Geosynthetic-encased Stone Column Research Articles

Optimizing ground improvement with encased stone columns: a 3D numerical analysis in very soft clay

Purpose This study aims to investigate the behavior of different types of stone columns, including the short and floating columns, as well as the ordinary and the geosynthetic encased stone columns (OSC and GESC). The effectiveness of the geosynthetic encasement and the impact of the installation using the lateral expansion method on the column performance is evaluated through a three-dimensional (3D) unit cell numerical analysis. Design/methodology/approach A full 3D numerical analysis is carried out using the explicit finite element code PLAXIS 3D to examine the installation influence on settlement reduction (ß), lateral displacement (Ux) and vertical displacement (Uz) relative to different values of lateral expansion of the column (0% to 15%). Findings The findings demonstrate the superior performance of GESC, particularly short columns outperforming floating counterparts. This enhanced performance is attributed to the combined effects of geosynthetic encasement and increased lateral expansion. Notably, these strategies contribute significantly to decreasing lateral displacement (Ux) at the column’s edge and reducing vertical displacement (Uz) under the rigid footing. Originality/value In contrast to previous studies that examined the installation effect of OSC contexts, this paper presents a comprehensive investigation into the effect of geosynthetic encasement and the installation effects using the lateral expansion method in very soft soil, using 3D numerical simulation. The study emphasizes the significance of the consideration of geosynthetic encasement and lateral expansion of the column during the design process to enhance column performance.

Model tests on ordinary and geosynthetic encased stone columns with recycled aggregates as filler material

PurposeSincethe availability of natural aggregates is very sparse, recycled industrial and construction waste provides a sustainable alternative to ground improvement using vibro replacement method. Utilizing recycled building waste caters the requirement for its disposal and offers an effective remedy for the scarcity of natural resources. The aim of this study was to give a sustainable alternative for the natural aggregates as the material for stone column.Materials and methodsA good stone column material should be hard, dense, chemically inert and must comply with the size requirement. The utilization of construction debris and spent railway ballast as column material has been the subject of numerous researches. This work focuses on finding the suitability of railway ballast and concrete debris as alternatives for stone column material. A detailed laboratory testing of these materials has been carried to judge their strength requirements as the material for both Ordinary Stone Columns (OSCs) and Geosynthetic Encased Stone Columns (GESCs). The improvement in capacity of both OSCs and GESCs is evaluated by performing California Bearing Ratio (CBR) test in laboratory by creating unit cell stone column models of different recycled aggregates and comparing their load settlement behavior with natural aggregates.Results and discussionRailway ballast, natural aggregates, concrete debris and virgin soil were found to show decreasing order in CBR test results. Loading required for causing settlement in both OSCs and GESCsshowed remarkable increase as compared to that of virgin clay and the maximum load settlement improvement was observed for railway ballast in both the types of stone columns. The CBR values for GESC made from railway ballast, natural aggregates and concrete debris were 54, 49 and 38% respectively. On the other hand, CBR for OSC made from railway ballast, concrete debris and natural aggregates were found to be 25.5, 20.4 and 24% respectively and CBR of virgin clay was found to be just 11%.ConclusionThe demonstrated application of sustainable sources in place of natural aggregates provides a crucial pathway for utilizing the recycled aggregates as stone column filler material. Up on encasing the OSC with geotextile the performance of stone columns has improved appreciably in terms of load capacity. Railway ballast and concrete debris can be adopted as an alternate for the natural stone column materials to improve the bearing capacity of site consisting mainly of soft clays.

Shaking table tests on the influence of geosynthetic encasement stiffness on the shear reinforcement effect of GESC composite foundation

This paper presents an experimental study of shaking table tests on two geosynthetic encased stone columns (GESC) composite foundation models with different geosynthetic encasement stiffness to investigate the influence of geosynthetic encasement stiffness on the shear reinforcement effect. The reduced-scale GESC composite foundation models were designed according to the similitude relationships by scaling the model geometry, geosynthetic encasement stiffness, and input motions for shaking table tests in a 1 g gravitational field. The GESC composite foundation models were constructed using poorly graded sand, gravel, and geotextile encasement, and then were excited using a series of sinusoidal input motions with increasing peak acceleration. The acceleration amplification factors for the GESC composite foundation model with higher geosynthetic encasement stiffness are larger than those of the lower geosynthetic encasement stiffness model due to the increased stiffness of the composite foundation. The higher geosynthetic encasement stiffness composite foundation has smaller settlements and lateral displacements under the same input motions compared to the lower geosynthetic encasement stiffness composite foundation. The incremental geosynthetic encasement tensile strains increase with increasing input acceleration for both models. The longitudinal tensile effect of geosynthetic encasement plays an important role on the shear reinforcement mechanism of GESC.

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.

Centrifuge investigation on behavior of geosynthetic-encased stone column supported embankment under freeze-thaw cycles

In this paper, two centrifuge tests were conducted on geosynthetic-encased stone column (GESC) supported embankment to investigate the behavior under freeze-thaw cycles considering the effects of initial degree of consolidation and number of freeze-thaw cycles. The embankments with a height of 4.5 m supported by GESC-reinforced foundation with a thickness of 9 m in prototype were first consolidated to initial degrees of consolidation of 30% and 60%, respectively, and then subjected to three freeze-thaw cycles. The results showed that the foundation soil under the embankment toe experienced a higher degree of freezing than the soil outside the embankment with non-uniform frozen depths observed at different positions in soil under the embankment, and that multiple freeze-thaw cycles contributed to extra settlement in the foundation. Both the frost heave and thaw settlement appeared on GESCs prior to soil. During freezing, the stress on GESC increased first, while decreased rapidly when soil began freezing. The stress concentration ratio (SCR) reached nearly 1.0 before complete freezing. After the complete thawing of GESCs, the stress on GESC increased significantly. As the number of freeze-thaw cycles grew, the variation in settlement, stress and pore pressure was reduced. The final settlement on soil was greater in the foundation with lower initial degree of consolidation, as well as the SCR. The GESCs had larger deformation and showed outward bending under lower initial degree of consolidation, but inward bending appeared on GESCs under higher one.

Numerical Analysis on the Behavior of Floating Geogrid-Encased Stone Column Improved Foundation

The ordinary (OSC) and geosynthetic-encased stone column (ESC) with different bearing strata significantly influenced its behavior. The paper established seven models for studying the behavior of floating stone columns using the finite difference method (FDM). The effect of geogrid and column length on the load-settlement behavior, bulging deformation, failure mode, and load transfer coefficient were also analyzed based on proposal models. The results showed that the bearing capacity of F-OSCs and F-ESCs increased with the increase in column and encasement length, respectively, and a critical length (i.e., 4D, where D was the column diameter) was found in settlement improvement. The bulging deformation was significant in F-OSCs and was observed at the top of a long column and the full length of a short column. The geogrid encasement could constrain the OSC to decrease the bulging deformation. The failure mode in F-OSCs was mainly a punching failure with bulging deformation for a short column (e.g., less than 4D), and was relative to the vertical pressure for a long column. The failure mode in F-ESCs was a punching failure, and the punching degree increased with an increase in encasement length. The load transfer coefficient of F-OSCs or F-ESCs was relatively stable as the column length increased to a critical value (e.g., 4D) or the encasement length increased to a critical value (e.g., 4D).

A DEM Study on Bearing Behavior of Floating Geosynthetic-Encased Stone Column in Deep Soft Clays

The use of geosynthetic-encased stone columns has been proven to be an economical and effective method for soft soil foundation treatment. This method is widely used in civil engineering projects at home and abroad. When the geosynthetic-encased stone columns are applied to deep soft clays, they are in a floating state. The load-bearing deformation mechanism of geosynthetic-encased stone columns has changed. The interaction between the aggregates, geogrid, and soil is worth studying, especially at the bottom of the column. In this paper, the discrete element method is used to simulate a floating geosynthetic-encased stone column with a 280 mm encasement depth in deep soft clays. The load-bearing deformation characteristics and mesoscopic mechanism of the floating geosynthetic-encased stone column are studied. The results show that there are large vertical and radial stresses in the top region. Moreover, the porosity and sliding fraction of aggregates in this region increase with settlement, and the coordination number decreases with settlement. The vertical and radial stresses of the soil near the column body are not affected much by the column body. When the encasement depth exceeds 280 mm, the bearing capacity of the FGESC does not increase much. The encasement depth controls the failure mode of the floating geosynthetic-encased stone column. As the encasement depth increases, the failure mode of the floating geosynthetic-encased stone column gradually transitions from swelling deformation to penetration failure.

Numerical Analysis of a Dual-Layer Geosynthetic-Encased Stone Column Installed in Soft Soil

Stone columns are being used to reduce soft soil settlement and increase load-carrying capacity. Since there is inadequate lateral support from the local native soil, soft soil undergoes excessive settlement under vertical loading. This issue is effectively resolved by suitably encasing stone column material by geosynthetic with significant axial stiffness, which provides the required additional confinement reported in the literature. In the current study, an effort has been made to examine the load settlement behaviour of the dual-layered geosynthetic-encased stone column (DL-GESC) under vertical loading. In order to simulate the behaviour of stone column-reinforced soft soil, a FEM analysis was performed using PLAXIS-3D and three-dimensional (3D) models made utilising the unit cell idealisation technique for a single column. The stone column diameter, spacing to diameter (s/d) ratio, and encasement layers were varied to determine their influence on load-settling behaviour. The vertical load-carrying capacity of the ground was significantly improved when an additional layer of geosynthetic encasement was inserted into the stone column as compared to SL-GESC. Improvement of 15–25% was observed for the analysis of a single column installed in soft clay, according to the result obtained. Improvement ratios have been discussed in detail for various encasement conditions.

Construction and Demolition Waste as Valuable Resources for Geosynthetic-Encased Stone Columns

Construction and Demolition Waste as Valuable Resources for Geosynthetic-Encased Stone Columns

Bearing Characteristics of Composite Foundation Reinforced by Geosynthetic-Encased Stone Column: Field Tests and Numerical Analyses

In order to study the bearing characteristic of the geosynthetic-encased stone column (GESC) on the composite foundation, a series of field tests and numerical simulation were carried out on the composite foundations reinforced by the traditional stone column and the GESC. The pile–soil stress ratio, excess pore water pressure and lateral displacement of two kinds of composite foundations were monitored. The effects of geotextile stiffness, geotextile wrapping length and gravel internal friction angle on the composite foundation with the GESC were analyzed by establishing different numerical models. The results show that the GESC can bear larger loading compared with the traditional stone column. The pile–soil stress ratio of the composite foundation with the traditional stone column gradually increases from 1.1 to 1.5 with the increasing of the embankment height. However, the pile–soil stress ratio of the composite foundation with the GESC reaches 1.5 at the initial filling stage and increases to 1.7 with the filling construction. The drainage effect of the GESC is better than that of the traditional stone column, and the GESC can effectively improve the overall stiffness of stone column, so as to reduce the lateral displacement of soil mass. The increases of geotextile stiffness, geotextile wrapping length and gravel internal friction angle can improve the bearing performance of the composite foundation with the GESC. However, after geotextile stiffness and wrapping length reach a certain value, the influence of its lifting amount on the composite foundation will be reduced.

Study on Bearing Capacity and Failure Mode of Multi-Layer-Encased Geosynthetic-Encased Stone Column under Dynamic and Static Loading

The “method of overlap” replaces traditional welding to solve the problem of how the geosynthetic-encased stone column is limited by the welding frame during site construction, making the site construction simplified and economical, but its bearing mechanism is not clear. Therefore, the bearing mechanism and failure mode of the stone column was studied through the compression test of the multi-layer geosynthetic-encased stone column under dynamic and static loading. The research shows that the multi-layer encasement improves the modulus and lateral restraint of the stone column, which increases the stress transfer rate and reduces the damage degree of the stone column. The vertical ultimate bearing capacity increase in multi-layer geosynthetic-encased stone columns under dynamic and static loading is significantly different, and the difference can be up to 47.1%; the corresponding number of encasement layers should be selected according to the actual situation. The influence of the difference between dynamic and static loading on the location of the main radial strain of the stone column can be ignored, but the lateral restraint of the stone column under dynamic loading is weakened, the stress transfer rate is reduced, and the radial strain is reduced and more uniform along the stone column height. The vertical ultimate bearing capacity of the one- and two-layer geogrid-encased stone column under dynamic loading is lower than that of static loading. When treating soft foundations, the influence of traffic loads should be considered, and the bearing capacity of the geosynthetic-encased stone column should be appropriately increased in design value.

Contribution of geosynthetic to the shear strength of geosynthetic encased stone columns

This paper presents a numerical study to evaluate the contribution of geosynthetic to the shear strength of geosynthetic encased stone column (GESC) under direct shear loading conditions. The backfill soil was characterised using the linearly elastic–plastic Mohr–Coulomb model. The geosynthetic encasement was simulated using linearly elastic liner elements. The interaction between the geosynthetic encasement and soils on both sides was modelled through two interfaces. The three-dimensional numerical model was validated using experimental data from direct shear tests of GESC models. The shear stress–strain response and the development of longitudinal and circumferential strains of GESC during the shear process were first discussed, and then a parametric study was conducted to investigate the effects of various design parameters on the shear strength of GESC and the contribution of geosynthetic. Results indicate that the shear resistance provided by the geosynthetic encasement develops slowly, which depends on the mobilisation of tensile strains. At the failure condition, the longitudinal strains are larger than the circumferential strains, which indicates that the longitudinal tensile rupture is more critical for GESC under shear loading. The vertical stress, geosynthetic encasement stiffness and stone column diameter and spacing have the most important influences on the shear strength contribution of geosynthetic encasement.

Static and Dynamic Load Transfer Behaviors of the Composite Foundation Reinforced by the Geosynthetic-Encased Stone Column

An accurate description of the load transfer behaviors of the geosynthetic-encased stone column (GESC) is of great importance for revealing the bearing capacity of GESC. Static load tests and shake table model tests were performed to characterize the static and dynamic load transfer behaviors of the composite foundation reinforced by the GESC. Under static loading, static load tests were conducted on a fully geosynthetic-encased stone column (FGESC), partially geosynthetic-encased stone column (PGESC) and traditional stone column (TSC). The influence of length and stiffness of the encasement on the stone columns were investigated. Under seismic loading, the shake table model tests were performed to analyze the differences of the dynamic pile-soil stress responses between the composite foundations with the GESC and the TSC. The results show that the static pile-soil stress ratios of the composite foundation with the FGESC are about three to six times of those of the composite foundation with the TSC, and the difference increases with the increase in the stiffness or length of the encasement. The static vertical stress of 60% acting on the pile top can be transferred to the pile bottom for the FGESC, while only 27~45% for the TSC. The dynamic pile-soil stress ratios of the GESC and the TSC first decrease and then increase slightly with the increase of the input peak acceleration. The dynamic pile-soil stress ratio of the GESC is about three times that of the TSC under seismic excitation with the same type and peak acceleration. The attenuation rate of dynamic stress along the pile body under dynamic loading is much faster than that under the static loading. Under the static and dynamic conditions, the load transfer capacity and pile efficacy of the GESC are always better than those of the TSC.

3-Dimensional Numerical Evaluation of Geosynthetic Encased Stone Columns in Unsaturated Soils

Geosynthetic encased stone columns are often designed using the conventional framework of saturated soils ignoring the influence of in-situ unsaturated soil conditions. This article evaluates the performance of stone columns with and without geosynthetic encasement extending the mechanics ofunsaturated soils. The focus of numerical simulations was directed towards understanding the influence of matric suction on the confining support offered by the surrounding soil to stone columns with and without geosynthetic encasement. Investigations were extended considering the stiffness and the length of the geosynthetic encasement. The numerical studies suggest the load-carrying capacity of stone columnincreased with an increase in the matric suction in the boundary effect and the primary transition zone. However, the contribution of matric suction towards load-carrying capacity starts reducing from the secondary transition zone. The information on boundary effect and transition zones can be derived fromthe soil-water characteristic curve, which is a relationship between the water content and soil suction. In addition, the effect of stiffness and length of encasing material in unsaturated soils was found to be in contrast with saturated soils. The results of the study are promising towards developing procedures thatcan be used in the rational design of stone columns in unsaturated soils.

Numerical Study on the Deformation Behavior of Geosynthetic-Encased Stone Columns Supporting Embankments

The method of encasing the stone column with a proper type of geosynthetic material is a widely used technique to provide the required lateral confinement and to avoid the dispersion of granular column material into soft clay. Along with the improved ultimate load capacity and the reduced settlement and bulging, the geosynthetic encasement preserves the easy drainage ability of stone columns. This paper presents the finite element analysis results of a hypothetical embankment on a soft soil deposit which is improved by geotextile encased stone columns and geogrid reinforced sand mat on top. At first, numerical results of three dimensional (3D) finite element model (FEM) were validated via the experimental data of previous field studies. Afterward, parametric studies were carried out on the FEM considering the effect of the sand mat thickness, the stiffness of the geosynthetic reinforcement, and the geosynthetic encasing length and encasement stiffness on both the horizontal and vertical deformation of stone columns. Settlement differences between columns and soft soil in both the short term and long term were also determined. The optimum values of sand mat layer thickness, vertical encasement length, and geosynthetic stiffness are recommended to be used for preliminary designs.

Shear performance of geosynthetic-encased stone column based on 3D-DEM simulation

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.

Centrifuge tests on geosynthetic-encased stone column supported embankment on seasonal frozen soil

Geosynthetic-encased stone columns (GESCs) have been widely applied into soft foundation. This paper aimed to evaluate the availability and efficiency of GESCs in seasonal frozen ground. Four centrifuge tests were conducted on GESCs supported embankment on seasonal frozen soil under embankment load and thawing, where the foundation was frozen before the construction of embankment. The effects of encasement stiffness and lateral reinforced cushion were investigated. Three phases could be distinguished in the tests by two timing nodes due to the complete thawing of columns and soil, and analysis were made based on the three phases. It is found that high-stiffness encasement can effectively reduce the differential settlement between soil and columns before complete thawing of soil. The GESCs appeared a deformation mode of inward bending, which is in inverse to that in common composite foundation. The reinforced cushion rearranged stress on columns and soil, and influenced the development of pore pressure. The stress concentration ratio (SCR) first decreased to less than 1 due to column thawing, and experienced a steady stage until soil was completely thawed. The SCR rapidly increased after complete thawing of soil, and decreased to a constant value due to soil consolidation.

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.

Geosynthetic Encased Concrete Debris Stone Columns

Rapid urbanization has compelled the building industry to improve the soft soil grounds which otherwise are unsuitable for construction activities. Ground improvement helps achieve better performance by avoiding several problems like excessive settlements, large lateral flow of soft soil beneath the structures, and loss of global or local stability under operational loading conditions by modification of foundation soils. The growing quantities and types of waste materials, shortage of landfill spaces, and lack of natural earth materials highlight the urgency of finding innovative ways of recycling and reusing waste material additionally can reduce the demand for natural resources creating sustainable ecosystem. The aim of this experiment is to use crushed concrete debris (CCD) in soil stabilization. A series of experiments are to be carried out to develop an understanding on the performance of soft clay foundation beds reinforced using geosynthetic encased stone-concrete debris column. Model tests was conducted on clay reinforced with encased vertical columns filled with both CCD and aggregates. A comparative study is to be done and an assessment of ultimate bearing capacity of the composite ground is to be done. The column failure mechanism in the both the cases are to be carefully studied. Modal tests and later numerical analysis is done in PLAXIS 2D for analyzing the behavior of geosynthetic encased stone column stabilized clay bed numerically. Analyses were carried out using Mohr-Coulomb’s criterion. Lab conditions were simulated in PLAXIS for accurate comparison for same model tests.

Numerical modeling of floating geosynthetic-encased stone column–supported embankments with basal reinforcement

The performance of the floating geosynthetic-encased stone column–(GESC)-supported embankments with basal reinforcement was examined using a 3-dimensional (3D) hydro-mechanical coupling finite element model. Comprehensive parametric analyses were performed on the governing factors such as consistency of substratum soil, tensile stiffness of basal reinforcement and encasement, and embankment height. The results indicated that a higher embankment load is transferred to the surrounding soil when a GESC was constructed on a weaker substratum. This causes larger increases in the settlement and lateral displacement of the GESC on the weaker substratum. The tensile strain of the basal reinforcement and hoop strain in the encasement also increases. In addition, high tensile stiffness in basal reinforcement and encasement is necessary to ensure feasible settlement reduction in a floating GESC-supported embankment with basal reinforcement.