Geosynthetic Encased Columns Research Articles

Column Supported Embankments with Geosynthetic Encased Columns: Validity of the Unit Cell Concept

Column supported embankments (CSEs) are used to overcome common problems associated with the construction of embankments over soft compressible soils. The use of granular columns as deep foundation elements for CSEs can be problematic in soft soils due to the lack of adequate lateral confining pressure, particularly in the upper portion of the column. Using a high-strength geosynthetic for granular column confinement forms geosynthetic encased columns (GECs); the confinement imposed by the geosynthetic increases the strength of the column, and also prevents its lateral displacement into the soft surrounding soil. This paper presents the results of finite element analyses of a hypothetical geosynthetic reinforced column supported embankment (GRCSE) (i.e., a CSE underlain by geosynthetic reinforcement) that is constructed with GECs as the deep foundation elements. Full three-dimensional (3-d), 3-d unit cell, and axisymmetric unit cell analyses of the GRCSE were carried out to investigate the validity of the unit cell concept. The effect of the degree of nodal constraint along the bottom boundary when numerically modeling GRCSEs was also studied in this paper. Numerical results show that a full 3-d idealization is required to more precisely determine the tension forces that are produced in the geosynthetic reinforcement that underlies the GRCSE. A number of design parameters such as the average vertical stresses carried by the GECs, lateral displacement of the GECs, and the maximum settlement of the soft foundation soil, however, can be successfully calculated using unit cell analyses.

Технические решения по замене железобетонного свайного фундамента на основание, улучшенное армированными грунтовыми сваями

Технические решения по замене железобетонного свайного фундамента на основание, улучшенное армированными грунтовыми сваями

Column Supported Embankments with Geosynthetic Encased Columns: Parametric Study

Three-dimensional finite element analyses were performed to investigate different factors that affect the behavior of column supported embankments (CSEs) that are constructed using geosynthetic encased columns (GECs) as the deep foundation elements. Analyses were performed to study the influence of the geosynthetic encasement on the behavior of granular columns (GCs) in CSEs. Stress reduction ratios (SRRs) obtained from finite element analyses were compared to those calculated from ten different analytical solutions. Parametric analyses were also carried out to study the effect of variations in the stiffness of the encasement, the area replacement ratio, and the length of the geosynthetic encasement on the performance of CSEs. Finally, the sensitivity of the numerical results, particularly the lateral displacement of GECs, to the constitutive model that was used to simulate the behavior of the granular column material was examined. Numerical results showed that encasing a GC in a CSE not only improves the performance of the CSE but also enhances the behavior of the GC. No agreement was found between the calculated values of the SRR from finite element analyses and those from existing analytical solutions.

Performance of a geosynthetic-encased column (GEC) in soft ground: numerical and analytical studies

ABSTRACT: Stone columns with granular material are often used to support embankments over soft soil. In extremely soft soils, encasing the granular column with geosynthetic material improves their stiffness and strength. Based on the soil and column properties, settlements and stresses can be determined by means of analytical solutions or numerical analyses, and this paper compares the results of these two procedures by means of parametric analyses. Two-dimensional finite-element analysis (FEA) was used to simulate the behavior of a geosynthetic-encased stone column in soft clay loaded by an embankment. Parametric studies are then carried out by varying the thickness of the soft soil layer and the tensile stiffness of the geosynthetic encasement for a given set of soft clay and column parameters. The results of FEA have shown that the influence of the geosynthetic encasement on settlement improvement increases in shallow soft soil layers. It was also observed that the dependence of stress concentration on the thickness of the soft soil increases with geosynthetic stiffness. The critical embankment height where ground settlement is negligible was also investigated.

3D numerical analyses of geosynthetic encased stone columns

Stone columns are commonly used as ground improvement elements since they act as reinforcing inclusions. However, due to the lack of sufficient lateral confinement for the columns, this technique is not applicable for the improvement of grounds that consist of very soft soils. In order to provide lateral confinement and increase the load bearing capacity of stone columns installed in very soft clay soils, they are usually encased with suitable geosynthetic materials, forming geosynthetic-encased columns (GECs). In this paper, a 3D numerical approach is used to study the effect of varying the encasement length of different columns of a group of GECs on the overall group behavior. These results are compared with those obtained from a group of fully encased columns, through comparison of the settlements and lateral deformations (bulging) of the columns. The analyses are calibrated through modeling the behavior of GECs used in a ground reclamation project in Hamburg, Germany. Parametric studies are also carried out to investigate the effects of factors such as stiffness of the geosynthetic encasement, column diameter, and modulus of elasticity and friction angle of the column material on the overall behavior of the GEC group. The results indicated that encasing only the outer columns of the stone column group is sufficient in providing an optimal design. It was also shown that increasing the stiffness of the encasement and the column diameter enhance the overall behavior of the GEC group through increasing the overall stiffness of the stone columns and the ratio of the soft soil replaced by the stone columns (i.e. the area replacement ratio), respectively. Moreover, it was observed that the performance of GECs is comparatively less sensitive to the internal friction angle of the column material, and that, in general, the modulus of elasticity of the column material has only a small effect on the group behavior.

Special Section on Geomechanics and Geosynthetics

This special section on geomechanics and geosynthetics began at the end of September 2009 with targeted invitations to selected individuals with recognized expertise in the interdisciplinary fields of geomechanics and geosynthetics. Each submission was peer reviewed in strict accordance with the Journal’s policy. The result is the seven papers that appear in this special section. These seven papers address different uses of geosynthetics with a primary focus on their reinforcement function in geotechnical and transportation applications. These applications include geosynthetic-encased columns, geogrid-soil and geogrid-aggregate interactions in reinforced soil structures and reinforced road bases, embankments over rate-sensitive soils, geosynthetic-reinforced piled embankments, seismic behavior of reinforced soil walls, and load and resistance factor design for internal stability of reinforced soil walls. These papers use a range of modeling approaches in geomechanical analysis, including three-dimensional and two-dimensional finite-element modeling, mechanistic-empirical modeling, discreteelement modeling, and probabilistic methods. The breadth of the geosynthetics applications addressed in this collection and the range of analytical, computational, and validation methods employed by the authors demonstrate their value to improve our understanding of the complex mechanics of geosynthetic-geomaterial systems and to advance analysis and design methods. We hope that the readers of this issue will find this special section to be a valuable collection of papers, especially for those interested in numerical modeling of geosynthetics applications, verification of analytical and probabilistic-based design methods, and researchers developing new constitutive models for soils, aggregates, and geosynthetics. We congratulate and thank all of the authors who contributed to this special section. We would also like to acknowledge the valuable contribution of the anonymous reviewers. Finally, we would like to express our appreciation to Professor Musharraf Zaman, the Editor-in-Chief of the International Journal of Geomechanics, for his support and patience in seeing this special section through to completion.

Theoretical elastoplastic analysis for foundations with geosynthetic-encased columns

As a new technique in ground improvement, geosynthetic-encased columns (GECs) have promising applications in soft soil foundation. By assuming yielding occurs in the columns while the surrounding soil and the geosynthetic remain elastic, an elastoplastic analytical procedure for foundations improved by GECs is proposed. The radial stresses that the geosynthetic provides and the elastoplastic deformations of the foundation resting on a rigid base are derived. A comparison with finite element analysis shows that the proposed method is effective and can provide a reasonable prediction of a GEC’s deformation. Subsequent parametric analysis indicates that higher geosynthetic stiffness leads to better performance of the composite foundation. The optimum length of encasement is related to the load acting on the foundation and the permissible vertical and radial displacements of the column. Moreover, as the dilation angle of the column increases, the settlement decreases, especially under high loading. The influence of the encasement is more significant in soils with smaller elastic modulus.

Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns

Past numerical simulations of geosynthetic encased columns (GECs) using different versions of the quasilinear elastic hyperbolic model for the encased granular material have, in certain cases, yielded unrealistic results. In this paper the cause of such results is investigated by performing three-dimensional finite element analyses of GECs in soft clay, utilizing three common functional forms of the hyperbolic model for the encased granular material. Results indicate that one form of the hyperbolic model can predict an unrealistic lateral response for GECs during application of load to the column. In addition, the inability of hyperbolic models to properly account for soil behavior near failure compromises their ability to realistically capture the behavior of encased granular soil in GECs. Modeling the behavior of soil near failure is essential for properly simulating the behavior of GECs, as soil shear failure is necessary to mobilize the tensile stresses in the encasement and improve the stress–displacement response of the GEC. Although this type of hyperbolic model behavior was demonstrated for the specific case of encased soil in a GEC, the limitations of the hyperbolic model described herein apply equally to other geotechnical problems in which some portion of the soil mass is at or near failure.

Theoretical elastic solutions for foundations improved by geosynthetic-encased columns

ABSTRACT: Based on an axisymmetric elastic model, a theoretical solution is obtained for a foundation improved by geosynthetic-encased columns (GEC). The expressions for stress, displacement, and hoop force of the geosynthetic, as well as stress ratio and improvement factor of the GEC composite foundation, are derived in detail. To validate the current analysis, a comparison with the solution proposed by Balaam and Booker is carried out. It is shown that the solution developed by Balaam and Booker confirms the presented solution for the case of an unreinforced column. Using the proposed method, the radial displacement and hoop force of the geosynthetic, which are affected by its stiffness, area replacement ratio and oedometric modulus ratio, are discussed. The results indicate that the main function of the geosynthetic is to provide sufficient lateral support to the columns. Decreasing the area replacement ratio is advantageous in maximising the confining effect of the geosynthetic. The oedometric modulus ratio has little influence on the confining effect provided by the geosynthetic encasement.

Modeling the Behavior of Geosynthetic Encased Columns: Influence of Granular Soil Constitutive Model

AbstractUsing a high-strength geosynthetic for encasement of granular columns increases the strength of a given column and improves its stress-displacement response. This paper describes the results from a series of three-dimensional finite-element analyses that were performed to simulate the behavior of a single geosynthetic-encased column in soft clay. To examine the sensitivity of the results to the constitutive model that was used to simulate the behavior of the encased granular soil, analyses were performed using models for the encased soil possessing various levels of sophistication. For each model that was studied, comparative analyses were performed to simulate the behavior of a dense and a loose granular soil within the encasement. The results demonstrate the importance of selecting a constitutive model that accurately captures the shear-induced volume-change behavior of the encased granular soil. Additional findings provide guidance for other researchers seeking to model the behavior of geosynth...

Behaviour of geosynthetic-encased granular columns in porous collapsible soil

ABSTRACT: The construction of embankments on soft saturated soils can be a challenge due to the low shear strength and high compressibility of these soils. The same may be true for constructions on porous collapsible soils, because the collapse of the soil structure can lead to large foundation soil deformations leading to damage to the embankment, particularly in the case of bridge abutments. Conventional piles and granular columns have been used to stabilize such embankments when there are limits for embankment settlements. More recently, geosynthetic-encased granular columns have been increasingly used as an alternative solution. This study examined the performance of geosynthetic-encased columns for the stabilization of embankments on porous collapsible structured unsaturated soil. Field load tests were performed on conventional sand and gravel columns and on geotextile-encased sand columns and on geogrid-encased gravel columns. During testing, the collapse of the surrounding soil was induced by water injection through the granular column top to investigate the influence of the casing on the column performance. The results obtained show that significant increases on pile load capacity can be obtained depending on the materials used and construction conditions. The effects of foundation collapse can also be reduced with the use of a geosynthetic casing in the granular column.