Settlement Of Municipal Solid Waste
Authors:   ,   Cork Institute of Technology, Burke Emmet 
Published: September 2024

"The mechanical properties of waste (compression index, elastic modulus, stresses) are an important aspect for the design, operation and post-closure uses of landfills. In recent years the requirement to utilise the acreage that the landfill occupies to its maximum potential has necessitated the landfill structure to accommodate a greater volume of waste over a smaller plan area. This pushes the boundaries of design relating to side-slope and infrastructural elements of landfill structures. The objectives of this research were identified from significant gaps in the knowledge relating to the settlement and mechanical properties of municipal solid waste. The main objectives are as follows; • To predict long term settlement of municipal solid waste as a function of placement conditions, elevation & stresses exerted due to self weight and overburden. • To estimate the shear strength properties of municipal solid waste without using invasive sampling techniques or disturbance of final barrier (capping) systems. • Make estimates on the mechanical properties (elastic modulus, stress-strain relationship & compression characteristics) of waste so that a finite element model could be used to predict displacements. For this study, continuous monitoring data and topographical surveys from the Kinsale Road Landfill (KRL), Co. Cork Ireland is used to estimate final settlements and the time to reach final settlement. Settlements measured over a period of 8 years were assessed for varying levels of strain and horizontal displacements. Based on the characteristics of waste being best described by a non-linear elastic hyperbolic or soil hardening model, the hyperbola shape was adopted to determine both predictions for long term displacements and mechanical properties such as shear strength and elastic modulus. The settlements were assessed by three methods. Two methods of which were secondary compression indices where vertical displacement is presented in terms of actual change in elevation or axial strain plotted against the logarithm of time. The other method adopted was the hyperbolic function whereby final settlements were expressed in the form of a hyperbolic curve, the asymptote of the curve being the value determined for ultimate settlement and time to ultimate settlement. The elastic properties of the waste were calculated from initial estimates on in-situ densities, this permitted figures for effective stresses (vertical & confined) to be estimated. These values were then entered into computer programmed triaxial soil tests where effective stress strain properties could be determined at various depths within the landfill. A statistical analysis of the stress strain data revealed estimations for shear strength properties based on the hyperbolic function elastic model. The results for this exercise were validated through a finite element study, whereby the estimated elastic properties were entered into a scaled down model of the landfill structure. The results of the study were found to be in close agreement with the calculated values for both displacement and elastic strain. The study found through the various methods of calculation for secondary settlement that the hyperbolic function replicates the mechanical compression of the waste accurately. Specifically related to this study it was found that by integrating both the hyperbolic function and modified secondary compression index, the time taken to achieve final settlement could be accurately predicted. It was possible to confirm this due to the long period of monitoring data available."