Geosynthetic-reinforced Column-supported Embankments Research Articles

Progressive failure mechanisms of geosynthetic-reinforced column-supported embankments over soft soil: Numerical analyses considering the cracks-induced softening

Cement-based columns in combination with geosynthetic reinforcement is a well-established soft ground improvement technique to enhance embankment stability. This paper aims to present a finite-element (FE) study based on a case history of a geosynthetic-reinforced column-supported (GRCS) embankment over soft soil. In this study, the columns are simulated with an advanced Concrete model to simulate the development of possible cracking and induced strain-softening. Numerical results are compared against published centrifuge tests, giving confidence to the established FE model with the Concrete model. New insights into the progressive failure mechanisms of GRCS embankments over soft soil are then discussed by examining the stress paths, internal forces, and cracks, as well as the plastic failure zones of columns. In addition, the role of columns and geosynthetics on the progressive failure mechanisms (failure loads and sequences) is also examined by an extensive parametric study. The results suggest that provided the optimization of compressive and tensile forces in the columns combined with the tensile stiffness of the geosynthetics is put in place, more columns can be mobilized to resist global sliding failure and to improve the bearing capacity of GRCS embankments.

A spreadsheet based stability calculation for geosynthetic reinforced column-supported embankments

Design procedures for Geosynthetic Reinforced Column-Supported (GRCS) embankments should consider both internal stability arising from failure of single columns and external stability arising from failure of the embankment as a whole unit. A number of possible failure mechanisms for external stability due to slip circle failure and sliding failure of group of columns as a single unit are discussed in the literature. However, internal stability issues due to failure of individual columns have not been received much attention. Therefore, an in-depth understanding of individual column failure and subsequent progressive failure of embankments is an important topic, which needs attention when investigating the stability of GRCS embankments. In this paper, two-dimensional finite element modelling incorporating strain-softening behaviour of cement mixed soils has been used to identify the individual column failure of GRCS embankments at the Ultimate Limit State. Results show that the bending failure of individual columns progressively leads to develop a failure plane within the column improved ground causing internal stability issues. For the identified failure mode, an analytical equation is proposed to calculate the factor of safety against shear failure due to progressive bending failure of columns based on the Coulomb’s wedge theory. Since the proposed approach can be conveniently implemented in an EXCEL spreadsheet program, it is useful for practicing geotechnical engineers.

Field study and numerical modelling for a road embankment built on soft soil improved with concrete injected columns and geosynthetics reinforced platform

Generally numerical modelling can provide an accurate and cost-effective approach to understand the behaviour of geosynthetic-reinforced column-supported embankment. When the problem geometry cannot be simplified to the two-dimensional plane-strain or axisymmetric, a full three-dimensional solution is required to obtain sensible results. This study presents a modelling of the geosynthetic-reinforced composite ground supporting a road embankment. Response of soft soil is captured by adopting Modified Cam-Clay model. In addition, Hoek-Brown constitutive model is considered to simulate non-linear stress-dependent yield criterion for Concrete Injected Columns (CIC) that describes shear failure and tensile failure by a continuous function. To assess whether the proposed numerical model can capture real behaviour of composite ground, field monitoring data of deep soft clay deposit improved by CIC from Gerringong Upgrade is used to validate the model. The settlement and lateral displacements of ground, stress transferred to column, and pore water pressure results for the embankment during and after the construction, measured using the field instrumentations including settlement plates, inclinometers, earth pressure cells on CIC, and pore pressure transducers, are compared with numerical predictions. In addition, the numerical results provide insights to investigate load transfer mechanism in the composite ground, capturing response of soil – column - embankment system.

Geosynthetic reinforced column supported embankments and the role of ground improvement installation effects

For geosynthetic reinforced column supported embankments (GRCSE) supporting a high embankment, lateral forces associated with lateral sliding and embankment stability often govern the acceptability of a given design under serviceability conditions. Frequently, the complex soil–structure–geosynthetic interaction, the size, and the three-dimensional nature of a GRCSE necessitate the use of numerical analysis to assess embankment performance relative to serviceability criteria. However, traditional finite element method techniques used to model serviceability behaviour are limited in their ability to model the geotechnical mechanisms associated with column installation, equilibration, and group installation effects. These installation effects are examined herein based on a GRCSE field case study located in Melbourne, Australia, that has been extensively instrumented. The role that these installation effects have on the performance of the GRCSE is highlighted and the behaviour of the columns supporting the embankment is emphasized. It is shown that cracking of the unreinforced columns supporting the embankment is likely inevitable and that the reduction of lateral resistance provided by the columns should be accounted for in design. The suitability of various numerical approaches currently used in design to model the columns supporting the GRCSE, and the embankment itself, are discussed and recommendations are made.

Behaviour of geosynthetic reinforced column supported embankments

PurposeThis paper aims to investigate the behaviour of geosynthetic reinforced deep cement mixed (DCM) column-supported embankments constructed over soft soils.Design/methodology/approachCoupled consolidation analyses based on the finite element method are carried out assuming that the soil and DCM columns are fully saturated porous mediums. In the first part of the paper, a case study of an embankment constructed over a very soft soil deposit in Finland is presented. Two- and three-dimensional finite element models for the case study are developed including isolated and attached DCM columns beneath the embankment to capture the arching mechanism between DCM columns. The model simulations were carried out considering the actual staged construction procedure adopted in the field. Finite element predictions show good agreement with field data and confirm that the load transfer is mainly between attached columns beneath the embankment. Next, the significance of geosynthetic reinforcement on the load transfer mechanism is investigated. Finally, the influence of permeability of columns and soft soil on the performance of geosynthetic reinforcement column-supported embankments is studied.FindingsResults demonstrate that the excess pore pressure dissipation rate is fast in DCM column-improved ground compared to the same case without any columns, although the same permeability is assigned to both DCM columns and surrounding soft soil. When DCM column permeability exceeds soil permeability, excess pore pressure dissipation rate shows a remarkable increase compared to that observed when the DCM column permeability is less than or equal to the permeability of surrounding soft soil. [ ]Originality/valueThis paper investigates the contribution of permeability and geosynthetic layer on the vertical load transfer mechanism of the embankment and modelling issues related to application of the embankment load and the properties of the cement-improved columns.

Load-transfer platform behaviour in embankments supported on semi-rigid columns: implications of the ground reaction curve

Post-construction data from an instrumented geosynthetic reinforced column supported embankment (GRCSE) on drilled displacement columns in Melbourne, Australia, show the time-dependent development of arching over the 2 year monitoring period and a strong relationship between the development of arching stresses and subsoil settlement. A ground reaction curve is adopted to describe the development of arching stresses and good agreement is found for the period observed thus far. Predictions of arching stresses and load-transfer platform behaviour are presented for the remaining design life. Four phases of arching stress development (initial, maximum, load-recovery, and creep strain phases) are shown to describe the time-dependent, and subsoil-dependent, development of arching stresses that can be expected to occur in many field embankments. Of the four phases, the load-recovery phase is the most important with respect to load-transfer platform design, as it predicts the breakdown of arching stresses in the long term due to increasing subsoil settlement. This has important implications in assessing the appropriate design stress for the geosynthetic reinforcement layers, but also the deformation of the load-transfer platform in the long term.

Serviceability design for geosynthetic reinforced column supported embankments

In recent years, geosynthetic reinforced column supported embankments (GRCSEs) have become an increasingly popular design solution for road and rail infrastructure constructed over soft soil sites. However, the serviceability behaviour and deformation that often govern the suitability of their design is not well understood. This is due, in part, to the difficulties in describing the arching stress development in the load transfer platform (LTP). This paper highlights the need for coupled arching stress-deformation models to describe accurately serviceability behaviour. This approach contrasts the widely adopted two-step design approach, which uses limit-equilibrium models that de-couple the arching stress-deformation relationship to describe ultimate limit state behaviour. Using an analytical example, an arching stress/deformation model and an empirical relationship (developed by others) relating base LTP settlement to surface settlement, the relationship between serviceability behaviour and soft soil parameters is highlighted and the conditions leading to progressive collapse in GRCSEs are described. The approach presented provides a means to predict serviceability behaviour, and at the same time, raises questions about the long-term performance and the manner in which acceptable performance has been achieved in the short-term in several field case studies. In particular, those constructed at, or near, a minimum embankment height.

Modelling of geosynthetic-reinforced column-supported embankments using 2D full-width model and modified unit cell approach

Soil-cement deep mixing (DM) columns combined with geosynthetic basal reinforcement are an accepted technique in geotechnical engineering to construct road and railway embankments over soft foundations. Both full-width and unit cell models have been used to numerically simulate the performance of geosynthetic-reinforced and column-supported (GRCS) embankments. However, the typical unit cell model with horizontally fixed side boundaries cannot simulate the lateral spreading of the embankment fill and foundation soil. As a result, the calculated reinforcement tensile loads using typical unit cell models are much less than those from matching full-width models. The paper first examines GRCS embankments using a full-width model with small- and large-strain modes in FLAC and then compares the calculated results from the full-width model with those using a typical unit cell model, a recently proposed modified unit cell model, and a closed-form solution. The paper also examines the influence of the soft foundation soil modulus, reinforcement tensile stiffness, and DM column modulus on the reinforcement tensile loads. Numerical analyses show that the reinforcement tensile loads from the modified unit cell model are in good agreement with those from the full-width model for zones under the embankment crest for all cases and conditions examined in the paper. Both the full-width model and modified unit cell model perform better than the typical unit cell model for the prediction of the reinforcement tensile load when compared to the closed-form solution. However, while the modified unit cell developed by the writers is shown to be more accurate than the typical unit cell when predictions are compared to results using full-width numerical simulations, the benefit of using this approach to reduce computation times may be limited in practice.

Modified unit cell approach for modelling geosynthetic-reinforced column-supported embankments

Geosynthetic-reinforced and column-supported (GRCS) embankments have proven to be an effective construction technique for fills on soft foundations. The paper introduces a modified unit cell approach to model GRCS embankments supported by deep mixed column walls. The modified unit cells include linear elastic springs at one or both vertical boundaries to simulate lateral displacements of the embankment fill and foundation soil. The finite difference program FLAC is used to compare numerical outcomes using the modified unit cells with those using the typical unit cell arrangement with lateral rigid side boundaries. Numerical results demonstrate good agreement between simulations using small-strain and large-strain modes in some cases and large differences in other cases. Lateral displacements of the embankment fill and foundation soil using the modified unit cells are shown to have large influence on reinforcement loads. Finally the paper demonstrates that calculated reinforcement loads are sensitive to choice of small-strain or large-strain mode when using program FLAC.

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.

Two-dimensional parametric study of geosynthetic-reinforced column-supported embankments by coupled hydraulic and mechanical modeling

Geosynthetic-reinforced column-supported (GRCS) embankments have been increasingly constructed over soft soil. Current design methods were developed based on different assumptions and simplifications, which lead to great variations among them. Numerical studies have been conducted to improve the understanding of the GRCS embankment system. Most of the past numerical studies were limited to mechanical modeling only (i.e., not coupled with hydraulic modeling), which can be used to evaluate long-term or short-term behavior by using appropriate mechanical properties. However, the generation and dissipation of excess pore water pressure under GRCS embankments during and after construction make their behavior time-dependent. A two-dimensional (2D) parametric study based on coupled hydraulic and mechanical modeling was conducted to study the time-dependent behavior of GRCS embankments under various conditions. Special attention was paid to the influence of several key factors on the performance of the embankments indicated by maximum and differential settlements, tension in the geosynthetic, and degree of consolidation. The investigated key factors include modulus and permeability of the soft soil, modulus and spacing of columns, tensile stiffness of the geosynthetic, and average construction rate. The influence of these factors are compared and rated in terms of the degree of importance in this study.

3D coupled mechanical and hydraulic modeling of a geosynthetic-reinforced deep mixed column-supported embankment

Numerical studies were conducted to improve the understanding of the behavior of geosynthetic-reinforced column-supported embankments. Due to the complexity of the problem, so far, consolidation process and three-dimensional patterns of columns have not been well simulated in most published numerical studies. As a result, the time-dependant behavior and the serviceability of this system have not been well evaluated. In this study, a three-dimensional coupled mechanical and hydraulic modeling was conducted using FLAC3D to consider consolidation and three-dimensional arrangement of columns. This study was based on a well-documented bridge approach embankment reinforced by a layer of geotextile and supported by deep mixed (DM) columns. The foundation soils including soft clay and silt, the embankment fill, and the deep mixed columns were modeled as linearly elastic-perfectly plastic materials with Mohr–Coulomb failure criteria. The geotextile reinforcement was simulated by geogrid elements incorporated in the FLAC3D software, which can sustain in-plane tensile force only. The staged construction was simulated by building the embankment in lifts. The duration of each lift was the same as the actual construction time plus the lapse time between two consecutive stages. The development of settlement and tension in the geotextile with time is compared with the long-term monitoring data and yields good agreement. The generation and dissipation of excess pore water pressure during and after construction are presented and discussed.

Coupled Mechanical and Hydraulic Modeling of Geosynthetic-Reinforced Column-Supported Embankments

Geosynthetic-reinforced column-supported (GRCS) embankments have increasingly been used in the recent years for accelerated construction. Numerical analyses have been conducted to improve understanding and knowledge of this complicated embankment system. However, most studies so far have been focused on its short-term or long-term behavior by assuming an undrained or drained condition, which does not consider water flow in saturated soft soil (i.e., consolidation). As a result, very limited attention has been paid to a settlement-time relationship especially postconstruction settlement, which is critical to performance of pavements on embankments or connection between approach embankments and bridge abutments. To investigate the time-dependent behavior, coupled two-dimensional mechanical and hydraulic numerical modeling was conducted in this study to analyze a well-instrumented geotextile-reinforced deep mixed column-supported embankment in Hertsby, Finland. In the mechanical modeling, soils and DM columns were modeled as elastic-plastic materials and a geotextile layer was modeled using cable elements. In the hydraulic modeling, water flow was modeled to simulate generation and dissipation of excess pore water pressures during and after the construction of the embankment. The numerical results with or without modeling water flow were compared with the field data. In addition, parametric studies were conducted to further examine the effects of geosynthetic stiffness, column modulus, and average staged construction rate on the postconstruction settlement and the tension in the geosynthetic reinforcement.