Geosynthetic Encased Columns Research Articles

Evaluation of geosynthetic encased columns in Zambian heterogenous soils

The development of Zambia has brought increased large-scale infrastructure construction that has necessitated the need for improved foundation techniques that are both economical and adequate in capacity. Clay and soft soils with low bearing capacities and high compressibility could render structural foundations to perform poorly and shorten the design life of the bridges and structures. This study used a bridge case study in Northern Province, Zambia to investigate the use of geosynthetic encased columns (GECs) to support the bridge embankments to reduce differential settlements. End bearing fully encased columns were compared to floating columns of varying lengths by numerical modelling in PLAXIS 3D. The Hardening Soil and Mohr–Coulomb soil models were used for the column surrounding soil and the GECs in the finite element analysis. The results showed that the end bearing columns had the least differential settlements at the soil surface, whilst the reduction in floating column length increased the punching settlements. Moreover, the shear stress along the interface of the GECs and surrounding soil varied from 20 kN/m2 to 142 kN/m2, where the end bearing GEC had the least shear stress.

Geosynthetic-encased columns (GEC): A proven case study over 30 years

The construction of embankments on very soft soils is a challenge due to the low shear strength, low permeability, high compressibility, and high-water content of the subsoil. The surcharge load imposed by the embankment can result not only in a local or total loss of stability but also in unacceptable settlements and horizontal deformations or thrusts, which could endanger nearby structures. To overcome these problems, different construction methods are applicable. One particularly interesting and still innovative foundation system is the use of geotextile encased granular columns, which can be considered “State-of-the-Art” in Europe. It was developed and introduced in 1994, and since then many major projects have been realised using geotextile encased columns (GEC). The overall loads and stress concentrations above the column heads induce outwardly directed radial horizontal stresses in the columns. The particularity of the GEC system is that these stresses are counteracted not only by the inwardly acting pressure of the soft soil but also –most importantly – by the radial resistance of the tensile stiff geotextile casing. This paper briefly describes the GEC system and includes the description of a few notable projects using GEC as a foundation system: land reclamation at the Airbus site in Hamburg, Germany, and the Second Niger Bridge in Africa. The geotechnical challenges, design basis, and what can be considered the most important findings of each of these projects are also discussed in this paper.

Determination of Mechanical Properties of Soils Based on CPTU Data

Abstract The study aimed to interpret the results of cone penetration tests with pore pressure measurement (CPTU). Tests were performed on a section of the designed road about 200 m long. The subsoil under consideration was characterized by organic soils deposited to a depth of approximately 8 m. Due to the problematic soil and water conditions, improvement with geosynthetic encased columns (GEC) was performed. According to the Recommendations for Design and Analysis of Earth Structures using Geosynthetic Reinforcements – EBGEO, the constrained modulus Eoed and effective friction angle φ’ of soil below the GEC column base should be higher than 5 MPa (Eoed > 5 MPa) and 30° (φ’ > 30°), respectively. In this study, the Eoed and φ’ parameters were determined based on CPTU tests conducted in 12 locations. Analysis of the CPTU data showed that the constrained modulus and effective friction angle of the soil deposited below the GEC column base did not meet EBGEO requirements in most locations. It was also concluded that the minimum value of the constrained modulus required by EBGEO is too low compared with the requested value of the effective friction angle.

Ground improvement techniques applied to very soft clays: state of knowledge and recent advances

Soft ground improvement techniques have evolved substantially in Brazil in recent years. However, their application in soft and very soft clays requires a good understanding of the fundamentals of ground improvement techniques suited to the problem as well as the actual field behavior when implemented on a real scale. This paper describes some of the most widely used ground improvement techniques in the context of very soft clays in Brazil. The techniques described in the paper use prefabricated vertical drains (PVD) such as vacuum preloading; or combine PVD and rigid inclusion, such as CPR grouting; or are purely column-like elements such as piled embankments (including those executed with the deep mixing technique, DSM); or combine column-like elements with the drainage function, such as stone columns and geosynthetic encased columns; or use cementitious binders such as shallow soil mixing. The paper reference condition is a soft clay foundation in which no strengthening is implemented, such as, an embankment with basal reinforcement or soft clay with vertical prefabricated drains, or the use of vacuum preloading to speed up the consolidation rate. The applications of the ground improvement techniques are illustrated by case histories, numerical analyses, or physical models. Different types of measurements are used to evaluate the performance of each technique, including settlements, horizontal displacements, excess pore pressures, embankment applied stresses, stress concentration factors, and clay strength following the ground treatment. The settlement improvement factor β, the ratio between the settlements for untreated and treated conditions, is shown to be a suitable parameter to assess the degree of improvement imposed in the soft foundation by ranking the various methods in increased order of strengthening effect.

Geosynthetic encased column-supported embankment: behavior with and without basal geogrid

Centrifuge model tests were performed to investigate the beneficial effect of the basal geogrid in improving the performance of geosynthetic encased column (GEC)-supported embankments. The digital image correlation (DIC) technique was adopted to understand the deformation behavior of GEC-supported embankment models. The mobilized tensile strain at various locations on the model basal geogrid was quantified using strain gauge-based instrumentation. The results from the centrifuge test indicate that vertical and lateral displacement of the GECs and surrounding soil has been significantly reduced with the inclusion of basal geogrid in the embankment. Moreover, the differential settlement between the GECs and surrounding soil due to embankment loading were significantly reduced with the inclusion of basal geogrid, thereby the distress in the embankment resting on soft foundation can be avoided. The basal geogrid in the embankment has enhanced the stability of the embankment and improved the load transfer between the GECs and surrounding soil. The maximum mobilized tensile strain in the basal geogrid occurred near the shoulder of the embankment, which indicates the resistance offered by the basal geogrid in preventing the differential settlement between the GECs and surrounding soil.

Preliminary experimental investigation on the inundating-induced collapse in collapsible soils improved by encased sand column

ABSTRACT The present research aims to explore experimentally the performance and modes of failure of the wetting-induced collapse of fully penetrating reinforced and unreinforced models of a short single sand column in a collapsible soil subjected to partial inundation through a series of model tests under high constant total stress of more than 200 kPa. A specific loading frame and a test container were designed to apply vertical pressure and measure vertical movements of the columns . The experimental test results demonstrated that providing only sand columns in collapsing soil have no significant contribution in reducing the large volume change and sudden collapse when wet. In contrast, geosynthetic-encased columns prevented the occurrence of wetting-induced collapse and highly reduced the sudden vertical movements to a gradual process under relatively high applied pressure. Additionally, a slight change in the dry density of the soil had a dramatic influence on the behaviour of the wetting-induced collapse and modes of failure. In this study, the experimental test results and theoretical results from different analytical approaches in the literature have confirmed that it is practically possible to apply high total stress of more than 300 kPa on a single footing on collapsible soils subjected to wetting condition.

Deformation behaviour of granular column reinforced by geosynthetic encasement

In Vietnam, the overpopulation and strong economic development require the synchronous development of infrastructure such as roads, urban areas, industrial parks, export processing zones, etc. With such requirements, the development of land fund for infrastructure construction is an indispensable need. Meanwhile, the appropriate land fund is very limited. Therefore, the land fund must be developed for areas with little value for agriculture, such as swamps, estuaries, and coastal areas, etc. These areas often have weak geological conditions; hence, to meet the requirements of infrastructure construction on the soft ground, it is necessary to carry out soil improvement to ensure load bearing capacity, total settlement, and consolidation settlement but still ensuring economic effectiveness. Beside several conventional methods widely used for soft soil improvement in order to increase bearing capacity and accelerate consolidation settlement of the ground, geosynthetic reinforced granular column is one of the new methods that has been applied to improving soft ground in designing practice in the recent years due to the many advantages of this method compared with other methods. In this paper, based on the unit cell model, the authors research on deformation behavior of granular column reinforced by geosynthetic encasement through the analytical analysis by varying external loadings corresponding to column diameter, stiffness of geosynthetic encasement. The settlements of a single geosynthetic encased granular column and load bearing capacity of the composite foundation are calculated on geological conditions of Ash Pond Area of Song Hau 1 Thermal Power Plant located in Hau Giang Province. The relationship between settlement and load bearing capacity with external loadings for different column diameters and geosynthetic stiffnesses are shown schematically. Other considerations related to factor of safety are also presented. The future researches are also proposed.

Analytical and numerical evaluation of the effect of stiffening with the rigidity of bearing stratum on the reduction of settlement for a partially stiffened granular pile

ABSTRACT A traditional approach of using granular pile (GP) is an economical and effective technique to achieve the objectives of ground improvement. If GP is partially stiffened from the top of GP, their overall performance gets improved. Stiffening of GP can be achieved by replacing partially the upper portion of GP with a material having superior mechanical properties, e.g. by geosynthetic encased columns, SDCM (stiffened deep cement mixing), etc. Analyses of a single partially stiffened (SPS) floating and end-bearing GP is presented in this paper in terms of settlement influence factor (SIF) for the top of GP, settlement reduction factor (SRF), percentage load shared by the base (PLSB) of GP. Analytically the variation of normalised shear stress (NSS) distribution along the length of the partially stiffened GP is presented in this paper. SIF for the top of GP was found to decrease, while the PLSB of GP was found to increase, with an increase in the stiffening parameters viz. stiffening factor (SF) and the percentage length of stiffening (PLS) from top of GP for a partially stiffened GP.

Bearing capacity of floating geosynthetic encased columns (GEC) determined on the basis of CPTU penetration tests

Abstract Floating geosynthetic encased columns (GEC) are an increasingly popular method of strengthening weak subsoil. Design of floating columns is a difficult and not fully recognized issue. This paper treats the floating GEC column as a special kind of “pile” and its bearing capacity is calculated using five selected methods for calculating the bearing capacity of piles based on CPTU penetration tests. The calculations were done on the basis of insitu tests carried out on one of the sections of the Bargłów Kościelny bypass. The paper contains a comparison of the bearing capacities of floating GEC columns calculated with different methods based on CPTU penetration tests.

A laminar box apparatus for 1g testing of granular columns embedded in soft clay

Ordinary stone columns and geosynthetic-encased columns are widely used as a method of soil improvement for soft clay deposits. The behaviour of encased and unencased columns under the action of dynamic loads are not thoroughly understood. In order to shed light into the dynamic behaviour of granular columns and column-enhanced soft soil deposits, a novel large-scale laminar box assembly is designed together with provisions for instrumentation, consolidation of clay slurry and casing pipe pushing units for column installation. The laminar box is comprised of 16 individually supported laminates. The laminar box has inner areal footprint dimensions of 900 × 900 mm and a height of 1932 mm. The laminar box is utilised to study the behaviour of granular columns of varying reinforcement stiffnesses under dynamic loads. In order to make a thorough comparison of reinforcement stiffnesses on column behaviour, columns with different reinforcement arrangements are installed together in the same clay bed, and scaled earthquake accelerograms and sinusoidal base excitations are applied. Column response is studied through the reinforcement strains occurring on the encasement and the amount of vertical pressure sustained on the column during the dynamic load cases.

Large-scale load capacity tests on a geosynthetic encased column

Stone columns have been used to minimize the settlement of embankments on soft soils but their use in very soft soils can become challenging, partly because of the low confinement provided by the surrounding soil. Geosynthetic encased columns (GECs) have been successfully used to enhance to reduce settlements of embankments on soft soils. This paper describes an investigation on the performance of encased columns constructed on a very soft soil using different types of encasement (three woven geotextiles with different values of tensile stiffness) and different column fill materials (sand, gravel and recycled construction and demolition waste, RCDW). The results of load capacity tests conducted on large-scale models constructed to simulate the different types of GECs indicate that the displacement method adopted during column installation can lead to an enhanced shear strength in the smear zone that develops within the very soft soil. In addition, breakage of the column fill material was found to affect the load-settlement response of gravel and RCDW columns. Furthermore, the excess pore water pressure generated in the surrounding soil during installation, was found to remain limited to radial distances smaller than three times the GEC diameter.

Book review

Geosynthetic Encased Columns for Soft Soil Improvement

Field-measured Poisson's ratio of geotextile-encased granular column

The performance of a geosynthetic-encased column (GEC) supporting an embankment is often analysed by measurement of the total deformations under the embankment, the hoop tensile force in the geosynthetic encasement and the vertical stresses shared between the GEC and the surrounding soft soil. Several analytical solutions are available in the literature for calculation of the stresses and deformations of a GEC under an embankment. Generally, the existing formulations use a constant value of Poisson's ratio for the granular column material in their computation. This study describes the variation of Poisson's ratio of the composite material GEC with computations based on the real-time measurement of the radial and vertical deformations of a GEC. Accordingly, specific equipment was developed to allow the determination of changes in the GEC diameter. The vertical deformation was also recorded using a settlement sensor placed on the top of the GEC. The results showed a significant variation of the field Poisson's ratio due to the increase in vertical stress caused by the embankment construction and also due to the gain in the confining stress of the surrounding soft clay due to its higher degree of consolidation.

On the shear failure mode of granular column embedded unit cells subjected to static and cyclic shear loads

Since the initial conception of geosynthetic encased columns (GECs), exhaustion of column capacity due to vertical loads in bulging and punching failure modes were readily recognized. This lead to a vast majority of the available research on GECs to be about the behavior of columns under the action of vertical loads. Recently, two other likely and perhaps more dominant failure modes for granular columns namely, shear and bending failure modes, were identified. The purpose of this paper is to study the behavior of unit cells containing ordinary stone columns (OSCs) and GECs under static and cyclic lateral loads where shear failure of the column is imminent. 1-g physical tests are conducted with a novel apparatus, designated as Unit Cell Shear Device (UCSD), to model the behavior of the unit cells located close to the toe of an embankment where OSCs and GECs experience significant lateral loading. Overall failure envelope and strength parameters for GECs with varying reinforcement stiffnesses are quantified under static and cyclic lateral loading conditions. The distribution and magnitude of reinforcement strains in horizontal (hoop) and vertical direction of the columns are also considered.

Numerical modelling and validation of geosynthetic encased columns in soft soils with installation effect

If the bearing capacity of the soil is not sufficient an improvement method has to be considered. In case of soft and cohesive soils the vibroreplacement technique can be used. This paper describes the numerical simulation of a group of encased granular columns under an embankment based on a real life project situated to the north of Hamburg, Germany. The soft soil creep model and the hardening soil model were used to model the behaviour of the soft clay and granular material respectively. The material parameters were determined based on laboratory tests conducted on test samples from the field. The installation effect of columns in numerically modelled based on the cavity expansion method in a 2D axis symmetric model. The results of the installation effect in terms of stress state changes in the soft soil after complete consolidation are then imported to the 3D model involving group of columns. The results of the numerical simulations are validated against field measurement data in form of vertical settlement of the ground at various locations with respect to time and horizontal deformations in the encased columns with depth.

Shaking table tests on geosynthetic encased columns in soft clay

This paper presents the results of an experimental research on the behavior of geosynthetic encased stone columns and ordinary stone columns embedded in soft clay under dynamic base shaking. For this purpose, a novel laminar box is designed and developed to run a total of eight sets of 1-G shaking table tests on four different model soil profiles: Soft clay bed, ordinary stone column installed clay bed, and clay beds with geosynthetic encased columns with two different reinforcement stiffnesses. The geosynthetic encased columns are heavily instrumented with strain rosettes to quantify the reinforcement strains developing under the action of dynamic loads. The responses of the columns are studied through the deformation modes of the encased columns and the magnitude and distribution of reinforcement strains under dynamic loading. The response of the granular inclusion enhanced soft subsoil and embankment soil and the identification of the dynamic soil properties of the entire soil body are also discussed in this article. Finally, to determine the effect of dynamic loading on the vertical load carrying capacity, stress-controlled column load tests are undertaken both on seismically loaded and undisturbed columns.

Lateral pressure relief at a bridge by Geosynthetic Encased Columns (GEC): latest experience

This article is not available for publication.

A material which started a new era in geotechnical engineering: geosynthetics

In this paper, the opportunities that can be provided by using geosynthetics are introduced. Many examples mentioned in the text show explicitly that geosynthetics can provide effective engineering solutions for a variety of projects ranging from extremely important to very simple projects. In order to illustrate the benefits of the geosynthetics, two applications were chosen as example applications. These were chosen from the applications where geosynthetic products are used most frequently, namely: reinforced walls and barrier systems. The most important research papers were referenced in these two subjects and examples from projects in Turkey were given. New geosynthetic products are brought to the market every day. To highlight this aspect, a relatively new, however, well-established technique was chosen: geosynthetic encased columns. Basic concepts of this technology is given. Many similar developments in the geosynthetic industry necessitates that engineers are up to date with the new developments in this field. It is well known that besides providing very efficient engineering solutions, use of geosynthetics also allows to reduce the construction time and cost. Recently, survivability issues are becoming as important as the other concerns. Therefore, at the very end of the paper examples are given of how the use of geosynthetics can reduce the carbon dioxide footprint.

Numerical Analyses of Soft Soil Reinforced with GEC

Construction of embankments on soft soil may be a challenge, owing to its low shear strength and high compressibility. Among the different methods of ground improvement, the geosynthetic encased columns (GEC) are often used to support embankments over soft soil. In order to evaluate the mechanical response of the geosynthetic encased columns, based on the soil and column properties, settlements and stresses can be determined by means of analytical solutions or numerical analyses.The paper compares the results of these two approach by means of parametric analyses. The German regulation EBGEO proposed an analytical model for predicting the pressure-settlement response of an axisymmetric unit cell of clay with a GEC. The numerical simulation of the behavior of a GEC in soft clay loaded by an embankment has been carried out by means of a two-dimensional finite-element commercial code.

Discussion of “Column Supported Embankments with Geosynthetic Encased Columns: Validity of the Unit Cell Concept” [Geotechnical and Geological Engineering 33 (2015) 425–442

The authors present numerical analyses of an embankment underlain by geosynthetic reinforcement and supported by geosynthetic encased columns (GRCSE). In the last part of the paper, the transverse and longitudinal tensile forces in the geosynthetic reinforcement, which underlays the embankment, are compared using a 3-d strip model and the Unit-Cell concept, both in a 3-d model and in a 2-d axisymmetric model. The results of both Unit-Cell models are very similar. The authors show that the Unit-Cell models are not able to capture the transverse force (Tx), which seems obvious, nor the longitudinal force (Ty), which may not be so obvious. The discusser would like to explain that in the authors’ case the longitudinal force (Ty) is not properly captured by the Unit-Cell models because of the ‘‘socalled’’ Poisson effect. A measure of this effect is the Poisson’s ratio (m = -ey/ex) (Fig. 1). In the 3-d strip model, Tx is notably higher than in the Unit-Cell models, as expected. The authors use a Poisson’s ratio of m = 0.3 for the geosynthetic reinforcement, and then, the higher value of Tx causes an additional tensile force in the perpendicular direction (Ty) because of the Poisson effect. Assuming plane strain conditions in the longitudinal direction (ey = 0), ey 1⁄4 0 1⁄4 1 E ry mrx 1⁄4 1 E Ty t m Tx t ð1Þ