Settlement-The Short and the Long of it

Total settlement in municipal solid waste (MSW) landfills can be considerable, from 25% to 50% of original fill thickness (Bjarngard and Edgers, 1990). Some occurs very quickly, i.e. during the filling phase (or short term), but there remains a significant amount of long-term settlement. Deformations of this magnitude and duration impact on the design, operation, closure and future use of landfills. Long-term settlement defines the pre-closure fill level and hence chargeable void space. If the eventual settlement is excessive or unforeseen, the integrity of the capping layer and any infrastructure for the collection of gas or recirculation of leachate may be compromised. Even if the capping layer remains intact it is necessary to avoid depressions on the upper surface where rainwater could pond. Finally, since settlement can continue for many years, it will pose a constraint on future development, including vertical expansion of the site. So, for engineers charged with the design of a landfill facility or site restoration, an understanding of, and ability to analyse, landfill settlement is required. It is well known that landfill settlement is the product of a combination of processes, including load and biodegradation-related phenomena. Whilst existing geotechnical settlement models can handle load effects, they are limited in their ability to deal with biodegradation. There already exist a number of good reviews of the form and capability of these models, see for example, El-Fadel and Khoury (2000); Elagroudy et al. (2008). In this chapter conventional methods of geotechnical settlement analysis are reviewed as the background to recent work to develop a more fundamental approach to landfill settlement analysis. In addition to the review papers, there are a number of research papers that have contributed to the preparation of this chapter but which have not been expressly cited; these papers are collected in the bibliography.