In view of the vast numbers of used tyres stockpiled across the USA and their problematic disposal characteristics, it makes environmental and financial sense to find ways of recycling them. This briefing paper describes a research project which was undertaken in Indiana to construct and test a soil–tyre enbankment. Findings are outlined and recommendations made.
1. INTRODUCTION
It is estimated that there are more than 500 million tyres stockpiled across the United States, and 270 million more are generated each year. A significant number of them are disposed of in landfills, left in empty yards and even dumped illegally. Tyres have characteristics that render them difficult to dispose of, and potentially combustible. For these reasons, there is a strong need to find beneficial ways to recycle or reuse tyres. Civil engineering applications constitute one of the major markets for scrap tyres. They have been used in various areas such as leachate collection systems, landfill cover, artificial reefs, clean fill for road embankments, road bed support and similar projects. Using tyre shred material for civil engineering application has several advantages, the most important property being that it is a lightweight material. There are many benefits of using tyre shreds as lightweight fill in embankments or retaining walls since tyre shreds are non-biodegradable and thus more durable. Also they are inexpensive compared to other lightweight materials.
The present research project consists of construction of a test tyre shred and soil embankment as well as basic laboratory tests for material property characteristics and instrumentation of the embankment. The instrumentation includes settlement monitoring using nine settlement plates, vertical and horizontal inclinometer monitoring, temperature monitoring and ground-water quality analysis. The objective of this project is to evaluate the feasibility of using a mixture of tyre shreds and soil as fill material for embankments on the basis of field instrumentation and tests.
2. FINDINGS
Based on the investigation and monitoring of the tyre shred and soil embankment test site, the following observations and conclusions are made.
At State Road 31 in Lakeville, Indiana, a tyre shred and soil embankment was constructed. The fill material was a 50/50 mixture by volume of tyre shred and soil. The total volume of the embankment after compaction was approximately 813 m3. The height and length of the embankment was about 2 m×20 m.
Monitoring, using nine settlement plates at three different sections, was conducted for a year after opening the road for traffic. The maximum settlement was approximately 12 mm and the settlement stabilised after 200 days of traffic.
Vertical and horizontal inclinometer monitoring was conducted to check lateral movement and differential settlement for one year. Maximum lateral movement was about 5 mm. No evidence of significant differential settlement has been observed.
Samples of groundwater were analysed for metals which apply to a secondary drinking water standard and standard of maximum contaminant level for drinking water according to Indiana Department of Environmental Management (IDEM). With the exception of manganese, all levels have been well below the standard limits. However, manganese is not a health concern.
To check for the possible development of exothermic reactions that might lead to the initiation of fires in the embankment, the temperature was observed for a year. No evidence of internal heat generation has been detected. This confirms that the use of a tyre shred and soil mixtures is an effective method for preventing self-heating of the tyre shreds.
Observations show no signs of slope stability problems, cracking on the road or erosion.
3. IMPLEMENTATION
Following this study, the following recommendations can be made.
Based on the above findings and observations, using a mix of tyre shreds and soil in embankments or fills is very promising and should be promoted. Performance of the test embankment was quite satisfactory. Advantages of this material include the fact that it is lightweight, relatively cheap, easy to compact, free-draining and relatively compressible. Additionally, this use is beneficial to the environment in that a waste material is recycled.
Given the characteristics of soil–tyre shreds mixtures, they can be used successfully as backfill material for retaining structures. Since tyre shreds are lightweight, they induce low horizontal stresses, thus reducing the thickness of retaining structures. Similar cost savings will also be possible for mechanically-stabilised earth (MSE) walls.
To prevent self-ignition, floating of the tyre shred in a soil matrix is desired. The minimum mixing ratio that produces such an arrangement can be determined by vibration compaction tests. The mixing ratio producing a minimum overall void ratio is the minimum mixing ratio leading to this arrangement. Other mixing ratios can be explored in order to increase the strength of other experimental mixtures.
