Stone Column-supported Embankments Research Articles

Evaluation of Critical Length of Encasement for Stone Column-Supported Embankment with Basal Geocell

Evaluation of Critical Length of Encasement for Stone Column-Supported Embankment with Basal Geocell

Numerical study on stability of geosynthetic-encased stone column-supported embankments based on equivalent method

The application of geosynthetic-encased stone column-supported embankments (GESC-supported embankments) has garnered significant attention in geotechnical engineering, with limited investigations into their slope stability. In this paper, an equivalent method was proposed to establish GESC-supported embankment models through three-dimensional finite difference method (3D-FDM). Responses of factor of safety (FS) to arrangement tightness of GESCs (e.g., various column diameters, spacings, and quantities under constant area replacement ratio) and hoop effect of geosynthetic encasements (e.g., various geogrid strengths under different shear strengths of soft soil) were studied. For a given area replacement ratio, FS increased non-linearly with the increasing column quantity or the decreasing ratio of column spacing to diameter at high area replacement ratios. GESCs with smaller diameter and spacing proved to be more effective in improving slope stability, and an optimum ratio of column spacing to diameter was observed for practical use. Besides, FS increased linearly with geogrid strength at low geogrid strength levels, but an extremely lack of the shear strength of soft soils or the column quantity was unfavorable for the application of GESCs. Bending failure was the primary failure feature of GESCs under embankment loading, and the geogrid strength influenced slope stability through the bending capacity of GESCs.

Geosynthetic-Reinforced and Stone Column-Supported Embankments: Numerical and Stability Study

Geosynthetic-Reinforced and Stone Column-Supported Embankments: Numerical and Stability Study

Field tests on partially geotextile encased stone column-supported embankment over silty clay

Various researchers have highlighted that geosynthetic-encased stone columns could be used in place of stone columns (SCs) in very soft soil. In this study, field load tests are performed on individual partially geotextile-encased stone columns (pESCs), individual stone columns (SCs), and a field embankment reinforced with pESCs and SCs in silty clay. Although the material cost of pESCs is designed to be comparable to that of SCs, the tests reveal that pESCs display a higher allowable bearing capacity and stress concentration ratio and less residual settlement than SCs. The embankment reinforced by pESCs is found to incur fewer lateral displacements, while there is no improvement in the drainage performance during the construction period compared to the SCs. For the pESCs, 51–66% of the pressure acting on the top of the column propagates to a depth of z = 1.2 m, wheras, for the SCs, it is only 21–55%. Furthermore, to estimate group behaviour, the ultimate bearing capacity of an individual pile composite ground can be extrapolated. However, individual pESCs achieve much higher stress concentration ratios than the pESC group.

Limit equilibrium slope stability analysis of column-supported embankment on weak ground

PurposeThe study aims to analyse the stability of embankments over the improved ground with stone column (SC) and pervious concrete column (PCC) inclusions using limit equilibrium method. The short-term stability of PCC-supported embankment system is rarely addressed. Therefore, the factor of safety (FOS) of column-supported embankment system is calculated using individual column and equivalent area models.Design/methodology/approachThe stability analysis of column-supported embankment system is conducted using PLAXIS LE 2D. The various geometrical and shear strength parameters influencing the FOS of these embankment systems such as diameter of columns, spacing between columns, embankment height, friction angle of column material, undrained cohesion of weak ground and cohesion of PCC are considered.FindingsThe critical failure envelope of PCC-supported embankment system is observed to be of toe failure, whereas the failure envelope of stone column-supported embankment system is generally of deep-seated nature.Originality/valueIt is found that for PCC embankment system, FOS and failure envelope are not influenced by the geometrical/shear strength parameters other than height of embankment. However, for stone column-supported embankment system, FOS and failure envelope are dependent on all the shear strength and geometrical parameters considered in this study.

Three dimensional analyses of geocell reinforced encased stone column supported embankments on lithomargic clay

ABSTRACT Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. However, numerical modeling of geocell is always challenging due to its three-dimensional honeycomb structure. The limitations of the equivalent composite approach (ECA) led to the recent development of full 3D numerical models, which consider geocell-infill material interaction. This paper discusses the time-dependent performance of geocell-reinforced encased stone column-supported embankment considering the actual 3D nature of geocells using the finite element program ABAQUS. Parametric studies were carried out to study the stress transfer mechanism, vertical deformation of the foundation soil, and stress-strain variation inside the geocell pockets. It is found from the analyses that with the provision of a geocell layer on top of Geosynthetic Encased Stone Columns (GESC), the stress concentration ratio improved by 47% at the end of consolidation compared to GESC alone. Also, an 80% reduction in foundation surface settlement is observed with geocell-sand mattresses. The geocell-sand mattress decreased the bulging of the stone columns, and almost 80% of the stone column bulging occurred by the end of the embankment construction. The proposed model’s numerical results show that the equivalent composite approach overestimated the stress concentration ratio and bearing capacity. The tensile stresses are non-uniformly distributed in the geocell pockets, and the maximum tensile force was mobilised at the geocell mid-height. Among the various geocell infill materials analysed, the aggregates were best suited considering the stress concentration ratio and vertical settlement. The numerical results supported the idea that encased stone columns with geocells at the embankment base can perform similarly to a geosynthetic reinforced piled embankment system, which is costlier but very efficient. When the modular ratio is more than 40, geocell-reinforced encased stone column-supported embankment is similar to GRPES.

Effect of basal reinforcement on performance of floating geosynthetic encased stone column-supported embankment

In this paper, two centrifuge modeling tests were performed to investigate the influence of basal reinforcement on the global performance of floating geosynthetic encased stone column (GESC)-supported embankments. Based on the centrifuge tests, a 3-dimensional (3D) numerical modeling was carried out to investigate the influence of basal reinforcement on the deformation behavior of the floating GESC-supported embankment. The centrifuge and numerical modeling results showed that the basal reinforcement reduced total and uneven settlement at the embankment crest and base significantly. Moreover, the inclusion of the basal reinforcement significantly reduced the lateral displacement on top of the column, preventing outward bending of the floating GESCs below the embankment toe. However, the basal reinforcement increased the lateral displacement at the bottom of columns.

Analytical solutions for geosynthetic-encased stone column-supported embankments with emphasis on nonlinear behaviours of columns

This paper presents an analytical approach to predict the behaviours of geosynthetic-encased stone column (GESC)-supported embankments. The soil arching in the embankment and the nonlinear behaviours of stone columns are considered. Based on nonlinear elastic and elastoplastic constitutive models of stone columns, the nonlinear behaviours of GESCs, including settlement and radial deformation, are analysed. The deformations of GESCs, the surrounding soil, and the overlying embankment fill are compatible by applying stress continuity and volume deformation continuity at the bottom of the embankment fill. This method is verified via comparison with literature data and numerical analysis. The influences of parameters of the GESC, including encasement stiffness and column friction, on the performance of the embankment are investigated. Without considering the nonlinear behaviours of the column, the column-soil stress ratio is overestimated. It is more appropriate that the nonlinear characters of the column be considered in the analysis of GESC-supported embankments.

Analysis of Geocell-Reinforced Stone Column-Supported Embankment Considering Soil-Structure Interaction

Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. The effect of the basal geocell layer on the performance of the stone column-supported embankment was evaluated in this paper by carrying out time-dependent coupled analyses considering the geocell-infill soil interactions. The various parameters, such as arching, stress concentration ratio, surface settlement, column bulging characteristics, and the influence of various types of infill materials and drainage blanket thickness on column-supported embankment behavior, were analyzed. The load transfer mechanism was quantified using the term stress concentration ratio, and with the use of geocell mattress above stone columns, the stress concentration ratio increased by 1.5 times that of ordinary stone columns. Also, it was observed that compared to ordinary stone column-supported embankment, the combination of stone columns with geocell-sand mattress resulted in a further reduction of the foundation settlement by 15%. Analysis results showed that the arching behavior is not predominant in geocell-reinforced columnar structures. The various analytical methods like Guido et al. (1987), Low et al. (1994), and Abusharar et al. (2009), which were developed for geosynthetic reinforced columnar embankments, were found to significantly under-predict the stress reduction ratio values with geocells.

3D coupled mechanical and hydraulic modeling of geosynthetic encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proved as an effective alternative to reinforcing soft marine clay soils with low undrained shear strength. In this paper, three-dimensional (3-D) coupled mechanical and hydraulic modeling based on centrifugal model test was carried out to investigate the performance of GESC-supported embankment over soft clay with two different stiffness of encasement (i.e., 450 kN/m and 3300 kN/m). The numerical modeling results agree well with the centrifugal model test recordings. This investigation reveals that with the increase in the geogrid stiffness the differential settlement between the GESC and the surrounding soft marine clay soil was found to increase. Under the embankment load, the encased columns located below the embankment slope have experienced significant vertical and lateral stress ratio. Moreover, columns below the embankment slope tend to experience significant hoop strain below the top of the column.

Deep-Seated Slope Stability Analysis and Development of Simplistic FOS Evaluation Models for Stone Column-Supported Embankments

Inclusion of stone columns in the underlain soft soil is one of the most prominent methods for improving the stability of embankments. The stone columns are encased with a geosynthetic material to further enhance the stability. The influence of this partial replacement of weak foundation soil with stone columns on the performance of embankments needs to be quantified. In this study, performance of ordinary stone column (OSC) and geosynthetic-encased stone column (GESC)–supported embankments is carried out using a three-dimensional finite element programme (PLAXIS3D). A parametric study was conducted to quantify the influence of various factors viz. spacing to diameter ratio (S/D), stiffness of encasement, cohesion of soil, friction angle of stone column and friction angle of embankment on the factor of safety against deep-seated failure. The results show that encasing the stone columns enhances the stability of embankments. Decreasing the column spacing (S/D) enhances the stability, reduces the excess pore pressure development and average settlement. Increase in geotextile encasement stiffness, cohesion of underlain soft soil, friction angle of stone column and friction angle of embankment improves the performance of embankments and also reduces the average settlement of the ground under embankment. Results of the parametric study were used to develop two types of data-driven models viz. multiple linear regression (MLR) and artificial neural networks (ANN) to simplify the evaluation of the factor of safety (FOS) of embankments. Among the two approaches, ANNs were able to predict the factor of safety values more accurately.

Performances of geo-reinforced and stone column-supported embankment by a mounting-beams model

This study aims to develop a mounting-beams model predicting the behavior of geosynthetic reinforced and stone column-supported embankment with upper pavement under vehicle loads. Existing models are made a comparison with the proposed model, which can be deemed as their extension. Multiple factors, such as modified stiffness for tensionless granular mattress, the nonlinearity and consolidation of soft soils and the beam-soil interface resistance, are taken into account. On this basis, a parametric study is carried out to quantify the influence of bending stiffness of the upper pavement structure, the tensile stiffness of geosynthetic reinforcement, the vertical stiffness of a stone column wall, and the consolidation of soils on the response of the embankment system reinforced with geosynthetic and stone columns.

Response of stone column-improved ground under c-ϕ soil embankment

Problems with stone column-supported embankments are often addressed considering the pure granular soil properties of the embankment material. In practice, however, the embankment material may contain a certain percentage of fines which makes the material c-ϕ in nature. In the present paper, experimental and 3D numerical studies are conducted to investigate the effect of the fines content present in the embankment soil on the response of a stone column-improved ground. The numerical study is carried out using the finite difference software package FLAC3D. The time-dependent behavior of the improved soil is modeled. It is observed that an increase in the fines content in the embankment soil brings about a decrease in the stress concentration ratio and an increase in the vertical displacement and the pore pressure in the soft soil. However, the vertical displacement of the soft soil does not change significantly above a fines content of 20%. The lateral displacement of the stone column increases at the top with an increase in the fines content and the point of the maximum lateral displacement shifts in the downward direction. It is also observed that the differential settlement increases at the ground level as well as at the embankment top with an increase in the quantity of fines in the embankment soil. Thus, the critical height of the embankment (a height where no differential settlement occurs on the embankment surface) increases due to the increase in the fines content.

Modeling of Stone Column-Supported Embankment Under Axi-Symmetric Condition

In the present work, a simplified model has been developed to study the behavior of stone column-supported embankment under axi-symmetric loading condition. The rate of consolidation of stone column-reinforced soft ground under axi-symmetric condition has also been presented in the paper. Mechanical model elements such as Pasternak shear layer, spring–dashpot system are used to model the different components such as granular layer, soft soil, stone columns etc. The governing differential equations are solved by finite difference technique. Parametric study has also been carried out to show the effect of different model variables on the settlement, stress concentration ratio of the foundation system. It is observed that for lower diameter ratio, at a particular time, the degree of consolidation predicted by the present method for axi-symmetric loading condition is almost same or lower than the degree of consolidation obtained by unit cell approach, but as the diameter ratio increases present analysis predicts higher degree of consolidation as compared to the unit cell approach. The maximum settlement decreases as the modular ratio increases and beyond the modular ratio value 30, the reduction rate of settlement decreases.

Numerical investigation on factors for deep-seated slope stability of stone column-supported embankments over soft clay

Stone columns have been commonly used as an alternative to solve deep-seated slope stability problems. Due to the complexity of a three-dimensional (3-D) arrangement of multiple columns, a 3-D problem has been commonly converted into a two-dimensional (2-D) model which has equivalent properties and dimensions, by the column-wall method and the equivalent area method. In this paper, two column-wall approaches based on matching either column geometry or column properties were compared and verified by 3-D numerical results in the stability evaluation of the stone column-supported embankment over soft soils. This study also investigated the 2-D numerical models using the column-wall method and the equivalent area method considering the factors of stress concentration, area replacement ratio, and soil conditions under short-term and long-term conditions. The numerical results show that the equivalent area method resulted in a continuous critical slip surface in the stone column-supported embankment over soft soil; however, no continuous slip surface developed using the column-wall method. Under the short-term condition, the computed factor of safety by the equivalent area model with or without considering the stress concentration effect was greater than that computed by the column-wall model. However, their difference became smaller under the long-term condition. The columns at certain locations along a prescribed slip surface from the equivalent area method did not mobilize their shear strengths under the short term condition. A reduction factor of 0.9 is suggested to correct the calculated factor of safety by the equivalent area method without considering the stress concentration ratio to that by the column-wall method under the short-term condition. No reduction factor (or the reduction factor of 1.0) is proposed under the long-term condition.

Evolutionary approach for optimal stability analysis of geosynthetic-reinforced stone column-supported embankments on clay

Stone columns and geosynthetics are being used to increase the stability of embankments constructed over soft clays. The column into the soft soils acts as flexible vertical reinforcing element that increases the bearing capacity; reduces the settlement of the soil and also increases the stability of the embankments. Use of geosynthetic reinforcements is also very useful to reduce the settlement, increase the bearing capacity and improve the stability of the embankments resting on soft foundation soil. The present paper deals with the combined use of stone columns and geosynthetic reinforcements to improve the stability of embankments constructed over clays. A combined simulation-optimization based methodology has been developed for analyzing the stability of geosynthetic-reinforced embankments resting on stone column-improved ground by using an Evolutionary Genetic Algorithm NSGA-II (Non-Dominated Sorted Genetic Algorithm-II) to locate the critical surface and optimize the corresponding factor of safety under various conditions. In order to obtain critical failure surface, some constrains have been imposed on design variables. It has been shown that a genetic algorithm can be successfully employed to locate the critical failure surface in geosynthetic-reinforced stone columnsupported embankments. The variation of optimum factor of safety with area ratio, stiffness of the geosynthetics under various conditions has been studied. The proposed methodology has been validated by comparing the results of the present methodology with the results obtained by various other methods.

Analysis of stone column-supported geosynthetic-reinforced embankments

In the present paper, the behavior of stone column-supported geosynthetic-reinforced embankments has been studied. The soil arching effect is incorporated in the study to determine the stresses acting on stone columns as well as soft soil. The effect of stiffness of the stone column is also incorporated in the present study. Based on the stress action in the improved ground, stress concentration ratio, axial strain of geosynthetic reinforcement, tension developed in it and settlement of the improved soft ground are determined by using the developed methodology. Present analytical method is also verified with several current design methods. It is observed from parametric studies that modular ratio or stiffness of the stone columns, spacing to diameter ratio, height of the embankment, depth of the soft soil, stiffness of the geosynthetic reinforcement significantly affect the behavior of geosynthetic-reinforced stone column-supported embankments resting on soft soil.

A mathematical model to study the soil arching effect in stone column-supported embankment resting on soft foundation soil

Soil arching is a common phenomena in pile or columnar (vibroconcrete columns, soil–cement columns by mixing or grouting, stone columns) supported geosynthetic-reinforced or unreinforced embankments resting on soft soil. Due to soil arching, stress acting on soft soil or geosynthetic reinforcement decreases and stress on piles or columns increases. In this paper, using mechanical elements (such as spring, dashpot), a generalized mathematical model has been developed to study the soil arching effect in stone column-supported geosynthetic-reinforced and unreinforced embankments resting on soft soil. Pasternak model concept has been used to model the embankment soil. The soft soil has been idealized by spring-dashpot system to include the time-dependent behavior. The stone columns and geosynthetic reinforcement are idealized by stiffer nonlinear springs and rough elastic membrane, respectively. The consolidation effect of soft soil due to inclusions of stone columns has also been included in the model to study its effect on soil arching. Plane strain condition has been considered in the analysis. A finite difference scheme has been used to solve the governing differential equations and results are presented in non-dimensional form. It has been observed that the height of embankment, degree of consolidation of soft soil, stiffness of the stone column material, spacing between the stone columns, use of geosynthetic reinforcement and properties of soft and embankment soils (such as ultimate bearing capacity of soft soil, shear modulus and ultimate shearing resistance of embankment soil) significantly influence the degree of soil arching.