Investigation into the use of waste tyre shreds for reinforcement of sandy soils in South Africa.

Abstract

End-of-life tyres are a disposal problem resulting from the large volumes produced worldwide every year. Waste tyres are difficult to manage because of their sheer volume and the potential impacts on human health and environment. These discarded tyres, currently stockpiled and dumped in the open, are a source of fire hazards and provide a prolific breeding ground for mosquitoes and other pests. The use of waste tyre shreds as fill material in geotechnical applications can help to mitigate the waste tyre disposal problems. Specifically, when tyre shreds are used as lightweight fill material in the construction of highway embankments, a considerable volume of waste tyres is consumed. With regard to this, an investigation into tyre shreds mixed with sandy soils of South Africa was undertaken to assess the shear strength behaviour of the sand-tyre shred composite and to propose an alternative use of the scrap tyres produced every year. A series of shear strength tests were performed using a large-scale direct shear box on the mixtures composed of tyre shreds with two relative sizes, i.e. 10-15 mm and 50-60 mm sizes, combined separately with Cape Flats and Klipheuwel sands. The shear strength tests started from unreinforced material (control tests) followed by those on tyre shreds-sand composites at different shred dosages such as 10, 20, 30, 40 and 50% by dry weight, ending on pure tyre shreds. The results showed that this inclusion generally improved the shear strength of sand. The angle of friction reached its maximum at 10% tyre shred content and reduced for increased shred content in the mixtures. The cohesion was improved for dosage up to 30%, then decreased at higher concentrations. Despite the shred size in the composite, a concentration of 30% by dry weight was considered as optimum shred content to reinforce granular soils of South Africa, but the long shreds showed better improvement compared to the small tyre pieces. Based on the findings of this study, the use of lightweight tyre shreds sand composite material was recommended in the construction of road embankments and the guidelines to be followed were proposed. Because of the complexity in the preparation of the mixture based on density, the optimum shred dosage by weight was converted into volume ratio and this corresponded to 55% shred content by volume which can easily be used on site. For workability and simplicity of quantity measurements in the field, 50% tyre shred content by dry volume was suggested in the proposed guidelines to be mixed with sand in the preparation of embankment material.