A numerical modelling technique for geosynthetics validated on a cavity model test

https://doi.org/10.1016/j.geotexmem.2017.04.006Get rights and content

Abstract

Numerical modelling approaches can aid in designing geotechnical constructions involving geosynthetics. However, the reliability of numerical results depends on how the model is developed, the constitutive model, and the set of parameters used. By comparing the numerical results with experiment, the present work verifies a numerical modelling technique developed to model multilayered geosynthetic lining systems for landfills. The numerical modelling technique involves strain softening at interfaces and allows the axial stiffness of the geosynthetics to evolve as a function of strain. This work focuses on a two-dimensional finite-difference model, which is used to simulate three types of experimental tests: conventional uniaxial tensile tests, direct shear tests, and a large-scale test that was used to assess the overall mechanical behaviour of a reinforced geosynthetic system that spanned over a cavity. This reinforced geosynthetic system consisted of a 50 kN/m polyvinyl alcohol geogrid reinforcement embedded in a layer of sand, a geosynthetic clay liner, a high-density polyethylene geomembrane, and a non-woven needle-punched geotextile. The uniaxial tensile tests, direct shear tests, and the large-scale test were numerically modelled and the numerical results were compared with experimental results. The results of the numerical modelling technique presented very closely match the results of the three experimental tests, which indicates that the numerical model correctly predicted the measured data.

Introduction

Geotechnical constructions that involve geosynthetics, such as landfills, are traditionally designed by using limit equilibrium methods (Giroud and Beech, 1989, Koerner and Hwu, 1991). However, these methods cannot be used to assess the integrity (e.g., strain or tensile forces) (Long et al., 1995) of the construction components and do not consider whether stresses are compatible with strains and displacements (Villard et al., 1999). As an alternative, such constructions may be designed by using numerical modelling methods (Fowmes et al., 2008); these methods not only account for the above-mentioned aspects but also account for the multiple interactions between geosynthetics.

Numerical modelling techniques are becoming ever more sophisticated because today's software allows designers to consider the key aspects of the mechanical characteristics of geosynthetics (e.g., the nonlinear stiffness) and of the interfaces (e.g., strain softening). However, the reliability of such numerical results depends on the numerical modelling technique used, which in turn rests upon how the model is developed, the constitutive model, and the set of parameters used.

Whichever numerical modelling technique is used, questions exist with respect to (i) the relevance of the numerical modelling technique and therefore (ii) the reliability of the numerical results. Consequently, to answer such questions, numerical results should be confirmed by comparing them with experimental data. In the context of landfills, and particularly for piggy-back landfill expansions where a new landfill is built over an older one, such verification is essential because of the interactions between the various materials, such as clay, sand, gravel, geosynthetic, and waste (Tano and Olivier, 2014).

Unfortunately, limited studies that addressed the comparison of the experimental behaviour of multilayered geosynthetic lining systems with that predicted by numerical models are available (Fowmes et al., 2008). To the best of our knowledge, only three studies (Villard et al., 1999, Fowmes et al., 2008 and Zamara et al. (2014)) compare experimental results of multilayered geosynthetics with numerical results of models of landfill lining systems.

This work aims to verify a numerical modelling technique by comparing it with three experimental tests: a tensile test, a direct-shear test, and a large-scale large test. These tests were developed to assess the mechanical behaviour of a reinforced geosynthetic lining system.

Prior to discussing the details of the verification process, the previous studies of Villard et al., 1999, Fowmes et al., 2008 and Zamara et al. (2014) are further discussed in the following section. The benefits and limitations of these studies provide a framework for the present study and lead us to develop a new modelling technique.

Section snippets

Background

Villard et al. (1999) applied finite-element modelling to describe a veneer cover of a landfill and to better understand the distribution of forces and strains within a geotextile (GTX) and geomembrane (GMB) placed at the bottom and on side slopes of the landfill. The forces within the GTX were measured by force sensors positioned at the top of the slope. A cable-type displacement (extensometer wires) was used to measure the geosynthetic displacements and then the strains were calculated from

Uniaxial tensile test device

The UTT is a simple test used to assess tensile behaviour of a material. Geosynthetic manufacturers generally systematically subject all products to UTTs to determine their quality. However, complete tensile curves are rarely provided. Therefore, the authors performed additional tensile tests to obtain the complete tensile curves. In these tests, a geosynthetic specimen is subjected to a constant tensile deformation rate along a given axis and the resulting forces and strains within the

Material, geosynthetic and interface properties

This study considers a lining system made of a GTX, a GMB, a geosynthetic clay liner (GCL), and a GGR reinforcement embedded in a dense material. Sand was used to simulate this dense material. The full properties of these geosynthetics, materials, and of the interfaces between the various materials are presented below.

Numerical modelling technique

As mentioned in section 2, Tano et al. (2016a) proposed a numerical model technique to simulate the behaviour of a multilayered geosynthetic lining system in the context of piggy-back landfill expansions. This technique was based on five criteria (CR1 to CR5). In the present study, the authors use this same numerical modelling technique (Tano et al., 2016a) to model the three experimental tests (UTT, DST, and LSTA). However, CR2 is not considered herein because none of the three experimental

Numerical modelling versus experimental results

This section compares the results of numerical modelling with experimental results and discusses these comparisons for the three experimental tests (UTTs, DSTs, and LSTA).

Conclusions

Numerical modelling can be used to aid in the design of geotechnical constructions involving geosynthetics (geosynthetics), such as landfills. However, the reliability of the numerical results depends on how the model is developed and on the parameter set that is used.

The present work develops a procedure to analyze multilayered-lining systems by using a 2D finite-difference method. The accuracy of this numerical modelling technique is then verified by comparing its results with those obtained

Acknowledgments

The authors thank Didier Croissant (Irstea, Antony, France) and P. Mailler (IFTH, France) for their assistance during in carrying out the various tensile tests. Special thanks are also due to H. Mora and J.-M. Miscioscia (LTHE, Grenoble, France) for manufacturing the large-scale experiment apparatus used in this study. Finally, the authors thank J. Bruhier at Huesker, France for providing us with the geogrid, geotextile, and geosynthetic clay liner products used in this study.

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