media using the finite difference method. The technique was validated using synthetic data from finite elements simulations of an excited soil deposit. These numerical simulations were performed using an advanced multi-yield surface elastoplastic model. Additionally, the technique was used to analyze a series of high-quality dynamic centrifuge tests performed on Ottawa F-65 sand as part of the LEAP- 2020 project. A comparative analysis between recorded and identified bulk modulus values highlights the effectiveness of the proposed technique across a wide range of conditions.

The present experimental study evaluates the overburden correction factor (K6) of different pond ash samples under earthquake loading for liquefaction analysis. A series of 54 stress-controlled cyclic simple shear tests was conducted on pond ash specimens at different overburden pressures and cyclic stress ratios. Cyclic resistance ratio (CRR) was evaluated for each pond ash sample at different overburden pressures using two criteria based on maximum excess pore water pressure and double amplitude shear strain to evaluate the K6. The K6 values obtained for the pond ash were compared with the K6 values for natural soils (clean sand and sand-silt mixtures). The cyclic resistance ratio (CRR) and K6 values were observed to decrease with an increase in overburden pressure from 50 kPa to 100 kPa, and a further increase in overburden pressure to 150 kPa led to an increase in CRR and K6 values for pond ash specimens with fine particles dominated matrix. However, an opposite trend was observed for pond ash specimens with coarse particles-dominated matrix. The unique response of K6 values for pond ash was found to be significantly different from the already available K6 response for natural cohesionless soil (clean sand and sand-silt mixtures) as it unavoidably included the effect of OCR and void ratio along with the vertical overburden pressure.

In order to gain a more accurate understanding of the dynamic characteristics of soil, vibration triaxial tests were conducted on representative sand and clay samples from the Beijing area. The study investigated the influence of varying loading frequencies, cyclic stress ratios, and confining pressures on soil strength and liquefaction resistance, while also analyzing changes in shear modulus and damping ratio. The dynamic shear modulus of both sand and clay decreases with increasing shear strain, with higher confining pressures resulting in larger shear moduli. For sand, the damping ratio decreases as shear strain increases; however, for clay it initially increases before decreasing. Overall, clay exhibits a larger dynamic shear modulus but smaller damping ratio compared to sand. The number of cycles experienced by both sand and clay samples decreases with increasing confining pressure or deviational stress. As loading frequency increases, the number of cycles gradually rises for sand samples but first increases then decreases for clay samples. The damping ratio of sand gradually declines with an increase in cycle count while that of clay remains relatively stable. The variations observed in shear modulus and damping ratio are influenced by factors such as loading frequency, confining pressure, and stress.

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).

Despite over six decades of field and laboratory investigations, theoretical studies, and advances in constitutive modeling, questions remain on the fundamental issues concerning liquefaction mechanisms, the collective influence of multiple factors on excess pore water pressure (EPWP) generation, and liquefaction triggering criteria. This paper presents the general apparent viscosity-and average flow coefficient-based methodology for quantifying the solid-liquid phase-change process of liquefiable soil under undrained cyclic loading. The analysis reveals that the evolution of the soil particle-fabric system is the fundamental physico-mechanical mechanism behind EPWP generation in a liquefiable soil, with the accompanying change in soil physical state serving as the intrinsic mechanism driving EPWP generation. The study further identifies the physico-mechanical foundations of EPWP generation, as well as the inherent causes and a unified quantitative characterization of the coupled influences of multiple factors on EPWP generation. This work presents the novel observation that the marginal peak excess pore pressure ratio (ru,pm) between the solid-liquid mixed phase and the liquid phase of liquefiable soil can be identified accurately and that ru,pm is characterized by its inherent robustness. A ru,pm value of 0.90 can be used as a liquefaction triggering criterion for soils both in laboratory element tests and in the field. Another original finding is that the liquefaction triggering resistance curve is the threshold state curve between solid-liquid mixed phase and transiently liquid phase of a liquefiable soil and is unique for a specific initial physical state. The definitions of liquefaction triggering and corresponding liquefaction triggering resistance are clear and unambiguous and have the same physico-mechanical basis. The insights obtained in this paper will potentially enable the scientific and engineering communities to reinterpret the liquefaction mechanism, its evaluation, and liquefaction mitigation strategies.