Numerical modelling of the behaviour of synthetic barrier layer in a landfill cap

Mehdi Qoreishi, Muhammad Khalil

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Geosynthetics

ABSTRACT: FLAC software was used to develop a numerical model of a synthetic cap to be constructed at an existing landfill in Alberta, Canada. A parametric study was completed to evaluate the response of the geosynthetic barrier layer (LLDPE) to the variations in the properties of the model components. Heterogeneity of waste (Municipal Solid Waste) material was incorporated in the model by defining zones with different mechanical properties within the waste material. It is shown that heterogeneity may increase the tensile forces and strains in the LLDPE barrier layer several times due to non-uniform settlement patterns. It is concluded that stabilizing the cap with surface gravel and geogrid significantly reduces the maximum in-plane strain experienced by the LLDPE barrier layer. An optimum cap thickness has been determined to protect the geosynthetic barrier layer from excessive stresses and strains induced by truck loading and weight of the cover soil. RÉSUMÉ Un modèle numérique a été développé par le logiciel FLAC pour analyser le recouvrement d'une déchetterie situé en Alberta, Canada. Une étude paramétrique a été réalisée pour évaluer le comportement de la couche géosynthétique (LLDPE) aux variations des paramètres du modèle. L'hétérogénéité des déchets est considérée par le modèle en définissant des zones aux propriétés mécanique distinctes. Il a été démontré que cette hétérogénéité pourrait augmenter les forces de traction ainsi que les déformations de la couche géosynthétique (LLDPE) à de nombreuses reprises, à cause des tassements non-uniformes. On en conclut que la stabilisation du recouvrement par une couche de géogrid et de graviers pourrait limiter les déformations en plan subies par la couche géosynthétique (LLDPE). Une épaisseur optimale du recouvrement a été calculée pour protéger la couche géosynthétique contre les contraintes excessives induites par le chargement des camions ainsi que le poids des sols de recouvrement. 1 INTRODUCTION Conventional caps for landfills generally include a low permeability compacted clay barrier layer that reduces percolation of surface water into the waste. Use of geosynthetic material in place of the natural compacted clay barrier layer in landfill caps has been successfully accomplished as some synthetic materials have much better performance and durability as compared to clay barrier layers. A synthetic cap usually consists of four main components: 1) landfill gas (LFG) venting layer; 2) barrier layer with very low hydraulic conductivity in order of 10-15 m/s; 3) a drainage layer; and 4) cover soil for the protection of the barrier layer. In this project, the barrier layer refers to a layer of Linear Low Density Polyethylene (LLDPE) geomembrane installed between two layers of geocomposite material for drainage above and LFG venting below the barrier layer. The drainage and LFG venting layers are not considered to resist any forces. The LLDPE geomembrane has been selected due to its highest multi-axial strain resistance property amongst the available geomembrane materials. The cover soil is designed to protect the barrier layer from damage caused by elements of weather, static and dynamic loads. The cover soil needs to be capable of distributing the load over a large area to mitigate the high stresses that may be induced into the geosynthetic barrier layer. Numerical modelling of soil-structure interactions of geosynthetic layers is a useful tool to study the behaviour of the cover soil system. There are limitations to the application of numerical models in this type of geotechnical problem that includes unknown behaviour of waste material and limited test data on both waste and geosynthetic layers to estimate input data for advanced constitutive models. Considering these limitations are important and simulations should be used as a tool to understand the general behaviour of the cap structure by performing parametric studies and sensitivity analysis rather than predicting the exact values of settlements or stresses in the field for this type of material with high heterogeneity and unknown variables. In this study the behaviour of the barrier layer of the Utility pad cap was analysed using a numerical model considering heterogeneity of the waste material. Simple constitutive laws were chosen in the analysis and the model was run several times to study the behaviour of the geosynthetic layer in different possible scenarios. An optimum cover system was determined to mitigate high stresses that resulted from heavy trucks passing over the Utility pad of the landfill. 1.1 Project overview The studied waste landfill has been operational since 1968. It occupies 2.6 square kilometer of land. As shown on figure 1, the current footprint of the landfill is divided

RÉSUMÉ: ical modelling of the behaviour of synthetic

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Mehdi Qoreishi; Muhammad Khalil (2014) Numerical modelling of the behaviour of synthetic barrier layer in a landfill cap in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper471,author = Mehdi Qoreishi; Muhammad Khalil,title = Numerical modelling of the behaviour of synthetic barrier layer in a landfill cap ,year = 2014}