Compressibility of tyres for use in landfill drainage systems

Abstract

Whole or shredded scrap tyres are sometimes proposed as an alternative to conventional aggregates in landfill drainage systems. Landfill basal drainage systems are, however, subjected to large overburden stresses from the overlying waste, which may compress a tyre drainage layer reducing its porosity and hence its effectiveness. Previous work has indicated that tyre drainage layers will remain effective under high stresses, but tests have in the main been restricted to small (<100 mm) shred sizes. The use of coarser shreds or even whole tyres for landfill drainage systems may be advantageous as they are more economical to produce and may be less prone to clogging than smaller shreds. In this paper, the results of largescale (2 m sample diameter) tests to investigate the compressibility of 50, 200 and 450 mm nominal-size tyre shreds are compared with data from the literature for smaller-size shreds and whole tyres. Tests were carried out at vertical stresses up to 600 kPa, representing landfill depths of up to about 60 m. Generally whole tyres and larger-size segments compress more in overall volume terms on initial loading than smaller size shreds. At vertical stresses greater than about 80 kPa, however, the differences on a graph of dry density or porosity plotted against vertical stress are much smaller. The proportion of voids that are not free-draining decreases with compression for whole tyres, as cup-like structures capable of retaining water flatten out. For tyre chips, the proportion of non-drainable voids increases with compression, suggesting that flow pathways are being lost. The dependence of the compressive behaviour of tyres and tyre chips on the particle shape is discussed, and the need for a more detailed and rigorous way of characterising particle size and shape is highlighted.