Extended phase relations and load effects in MSW

Abstract

The moisture retention and compression characteristics of municipal solid waste under self-weight are likened to those of an unsaturated soil. By assuming that the solid organic fraction in waste retains a relatively immobile micropore moisture and that deformation at low confining stress occurs at the expense of a relatively large macropore system, an insight into the variation of density and moisture with depth can be gained. With data on the composition of the waste, the phase composition can be extended to distinguish between solid organic and solid inorganic fractions, resulting in a four phase material model. The model is developed using detailed moisture and waste composition data from the Lyndhurst Sanitary Landfill site in Victoria, Australia. Finally, comparison of the model with large scale compression test results provides an insight into the nature of waste compression and moisture content data at low confining stress.

Introduction

To simulate the hydro-mechanical behaviour of a soil requires an appropriate soil model and a knowledge of the initial conditions. The model defines the constitutive relationships between the volumetric state variables (the phase composition) and the stress state variables, whereas the initial conditions provide information about the initial stress and initial volumetric states upon which predictions of some future condition can be made. In a partially saturated inert soil, the phase composition comprises bulk solid, liquid and gas volumes; variations with depth of all these phases commonly exist.

By comparison with inert soils, municipal solid waste is more heterogeneous and more compressible; these characteristics both complicate and amplify the variation of phase composition with depth. However, landfill site inventories together with simple laboratory tests and/or field monitoring provide only a cell-averaged estimate of the phase composition. By definition, these data conceal internal variations of density and moisture content. Whilst measurement of phase composition in the field is not easy, it has been investigated in waste refuse by a number of workers. For example, Beaven and Powrie (1995) used a large-scale compression cell to examine the relationship between waste density and absorptive capacity with load. Kavazanjian et al. (1994) used surface shear wave analysis to obtain in-situ waste densities and Yuen et al. (2000) described the use of neutron probe techniques for in-situ moisture content measurement.

When the effects of biodegradation are considered, particularly in relation to landfill settlement, the three phase model appears inadequate. The decomposition of organic matter in landfilled waste is controlled by what might be referred to as ‘biochemical state variables’, i.e. volatile fatty acid concentrations and methanogenic biomass, which produce a change in the phase composition, principally in the solid phase volume. An extended phase description, i.e. one that can distinguish between solid organic and solid inorganic fractions, would provide a convenient framework within which to associate correctly biochemical state variables and volumetric state variables.

The aims of this paper are, firstly, to interpret the moisture retention and mechanical compression behaviour of landfilled waste as a two-tier porous medium, of the kind postulated in unsaturated soil mechanics; and secondly, to describe a simple method of back-calculating the in-situ phase composition of waste with depth using cell-averaged mass, volume and moisture data. The in-situ phase calculations are based on data obtained at the Lyndhurst Sanitary Landfill in Victoria, Australia. Then, using more generic waste composition data, the phase composition is extended to distinguish between the organic and inorganic fractions of the solid phase. Finally, some aspects of the proposed waste model are examined against large-scale test results.