Three-dimensional behavior of geosynthetic tubes

Geosynthetic tubes filled with pressurized slurry are used for various purposes (e.g., as dikes or breakwaters). The three-dimensional behavior of such structures is investigated. The tubes are assumed to be comprised of two rectangular sheets connected at their edges, and to rest on a tensionless elastic foundation. As the slurry is pumped in, the top surface rises and the outer portion of the bottom surface lifts off the foundation. For tubes with aspect ratios of 2:1 and 5:1, the deflected shapes, mid-surface stresses, and contact regions with the foundation are determined with the use of the finite element method and thin shell elements. Wrinkling occurs close to the long edges of the tubes.     

3-Dimensional numerical modeling of geosynthetic-encased granular columns

In this paper, series of three-dimensional (3-d) numerical modeling of geosynthetic-encased granular columns were performed both in model and prototype scale using FLAC^3D software to understand the lateral load carrying capacity of ordinary and geosynthetic encased granular columns (OGC and EGC). In the first part of the study, numerical modeling of direct shear tests were carried out. The soil in the direct shear box was reinforced with two different diameters of granular columns (50 mm and 100 mm) and three different patterns of arrangement (single, triangular and square) to study the effect of group confinement. The numerical simulations were carried out at four different confining pressures namely 15, 30, 45 and 75 kPa. From the numerical simulations it was observed that higher shear stresses are mobilized inside the granular column due to geosynthetic encasement and the magnitude of shear stress increases with increase in the normal pressure. It was found that the tensile forces in the geosynthetic encasement were mobilized both in circumferential and vertical directions, which helps in mobilizing additional confinement in the granular column. In the second part, the influence of the geosynthetic encasement of granular column treated soft ground was demonstrated through 3-dimensional slope stability analyses.     

Experimental evaluation of wicking geotextile-stabilized aggregate bases over subgrade under rainfall simulation and cyclic loading

Wicking geotextile has been increasingly utilized in field projects to mitigate water-related roadway problems. The previous studies showed that the wicking geotextile could provide mechanical stabilization, serve as capillary barrier, and enhance lateral drainage. The wicking geotextile differentiates itself from non-wicking geotextiles by providing capillary or wicking drainage in unsaturated conditions, whereas non-wicking geotextiles only provide gravitational drainage under saturated or near-saturated conditions. Although the previous studies have demonstrated the benefits of soil water content reduction by the wicking drainage, it is not well understood how the wicking geotextile stabilization improves overall performance of aggregate bases over subgrade under traffic or cyclic loading. This paper presents an experimental study where large-scale cyclic plate loading tests were conducted under different conditions: (1) non-stabilized base, (2) non-wicking geotextile-stabilized base, and (3) wicking geotextile-stabilized base, over soft and moderate subgrades. Rainfall simulation was carried out for each test section. After each rainfall simulation, a drainage period was designed to allow water to drain from the section. The amounts of water applied and exiting from the test section were recorded and are compared. Cyclic loading was applied after each drainage period. The test results show that the combined hydraulic and mechanical stabilization effect by the wicking geotextile reduced the permanent deformation of the aggregate base over the subgrade as compared with the non-stabilized and non-wicking geotextile-stabilized sections.     

Numerical analysis of a geosynthetic-reinforced piled load transfer platform – Validation on centrifuge test

Soft soil improvement techniques using a network of rigid inclusions and geosynthetic reinforcement are investigated to improve our understanding of load transfer mechanisms towards piles. The physical modelling of the system consists in simulating fictional soft soil settlement through downward displacement of a perforated tray above a network of rigid piles placed in the centrifuge swinging basket. Tests are used to validate the results of the numerical study.Elasto-plastic and hypoplastic constitutive models have been used to predict the behaviour of the granular mattress, which simulates a Load Platform Transfer (LPT). A two-dimensional, axisymmetrical model has been adopted, which fulfils the validation on the experimental test and the time needed for calculation.The results of the parametric studies show that load transfer increases with mattress thickness and closer pile spacing. Geosynthetic deflection is reduced when load transfer is high.     

Hydraulic conductivity of bentonite-polymer geosynthetic clay liners to coal combustion product leachates

Hydraulic conductivity of seven geosynthetic clay liners (GCLs) to synthetic coal combustion product (CCP) leachates were evaluated in this study. The leachates are chemically representative of typical and worst scenarios observed in CCP landfills. The ionic strength (I) of the synthetic CCP leachates ranged from 50 mM to 4676 mM (TCCP-50, LRMD-96, TFGDS-473, LR-2577, HI-3179 and HR-4676). One of the GCLs contained conventional sodium bentonite (Na–B) and the other six contained bentonite-polymer (B–P) mixture with polymer loadings ranging from 0.5% to 12.7%. Hydraulic conductivity tests were conducted at an effective confining stress of 20 kPa. The hydraulic conductivity of the Na–B GCLs were >1 × 10⁻¹⁰ m/s when permeated with all six CCP leachates, whereas the B–P GCLs with sufficient polymer loading maintained low hydraulic conductivity to synthetic CCP leachates. All the B–P GCLs showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) to low ionic strength leachates (TCCP-50, I = 50 mM and LRMD-96, I = 96 mM). B–P GCLs with P > 5% showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) up to HI-3179 leachates. These results suggest that B–P GCLs with sufficient polymer loading can be used to manage aggressive CCP leachates.     

Responses of single and group piles within MSE walls under static and cyclic lateral loads

To investigate the behavior of piles and the performance of the mechanically stabilized earth (MSE) walls under static and cyclic lateral loading, six reduced-scale model tests of single and group piles within the MSE walls were conducted inside a test box. In the single pile tests, a hollow aluminum tube as a pile was placed at a distance of 2D or 4D (D is pile diameter) behind the wall facing, while in the group pile tests, the piles were only placed at the distance of 2D with a spacing of 3.3D between the piles. The piles were subjected to static lateral loading only and cyclic lateral loading followed by static loading until failure. The test results showed that the lateral load capacity of each pile in the group pile test was approximately 60% that of the single pile, while the wall facing displacements and the geogrid strains in the group pile test were larger than those in the single pile test. The lateral pile capacity, the wall facing displacement, the strain in the geogrid, and the lateral earth pressure behind the wall facing in the static and cyclic loading tests were evaluated at the pile head displacement equal to 20%D.     

Numerical parametric study of geosynthetic reinforced soil integrated bridge system (GRS-IBS)

A 2-D finite flement model was developed in this study to conduct a FE parametric study on the effects of some variables in the performance of geosynthetic reinforced soil integrated bridge system (GRS-IBS). The variables investigated in this study include the effect of internal friction angle of backfill material, width of reinforced soil foundation (RSF), secondary reinforcement within bearing bed, setback distance, bearing width and length of reinforcement. Other important parameters such as reinforcement stiffness and spacing were previously investgated by the authors. The performance of GRS-IBS were investgated in terms of lateral facing displacement, strain distribution along reinforcement, and location of potential failure zone. The results showed that the internal friction angle of backfill material has a significant impact on the performance of GRS-IBS. The secondary reinforcement, setback distance, and bearing width have low impact on the performance of GRS-IBS. However, it was found that the width of RSF and length of reinforcement have negligible effect on the performance of GRS-IBS. Finally, the potential failure envelope of the GRS-IBS abutment was found to be a combination of punching shear failure envelope (top) that starts under the inner edge of strip footing and extends vertically downward to intersect with Rankine active failure envelope (bottom).     

Comparison and evaluation of analytical models for the design of geosynthetic-reinforced and pile-supported embankments

Geosynthetic-reinforced and pile-supported (GRPS) embankments are becoming more and more popular as this technique showed good performances in practice. Various design methods were introduced to analyze GRPS embankments. However, the applicability of these design methods was not always fully validated. This paper focuses on the review of projects containing field observations of GRPS embankments. The comparison results showed that the assumptions related to the subsoil support, geosynthetic, arching shape, and its evolution are not consistent in the analytical methods. Comparison results with twenty-five full-scale cases and six series of experiments emphasize that these available design methods produce significantly different results in predicting loads transfer mechanism. The analytical models predict arching for cohesionless fill better that for cohesive fill soils. Besides, the analytical methods which consider subsoil support such as the CUR226 and EBGEO methods give results that are in a better agreement with experimental data as compared to other methods which do not consider the subsoil support. The CUR226 (2016) analytical model seems to be able to give the best performance with measured data when compared to other design methods. Finally, the results pointed out that the limit equilibrium model is adequate and has good performance.     

Numerical evaluation of a granular column reinforced by geosynthetics using encasement and laminated disks

Structures built on soft strata may experience substantial settlement, large lateral deformation of the soft layer and global or local instability. Granular columns reinforced by geosynthetic materials reduce settlement and increase the bearing capacity of the composite ground. Reinforcement is more common in the form of geosynthetic encasement, but laminated disks can also be used. This paper compares these two forms of reinforcement by means of unit cell finite element analyses. Numerical results were initially validated using field and experimental data, and parametric studies were subsequently performed. The parametric studies varied the geosynthetic interval and the geosynthetic tensile stiffness of the laminated disks as well as the length of the reinforced column. The analyses showed that in both modes; encasement and laminated disks; the geosynthetic increases the vertical stress mobilized on the reinforced column and reduces settlement on soft soil. It was also observed that in order to achieve the same performance as with encased column, the optimum interval between laminated disks is dependent on the stiffness of the geosynthetics and the column reinforced length.     

Performance-based indicators for controlling geosynthetic clay liners in landfill applications

The results of a project aimed at identifying performance-based indicators that can be used by landfill operators to check the suitability of GCLs for bottom barrier applications are presented. The general methodology consisted of performing detailed characterization of the prevalent GCLs used in France for landfill barrier applications, before and after prolonged contact with several fluids during oedo-permeameter tests. Results of mineralogical analysis illustrate the variety of composition of the tested bentonites, which in addition to smectite clay contain a large number of accessory minerals. For one of the GCLs tested, the proportion of smectite was lower than 30 wt%, which highlights the limitations of the generic designation “bentonite” when referring to GCLs destined to landfill applications. Results also underline the correlation between cation exchange capacity (CEC) and smectite content, the correlation between free swell volume and proportion of exchangeable sodium and the influence of the bentonite's calcium carbonate fraction on hydraulic conductivity. Transmission electron microscopy (TEM) photographs illustrate the effect of cation exchange on clay microstructure, with the formation of clay particles which lead to increased hydraulic conductivity. The exchange is also documented by exchangeable cation analyses. Results of isotopic analyses indicate that information provided by suppliers with respect to the “natural” versus “activated” nature of the bentonite, may sometimes be arbitrary and related to factors that are very difficult to check in practice, even by the suppliers themselves. This further underlines the need for performance-based indicators, rather than generic designations, to provide objective information regarding GCL suitability for landfill applications. Several performance-based indicators are selected in order to provide practical tools for checking the suitability of sodium-bentonite GCLs in bottom barrier applications and limit values are proposed.