No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Finite-difference solution for dissipation of excess pore water pressure within liquefied soil stabilized by stone columns with consideration of coupled radial-vertical seepage
... The diameter of drainage pipes is usually only 7~8 cm, which is several orders of magnitude smaller than the size of engineering structures. When simulating with the finite element method [8] or the finite difference method [9], it is necessary to divide the relevant areas of drainage pipes into dense mesh to capture the outflow conditions through the pipe wall. However, this will lead to a significantly large and time-consuming calculation scale, which is even more challenging to perform. ...
... h(x, t) = N T h e (9) where N is the shape function array of the element and h e is the nodal head array of the element. There is also a one-dimensional local coordinate ζ(−1 ≤ ζ ≤ 1) along the direction of the drainage pipe, as shown in Figure 2. ...
... Due to the water flow exchange and the difference between internal and external water heads on the surface of , it can be treated as a mixed boundary condition, as shown in Equation (7). Assuming that the infiltration zone is well consolidated, according to the structural characteristics of the calculation region, it is discretized into a finite element mesh, and the head interpolation function of a certain element is as follows: (9) where N is the shape function array of the element and e h is the nodal head array of the element. There is also a one-dimensional local coordinate ( ) ...
Drainage pipes are often positioned downstream of embankments to mitigate pore pressure, thereby reducing the risk of dam failure. Considering that the size of drainage pipes is much smaller than that of embankment dams, directly discretizing the drainage pipes will generate a huge number of elements. Therefore, this paper proposes a seepage element containing drainage pipes. In this element, the permeability of the drainage pipe is taken as the third type of permeable conductivity condition, and it is considered in the energy functional. The governing equations for the steady-state and the transient seepage element containing drainage pipe are derived using the variational principle, and the infiltration matrix, equivalent nodal seepage array, and water storage matrix of the seepage element containing drainage pipe are obtained. In conjunction with the user-defined element module UEL of ABAQUS 2016 software, the established seepage element containing drainage pipe is programmed. The accuracy and efficiency of the proposed seepage element containing drainage pipe are verified through seepage field simulations of three examples. Finally, the influence of the permeable conductivity of drainage pipes on the pressure reduction effect is investigated, providing a reference for the layout of drainage pipes in embankment defense systems.
... Among various techniques available (Ghorbani and Rabanifar 2021;Krishnan and Shukla 2021;Seyedi-Viand and Eseller-Bayat 2022;Chavan et al. 2022;Zhang et al. 2024), the implementation of Stone Columns (SCs) has emerged as a cost-effective and widely adopted method to mitigate the risk of liquefaction in saturated sandy deposits (Mitchell et al. 1995;Adalier and Elgamal 2004). The use of gravel drains has captured the attention of numerous researchers, including Noorzad et al. (1997Noorzad et al. ( , 2007, Brennan and Madabhushi (2002), Adalier et al. (2003), , Rayamajhi et al. (2016), Li et al. (2018), Agah Nav et al. (2020), Ardakani et al. (2020), Gholaminejad et al. (2020Gholaminejad et al. ( , 2021, Thakur et al. (2021), Kumar and Takahashi (2022), Chen et al. (2022), Bhochhibhoya et al. (2023), Abdelhamid et al. (2023), Chakraborty and Sawant (2023), Chaloulos et al. (2023), and Sun et al. (2024). The SCs effectively enhance soil density, improve drainage capacity, and provide a shear reinforcement effect to the ground (Shenthan et al. 2003;Zhou et al. 2021). ...
One effective technique for mitigating the earthquake-induced liquefaction potential is the installation of stone columns. The permeability coefficients of stone columns are high enough to cause a high seepage velocity or expedited drainage. Under such conditions, the fluid flow law in porous media is not linear. Nevertheless, this nonlinear behavior in stone columns has not been evaluated in dynamic numerical analyses. This study proposes a dynamic finite element method that integrates nonlinear fluid flow law to evaluate the response of liquefiable ground improved by stone columns during seismic events. The impact of non-Darcy flow on the excess pore pressure and stress path compared to conventional Darcy law has been investigated numerically in stone columns. Furthermore, the effects of different permeability coefficients and stone column depths have been studied under near and far field strong ground motions. The results indicate that the non-Darcy flow increases the excess pore water pressure as high as 100% in comparison to the Darcy flow.
Foundations supporting infrastructure built on soft and compressible marine soil are unlikely to sustain due to possibility of undrained shear failure or excessive settlement of the supporting soil. This necessitates the importance of implementing an adequate ground improvement strategy. Among different techniques, soft soil reinforcing by the installation of stone columns is one of the most successful methods in terms of long-term stability of foundations. To investigate the load-settlement characteristics of such reinforced soil, a group of closely spaced stone columns was constructed at a location along the eastern coast of Australia. The site geology revealed thick layers of soft, compressible marine clay deposit. These stone columns were loaded by constructing earthen embankment and the resulting load-settlement characteristics were measured by an array of sensors. A two-dimensional plane strain analysis was performed using finite element modeling simulations. Comparison of numerical results with the field data demonstrated accuracy of the numerical model. Additional studies were carried out to investigate the efficiency of the model. This paper integrates the new findings from the full-scale field study and advanced numerical simulations while drawing pertinent conclusions.
This study aims to predict the effect of liquefaction on an embankment resting on liquefiable foundation soil. A numerical model has been simulated in PLAXIS 2D with plane strain idealization. An effective stress-based elastoplastic UBC3D-PLM model has been used to represent the constitutive behavior of foundation sandy soil. The embankment soil has been modeled using the Mohr-Coulomb material model. Initially, the pore pressure and settlement response have been derived for the model without a stone column. The top surface of the loose foundation soil experiences excessive heaving near the embankment toe toward the free surface beside the embankment on either side. Subsequently, a parametric study has been conducted on the mitigation of liquefaction beneath the embankment and liquefaction-induced settlement considering stone columns as a mitigation measure. Stone columns have been modeled assuming equivalent plane strips by considering the equivalent permeability and bulk modulus of stone columns. The efficacy of stone columns in controlling the heaving has also been revealed from this study in addition to the reduction in excess pore pressure beneath the embankment toe and the settlement of the embankment. The parametric study has also investigated the effect of diameter and spacing of the stone column. It has been observed in the case of cyclic loading input that with increasing the amplitude of loading, the effectiveness of stone columns reduces, and this leads to an increase in the crest settlement. Moreover, a seismic study of the embankment model has been carried out for 10 different ground motions to examine the effect of the stone column. The study reveals that for moderate-intensity ground motions, the stone column shows an effective mitigation of excess pore pressure near the embankment toe along with a reduction of embankment crest settlement.
To optimize the design of stone columns composite foundation for liquefiable ground improvement in the Tibar Bay Port Project, a 3D Finite Element (FE) analysis is implemented on the earthquake response and liquefaction mitigation effect. Nine improvement schemes are designed with the orthogonal design method. Taking peak ground acceleration and peak excess pore pressure ratio as the target indicators, the influences of four factors, including diameter, replacement ratio, stiffness, permeability ratio, of stone columns are analyzed by means of range analysis, and subsequently, the optimal ground improvement design is obtained. The analysis results indicate that the responses of ground acceleration and excess pore pressure ratio are relatively sensitive to stone columns’ permeability ratio and a little sensitive to the replacement ratio. The stiffness and diameter ranging in the prescribed boundary only have negligible effect. The mitigation effect of drainage is rather significant when the ratio of the stone columns’ permeability to the soils’ permeability is greater than 100.
Installation of a stone column in loose soil is one of the robotics ground enhancement techniques that reduce the risk liquefaction
and related ground deformation. However, the experimental investigation is costly and it is time-consuming to check the feasibility of this technique for the evaluation of ground subsidence and fluctuations in excess pore water pressure concerning different area ratios of a high modulus column. This paper presents a numerical investigation into shallow foundations underlain by a stone column as a liquefaction countermeasure. The Biot’s consolidation equations are coupled with stress equilibrium equations in finite element modeling for cyclic loading conditions. The critical state model based on generalized plasticity theory was applied to model the nonlinear behavior of soil. The unknown displacement and pore pressure are evaluated using code written in FORTRAN 90. The granular column installation effect is also considered to predict displacements and excess pore pressure (EPP) at each time step. With the introduction of a granular column, considerable reductions were observed in displacements and EPP. The results underline the effectiveness of granular column installation as an effective measure in the mitigation of liquefaction.
The death toll and economic impact of an earthquake can be greatly exacerbated if seismic ground shaking triggers landslides. Earthquake-triggered landslides typically occur in two different contexts: localized failure of steep slopes and resulting landslides that pose a major threat to life in areas below; and lateral spreading of nearly flat sediment plains due to shaking-induced liquefaction, which can damage large areas of critical infrastructure. Unexpected catastrophic landsliding triggered by the 28 September 2018 earthquake at Palu, Indonesia did not occur in either typical context, but produced both destructive outcomes. Here, we show that alluvial ground failure in the Palu Valley was a direct consequence of irrigation creating a new liquefaction hazard. Aqueduct-supported cultivation, primarily of wet rice, raised the water table to near ground level, saturating sandy alluvial soils that liquefied in response to strong ground shaking. Large-displacement lateral spreads occurred on slopes of 1°. Slopes steeper than 1.5° sourced long-runout landslides and debris flows that swept through villages occupying the gentler slopes below. The resulting damage and loss of life would probably not have occurred in the absence of a raised water table. Earthquake-triggered landsliding of gentle, irrigated alluvial slopes is an under-recognized, but avoidable, anthropogenic hazard.
Stone columns act as vertical drains, and due to their high permeability, allow for the quick dissipation of earthquake induced excess pore water pressure. When water flows through a loose sandy soil, it washes away fine soil particles. The fine sand particles get detached when the hydrodynamic force applied on the soil particles breaks the inter particle bonds between soil grains. These detached soil particles are then migrated by the seepage water. Based on the concentration of the soil particles in the seepage water, these may be captured at the pore constriction of gravels during the flow of water through the stone column. Thus, the clogging of stone column initiates which reduces of the permeability of column. The rate of dissipation of pore water pressure during earthquake is affected due to the clogging of column. In this paper, a mathematical model is proposed to determine the rate of dissipation of pore water pressure of stone column-reinforced ground by considering the clogging effect of column. The result obtained from the proposed model is verified with the available in-situ experimental data. A parametric study is also performed to investigate the effect of different parameters of the proposed model on the clogging of stone column. It is observed that when the permeability ratio, compressibility ratio and area ratio decrease, the possibility of clogging increases. The peak value of the excess pore water pressure ratio can increase up to around 50% due to clogging.
Estimating the possible region of liquefaction occurrence during a strong earthquake is highly valuable for economy loss estimation, reconnaissance efforts and site investigations after the event. This study identified and compiled a large amount of liquefaction case histories from the 2008 Wenchuan earthquake, China, to investigate the relationship between the attenuation of seismic wave energy and liquefaction distance limit during this earthquake. Firstly, we introduced the concept of energy absorption ratio, which is defined as the absorbed energy of soil divided by the imparted energy of seismic waves at a given site, and the relationship between the energy absorption ratio and the material damping ratio was established based on shear stress–strain loop of soil element and the seismic wave propagation process from the source to the site. Secondly, the threshold imparted seismic energy of liquefaction was obtained based on existing researches of absorbed energy required to trigger liquefaction of sandy soils and the ground motion attenuation characteristics of the 2008 Wenchuan earthquake, and the liquefaction distance limit of this earthquake was estimated according to the proposed magnitude–energy–distance relationship. Finally, the field liquefaction database of 209 sites of the 2008 Wenchuan earthquake was used to validate such an estimation, and the field observed threshold imparted seismic energy to cause liquefaction in recent major earthquakes worldwide was back-analyzed to check the predictability of the present method, and several possible mechanisms were discussed to explain the discrepancy between the field observations and the theoretical predictions. This study indicates that seismic energy attenuation and liquefaction distance limit are regional specific and earthquake dependent, and 382 J/m³ is the average level of threshold imparted seismic energy to cause liquefaction for loose saturated sandy soils, and the corresponding liquefaction distance limit is approximately 87.4 km in fault distance for a Mw = 7.9 event in the Chengdu Plain. The proposed regional energy attenuation model and threshold imparted seismic energy may be considered as an approximate tool in evaluating the liquefaction hazard during potential earthquakes in this area.
Dense granular columns are often used as a liquefaction mitigation measure to: 1) enhance drainage; 2) provide shear reinforcement; and 3) densify and increase lateral stresses in the surrounding soil during installation. However, the independent influence and contribution of these mitigation mechanisms on excess pore pressures, accelerations (or shear stresses), and lateral and vertical deformations is not sufficiently understood to facilitate reliable design. This paper presents the results of a series of dynamic centrifuge tests to fundamentally evaluate the influence of dense granular columns on the seismic performance of level and gently sloped sites, including a liquefiable layer of clean sand. Specific consideration was given to the relative importance of enhanced drainage and shear reinforcement. Granular columns with greater area replacement ratios (e.g., Ar greater than about 20%) were shown to be highly effective in reducing seismic settlement and lateral deformations in gentle slopes, owing primarily to expedited dissipation of excess pore water pressures rather than shear reinforcement. The influence of granular columns on accelerations (and hence, shear stress demand) in the surrounding soil depended on the column’s Ar and drainage capacity. Increasing the Ar from 0 to 10% was shown to reduce accelerations across a range of frequencies in soil due to the shear reinforcement effect alone. However, enhanced drainage simultaneously increased the rate of excess pore pressure dissipation, helping the surrounding soil regain its shear strength and stiffness faster. At short drainage distances or high Ar values (e.g., 20%), this could notably amplify accelerations and shear stress demand, particularly at greater frequencies. The experimental insight aims to improve our understanding of the mechanics of liquefaction and lateral spreading mitigation with granular columns and may be used to validate the future numerical models used in their design.
Among the most used ground improvement techniques is the mitigation of soil liquefaction by installation of stone columns. To minimize soil liquefaction, the area ratio of the stone columns should be evaluated properly. The available solutions for determining the area ratio of stone columns were developed without consideration of the effects of the stiffness of the columns. However, stone columns have a larger drained elastic modulus than the surrounding soil. The aim of the present paper is to provide a simplified solution for determining the drainage capacity of stone columns during soil liquefaction considering stiffness and limited permeability of the stone columns. Equal strain condition is considered at all depths, i.e., the same vertical deformation of the stone column and surrounding soil is considered at all depths. The present solution shows that the susceptibility of soil liquefaction increases due to the reduction of the stiffness and permeability of the stone column. It is also observed that the maximum pore water pressure ratio decreases by 20-60% due to the stiffness effect of the stone column. Design charts are presented based on the results of the developed solution. Charts can be used to determine the area ratio of the stone columns. Through the presentation of a design example, the present solution is compared with available design methods and the variances between these methods are discussed.
During many earthquakes, soil liquefaction causes dramatic damage. The use of a granular column is a ground-improvement technique used to mitigate soil liquefaction. The present paper studies the performance of reinforcement with granular columns as a method for mitigation of potential risk of liquefaction during cyclic loading. The generation and dissipation of excess pore pressure are analyzed by considering the granular-column installation effect. This effect is described by a decrease in horizontal permeability within a disturbed zone around the column, which is described by constant, linear, and parabolic variations. The evolution of excess pore pressure is studied as a function of the disturbed zone in terms of reduced horizontal permeability. Obtained results show that a granular-column installation has a significant influence on the mitigation of liquefaction.
In this study, an investigation has been performed on a small-scaled laboratory model and its numerical model by the code of PLAXIS to see the effect of stone columns (SCs) placed vertically in a soft soil slope in terms of slope stability, bearing capacity, and settlements. Also, several hypothetical cases have been examined by the code. Effect of s/D ratios (distance between the vertical axes of SCs/diameter of SCs) was also investigated on slope stability, ultimate bearing capacity, and settlement of a footing rested on top of the slope on the laboratory model. Firstly, ultimate bearing capacity and settlement properties of soil were determined for unreinforced soil that is no SCs were considered. Then, some values of soil were determined after the installation of stone columns with various ratios of s/D. The ratios of s/D were 2, 3, 3.5, and 4. The tests carried out on the laboratory model were simulated and numerically analyzed in two dimensions under plain-strain conditions by Mohr–Coulomb model. In the analyses, PLAXIS computer code, which is based on finite elements method, has been employed. Then, a parametric investigation was carried out to see the effect of SCs on the stability of the slope. In the parametric investigation, several hypothetical cases that were one layer of soil and two layers of soil with the presence of water in the reservoir side of the slopes were examined. The analyses in the investigation were performed by the PLAXIS code for various slope angles β, ratios of c/(γH), and ratios of s/D. From the test results of the laboratory model, and the results obtained from the numerical analyses, it was observed that the bearing capacity of the footing constructed on the top of the slope in soft soil was increased; settlements were decreased after the improvement with SCs. From the analyses performed, it was found that the SCs increased the stability of slope 1.18- to 1.62-fold as a relative effect of different parameters.
Cet article decrit les resultats de simulations numeriques visant a investiguer les effets de l'installation de colonnes ballastees dans de l'argile molle naturelle. La geometrie du probleme est simplifiee sous forme de symetrie axiale, en considerant l'installation d'une seule colonne. L'installation de la colonne ballastee est modelisee en tant qu'une expansion non-drainee d'une cavite cylindrique, ainsi la pression interstitielle excessive generee est dissipee vers la colonne permeable. Le processus est simule a l'aide d'un code par elements finis qui permet les grands deplacements. Les proprietes de l'argile molle correspondent a l'argile Bothkennar, qui est modelisee avec S-CLAY1 et S-CLAY1S, qui eux sont des modeles de type Cam-clay considerant l'anisotropie et la destructuration. L'installation des colonnes ballastees altere le sol environnant. L'expansion de la cavite genere des pressions interstitielles excessives, augmente les contraintes horizontales sur le sol, et modifie de facon importante la structure du sol. Les simulations numeriques effectuees ont permis d'evaluer la valeur du coefficient de pression laterale des terres post-installations ainsi que les variations dans la structure du sol causees par l'installation des colonnes. Ces effets et leur influence sur la conception des colonnes ballastees sont discutes.
Immediately following the 11th March 2011 Mw 9.0 Tohoku (Japan) earthquake, a field investigation was carried out around the Tokyo Bay area. This paper provides first-hand observations (before or just at the onset of repair) of widespread liquefaction and the associated effects. Observations related to uplift of manholes, settlement of ground, performance of buildings and bridges and the effects of ground improvements are also presented. Recorded ground motions near the Tokyo Bay area were analysed to understand their key characteristics (large amplitude and long duration). Lessons learnt are also presented.Highlights► Geotechnical field investigations of the 2011 Mw 9.0 Tohoku earthquake are reported. ► First-hand observations of widespread liquefaction around the Tokyo Bay area are presented. ► The effects of long-duration ground motions critical to liquefaction-related damage are analysed. ► Successful cases of ground improvement for reclaimed/filled land are discussed.
The shear performance of the geosynthetic-encased stone column (GESC) is of significance to the stability analysis of increasingly used GESC improved foundations, since the toe of the embankment is generally affected by lateral loads. However, systematic studies in this field are still scarce now. In this paper, a three-dimensional discrete element method (3D-DEM) was employed to study the shear performance of the unit cell with a single GESC and the surrounding soil. The influences of some physical geosynthetic parameters (e.g., geogrid strength, encasement aperture and column diameter) on the shear performance were examined. Higher shear strength of GESC was observed when increasing the geogrid strength. The reinforcement effect of geosynthetic encasement increased with the column diameter non-linearly, however, the encasement aperture was found insignificant. The failure features of GESCs were considerably affected by the geogrid strength and can be generally divided into three categories (e.g., bulging failure, shear failure and flexural failure). Moreover, the shear load transfer mechanism and the development of the shear stress ratio were analyzed. Correlations between the shear strength parameter of GESC (e. g., apparent cohesion) and geogrid strength were quantified for practical use.
Wind turbine structures can be susceptible to damage from earthquake loading due to large overturning moments at the foundation level. As the wind energy market expands to seismically active parts of the world, it becomes important to evaluate the seismic behaviour of these structures. Especially, earthquake-induced liquefaction can be important when wind farms are located on sites with marginal soil conditions such as loose, saturated sands or non-plastic silts. Reduction of stiffness during liquefaction can cause excessive foundation settlement and/or rotation. Thus, suitable mitigation methods should be investigated. This paper discusses the seismic behaviour of the wind turbine on liquefiable soil and the effectiveness of the stone column-based liquefaction mitigation method. Two geotechnical centrifuge tests were conducted with and without stone columns for a model wind turbine on a circular raft foundation. The results show that stone columns can significantly reduce both the settlement and rotation of the raft.
This paper presents a probabilistic model for evaluating the liquefaction-triggering hazard in level, layered, and saturated granular soil profiles improved with dense granular columns (DGCs). The model is developed using the results of a comprehensive numerical parametric study, validated with a dynamic centrifuge test, and subsequently tested with the available case histories involving DGCs as a liquefaction countermeasure. The numerical database includes a total of 30,000, three-dimensional (3D), fully coupled, nonlinear, dynamic finite-element simulations with a statistically determined range of layer-, profile-, DGC-, and ground motion-specific input parameters. The criteria for the predicted degree of liquefaction (i.e., full, marginal, and no liquefaction) are based on the peak values of excess pore pressure ratio and shear strain anticipated within each soil layer. A machine learning approach that performs multinomial logistic regression along with variable selection and regularization is used to develop a set of functional forms for estimating the probabilities of full-, marginal-, and no-liquefaction in sites improved with DGCs. The proposed probabilistic model is the first of its kind that explicitly considers variations in the area replacement ratio (A r), stiffness, and drainage capacity of the DGC; the thickness, depth, relative density, and hydraulic conductivity range of each layer; the evolutionary characteristics of ground motions; and the underlying uncertainty in the prediction of pore pressures and shear strains within each layer.
The installation of a drainage system offers an attractive and economical procedure for stabilizing an otherwise potentially liquefiable sand deposit. Such a procedure has already been used in one case involving the construction of stone columns in a relatively loose sand deposit, and it is being proposed for stabilization of a medium dense sand layer. The paper presents a simplified theory that provides a convenient basis for evaluating the possible effectiveness of a grain drain system in such cases. Where appropriate, additional analyses may readily be made using a computer program (LARF) based on the theory presented, but for most practical cases, it is believed that the charts presented in the paper will provide for effective stabilization of potentially liquefiable sand deposits.
Dense granular columns (DGCs) are generally known to mitigate the liquefaction hazard through a combination of (1) installation-induced ground densification, (2) enhanced drainage, and (3) shear reinforcement. However, the relative contribution of these mitigation mechanisms remains poorly understood. A recent case history of successful embankment performance on a liquefiable site treated with DGCs that had relatively low area replacement ratios (A r) (where drainage is not notably enhanced) suggested that shear reinforcement and installation-induced ground densification may be the two dominant mitigation mechanisms provided by DGCs. In this paper, we present a series of four dynamic centrifuge experiments designed and conducted to test this hypothesis under controlled conditions. Consistent with case history observations and supporting our initial working hypothesis, densification combined with shear reinforcement was shown to be primarily responsible for limiting the embankment's seismic deformations. Additional drainage led to minor improvements in terms of embankment settlement, while increasing its permanent lateral displacement. The results suggest that the combined effects of A r and ratio of maximum shear modulus of the DGCs to that of the surrounding soil (G r) can play a key role in the distribution of stress between DGCs and soil prior to shaking and the extent of softening and strain accumulation in various layers during shaking. For example, it was observed that densification of the liquefiable sand layer around DGCs shifted the generation of larger excess pore pressures to greater depths compared to the DGC-treated test without densification. This led to a base isolation effect that reduced accelerations, degree of softening, and accumulation of shear and volumetric strains at shallower depths, producing a notably improved performance for the soil-embankment system even when the DGC's drainage capacity was inhibited. These observations were attributed to the reinforcement effect of DGCs, the simultaneous reduction in G r due to densification, and a more even transfer of the embankment load onto the soil-column matrix, increasing the stiffness and strength and reducing shear strains in the shallower and looser layer. The presented experimental results point to the importance of accounting for pre-and postinstallation soil density and stiffness in relation to DGCs, confining pressure distributions, kinematic constraints, and activity of various mitigation mechanisms when evaluating the potential influence of DGCs on seismic demand, liquefaction triggering, and deformations near embankment structures.
This paper presents the seismic analysis of stone column (SC) improved liquefiable ground using a three-dimensional (3D) plasticity model with unified description of coarse-grained soil (CGS). The model is a modification of an existing plasticity model for large post-liquefaction deformation of sand, with improvements on the formulation for strength characteristics exhibited by CGS. According to requirements of FLAC 3D User-Defined-Model (UDM), the model is implemented into the finite difference code FLAC 3D , for which a method synchronizing the mapping centres and sharing necessary internal variables of subzones is developed to resolve the computational stability issue caused by the mixed discretization technique of FLAC 3D. The model is validated against monotonic/cyclic and undrained/drain laboratory tests under 8 different stress paths and initial states on a gravel material and Toyoura sand. A centrifuge shaking table test on SC improved liquefiable ground is simulated adopting the model. The results show that the constitutive model and analysis method are effective in reproducing the liquefaction behaviour of CGS ground under seismic loading. Parametric analysis for lique-faction mitigation effects of SCs is further conducted, highlighting the importance of densification and maintaining drainage efficiency of SCs. Results show that the SC's effective improvement area is approximately 2-3 times its diameter.
This paper provides an analytical solution for consolidation problem of a stone column-improved soft soil layer subjected to an instantly applied loading under free strain condition. The radial and vertical consolidation equations are solved in a coupled fashion for both the stone column and its surrounding soil. A general solution of excess pore water pressure at any point of a unit cell model in terms of a Fourier-Bessel series was achieved using the combination of separation of variables method and orthogonal expansion technique. The obtained solution can capture the drain (well) resistance effect and the space-dependent distribution of total vertical stress induced by the external loading. Indeed, since the permeability and size of the stone column are directly utilised in the governing equations and the analytical solution, the drain resistance is directly captured. The capabilities of the proposed solution are exhibited through a comprehensive worked example, while the accuracy of the solution is verified against a finite element simulation and field measurements of a case history with good agreements. To examine the effect of various factors on consolidation behaviour of the composite ground, a parametric study involving column spacing, modulus and permeability of soft soil along with distribution pattern of total stress and thickness of soil layer is also conducted. A decrease in the column spacing or an increase in the modulus or permeability of soft soil led to the acceleration of the consolidation process of the soil region, while the variation of the total stress with depth and the thickness of soil deposit primarily affected the consolidation rate of stone column. Under the free strain condition, the average differential settlement between the stone column and encircling soil was indeed considerable during the consolidation process.
Stabilizing the soft clay by stone column reinforcement is one of the most accepted methods of ground improvement techniques. Because of their larger diameter and higher hydraulic conductivity, drainage with stone columns is much faster than prefabricated vertical drains (PVDs) or sand compaction piles (SCPs). Still, a significant hydraulic gradient at the soil-column interface induces migration of clay particles into the column pores, leading to clogging, which adversely affects the consolidation rate of soft soil. The load bearing capacity that stone columns acquire depends primarily on the lateral confinement offered by the surrounding soft soil. Because of limited confinement at shallow depths, the stone column deforms laterally leading to bulging. This paper presents an in-depth study on the load settlement and consolidation characteristics of soft clay stabilized by stone columns with particular reference to clogging and lateral deformation via laboratory model tests and numerical analysis. The laboratory investigation includes one-dimensional consolidation tests with instrumented columns in reconstituted soft clay, X-ray computed tomography (CT) scanning to study the load transfer, column deformation and clogging characteristics, and a numerical analysis based on a fast Lagrangian finite-difference technique with associated subroutines. Previous solutions developed by the authors have been substantially modified to accommodate a time dependency of clogging, load transfer, and lateral deformation of columns. The proposed solutions are validated by experimental results, which demonstrate that the load settlement and column deformation pattern as well as the consolidation characteristics are significantly affected by clogging and lateral deformation of stone column.
The installation of stone columns is widely adopted to prevent liquefaction. The main improvement mechanisms of stone columns as a liquefaction countermeasure are drainage, stiffening, and densification. This paper investigates the effectiveness of stone columns as a liquefaction remediation. Twenty-four case studies, in which SPT and CPT tests are performed before and after stone column reinforcement, are used as a basis of the research. Densification and stiffening mechanisms are investigated and their individual and combined effects are analyzed. Comparative results indicate that considering the densification and stiffening effects considerably improve the assessment of liquefaction potential of reinforced soil by stone columns.
This paper presents two-dimensional (2D) nonlinear dynamic finite element (FE) modeling of a large-scale shake table test conducted at the E-Defense shake table facility in Japan. This study explores the efficiency of 2D effective stress analyses to predict the behavior of soil-pile systems subjected to liquefaction and lateral spreading using the library of existing constitutive models and the prescribed parameters. The coupled soil-water FE model was developed in OpenSees and the analysis results are compared with measured data from the shake table experiment with the main emphasis on the response of liquefied soil and the demand applied to the piles as well as the sheet-pile quay wall. By examining the numerical analysis results, it is demonstrated that the FE model was able to reproduce the shake table model behavior with reasonable accuracy. Lastly, a mitigation strategy was modeled to investigate its effectiveness to reduce the demand on the soil-pile system.
The installation of dense granular columns by various construction techniques can be used to mitigate liquefaction through a combination of densification, increase of lateral stresses, reinforcement, and drainage. The contributing mechanism of shear reinforcement is isolated and explored using nonlinear three-dimensional (3D) finite-element (FE) analysis. FE models representing both dry and saturated conditions were developed to evaluate cases with and without generation and dissipation of excess pore-water pressures. The shear stress and strain distributions between the granular columns and surrounding soil, and the level of shear stress reduction, were investigated for a practical range of treatment geometries, relative stiffness ratios, vertical stresses, and relative densities of the surrounding soil. A set of 10 acceleration time histories were used as input motions. The FE results show that granular columns undergo a shear strain deformation pattern that is noncompatible with the surrounding soil. As such, the achieved reduction in cyclic stress ratios imposed on the treated soil is far less than that predicted by the conventional shear strain compatibility design approach. Reductions in cyclic stress ratios are insensitive to the applied surface pressure, granular column length/diameter ratio (L/D), and relative density of the surrounding soil for the range of area replacement ratio and column-soil shear modulus ratio examined. A modified design equation to estimate the reduction in cyclic stress ratio provided by dense granular columns is shown to provide good agreement with the FE simulation results.
Installing stone columns is a convenient method of soft ground improvement. Although several analytical and numerical solutions are available to predict the performance of stone column improved soft ground, these models are incapable of capturing the influence of cyclic loading on transport corridors, such as highways and railways. The authors developed a novel finite-difference model adopting modified Cam clay theory to analyze the response of stone column reinforced soft soil under static and cyclic loadings. Apart from predicting excess pore water pressure dissipation and the resulting settlement, the model also captures the effect of cyclic loading by considering yield surface contraction. The solutions developed were validated using available field observations and laboratory test results. The model was successfully applied to a selected case study at the Australian National Field Testing Facility at Ballina, New South Wales, Australia. The limitations of the proposed model are highlighted as well. (C) 2015 American Society of Civil Engineers.
Installation of stone columns is one of the most popular ground improvement techniques used to improve the strength of soft soil. Owing to the presence of more pores in the stone column, it acts as a drainage path for seepage of water from the soil during consolidation. Thus, in addition to increasing the soil's strength, the stone column also increases the rate of consolidation of the soil. The water present in the pores of the soil is not pure, and some fine particles released from the soil are also present. During the seepage, these fines present in the fluid also move along with it. Depending upon the amount of concentration of fines present in the seepage water, the clogging of the stone column takes place which reduces the permeability of the stone column. Thus, the rate of consolidation of the ground improved by the stone column also decreases because of the clogging. In the present study, a mathematical model for the consolidation rate of soil improved with stone columns has been developed by considering the effect of clogging due to particle migration. The results are compared with the available rate of consolidation models for stone column-improved ground with or without clogging. The effects of various ranges of parameters of the developed model on the rate of consolidation are also studied. It is observed that as the diameter ratio and stress concentration ratio increase, the clogging also increases.
The risk of liquefaction and associated ground deformation may be reduced by using various ground-improvement methods, including the stone column technique. To examine the effects of stone columns, a shaking table experimental study using four models (two containing saturated sand and two containing stone column composite foundations) was conducted to measure the development and dissipation of excess pore water pressure and the acceleration response during a simulated earthquake. The test results demonstrate that the effectiveness of stone columns for mitigation of soil liquefaction during an earthquake depends on the following three aspects: 1) the densification of the surrounding soils, 2) drainage along the stone column, and 3) reduction in the total cyclic shear stress of the soil (because the cyclic shear stress is partially shared by the stone column). The first factor (the densification of the surrounding soils) is the most prominent factor among these three factors. The drainage and re-distribution of the shear stress can only develop fully for sand ground with a considerably higher density; thus, the effectiveness of the last two factors are only significant for dense sand ground.
LARGE SCALE SHAKING TABLE TESTS WERE PERFORMED ON THE GRAVEL DRAIN SYSTEM AS A COUNTERMEASURE TO LIQUEFACTION OF THE SAND DEPOSITS. THE SIZE OF THE SHAKING TABLE WAS 12 M * 12 M AND THE SIZE OF MODELS IS 12 M IN LENGTH, 3 M IN DEPTH AND 2 M IN WIDTH. THE MODEL SANDY GROUNDS WITH GRAVEL DRAINS AND THE HALF BURIED TYPE ROAD MODEL WERE HORIZONTALLY EXCITED TO INVESTIGATE THE EFFECTS OF THE GRAVEL DRAINS TO PREVENT LIQUEFACTION OF THE SANDY GROUND. THE CYCLIC LABORATORY SOIL TESTS AND THE FINITE ELEMENT ANALYSES WERE PERFORMED ON THE GENERATION AND DISSIPATION OF THE PORE WATER PRESSURE DURING CYCLIC LOADINGS.
A two-dimensional nonlinear dynamic finite element (FE) model was developed and calibrated against dynamic centrifuge tests to study the behavior of soil-pile-structure systems in liquefied and laterally spreading ground during earthquakes. The centrifuge models included a simple structure supported on pile group. The soil profiles consisted of a gently sloping clay crust over liquefiable sand over dense sand. The FE model used an effective stress pressure dependent plasticity model for liquefiable soil and a total stress pressure independent plasticity model for clay, beam column elements for piles and structure, and interface springs that couple with the soil mesh for soil-structure interaction. The FE model was evaluated against recorded data for eight cases with same set of baseline parameters. Comparisons between analyses and experiments showed that the FE model was able to approximate the soil and structural responses and reproduce the lateral loads and bending moments on the piles reasonably well.
After a subsurface layer of soil is liquefied completely, the excess pore water pressure therein is dissipated through the overlying layer of soil. The Terzaghi consolidation theory is applied to the problem to determine the excess pore water pressure, and numerical solutions are obtained using the finite element method for a variety of initial and boundary conditions. If the compressibility of the surface soil is one order of magnitude smaller than that of the initially liquefied soil, the maximum pore pressure in the surface soil is not significantly affected by the initial excess pore pressure in the surface soil, the relative thickness of the surface soil with respect to the initially liquefied soil, the drainage condition at the lower boundary of the initially liquefied soil, or by the depth of the water table. The smaller the permeability and compressibility of the surface soil, the smaller becomes the shear strength of the soil due to the upward seepage. The surface soil can even be liquefied if its permeability and compressibility are sufficiently low as compared to those of the initially liquefied soil.
The finite difference method is applied to numerical analysis of the effectiveness of gravel drains constructed to prevent liquefaction of sandy soils due to earthquake motion. The basic equation used here is the axisymmetric diffusion equation which includes the generation of excess pore water pressure. The arcsin function of the equivalent cycle ratio formulated by Seed and Booker is used as the pore pressure buildup curve under undrained conditions. A comparison with the results of in-situ experiments reveals that the well resistance of the drain wells should be considered in the gravel drain design, and that the numerical method is valid for the pore pressure prediction.
The analytical results are summarized as a set of diagrams in order to provide a convenient basis for the drain spacing design which takes well resistance into account. These diagrams permit drain spacing to be directly decided without the need for a computer. The application range of these diagrams is markedly wider than those of several existing diagrams currently used.
Mitigation of liquefaction potential of soils with fines contents greater than 15 to 20 percent usually requires special adaptation of the dynamic densification methods - Deep Dynamic Compaction, Explosive Compaction, Vibro-Compaction, Compaction Piles - that are ordinarily used to improve loose, clean cohesionless soils. However, these methods can be used to attain the needed improvement in many soils if closely spaced vertical sand, gravel, or prefabricated (wick) drains are installed prior to construction. Soil-cement columns or walls installed by deep soil mixing or jet grouting can be used to provide both reinforcement and containment of liquefied soil. Removal and replacement of liquefiable silty and clayey soils using compaction, admixture stabilization or substitution by another 'material; e.g., roller compacted concrete, is also possible; however, the cost may be high, and consideration must be given to dewatering needs and the stability of open excavations. Mitigation of the liquefaction potential of high fines content sandy and silty soils using permeation grouting is not feasible because the low hydraulic conductivity of such soils.
This study utilises the equivalent granular state parameter, *, as a key parameter for studying static and cyclic instability and their linkage. * can be considered as a generalisation of the state parameter as first proposed by Been and Jefferies so that the influence of fines content in addition to stress and density state can be captured. Test results presented in this study conclusively showed that * at the start of undrained shearing and ηIS , the stress ratio at onset of static instability, can be described by a single relationship irrespective of fines content for both compression and extension shearing. This single relationship is referred as instability curve. However, the instability curve in extension shearing is different from that of compression. In this paper, the capacity of the instability curve in predicting triggering of cyclic instability was evaluated experimentally. An extensive series of undrained one-way (compression) and non-symmetric two-way cyclic triaxial tests, in addition to monotonic triaxial tests in both compression and extension were conducted for this evaluation. Furthermore, a published database for Hokksund sand with fines was also used. Test results show that cyclic instability was triggered shortly after the cyclic effective stress path crossed the estimated IS-zone(s) as obtained from instability curve(s) irrespective of whether instability occurs in the compression or extension side.
In many cases densification with vibro-stone columns cannot be obtained in non-plastic silty soils. Shear stress re-distribution concepts [1] have been previously proposed as means to assess stone columns as a liquefaction countermeasure in such non-plastic silty soils. In this study, centrifuge testing is conducted to assess the performance of this liquefaction countermeasure. Attention is focused on exploring the overall site stiffening effects due to the stone column placement rather than the drainage effects. The response of a saturated silt stratum is analyzed under base dynamic excitation conditions. In a series of four separate model tests, this stratum is studied first without, then with stone columns, as a free-field situation, and with a surface foundation surcharge. The underlying mechanism and effectiveness of the stone columns are discussed based on the recorded dynamic responses. Effect of the installed columns on excess pore pressures and deformations is analyzed and compared. The test results demonstrate that stone columns can be an effective technique in the remediation of liquefaction induced settlement of non-plastic silty deposits particularly under shallow foundations, or vertical effective stresses larger than about 45 kPa (1000 psf) in free field conditions.
The devastating Mw 9.1 Sumatra earthquake on 26 December 2004 and subsequent tsunami caused severe damages to harbour structures which caused delay in supply of relief work in the earthquake and tsunami affected areas in Andaman Islands, India. Major structural damage was observed at the construction joints due to pounding of two portions of jetties and at the top of reinforced concrete piles, especially short piles. Inadequate structural design and reinforcement detailing along with poor maintenance of these structures were primarily responsible for the severe damages. Other geotechnical aspects, e.g. liquefaction of soils, slope-stability failure, etc., were also responsible for severe damage to these structures. Appropriate seismic design provisions in applicable codes and their implementation are necessary to ensure satisfactory structural response for uninterrupted services at harbours in seismically active zones, especially those in developing countries.