Suitability of shredded tires for use in landfill leachate collection systems

The suitability of shredded tires or "tire chips" for use in the leachate collection drainage layer of a municipal solid waste landfill was investigated in terms of the: (1) compressibility of the tire chips and resulting changes in hydraulic conductivity under varying applied loads, and (2) effect of leachate pH on the shredded tries compressibility and hydraulic conductivity behavior. A constant head hydraulic conductivity apparatus was fabricated to measure the hydraulic conductivity of the tire shred sample under different axial strains. Further, the fabricated assembly was capable of measuring hydraulic conductivity of the sample at various sample locations at a given strain level. One aim of this study was to provide supporting information for permission to use tire chips as an alternative to crushed stone in the leachate collection system of a landfill. Shredded tires from two different sources were used in this study to investigate any differences in the sensitivity of the shredding process to compressibility and hydraulic conductivity responses under varied applied loads. Under applied vertical loads resulting in average vertical stresses of up to 440 kPa, equivalent to over 50 m of waste, the maximum normal strain recorded in each type of tire chip was observed to plateau at a strain level near or slightly greater than 0.5. The results of the permeability testing indicated average hydraulic conductivity values ranging between 0.67 and 13.4 cm/s under average applied normal stresses ranging from approximately 60 to 335 kPa and strain increments between 0.3 and 0.5. These results are one to three orders of magnitude higher than the hydraulic conductivity typically specified for drainage layers in leachate collection systems of 0.01 cm/s. Additional tests were also carried out to identify how landfill leachate and varied pH levels may affect the compressibility and hydraulic conductivity of the shredded tires. Care should be exercised in extending these results to field conditions, as the results presented are based on limited experimental testing data and a limited time frame.