Hydraulic Properties of MSW

The science of understanding fluid flow in waste materials is both complicated and in a state of evolution. In general, however, it is possible to apply soil mechanics and hydrogeological principles to waste, although care needs to be taken with standard phase relationships for a number of reasons. Firstly, there is a great potential for confusion between terms, as landfill science has borrowed definitions from different disciplines which very often have slightly different meanings and derivations. Secondly, the impact of gas is a complicating factor, especially as it is being generated in situ. And finally ongoing degradation can cause a volume reduction in solids, a factor not dealt with in any other area involving fluid flow through porous media. There has been a reasonable amount of research and understanding of saturated waste hydraulics. Darcy's law is routinely applied and appears satisfactory for most eventualities. Changes in effective stress on first loading appears to be the dominant influence on hydrogeological properties, especially hydraulic conductivity, with secondary influences coming from gas, differences in waste composition and possibly degradation. Waste anisotropy and heterogeneity are also other important considerations. Less is known about unsaturated waste hydraulics, and indeed the applicability of standard unsaturated flow theory to waste has not been proven. For example there is considerable uncertainty about the nature and form of water retention curves for waste. Standard theory usually relates parameters to saturated conditions, but the link between saturated and unsaturated waste hydraulics is difficult to define, partly because ongoing gas generation in landfills probably mean that truly saturated conditions never exist. At present there is not enough data on the impact of gas generation, distribution and movement on unsaturated flow parameters.