Monitoring of a landfill side slope lining system: instrument selection, installation and performance

Municipal solid waste landfill barrier systems often comprise a combination of geosynthetics and mineral layers. Throughout the last twenty years there has been extensive research on the interactions between the materials and on performance of the geosynthetics including aspects of durability. This research has resulted in significant advances in the design and specification of landfill lining systems. However, to date there has been limited research carried out on in situ landfill lining system behaviour. Measured behaviour from field scale trials and of in service operation can provide valuable information on landfill lining system performance and allow a better understanding of composite material behaviour. Although many numerical modelling programs are applied to evaluate lining system stability and integrity, data to validate these models is currently limited. This paper highlights the data required to validate numerical models and instrumentation techniques that may be used to acquire this information. The paper focuses on geotechnical instrumentation deployed on the side slope lining system at the Milegate Extension Landfill, UK. The instrumented lining system comprises 1.0 m of compacted clay, a 2 mm double textured high density polyethylene geomembrane, a nonwoven geotextile and a sand cover soil layer. Instrument selection and problems associated with acquiring consistent, reliable and valuable data in a field environment are discussed, as are the challenges and problems that occur when preparing a full scale experiment. Sources of uncertainties within readings are highlighted. Additionally, initial results collected during sand veneer layer placement on the slope are presented. These demonstrate acceptable instrument performance over a 2 year period. Measured behaviour highlights the significance of geomembrane strains driven by temperature changes, generation of post peak strengths at interfaces during fill placement on the side slope due to relative displacement at interfaces between components, and mechanisms of stress redistribution in the geomembrane that result in time dependent changes in strain under constant load and temperature conditions.