Decomposition and settlement in landfilled waste and other soil-like materials

The term ‘soil-like materials’ is used to refer to particulate materials that contain organic or other non-inert solids. When these materials decompose, there is a reduction in solid phase volume and an induced change in void phase volume, the combination of which has consequences for subsequent mechanical behaviour. In the case of landfilled waste, the mechanical consequences of biodegradation can be troublesome with long-term settlements ranging between 25% and 40% of original waste height (Bjarngard & Edgers, 1990; El-Fadel & Khoury, 2000). Up to half of this settlement has been attributed to biodegradation (Coduto & Huitric, 1990; Wall & Zeiss, 1995), yet the process at field scale is not well understood. There is, however, a significant volume of research on decomposition in the area of public health engineering, and on biodegradation in waste refuse, but without a more fundamental description of solid phase loss and induced changes in void volume, geomechanical analysis of these soil-like materials is constrained to a highly empirical path. Through a review of two laboratory investigations into decomposition in clay–fibre mixtures and three recent landfill settlement monitoring projects, this note seeks to elucidate a more fundamental mechanism of decomposition-induced settlement