John McDougall

Unsaturated hydraulic conductivity of landfilled waste.

Unsaturated flow modelling of landfill hydraulics is a potentially valuable tool for the prediction of leachate discharge rates, design of leachate control systems and simulation of biodegradation in engineered landfills. Such an approach requires information on the moisture retention and hydraulic properties of waste, usually in the form of empirical functions such as those presented by van Genuchten (1980). However, there is a lack of information on moisture retention and conductivity properties. Recently, Kazimoglu et al. (2003, 2005) reported on moisture retention in MSW and presented a laboratory-scale method by which retention properties can be obtained. As with conventional inert soils, laboratory determination of unsaturated hydraulic conductivity is difficult and recourse is often made to analytical methods. The aim of this paper is to compare the unsaturated hydraulic conductivity of MSW obtained using Passioura’s (1976) one-step outflow test method with predictions using van Genuchten’s model. Good agreement is observed between the predictive model and experimental method for unsaturated hydraulic conductivities at low moisture contents but there is poor agreement at high degrees of saturation. This latter discrepancy is attributed to the difficulty of measuring retention properties of large pores at low suctions and the applicability of the van Genuchten model to such a material.

Moisture retention curve in landfilled waste

Moisture content and moisture movement are key factors in controlling the progress and rate of biodegradation within a landfill as it is the aqueous environment that facilitates the transport of nutrients and microbes. The modelling of infiltration and water movement requires information on the moisture retention and hydraulic conductivity properties, usually in the form of empirical functions such as those proposed by van Genuchten (1980). In waste however, the particle and pore size distribution, heterogeneity of waste composition and leachate chemistry complicate the determination of moisture retention and hydraulic conductivity. In this paper we describe the modification and use of a standard pressure plate apparatus to establish moisture retention properties of samples of 250 mm in diameter and the difficulties of using this method. Some initial results are presented.

Geomechanics and Long-term Landfill Settlement

Long term landfill settlement is commonly attributed to two phenomena: creep and biodegradation. Biodegradation, which may contribute the greater proportion of long‐term settlement, is a function of a range of factors, many of which are not easily associated with conventional geomechanical analysis. A more fundamental treatment of biodegradation effects within the constitutive framework of a hydro‐bio‐mechanical (HBM) model is presented in this paper. The role of a decomposition‐induced void volume change parameter Λ, and its ability to handle biodegradation‐related settlement, are revealed through the simulation of a long‐term, laboratory‐scale waste settlement test.

Estimating degradation-related settlement in two landfill-reclaimed soils by sand-salt analogues

Landfill reclaimed soil here refers to largely degraded materials excavated from old landfill sites, which after processing can be reinstated as more competent fill, thereby restoring the former landfill space. The success of the process depends on the presence of remaining degradable particles and their influence on settlement. Tests on salt-sand mixtures, from which the salt is removed, have been used to quantify the impact of particle loss on settlement. Where the amount of particle loss is small, say 10% by mass or less, settlements are small and apparently independent of lost particle size. A conceptual model is presented to explain this behaviour in terms of nestling particles and strong force chains. At higher percentages of lost particles, greater rates of settlement together with some sensitivity to particle size were observed. The conceptual model was then applied to two landfill reclaimed soils, the long-term settlements of which were found to be consistent with the conceptual model suggesting that knowledge of particle content and relative size are sufficient to estimate the influence of degradable particles in landfill reclaimed soils.