The long-term settlement of landfill waste

The biodegradation of solid waste materials, the main cause of secondary settlement in landfills, has a significant impact on the post-closure performance of landfill capping systems. Excessive settlement may in turn lead to water ingress and enhanced leachate generation, significantly increasing the risk of harm to the environment. Largescale long-term laboratory experiments were conducted to identify the rate and magnitude of waste settlement using three purpose-designed test reactors (consolidating anaerobic reactors (CARs)). This paper presents a detailed characterisation of the waste, its associated chemical and physical properties, and the overall volume changes associated with degradation over time. The total magnitudes of settlement after 919 days were 27.6% in CAR 1 (under 150 kPa pressure) and 25.0% in CAR 2 (50 kPa). Long-term secondary settlement was found to be dependent on waste depth-that is, stress dependent. Increased stress (150 kPa) led to a 20% increase in the rates of long-term secondary compression in comparison with a stress level of 50 kPa. Secondary settlement due to biodegradation was found to be of comparable magnitude to the component of secondary settlement caused by mechanical creep (values of 11.1-13.7%). If the settlement behaviour in the CARs approximates to full-scale observations, this data set could be used for validation of quantitative landfill settlement models.