Excess Pore Pressure Ratio Research Articles

Analysis of Liquefaction in Tailings Deposits by Fem Modeling of Undrained Cyclic Triaxial

In this article, a numerical calibration procedure for undrained cyclic triaxial tests is presented to evaluate the liquefaction potential in sand and silt samples from mining tailings in northern Chile. The numerical modeling of an axisymmetric specimen involved two stages: isotropic consolidation using the Hardening Soil Small (HSS) model and a cycling phase employing the UBC3D-PLM model to simulate the onset of liquefaction using the criterion that the excess pore pressure ratio Ru should exceed 0.8. The results demonstrate that the UBC3D-PLM modeling calibrated with experimental data from cyclic triaxial tests effectively represents the excess pore pressure in both sandy and silty soils from mining tailings. The accuracy of the modeling decreases when a single set of parameters is applied to the same soil at different cyclic stress ratios (CSR), highlighting the need for specific calibrations for each loading.

Experimental investigation of dynamic behaviour of pile-supported bridges in sandy deposits

This paper presents the results of centrifuge tests performed on reinforced concrete (RC) pile–supported simply supported girder bridge models in non-liquefiable (unsaturated) and liquefiable (saturated) slightly inclined sandy soil sites. The main objectives were to investigate the dynamic response characteristics of RC pile–supported simply supported girder bridges in non-liquefiable and liquefiable soils, examine the mechanisms causing beam collapse and damage to pile–pier yielding, and verify the effectiveness of cable restrainers. The centrifuge tests were conducted in non-liquefiable and liquefiable sites, each consisting of a three-span RC pile–supported simply supported girder bridge model and a control model with a superstructure equipped with a cable restrainer. First, the dynamic characteristic parameters of the soil and structural models in both sites were obtained. Subsequently, the conventional test results were interpreted, including the excess pore pressure ratio, acceleration, and displacement. The analysis included examining the moment demand of the pile–pier structure and seismic damage mechanisms. Finally, a correlation analysis was performed to evaluate the inertial and kinematic effects on the moment demand of the pile–pier structure. The results show that the acceleration response after soil liquefaction at the inclined sites is amplified unilaterally in the downslope direction and spikes appear. The liquefiable foundation is displaced to the downslope. However, major earthquake induces foundation displacement, and liquefaction leads to a longer duration and more significant soil displacement. The installation of cable restraints significantly increased the acceleration response of the superstructure. Still, it effectively reduced the relative displacement of the superstructure and prevented the beam from collapsing. Soil liquefaction decreases the bending moment of the pile–pier structure, and the cable restrainer causes the damage mode of the pile–pier structure to shift from the pile head to the pier bottom. In the non-liquefiable scenario, the bending moment demand at the pile head has a more significant influence on the inertial effect. Additionally, using the cable restrainer system can increase the kinematic effect after liquefaction. The test results can be used to validate numerical models and provide a reference for pile foundation design.

Cyclic liquefaction of granular materials with varied forms under multidirectional loads

Both the liquefaction resistance of granular materials under complex multidirectional cyclic loadings and the influence of particle form or morphology on that resistance are poorly understood. This paper presents an experimental investigation on this topic. Round, frosted, concave and convex glass beads as well as Hong Kong completely decomposed granite (CDG) sand were used to prepare samples with varying particle morphologies (i.e. roughness, sphericity, aspect ratio and convexity). Unidirectional and multidirectional cyclic simple shear tests were performed to examine the liquefaction susceptibility and highlight the specific role of various particle morphologies. Results show that the increase of shape irregularity and surface roughness both result in a considerable liquefaction resistance improvement. Moreover, overall shape has a more fundamental effect on liquefaction behaviour than surface roughness. An enhanced morphology index, providing a collective description for the particle geometry and surface texture, are proposed to optimise the correlation with liquefaction behaviour. In addition, it is discovered that multidirectional loading could exacerbate the generation of pore pressure and trigger a sharp increase in shear strain amplitude at a lower excess pore pressure ratio. Accordingly, a novel analytical procedure for estimating the liquefaction resistance, independent of cyclic loading patterns and particle morphology, is developed and validated by additional data in the literature.

Seismic Response Comparisons of Prefabricated and Cast In Situ Subway Station Structures in Liquefiable Site

Based on the engineering practice of the first prefabricated subway station (Shuangfeng Station) in Changchun, China, the applicability of prefabricated subway station structures (PSSSs) in liquefiable sites in seismically defended areas is investigated. In this paper, the finite difference software FLAC3D 5.0 is used to carry out the seismic response analysis of the PSSS in liquefiable ground, and the calculation results of the PSSS are compared with those of the same type of cast-in-place subway station condition. The results show that the trend of foundation excess pore pressure ratio (EPPR) in the PSSS condition is similar to that of the cast-in-place condition. For different ground vibration inputs, there is not much difference between the PSSS and the cast-in-place structure on the pore pressure (PP) of the surrounding liquefiable soil. The acceleration response of the PSSS is slightly smaller than that of the cast-in-place structure, and it has a better ability to adapt to ground deformation. The deformation of the upper part of the PSSS is slightly larger than that of the lower part, which is an important part of its deformation control, and the middle part is the key part of its strength control due to the presence of the center plate, which results in a significant increase in stiffness and stress. The flexible connection of the PSSS is easier to adapt to a larger vertical deformation than rigid connection, and its ability to resist overturning is better. Under the premise of ensuring static waterproofing, the PSSS can be constructed in liquefiable sites in earthquake-proof areas.

Numerical analysis of the post-installation consolidated response of skirted foundations

The effect of the installation process on the subsequent foundation consolidation response requires assessment when the preload is initially applied. In the framework of effective stress, the influence of large soil deformations during penetration is analysed in this study using a comparison of results from large deformation finite element (LDFE) analysis based on periodic mesh regeneration (Ritss technique) and a 'wished-in-place' small strain finite element (SSFE) model. The LDFE results demonstrate that considerations of the skirt penetration and soil plug bulge lead to different soil excess pore pressure and void ratio change distributions from that of the SSFE analysis, thereby affecting the foundation settlement and consolidated bearing capacity. The normalised fully consolidated undrained bearing capacity grows linearly with the preload ratio for preloads higher than the penetration resistance, and the capacity obtained from the LDFE is higher. The settlement ratio and the proportion of partially consolidated to fully consolidated capacity gain can be represented as a hyperbolic function of a revised dimensionless time factor. The methods integrated in this study can be used to predict the increase in the vertical undrained capacity of skirted foundations after consolidation, accounting for installation effects in engineering practice.

Model test and finite element analysis results of a monopile in very dense sand under unidirectional horizontal cyclic loading

This study is carried out to understand the response of a monopile for a large (8–9 MW) offshore wind turbine located at a 45m water depth under lateral cyclic loading. Firstly, a 1g large-scale model pile test was performed in saturated very dense sand under numerous cycles of lateral loading, N=2000. The test results show that the lateral pile head displacement and rotation increase while the secant stiffness of pile decreases with increasing N. The numerical model was then validated by comparing the pile head displacement results of the test and the finite element (FE) model analysis. The soil was modelled using the UBC3D-PLM constitutive model to understand the development of the excess pore pressure ratio (EPPR) around the monopile. The FE analysis was performed cycle by cycle up to N=5 for a large diameter “XL monopile” with diameter D=9m and embedded length L=50m. The results showed that the EPPR around the monopile is insignificant. Furthermore, the effect of the number of loading cycles on the evolution of the EPPR, pile head deformation and pile head rotation are insignificant. In addition, a relatively high maximum pile head rotation accumulation θmax≅0.13° was obtained during cyclic loading.

Response of embankment on liquefiable soil to sequential ground motions considering mitigation measures

The present study focused on gravel berm and stone column improvement techniques for mitigating the effects of liquefaction for an embankment resting on liquefiable ground. Three-dimensional finite element models are developed to evaluate the effectiveness of two countermeasure techniques for embankments resting on liquefiable soil. Three different embankment models are considered in this study as benchmark embankment model (BM), gravel berm embankment model (GBM), and stone column mitigation model (SCM). Foundation soil has been modeled using an elasto-plastic effective stress-based UBC3D-PLM model. Initially, the behaviour of three different embankment models (BM, GBM, and SCM) are evaluated under varying amplitude of cyclic input motion. Later, a seismic study is carried out considering 10 sequential earthquake motions to assess the effect of the aftershock following the main shock event. It can be seen that even small amplitude aftershocks can develop a high excess pore pressure ratio in the foundation soil, especially below the embankment toe. Moreover, a linear correlation has been observed between the input motion Arias intensity and the intensity at the embankment crest. In an overall observation, stone column mitigation was found to be a sound mitigation approach.

Dynamic response of a low plasticity silt deposit: comparison of in-situ and laboratory responses

This study compares the in-situ dynamic response of a low plasticity silt deposit subjected to multidirectional loading from vibroseis shaking and controlled blasting to a suite of element-scale, cyclic laboratory test specimens. The agreement between excess pore pressures and simple shear strain relationships over a wide range in strains is remarkable. Slightly larger excess pore pressures observed in-situ are attributed to three-dimensional loading and pore pressure migration/redistribution in the shallower portions of the deposit. Noted differences in shear modulus, G, are attributed to strain rate effects, spatial variability in the in-situ stiffness, and hydraulic boundary conditions. The variation in in-situ G/Gmax follows the trend from torsional shear specimens up to 0.4% shear strain; larger strains in the silt deposit imposed by controlled blasting yielded a stiffer response than that from cyclic torsional shear and direct simple shear specimens due in part to field drainage for deeper portions of the deposit. The in-situ cyclic resistance ratio for the deeper portion of the deposit in which plane body waves could be assumed and for the selected excess pore pressure ratio criterion was larger than that of stress-controlled cyclic direct simple shear (CDSS) test specimens, despite the detrimental effect of multidirectional shaking in the field. The effect of strain history, spatial variability, and drainage boundary conditions to drive differences between the in-situ and laboratory test specimens is identified.

Liquefaction Potential Analysis and Soil Improvement Method for Yogyakarta-Bawen Toll Road in Sleman, Yogyakarta

Based on soil investigation using SPT in the planned Yogyakarta-Bawen Toll Road in Sayengan District, Sleman Regency, the soil layers consist of sandy silt, fine-coarse sand, and gravel. Several studies that used the Idriss & Boulanger method showed the liquefaction potential in all observed locations is between high and very high. Therefore, the proposed soil improvement methods are stone columns and jet grouting. Further analysis is conducted to define the effective diameter of the column based on the excess pore pressure ratio (Ru) in both alternatives using the finite difference method (FDM). The result of Ru and CSR analysis in BH 78 showed that by using a 1.2 m diameter stone column in the triangular grid with 2.2 m spacing, Ru max is reduced by 54.75%, and SF liquefaction is improved to 1.202. Alternatively, soil improvement by using a 1.0 m diameter jet grouting in the rectangular grid with 3.0 m spacing showed Ru max is reduced by 11.41% and SF liquefaction is improved to 2.373. Therefore, improvement with the stone column simultaneously reduces Ru max and cyclic stress ratio (CSR), on the other hand, the jet grouting only reduces the cyclic stress ratio.

Seismically Evaluate Liquefaction-Induced Steel Tunnel–Sand–Pile Interaction (TSPI) Response

This paper presents the steel tunnel–sand–pile interaction (TSPI) response under scaled (0.05[Formula: see text]g and 0.1[Formula: see text]g) seismic excitations (Kobe and Loma Prieta) and sinusoidal wave (frequencies of 1, 5, and 7[Formula: see text]Hz) 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 143[Formula: see text]mm and 72[Formula: see text]mm, 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 61[Formula: see text]mm and 20[Formula: see text][Formula: see text]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.

Bearing capacity of shallow foundations accounting for seismic excess pore pressures

The method of characteristics has been used to develop a rigorous solution for the evaluation of seismic bearing capacity of shallow foundations resting on homogenous fully submerged soil accounting for the effect of earthquake-induced pore water pressures. The solution, derived with reference to both the Hill and Prandtl plastic mechanisms, is essentially referred to the bearing capacity factor Nγ, is of general validity and has been checked against finite element analysis results. An original empirical expression providing the corrective coefficient to be introduced in the usual trinomial bearing capacity formula has been proposed, accounting for the reduction in bearing capacity due to combined effect of soil inertia and earthquake-induced pore water pressures. It has also been demonstrated that, even considering earthquake-induced excess pore pressures, the soil and superstructure inertial effects are still decoupled. This allowed introducing an overall corrective coefficient accounting for both the inertial (soil and superstructure) and excess pore pressure effects, computed as the product of the corresponding corrective coefficients. Finally, original simple equations were derived giving a threshold value of the excess pore pressure ratio required to trigger a bearing capacity failure in the static post-seismic conditions because of the soil shear strength reduction.

Numerical Simulation of Piles in a Liquefied Slope Using a Modified Soil–Pile Interface Model

The liquefaction of soil surrounding a pile significantly affects the dynamic interaction between the soil and the pile. In particular, liquefaction of the sloping ground can induce permanent deformation and a bending moment on the pile due to the lateral displacement of the liquefied soil in the downslope direction. However, numerical analysis studies on piles installed in a liquefiable slope have been very limited and have not properly simulated the behavior of the pile. Therefore, a modified soil–pile interface model was proposed, which linearly decreases the interface friction angle with the increase in the excess pore pressure ratio. The proposed model was validated by comparing it with the centrifuge test results of Yoo et al. (2023). Simulation results on the slope crest settlement and the pile-bending moment showed good agreement with the centrifuge test results. A parametric study was conducted by applying the validated model to analyze the effect of slope inclinations and the amplitude of input motions on the slope displacement and the pile moment. The simulation results showed that the slope inclinations affected the area of the sliding mass, causing a larger pile-bending moment with a larger inclination. When the amplitude of the input motion was sufficiently large to trigger the failure of the liquefied slope, the slope displacement and the pile-bending moment did not increase any further.

Shaking table test on the dynamic responses of shield tunnel in liquefiable stratum

Seismic liquefaction affects badly the safety of the underground structures. For the dynamic responses of the tunnel in liquefied watered sandy stratum in one of the rail transit projects in Shanghai, this paper proposes a tunnel-foundation shaking table model test and analyses the dynamic responses of electrolytic desaturation technology on surrounding soil, which provides some theoretical guidance for seismic designs of Shanghai rail transit projects. The test result shows that the excess pore pressure ratio of soil decreases gradually with the increase of buried depth; sand is easier to liquefy than silt so the tunnel liquefaction and uplift during the sandy section should be paid more attention to; electrolytic desaturation technology works significantly on improving the liquefaction resistance abilities of sand and silt which can effectively reduce the damage caused by tunnel uplift due to seismic liquefaction.

Numerically liquefaction analysis of tunnel‐sand pile interaction (TSPI) under seismic excitation

AbstractLiquefaction is a vital issue for the tunnel‐sand pile interaction (TSPI) system under seismic excitation. Therefore the present study proposed a TSPI model for the liquefaction analysis numerically using the numerical code of Plaxis 3D under local seismic excitation. Validation results of the numerical code inform a convincing level of accuracy to ensure the analysis of the TSPI model using this code. In this research, the geometry of the TSPI model is considered smaller compared to the tunnel diameter to influence the probability of the liquefaction occurrence, and the only variable geometric dimension is the tunnel pile clear distance of the interaction zone while other geometric and material properties are considered to be the standard value. UBC3D‐PLM (two yield surfaces consisting of kinematic hardening rules) constitutive model is used to ensure the liquefaction effect in the TSPI model; however, the excess pore pressure ratio exceeds the limiting value of 1 to inform the existence of liquefaction in the TSPI model. In addition, the uplift of sand is exceeded from the settlement due to liquefaction. However, further investigations such as experimental and analytical studies may have to be performed in the future to enhance the present study.

Multidirectional Vibroseis Shaking and Controlled Blasting to Determine the Dynamic In Situ Response of a Low-Plasticity Silt Deposit

In this paper, efforts to characterize and compare the full-scale in situ three-dimensional (3D) dynamic response of a low-plasticity silt deposit to multidirectional loading from two different sources, a vibroseis shaker named T-Rex and controlled blasting, are presented. Horizontal vibroseis shaking at a frequency, f, of 10 Hz, produced dynamic responses in the silt that ranged from linear-elastic to nonlinear-inelastic, inducing maximum equivalent direct simple shear (DSS) shear strains, γDSS,max, up to 0.15% and residual excess pore pressure ratios, ru,r, of 14.1%. Blast-induced shear waves with predominant frequencies ranging from 9.6 to 14.6 Hz excited nonlinear-elastic and nonlinear-inelastic responses in the silt deposit, with γDSS,max of 1.14% and maximum ru,r of 61%. Importantly, these responses were observed to be minimally influenced by high frequency compression waves. Multidirectional loading, and excess pore pressure, ue, migration and impedance were identified as the predominant factors for achieving the large ru,r in the silt deposit from these two in situ testing techniques. The cyclic threshold shear strain, γtp, to trigger ru,r observed from the T-Rex shaking equaled 0.007% to 0.011% and varied with the initial soil stiffness. The two testing techniques demonstrated that the in-situ shear modulus, G, reduced to 90% of the maximum shear modulus, Gmax, at γDSS,max≈γtp, whereas by γDSS,max≈1%, G further reduced to 10 to 30% of Gmax corresponding to ru,r of ∼60%. Changes in soil fabric were quantified using small-strain shear-wave velocity measurements performed before and/or upon initiation and after each stage of dynamic testing, and were linked to the observed increase and decrease in γtp for the shallower and deeper 3D elements, respectively, following T-Rex shaking. The side-by-side comparison of the dynamic responses and soil properties derived from these two distinctly different field-testing techniques validate the use of controlled blasting for quantifying in situ dynamic soil properties and responses.

Effect of coefficient of uniformity on cyclic liquefaction resistance of granular materials

Using three-dimensional discrete element method, we analyze the particle size distribution (PSD) effect on the cyclic liquefaction resistance of spherical particle assemblies. For the same mean particle size and log-linear type PSD, the coefficient of uniformity (Cu) is chosen as a descriptor of the PSD. Samples with five levels of Cu are isotropically compressed to the same pressure and two relative densities (Dr) informed by the maximum and minimum achieved void ratios determined for each Cu. The ten samples are subjected to constant volume cyclic simple shearing at different cyclic stress ratios until reaching initial liquefaction, in 56 simulations. The simulations suggest that at each Dr the evolution pattern of excess pore pressure ratio against the number of loading cycles normalized by the number of cycles to liquefaction is minimally affected by the Cu. For the samples with lower Dr, increasing the Cu in the range 1–3 first increases and then decreases the liquefaction resistance; this trend reverses at the higher Dr. Two critical state parameters based on the void ratio and the coordination number at the pre-shearing state of the samples correlate well with the cyclic liquefaction resistance for the ranges of Cu and Dr considered in this study.

Experimental and Empirical Studies to Evaluate Liquefaction Resistance of Partially Saturated Sands

Partially saturated sands with occluded gas bubbles may occur naturally in the field or can be created artificially as a way of mitigating liquefaction effects. This study aimed to investigate the main parameters that influence the liquefaction resistance of partially saturated sands containing occluded gas bubbles. It also adopted a simplified approach and proposed empirical relationships that linked the liquefaction-resistance ratio to the bulk modulus, maximum volumetric strain, and excess pore-pressure ratio. Considering these objectives, a total of 60 stress-controlled dynamic triaxial tests were conducted. Nearly/fully saturated and partially saturated sand specimens prepared using sodium percarbonate were subjected to undrained cyclic loading with a loading frequency of 0.1 Hz. The loosely or densely packed sand specimens at different degrees of saturation (79–100%) were examined under different initial effective confining stresses (25–100 kPa) and back pressures (100–400 kPa). The analysis of the test results indicated that the liquefaction resistance of the partially saturated sands constantly increased as the degree of saturation decreased, and this effect was more prominent under higher initial effective confining stresses and lower back pressures. The adopted method satisfactorily captured this trend and showed reasonable matches between the empirical predictions and experimentally observed results from this study and previous research.

The p-y model of single pile foundation for OWTs under combined lateral environment loading and earthquake in saturated sand

A series of centrifuge tests were conducted on offshore wind turbines (OWTs) subjected to combined static lateral loading and the earthquake (LEAP wave) to obtain p-y curves (where p is the soil resistance and y is relative displacement between pile and soil), and a simplified beam on Winkler foundation (BNWF) model was proposed. The p-y curves are to investigate the influence of static lateral loading on the dynamic pile-soil interaction, and the BNWF model is to verify the p-y curves and to predict the after-shaking horizontal displacement. It's found that the dynamic p-y curve of shallow soil oscillated and moved to the right due to the influence of lateral loading; meanwhile the LEAP wave resulted in liquefaction of the surrounding soil and the p-y curve moved to the lower right, namely, the soil resistance decreased yet the displacement kept increasing. The BNWF model uses a hyperbolic function to fit the experimental p-y curves, and two parameters in the p-y curves, namely, the initial modulus of the subgrade reaction and the ultimate subgrade reaction, were connected with the excess pore pressure ratio (EPR). The comparison between the model and the test results verifies its effectiveness in predicting the after-shaking horizontal displacement.

Influence of Stone Columns on Seismic Response of Buildings Considering the Effects of Liquefaction

In this paper, the behaviour of a soil-foundation system supported on a stone column-reinforced liquefiable soil strata is investigated through finite element analysis. The numerical analyses are performed on a five story reinforced concrete moment resisting building supported on a raft foundation. The influence of stone column slenderness ratio on liquefaction mitigation is studied by varying the length of stone columns at a constant area replacement ratio. The results are obtained based on the excess pore pressure, free-field soil settlement, foundation settlement, acceleration response, superstructure's inter-story drift, and lateral story displacement for each ground motion. The results showed that the liquefaction of free-field soil had a major impact on the foundation settlement and building lateral deformation. With the inclusion of stone columns, excess pore pressure ratio in the free-field region reduced considerably, which had immediate effects on the building's lateral deformation.

Assessment of soil parameters during post-liquefaction reconsolidation of loose sand

Saturated deposits of loose sand may suffer liquefaction under earthquake shaking. Liquefaction-induced damage occurs during the earthquake loading and may continue during the post-liquefaction reconsolidation stage in which excess pore pressures gradually dissipate. An accurate evaluation of soil parameters is of great significance for the full depiction of this process. Consolidation-related problems are typically investigated using Terzaghi's one-dimensional consolidation theory, assuming a constant permeability, one-dimensional stiffness, and consolidation coefficient throughout this process. Some studies based on physical model tests suggest that an assumption of a single and constant value of soil parameter is not appropriate to study the reconsolidation of liquefied sand. This paper presents the results of three geotechnical centrifuge experiments to contribute to the current understanding of this phenomenon. The centrifuge data is combined with oedometer and fluidization test data, producing several empirical relationships to estimate the soil parameters related to the reconsolidation of liquefied sand. A simplified method is adopted to reproduce the time histories of excess pore pressure dissipation and ground surface settlement using the estimated soil parameters as the input. It is shown that the coefficient of consolidation and one-dimensional stiffness vary as a function of effective stress or excess pore pressure ratio, markedly increasing during the reconsolidation of liquefied sand. The predictions of the proposed methodology appear to match the experimental measurements reasonably well as long as appropriate soil parameters are used.