Excess Pore Pressure Ratio Research Articles

Numerical Analysis for Liquefaction Deformation of Low Embankment on Soft Foundation Based on Water Level Changes in Coastal Highway

Zhang, X.-G.; Zhu, J.-X.; Zeng, Y.; Jiang, P.; Yi, N.-P.; Yan, L.-E., and Li, Y.-J., 2019. Numerical analysis for liquefaction deformation of low embankment on soft foundation based on water level changes in coastal highway. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 321–326. Coconut Creek (Florida), ISSN 0749-0208.This paper mainly studies the water level change of coastal soft foundation by taking the soft foundation in the liquefaction zone of the Xiniujiao-Dafengjiang section of the coastal first-class highway in Guangxi, China as the research object. To this end, by arranging the saturated silt soft foundation of 6 1.5m inverted trapezoidal gravel wall low embankment structure in the vertical direction, the numerical simulation method was adopted to simulate its liquefaction deformation of the soft foundation problem under the traffic load in the working condition I (constant annual water level) and working condition II (temporary water level rise), and also judge its liquefaction potential, the development and dissipation of the pore water pressure, and the excess pore pressure ratio. Finally, the results show that under different water levels, the gravel-paving on embankment can effectively improve the liquefaction-resistant ability of the soft foundation and improve the uneven settlement of the original compacted fill embankment.

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.

3D Seismic Response Analysis of Shallow Foundation Resting on Sandy Soil

In the present study, an attempt has been made to study the response of a shallow foundation resting on medium dense sandy soil under seismic excitation. Numerical analysis of the soil-foundation system has been carried out using 3D finite element software OpenSeesPL. The effect of boundary conditions (shear beam and rigid box type) and the water table (0 m, 1 m and 2 m below the ground surface) on the response of soil-foundation system under seismic excitation have been analysed. The responses of the soil-foundation system are presented in the form of acceleration, displacement, excess pore pressure, excess pore pressure ratio and settlement variations at different locations in the soil domain. The results of the numerical analysis indicate that the peak acceleration, displacement, excess pore pressure and settlement values are found to be more in shear beam type boundary condition than that of a rigid box type boundary condition. Hence, rigid confinement and lower water table can reduce the liquefaction potential of the soil-foundation system under seismic excitation.

Cyclic Behaviors of Saturated Sand-Gravel Mixtures under Undrained Cyclic Triaxial Loading

ABSTRACT A few well-documented liquefaction case studies of gravelly soils during major earthquakes have increased concerns regarding the liquefaction susceptibility of gravelly sand soils. Understanding the failure mechanism of sand-gravel mixtures under undrained cyclic loading is important for better assessing their liquefaction susceptibility. The liquefaction mechanism and susceptibility of isotropically and anisotropically consolidated sand-gravel mixtures are investigated under undrained cyclic loading in a solid cylinder apparatus; and, the influences of the varying material properties, compactness states, and particle gradations are systematically studied. The authors introduce an index of the average flow coefficient (k a) that describes the fluidity of saturated sand-gravel mixtures, and then propose the elaboration of a new liquefaction mechanism of saturated sand-gravel mixtures. The specimens would experience cyclic liquefaction when the excess pore pressure ratio reaches 100% under undrained isotropic consolidation, whereas the cyclic mobility would be induced when the cumulative axial strain reaches 5% under undrained anisotropic consolidation. Particles with both sizes smaller than 0.25 mm and mass contents less than 30% in sand-gravel mixtures primarily play a role of the fillers of intergranular voids (fines in coarse). The skeleton void ratio (e sgk) is a unique physical state index to characterize the liquefaction susceptibility of saturated sand-gravel mixtures with the filler content less than 30%. The negative correlation between the cyclic resistance ratio (CRR) and e sgk can be expressed as the power function. Abbreviations: A cl: Closed-loop area of each loading cycle in Fig. 5; ACU: Anisotropically consolidated undrained; CRR: Cyclic resistance ratio; CRR 15: CRR in 15 cycle; CRR 25: CRR in 25 cycle; CSR: Cyclic stress ratio, Applied cyclic stress ratio in CTX test in the form of Equation (7); CTX: test Cyclic triaxial test; C u, C c: Coeffıcients of uniformity and curvature for the particle-size distribution curve; D, d: Size of coarse and small spherical particles in an idealized binary packing model; D 50: Median diameter of host coarse soils in binary mixtures; d 50: Median diameter; median diameter of host fine soils in binary mixtures; DA: Double-amplitude; D r: Relative density; initial relative density; D r0: Post-consolidation relative density; e: Global void ratio; initial void ratio; e 0: Post-consolidation void ratio; e min, e max: Minimum and maximum void ratios; e sgk: Skeleton void ratio; FC: Fines content for particles with the sizes less than 0.075 mm; FC th: Fines content threshold value; f f: Filler content by weight for particles with sizes less than 0.25 mm in the coarse-dominated grain contact sand-gravel mixtures; G c: Gravel content; G s: Specific gravityHCA Hollow cylinder apparatus; ICU: Isotropically consolidated undrained; k a: Index of the average flow coefficient; K c: Consolidation stress ratio, defined as the ratio of to m, n Fitting coefficients in Equations 11–13; N l: Number of cycles required to cause an r u = 100% for ICU CTX test or ε p = 5% for ACU CTX test; q: Deviatoric stress; q cyc: Peak cyclic deviatoric stress; q s: Initial static deviatoric stress; R: Roundness proposed by Power (1953); R d: Particle size disparity ratio in binary packing model; r u: Excess pore pressure ratio, defined as the ratio of Δ u to; Δu: Excess pore pressure change induced by the action of cyclic shear stressSA Single-amplitude; α d: Cyclic shear stress ratio of any plane in the specimen; β: Inclination angle to the horizontal axis in Mohr’s stress circle serve; : Shear strain rate; ,: Maximum and minimum shear strain rate of each loading cycle in Fig. 5; ε: Axial strain; ε p: Accumulated permanent axial strain; η: Apparent viscosity in fluid mechanics; ρ d: Dry densities; ,: Initial effective vertical and lateral consolidation stress; σ d: Applied sinusoidal cyclic axial stress amplitude; : Initial effective normal stress of any plane in the specimen; τ: Shear stress; τs : Initial shear stress of any plane in the specimen; τ d: Cyclic shear stress amplitudes of any cyclic shear effect plane; τ max, τ min: Maximum and minimum shear stress of each loading cycle in Fig. 5

Study on the Liquefaction Properties of Silt at Yellow River Delta

The seabed of the Yellow River Delta is formed by the rapid deposition of sediments from the Yellow River. Recent researches have shown that the geological hazards in the Yellow River Delta are mainly related to the liquefaction of silty seabed under cyclic loading. In this paper, based on the theory of Stokes Viscous Principle, a self-design dragging ball apparatus was used to study the fluid characteristics of liquefied and post-liquefied silt more thoroughly. Wave flume and shaking table were used to apply wave loads and vibration loads. The pore pressure, earth pressure, and moving parameters of the dragging ball were recorded during tests. The effect of cyclic loads and excess pore pressure ratio on viscosity of silty soil is discussed. The constitutive model of flow characteristics of liquefied silt is also established. The results indicate that the apparent viscosity of silty soil is much higher than sand. Cyclic loads accelerated the process of silt liquefaction, resulting in the decrease in apparent viscosity. The excess pore pressure ratio (ru) has effect on silt liquefaction. When the soil had not reached the completely liquefied state (ru < 1), the apparent viscosity decreased with the increase of excess pore pressure ratio; when the soil had reached the completely liquefied state (ru = 1), the shear stress decreased with the increase of strain rate. It was determined that silty soil characterized by shear thinning can be viewed as a type of non-Newtonian fluid. The equation of the shear stress and shear strain rates was established by fitting the test data.

Cyclic behaviour and liquefaction resistance of Izumio sands in Osaka, Japan

This article presents a study of liquefaction resistance behaviour of sand using a cyclic triaxial test. The site investigation was performed, and frozen undisturbed specimens were taken from the Izumio site in Osaka, Japan. According to the evidence in 1995 Kobe Earthquake, the first two sand layers are vulnerable to undergo liquefaction. The effect of deviatoric stress on liquefaction resistance was focused on in this study. The excess pore pressure ratio, hysteresis loop, and effective stress path from the cyclic triaxial tests were reported. A multispring element model was employed to simulate the soil behaviour under cyclic loading. The results showed that applied deviatoric stress could influence the liquefaction resistance of sandy soil samples. The plots of the cyclic stress ratio versus the number of cycles to generate liquefaction known as a liquefaction resistance curve can be then constructed and compared with other sands.

Wave induced instantaneously-liquefied soil depth in a non-cohesive seabed

Instantaneous liquefaction can be induced by the upward seepage force generated in a sandy seabed under wave troughs. The wave-induced pore-pressure distribution in the non-cohesive seabed with an instantaneously-liquefied layer is investigated analytically. Based on the analytical solution to Biot's consolidation equations for the response of a poro-elastic bed to water waves, the expressions of liquefied soil depth are derived with modifying the criterion for instantaneous liquefaction, which is verified with existing offshore field observations and multi-scale numerical simulations. Analytical investigation indicates that, in the instantaneously-liquefied soil layer, the buoyant weight of soil is essentially balanced by upward seepage force into a quasi-static state. Underneath the fully liquefied layer, the effective stress can be remarkably reduced. An excess pore-pressure ratio is then proposed to quantitatively evaluate the instantaneous liquefaction degree and its corresponding influential depth. Parametric studies indicate that, for a certain excess pore-pressure ratio, the influential depth decreases with increasing saturation degree, permeability of soil or water depth, but increases with increasing wave height. The influential depth of a non-cohesive seabed under waves increases significantly with reducing the threshold value of excess pore-pressure ratio, which may have much influence on the stability design for submarine structures.

Stability of seawalls using modified pseudo-dynamic method under earthquake conditions

By using the modified pseudo-dynamic method for submerged soils this paper explores the seismic stability of seawall for the active condition of earth pressure. Different forces such as seismic active earth pressure, seismic inertia forces of the wall, non-breaking wave pressure, hydrostatic and hydrodynamic pressures are considered in the stability analysis. Limit equilibrium has been used, and expressions for the factor of safety against sliding and overturning mode of failure have been proposed. The proposed methodology overcomes the limitations of existing pseudo-dynamic method for submerged soils. A detailed parametric study has been conducted by varying different parameters and results are presented in the form of design charts for computation of factor of safety against sliding and overturning mode of failures. It was noticed that the influences of soil friction angle, seismic acceleration coefficient, wall inclination and excess pore pressure are significant when compared to the other parameters. The value of factor of safety against the sliding mode of failure is increasing by about 62% when the value of soil frictional angle is increased from 30° to 40°. It was also found that the factor of safety against overturning mode of failure is decreasing by about 22% as the value of excess pore pressure ratio increases from 0 to 0.75. The proposed method with closed-form solutions can be used for the seismic design of seawalls.

Ground Motion Intensity Measures to Evaluate I: The Liquefaction Hazard in the Vicinity of Shallow-Founded Structures

When evaluating the liquefaction hazard within a performance-based framework, whether using simplified procedures or advanced numerical tools, the hazard and its effects on structures need to be evaluated under a range of ground motions. Choice of an optimum intensity measure (IM) in the selection and scaling of ground motions will reduce variability in the predicted response, dependence on source characteristics, and uncertainty in the prediction of the IM. This paper presents the results of a numerical parametric study, validated against centrifuge results, to evaluate the influence of different IMs on the liquefaction hazard in the far-field and near shallow-founded structures. Pore pressure redistribution and soil-structure interaction were considered in estimating the liquefaction hazard in terms of peak excess pore pressure ratio ( r u,peak). The IMs at the base rock, far-field soil surface, and foundation with the best combination of efficiency, sufficiency, and predictability were evaluated and identified as: (1) pseudo-spectral acceleration at the site's initial fundamental period ( PSA Base[ T So]) for predicting r u,peak in the far-field; (2) peak ground acceleration, ( PGA Base); and Arias intensity ( AI Base) for predicting r u,peak under the foundation.

Seismic passive earth resistance in submerged soils using modified pseudo-dynamic method with curved rupture surface

ABSTRACTThe sparsity of examination of seismic passive earth pressure acting on retaining wall holding soil backfill with full submergence, which is more common in waterfront areas, can be noticed from the literature. In the current study, a closed-form solution to compute the seismic passive earth pressure on nonvertical rigid retaining wall retaining a backfill with full submergence is proposed using the modified pseudo-dynamic approach. A nonlinear rupture surface (logarithmic spiral + straight line) in a submerged backfill of viscoelastic nature has been assumed. The presented modified pseudo-dynamic method overcomes the limitations of the existing pseudo-dynamic method for submerged soils. The proposed methodology has been thoroughly validated with the available literature. The influences of seismic acceleration coefficients, excess pore water pressure ratio, wall inclination, and soil and wall friction angles have been studied. It has been noticed that the consideration of excess pore pressure ratio leads to significant decrease in seismic passive resistance of the soil which in turn lead to extra hydraulic pressure acting on the wall in submerged backfill. There is a 57% decrease in seismic passive earth pressure coefficient as the wall inclination changes from −15° to 15°.

Liquefaction and Earthquake Response Analysis of Panipat Pond Ash Embankment in India

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)-[Formula: see text] 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.

Load-Settlement Mechanism of Shallow Foundations Rested on Saturated Sand with Upward Seepage

AbstractIn this study, the ultimate bearing capacity and settlement of square and strip foundations in the presence of upward seepage were assessed using physical modeling. The experiments were conducted in two series with different loading values of excess pore-water-pressure paths. The experimental results demonstrate significant degradation of bearing capacity resulting from excess pore-pressure development; however, a remarkable amount of soil strength was observed when the excess pore-pressure ratio was unity in the free-field ground. A correction coefficient (fγ) for the bearing-capacity factor Nγ was estimated based on the results. This factor is a function of upward-seepage magnitude. Based on the experimental results, dimensionless charts are presented to predict the settlement of square and strip footings rested on saturated sand with varied amounts of excess pore-water pressure. Results show strong dependency of settlement on the bearing-capacity safety factor presumed for the foundation design.

Generalized Seismic Active Thrust on a Retaining Wall with Submerged Backfill Using a Modified Pseudodynamic Method

The appropriate estimation of seismic earth pressures acting on retaining walls supporting submerged backfill is an important area of research in earthquake geotechnical engineering. There is a scarcity of research work on retaining walls with submerged soil under seismic conditions in the literature. In this paper, closed-form generalized solutions for computing seismic active earth thrust and its distribution on nonvertical rigid retaining walls with fully submerged backfill are proposed using the modified pseudodynamic approach. The seismic analysis of retaining walls was carried out with a planar rupture surface in viscoelastic submerged backfill. The proposed modified pseudodynamic analysis satisfied the zero-stress-boundary condition on a free surface and considered the standing shear and primary waves, soil damping, amplification in the backfill, and an excess pore-pressure ratio in the backfill. A detailed parametric study was performed to gain an understanding of the effects of different parameters, including the horizontal seismic acceleration coefficient, excess pore-water-pressure ratio, period of lateral shaking, damping ratio, wall inclination, and soil-friction and wall-friction angles. A comparison of the present study results and the existing analytical and experimental studies resulted in good agreement.

Experimental investigation on fluid characteristics of medium dense saturated fine sand in pre- and post-liquefaction

Liquefaction of saturated loose sand is a major cause of extensive damage to buildings and infrastructures during large earthquakes. A better understanding of the behaviour of liquefied soil is becoming increasingly necessary to mitigate earthquake damage, and the fluid method has become an increasingly popular means to study the behaviour of liquefied soils. The purpose of this study is to determine the fluid characteristics of liquefied fine sand. In this paper, the apparent viscosity was measured as an index of fluid characteristics using the shaking table tests of pre-liquefaction behaviour of saturated fine sand at approximately 45 % relative density; the relationship of apparent viscosity and shear strain rate on liquefying fine sand was indicated as a power-law shear-thinning non-Newtonian fluid; and liquefying fine sand has the alternating behaviour of shear dilatancy and compressibility during cyclic loading. Additionally, a series of a monotonic axial compression loading tests in an undrained manner were performed to measure the shear stress and excess pore pressure ratio relationship on the post-liquefaction saturated fine sand at approximately 50 % relative density. The fluid characteristics of post-liquefaction fine sand exhibits rate dependence and can be described by a combined fluid model of time-independent and time-dependent power-law functions; the time-independent viscous resistance is not relevant to the excess pore pressure ratio; but the time-dependent frictional resistance is closely related to the excess pore pressure ratio. Furthermore, the results of the verification tests demonstrate that the proposed fluid model has good applicability for the fluid behaviour of the post-liquefaction fine sand.

Seismic Performance of Shallow Founded Structures on Liquefiable Ground: Validation of Numerical Simulations Using Centrifuge Experiments

AbstractThe results of fully coupled, three-dimensional (3D), nonlinear finite-element analyses of structures founded on liquefiable soils are compared with centrifuge experiments. The goal is to provide insight into the numerical model’s capabilities in predicting the key engineering demand parameters that control building performance on softened ground for a range of structures, soil profiles, and ground motions. Experimental and numerical observations will also guide future analyses and mitigation decisions. The numerical model captured excess pore pressures and accelerations, the dominant displacement mechanisms under the foundation, and therefore building’s settlement, tilt, and interstory drift. Both experimental and numerical results showed that increasing the structure’s contact pressure and height/width (H/B) ratio generally reduces net excess pore pressure ratios in soil but amplifies the structure’s tilting tendencies and total drift. The settlement response of a structure with a greater pressu...

Some Aspects of the Impact of Multidirectional Shaking on Liquefaction of Level and Sloping Granular Deposits

AbstractA very limited number of computational studies have been presented for the analysis of the response of saturated granular soils to multidirectional shaking despite it being the realistic mode of loading that resembles an actual earthquake. Herein, this paper examines the capabilities of a recently developed, coupled lattice Boltzmann method (LBM)–discrete element method (DEM) of modeling level and gently sloped soil deposits when subjected to bidirectional shaking. The results of conducted simulations show that bidirectional shaking may increase surface settlement of level deposits by about 30% over unilateral shaking. The depth along the deposit that experiences excess pore pressure ratio close to unity increases as a result of bidirectional shaking compared to unilateral shaking. Bidirectional shaking also increases the magnitude of lateral spreading and associated shear strains in sloping deposits. Other aspects of the response include capturing soil dilative behavior and associated temporary i...

Shaking-table tests and numerical simulations on a subway structure in soft soil

Shaking table tests were performed to investigate the damage mechanisms of a subway structure in soft soil while experiencing strong ground motions. The seismic responses of the structure and soil were found to be more sensitive to input motions with richer low-frequency components. The excess pore pressure ratio of soil increased slightly, and the distribution of the excess pore pressure surrounding the structure showed clear spatial effects. The frequency spectrum characteristics of input ground motions clearly influenced the lateral displacement of the structure. In addition, the structure was most severely damaged at the top or the bottom of the interior columns. Finite element analyses were conducted by using the modified Martin–Seed–Davidenkov viscoelastic and the rate-independent plastic-damage constitutive models for soil and concrete, respectively. Satisfactory agreement was observed between the simulation and test results, the difference between these results was discussed in detail. The results provide insight into how the characteristics of strong ground motion might influence and present a simplified analysis method to quantitatively evaluate the damage of subway structures in soft soil.

Application of Effective Stress Model to Analysis of Liquefaction and Seismic Performance of an Earth Dam in China

Earthquake-induced liquefaction is one of the major causes of catastrophic earth dam failure. In order to assess the liquefaction potential and analyze the seismic performance of an earth dam in Fujian, Southeastern China, the in situ shear wave velocity test was firstly carried out. Results indicate that the gravelly filling is a type of liquefiable soil at present seismic setting. Then the effective stress model was adopted to thoroughly simulate the response of the soil to a proposed earthquake. Numerical result generally coincides with that of the empirical judgment based on in situ test. Negative excess pore pressure developed in the upper part of the saturated gravelly filling and positive excess pore pressure developed in the lower part. The excess pore pressure ratio increases with depth until it reaches a maximum value of 0.45. The displacement of the saturated gravelly soil is relatively small and tolerable. Results show that the saturated gravelly filling cannot reach a fully liquefied state. The dam is overall stable under the proposed earthquake.

Shaking table tests on a three-arch type subway station structure in a liquefiable soil

A series of large-scale shaking table tests were performed to investigate the damage mechanisms of a three-arch type subway station structure in a liquefiable soil experiencing strong motions. Methods to measure the displacement included the vision-based displacement test and the fiber Bragg grating test to measure the strain of the galvanized steel wire. Sand boils, waterspouts, ground surface cracks and settlements, and buoyancy movement of the model structure were observed. When the peak excess pore pressure ratios dramatically increased, the Arias intensity also dramatically increased. The peak acceleration of the model soil also almost coincided with liquefaction of the model soil. The seismic responses of the model structure and the soil were shown to be more sensitive to input motions with larger low-frequency components, the phenomenon of high frequency filtering and low frequency amplification effect of the liquefied soil were observed. The peak tensile strain located at the top and bottom of the center pillars was larger than that obtained at the subarch, while the peak tensile strain at the atrium arch was the smallest. The peak strain at the primary and secondary observation sections were remarkably affected by the spatial effect. The results can provide valuable insight into the seismic investigation of these subway structures.

Behavior of Nonplastic Silty Soils under Cyclic Loading

The engineering behavior of nonplastic silts is more difficult to characterize than is the behavior of clay or sand. Especially, behavior of silty soils is important in view of the seismicity of several regions of alluvial deposits in the world, such as the United States, China, and Turkey. In several hazards substantial ground deformation, reduced bearing capacity, and liquefaction of silty soils have been attributed to excess pore pressure generation during dynamic loading. In this paper, an experimental study of the pore water pressure generation of silty soils was conducted by cyclic triaxial tests on samples of reconstituted soils by the slurry deposition method. In all tests silty samples which have different clay percentages were studied under different cyclic stress ratios. The results have showed that in soils having clay content equal to and less than 10%, the excess pore pressure ratio buildup was quicker with an increase in different cyclic stress ratios. When fine and clay content increases, excess pore water pressure decreases constant cyclic stress ratio in nonplastic silty soils. In addition, the applicability of the used criteria for the assessment of liquefaction susceptibility of fine grained soils is examined using laboratory test results.