The EU landfill directive requires the amount of wastes going to landfills to be reduced signifi-cantly in compliance with the sustainable waste management principles. However, the disposal has been and will continue to play a significant role, and the overall aim is an improved design and operation of the landfill sites, and reduction of their negative environmental impact towards sustainable landfilling. Waste has been recognised by other researchers as the primary structural element in landfills; therefore for better understanding of its behaviour, the physical and engineering properties of its components must be well known. The main aspect of this research was focused on investigation of particle compressibility and its effect on the overall compressibility and settlement of the waste body. A methodology to measure particle compressibility in saturated conditions at various stress levels was developed, using synthetic deformable materials and mechanically-biologically treated (MBT) waste. MBT waste sample with particle size reduced to 9 mm showed a response to loading similar to soils, hence soil mechanics principles will be applicable. Simultaneously, a categorisation of different types of pre-treated wastes was carried out by some of their physical and geometrical properties. The results were interconnected into a newly developed waste classifcation system, which allowed an assessment and comparison of their geomechanical and flow properties, and predict to some extent their futurebehaviour in landfills. About a third of the MBT samples by mass comprised a matrix (�ne material of <5 mm) into which the larger particles were embedded. The large 2D elements (mainly presented by plastics, glass and metal foils) will play an important role for stability and flow transportation, taking about 25% by mass. On one hand, they will have a reinforcing effect but on the other, they will modify, divert, or impede the flow paths in the waste which may result in reduced permeability or preferential flows. Highly compressible synthetic materials were also used to simulate the deformable materials in landfills (such as hollow 3D elements). They tend to embed into each other and form a horizontal highly dense structure which reduces significantly or completely the volume of voids. In large scale, this will lead to modified or blocked the flow paths and hence reduce the flow rates and impede the flushing of contaminants in landfills. Compressible particles reach their maximum compressibility at certain stress thresholds and progressively change their shape from 3D compressible to 2D incompressible. At the end of the study, a simplified phase relationship model was suggested, considering changes in the solid phase due to both particle compressibility and decomposition. The applicability of the conventional effective stress theory on highly compressible materials was questioned as well.