Landfill settlement results in the lowering of the ground or surface of a landfill over time. Landfill settlement can vary considerably, but can be as high as ~40%. The surface of a landfill with an initial waste depth of 15 m which settled by 40% would drop by 6 metres.
What is is and what causes it?
Settlement in landfilled waste occurs due to loading and other processes including chemical and microbial actions. These processes appear as time-dependent, and are controlled by factors such as leachate composition, pH, temperature and moisture (both as a reactant and as a vector for species transport). As a result, settlement in landfilled waste, especially long-term settlement, is complex and difficult to predict in both magnitude and timing.
Practical approaches to landfill settlement identify two or three settlement mechanisms. However, discussion and interpretation of settlement is not helped by a lack of consistency in the terminology used to describe these mechanisms. Moreover, the start and/or the finish of the various processes can be difficult to distinguish.
For the purposes of this web-site, a framework of immediate, primary and secondary settlement has been adopted. Immediatesettlement is load-related, resulting from the compression of air voids and the deformation of readily compressible waste “particles”. It occurs very quickly. In saturated wastes, a period of primary compression due to consolidation may follow as water is expelled and the waste skeleton compresses. Primary or consolidation settlement is dependent on the rate of expulsion of liquid from the voids, i.e. the permeability of the waste. Secondary settlement is a long-term phenomenon, attributable to mechanical creep (including ravelling), and biodegradation.
Table 1 Classification of waste settlement mechanisms.