Geomembranes are used in waste containment barrier systems to limit to movement of contaminants. The geomembrane is placed over a clay liner, and a drainage blanket consisting of highly permeable coarse aggregate is placed above the geomembrane. The drainage blanket layer allows for removal of leachate to control the hydraulic head over the barrier system.  The large uniform aggregate ensures that the pore volume and the size of individual pores will both be sufficiently large so as to ensure a resilience against clogging.

Coarse uniform gravel is typically used as a drainage layer. Where such gravel is scarce or expensive, there has been a trend in recent years to use tire derived aggregate (TDA) to save cost, resources, and as a method of recycling the ever-growing stockpile of used tires. TDA is composed of vehicle tires that have been shredded. These pieces vary in size and contain rigid protruding wires that may puncture the geomembrane.

Differential loading of the geomembrane by the large TDA or gravel particles over a yielding compacted-clay subgrade leads to indentations and potential puncture. The high localised strain that occurs at and around the indentations may lead to stress cracking in the future.

To evaluate and compare induced localised strain from gravel and TDA, a procedure using photogrammetry was developed and performed on over 60 laboratory tests within a custom-built test device. Clay properties (including moisture content, density, plasticity, and silt content) play a key role in the development and magnitude of resulting strains. Fast loading rates (over less than 1 hour) resulted in higher strains when compared to slow incremental loading (over 6 months). The role of the clay in the deformation of geomembranes is a significant factor that must be taken into account when designing composite barrier systems.

When evaluating puncture, the wires in TDA caused immediate punctures at 500 kPa vertical load. The risk of puncture was a function of the proportion of wires, how the TDA particle lands, and the efficiency of the protection layer. Using a heavy nonwoven geotextile protection layer (>1000 g/m²) and strict quality control, the number of expected small holes (1mm) per hectare may be fewer than 20.

The final component of this study involved a comparison of tensile strain induced by TDA and gravel. High-localised strain from TDA was less than for gravel when evaluated using the method developed by Tognon (1999). However, since Tognon’s method does not accurately represent the deformation path, the estimated strains are incorrect. A new direct method for evaluating strains is presented. Through the new method, localised strain induced by gravel and by TDA were found to be higher and lower respectively compared with the estimations made using using Tognon’s (1999) method. It is concluded that, for different reasons, both TDA and gravel require heavy protection layers or soil protection layers to ensure long-term performance.