Narrow Results

This paper presents the steel tunnel–sand–pile interaction (TSPI) response under scaled (0.05g and 0.1g) seismic excitations (Kobe and Loma Prieta) and sinusoidal wave (frequencies of 1, 5, and 7Hz) along with the sand relative densities of 48% and 80%. For this reason, a scaled shake table test was performed in the laboratory, and the 3D numerical analysis was performed by the Plaxis for the simulation. No joint was considered along the length of the tunnel to avoid complexity. In most cases, the excess pore pressure ratio exceeds the unit value at bottom and top locations of the TSPI model to inform the occurrence of liquefaction. Maximum lateral and translational displacements of the tunnel were found to be 143mm and 72mm, respectively, at the crown. In addition, the maximum vertical translation of sand and root mean square (RMS) value of the pile strain were obtained to be 61mm and 20$\mu$m/m, respectively. However, close variations of numerical and experimental studies may convince the accuracy of the simulation. By the way, it has a scope to enhance this study in the future by varying the geometry of the TSPI model.

A coupled stress-flow finite element procedure, based on dynamic Biot equations, was used to analyze the behavior of pipe buried in liquefiable soil. The governing equations, soil constitutive model, finite element discretization and solutions were described. The results of analysis were compared with two cases of dynamic centrifuge test of soil deposit and pipe conducted at 30 g acceleration field. The horizontal soil deposit was analyzed followed by the deposit having a buried pipe of diameter 10 cm (3 m in prototype). The deposit was composed of loose Nevada sand that was saturated with a viscous solution in satisfying the similitude rules of time for the dynamic event and diffusion phenomena. The response of the ground, such as acceleration and excess pore water pressure, and the earth pressure and uplifting of the pipe, were presented and compared. The results of analysis indicated that a coupled stress-flow finite element procedure where the soil was expressed by Pastor–Zienkiewicz Mark-III model was able to simulate the dynamic response of the soil and pipe up to the stage of liquefaction. Several other issues related to the analysis were discussed.

In an earthquake, underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. Such uplift response of the buoyant structure is influenced by the soil it is buried in. In the case of a liquefiable soil deposit, the soil can lose its shear strength significantly in the event of an earthquake. If the soil liquefies fully, the buoyant structure can float towards the soil surface. However, a partly liquefied soil deposit retains some of its initial shear strength and resists the uplift. This paper discusses the different soil conditions and their influence on the uplift response of buoyant structures.

Offshore wind turbine (OWT) structures are subject to wave, wind, and seismic loading. Due to the cyclic nature of these loads, OWT foundations can be vulnerable to cumulative deformation and liquefaction triggered by waves and earthquakes. The effects of cumulative deformation and liquefaction on the monopile-supported OWT are not fully appreciated. This paper develops a three-dimensional numerical model for analyzing the seismic performance of large monopile-supported OWT under the long-term effect of cyclic loading. The numerical model was established employing FLAC3D and utilizing SANISAND constitutive model to simulate the soil behavior. The numerical model was validated by comparing its predictions with the results of dynamic triaxial tests and centrifuge tests. A simplified densification and subsidence site model was integrated into the numerical model to facilitate considering the long-term effect of cyclic loading. The numerical model was then used to conduct a comprehensive study to evaluate the influence of long-term cyclic loading on the natural frequency and seismic response of OWT structure. The results demonstrated that the densified subsidence zone around monopile increased the liquefaction resistance. However, the horizontal displacement of pile and the response acceleration of tower-top increased due to soil subsidence around monopile.

Shallow footings are the most preferred foundations for buildings due to low cost and ease of construction. However, under seismic loads, these foundations may suffer excessive settlements, particularly when there is a risk of soil liquefaction. This paper explores the effectiveness of granular columns in mitigating the liquefaction-induced ground deformations under shallow foundations, using FLAC2D program. PM4Sand, a critical state-based bounding surface plasticity model is used to simulate the stress–strain response of sand to cyclic loading. The responses of granular columns are also simulated using the same soil model. Validation of the numerical model is presented against the experimental results of mildly sloping ground. The application of granular column groups resulted in maximum reduction of the settlement of footing by 60% when soil densification is included along with drainage and stress redistribution effects. Though excess pore water pressure is relatively low in treated deposit compared to the untreated deposit, its contribution to the reduction in settlement of footing is found to be minimal.

Cyclic liquefaction of sands is influenced by many factors including the initial fabric. Yet, it is difficult to quantify the soil fabric using laboratory technology. In this study, discrete element method (DEM) is used to numerically simulate the process of liquefaction under undrained cyclic loading. Samples with the same void ratio and varying degrees of fabric anisotropy are prepared by the pre-shearing method. Fabric evolution before and after cyclic liquefaction is quantified by the coordination number, angular distribution and the principal direction of inter-particle contacts. The DEM study demonstrated that the coordination number decreases and the fabric anisotropy increases gradually when the sand is cyclically sheared to approach the initial liquefaction. In this process, the principal direction of the anisotropic fabric tensor is not coaxial with the stress tensor. After initial liquefaction, all samples with different initial fabric evolve towards a same fabric, which is strongly anisotropic. The principal direction of the fabric aligns with the principal direction of the stress in the post-liquefaction stage.

In this study, liquefaction and earthquake response analysis of Panipat pond ash embankment in India has been carried out considering saturated and natural water table conditions. Laboratory and field studies have been carried out to obtain the material properties required for the nonlinear response analysis of the Panipat pond ash embankment. Nonlinear finite element analysis has been carried out by using open system for earthquake engineering simulation (OpenSees). Three earthquakes namely Chamba, Chamoli and Uttarkashi earthquakes have been considered for liquefaction and earthquake response analysis. The standard penetration test (SPT)-$N$ value indicates that the ash deposit in Panipat pond ash embankment is in loose to medium dense state. The excess pore pressure ratio obtained from the analysis is found to be one or more than one below upstream and downstream locations of the pond ash embankment for both saturated and natural water table conditions. The horizontal and vertical displacement is found to be maximum near the toe and first rise slope of pond ash embankment. Hence, ash embankment in Panipat is prone to liquefaction under the excitation of moderate to high earthquake loading.

The objective of this study was to investigate the liquefaction response of soil using the spatial variability of the shear modulus by considering different values of the coefficient of variation (COV) and the horizontal scale of fluctuation (SOF). For this purpose, a Monte Carlo simulation, combining the digital generation of a non-Gaussian random field with finite difference analyses, was utilized. Parametric studies were performed from the perspectives of the liquefaction area, excess pore water pressure (EPWP), and displacement at the ground surface. We found that a larger COV of the soil shear modulus was correlated with a slower reduction of liquefaction area and a lower EPWP ratio. To explain the influence from the perspective of the spatial distribution characteristics of the shear modulus, a deterministic model test was carried out. Additionally, it was found that the displacement history and the differential settlement at the end of shaking were regularly affected by the COV and the horizontal SOF, especially for large COV and horizontal SOF.

During their propagation tsunamis often traverse continental slopes that are relatively steep compared to deeper oceans. Further due to the change of bed slope from offshore to near shore, there is every likely possibility that a tsunami might steepen and eventually break, thereby generating large pressure gradients that could enhance the likelihood of liquefaction of the seabed. In the drawdown, high shear stresses could trigger debris flow in submarine canyons and on steep ridges. Therefore estimation of the bed stress is important in estimation of the forces induced during tsunami wave propagation, both on the seabed as well as on the subsurface installations. Bed and shear stresses generated by wave forms that represent tsunami (a solitary wave and broken solitary wave in the form of a bore) are measured using a shear cell. This paper deals with the measurements and modeling of the bed stress under the solitary waves.

This paper presents the results of laboratory investigation carried out on Ahmedabad sand on the liquefaction and pore water pressure generation during strain controled cyclic loading. Laboratory experiments were carried out on representative natural sand samples (base sand) collected from earthquake-affected area of Ahmedabad City of Gujarat State in India. A series of strain controled cyclic triaxial tests were carried out on isotropically compressed samples to study the influence of different parameters such as shear strain amplitude, initial effective confining pressure, relative density and percentage of non-plastic fines on the behavior of liquefaction and pore water pressure generation. It has been observed from the laboratory investigation that the potential for liquefaction of the sandy soils depends on the shear strain amplitude, initial relative density, initial effective confining pressure and non-plastic fines. In addition, an empirical relationship between pore pressure ratio and cycle ratio independent of the number of cycles of loading, relative density, confining pressure, amplitude of shear strain and non-plastic fines has been proposed.

Several studies on liquefaction using physical model tests and numerical analysis have been conducted in recent years; however, few studies have investigated the effect of liquefaction-induced settlement on structures. Especially, this settlement seriously influences on gravity foundation during earthquake. This study aims to investigate the settlement of the surrounding ground of steel pipe sheet pile (SPSP) foundation during liquefaction by using an effective stress analysis. 2D numerical modeling was used in this study and the behavior of undrained soil was idealized using a cocktail glass model. The numerical results were compared with experimental results from a 1-G shaking table test with a scale of 1:60. The results indicate that the settlement of surface ground and SPSP foundation rapidly increase when the liquefaction occurs and is significantly influenced by permeability coefficient of ground.

There have been many costly damages of group-pile foundation of bridge-pier in soft or saturated-medium dense sandy ground after strong earthquakes. This study investigated the dynamic behavior of a 2 × 2 bridge-pier–group-pile foundation installed in a two-layer ground commonly found in Japan. A series of three-dimensional (3D) effective stress analysis, which adopted advanced cyclic elasto-plastic soil model and nonlinear axial force dependence reinforced-concrete model, has been implemented to demonstrate the interaction between the soil–group-pile–bridge-pier during an earthquake of the magnitude of the 1995 Kobe earthquake. The damages of the 2 × 2 group-pile foundation have been quantitatively found to be either caused by the inertial loading of the superstructure or the kinematic interaction of the soils.

Reducing the risk of structural damage due to earthquake-induced liquefaction in new and existing buildings is a challenging problem in geotechnical engineering. Drainage countermeasure techniques against liquefaction have been studied over the last decades with an emphasis on the use of vertical drains. This technique aims to allow a rapid dissipation of excess pore pressures generated in the soil during the earthquake thereby limiting the peak excess pore pressures and consequently improve the structural response. Rapid drainage in the post-earthquake period in the presence of these drains helps quick recovery of the soil strength. Recent studies propose different variations in the vertical drains arrangement to improve the excess pore pressure redistribution in the soil around structures. However, conventional arrangements for existing buildings do not achieve an adequate proximity from the drains to the soil below the foundation. To address this, the performance of inclined and vertical perimeter drain arrangements are studied in this paper. Dynamic centrifuge tests were carried out for the different arrangements in order to evaluate the excess pore pressure generation due to ground shaking and the following dissipation together with the foundation settlement and dynamic response.

Proper consideration of variations in soil properties and their effects is necessary to enhance the seismic safety of structures. In this study, the effect of spatial variations in the cyclic resistance ratio on seismic ground behavior was investigated. Initially, dynamic centrifuge model tests were conducted on sandy ground featuring a 20% mixture of weak zones with low relative density and on homogeneous sandy ground with no mixture of weak zones. Subsequently, an effective stress analysis was performed by modeling the distribution of weak zones in the centrifuge model tests. Finally, after confirming the validity of the parameter settings, several analytical models with different weak-zone distributions were generated and numerically analyzed using random field theory. The results indicate that a local mixing of approximately 20% weak zones has only a limited effect on overall ground behavior. However, differences were observed in the rate of increase and dissipation of the excess pore water pressure ratio and in the residual horizontal displacement.

Conventional methods of quantifying liquefaction hazards involve in-situ drilling techniques however, they are invasive and destructive on the sites and costly in terms of labor, time, and money. In this study, the Horizontal-to-Vertical Spectral Ratio (HVSR) of microtremors was used. It is a passive seismic geophysical technique, and we aim to evaluate its effectiveness in assessing liquefaction hazards. An array of single-station measurements was performed within the confines of selected experimental school sites in the coastal lowlands of the Greater Metro Manila Area (GMMA), Philippines. Measured predominant periods and their relative amplitudes were obtained to calculate the seismic shear strain, γ, of Nakamura and correlated with calculated liquefaction potential indices (LPI) from available downhole data. This study suggests that the employed methodology can be a quick, non-invasive, and cost-effective complement to existing downhole measurements in the assessment of liquefaction. Such a method can be adapted between borehole data gaps to extrapolate information where downhole data is limited or not always available.

In performance-based design of geotechnical works, accurate evaluation of deformation is often required. FEM is an effective method in this design process. However, not only the numerical models but also the detail of parameter setting is not unique, and it depends on the engineer. In this research, the difference of simulation results depending on the difference of the parameter set determined by several engineers is investigated. As a result, in liquefaction analysis, residual deformation of structure significantly depends on the parameter set for the liquefaction. And the coefficient of variation for the residual deformation is far larger than that of parameter set. Therefore, even a slight difference of parameter determination can cause a large difference on the estimated residual deformation.