Hydraulic conductivity of tyres 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, thereby 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 the current paper, the results of large-scale (2 m diameter) tests to investigate the variation in hydraulic conductivity (permeability) with stress 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. Potential errors in laboratory test procedures are highlighted and the influence of scale and the relative test chamber to particle size on the results is discussed. It is concluded that a chamber to particle size ratio of at least 6 is needed with a rigid walled permeameter, if errors owing to peripheral flow effects are to be avoided. Relationships between shred size and drainage properties are investigated: no clear correlation between tyre shred size and drainage properties in the particle size range 50–450 mm is found. Whole tyres, however, generally have a slightly higher hydraulic conductivity, while samples containing a significant proportion of material less than 20 mm in size have generally a lower hydraulic conductivity at a given vertical stress than clean samples of larger shreds. 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, thereby 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 the current paper, the results of large-scale (2 m diameter) tests to investigate the variation in hydraulic conductivity (permeability) with stress 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. Potential errors in laboratory test procedures are highlighted and the influence of scale and the relative test chamber to particle size on the results is discussed. It is concluded that a chamber to particle size ratio of at least 6 is needed with a rigid walled permeameter, if errors owing to peripheral flow effects are to be avoided. Relationships between shred size and drainage properties are investigated: no clear correlation between tyre shred size and drainage properties in the particle size range 50–450 mm is found. Whole tyres, however, generally have a slightly higher hydraulic conductivity, while samples containing a significant proportion of material less than 20 mm in size have generally a lower hydraulic conductivity at a given vertical stress than clean samples of larger shreds.