Eco-rubber GSI Foundation Systems

Project Overview

Introduction

Tyre recycling is the process of converting end-of-life or unwanted waste tyres into materials that can be utilized in new products or applications. End-of-life tyres (ELTs) typically become candidates for recycling when they become no longer functional due to wear or damage, and can no longer be re-treaded or re-grooved (Basel Convention Working Group, 1999).

In many countries, ELTs are a controlled waste under environmental regulations, which place a duty of care on waste producers to ensure safe disposal through licensed carriers to licensed sites. For instance, in Europe, since 2003, under the European Union Landfill Directive, waste tyres are prohibited from being used for landfill. Even shredded tyres are not accepted by landfill facilities. In contrast, at present no national regulations are in place in New Zealand to efficiently manage waste tyre recycling, and with the ever-growing volume of ELTs, environmental and socio-economic concerns are urging the reuse of waste tyres through large-scale recycling engineering projects.

ELTs disposal in New Zealand

The current rate of ELTs production in New Zealand is over 5 million per year, including passenger vehicle tyres (approximately 4 million) and truck tyres (approximately 1 million), and is expected to grow over time with increased population and number of vehicles on roads. An estimated 30% of such waste tyres are exported or recycled; yet, the remaining 70% are destined for landfills, stockpiles, illegal disposal or otherwise unaccounted for (Ministry for the Environment, 2015; Cann, 2017), giving rise to piles of waste tyres that do not readily degrade or disintegrate.

Figure 1 shows a typical example of illegal ELT disposal practices currently seen in New Zealand. The dumping of scrap tyres into open areas and landfills is clearly the least desirable option for the ELTs disposal. Vast piles of tyres or buried tyres are unsustainable management practices and cause substantial environmental and health problems. Not only do they occupy a large amount of land (up to 75% void space), but they also harbour many pests, mosquitos and other insects that spread contagious and unknown diseases (Torretta et al., 2015). Potential leaching of metals and other chemicals contained in the tyres is also a danger to the environment as they may be toxic and result in water and soil contamination (Basel Convention Working Group, 1999; Lynch et al. 2017). Another big threat of waste tyres landfilling and stockpiling is the potential for uncontrolled fires. Several tyre fire incidents have been reported in recent years by New Zealand mass media (e.g. Dangerfield 2018a, b and 2019a, b). Though tyres are relatively difficult to ignite, tyre fires are difficult to extinguish and the firefighting and clean-up from tyre fires are costly. More importantly, the gasses emitted from tyre fires are high in CO, CO₂, sulphur oxides, and are harmful to both people and the environment (Rowhani and Rainey, 2016). All these serious issues mean that there are great environmental and socio-economic benefits in moving away from waste tyres disposal and to find sustainable management practices.

Waste tyre recycling may be challenging, but it is not impossible to achieve. In Europe, Canada, USA, Japan and many other countries where strategic waste marketing, collection and management procedures have been put in place to effectively making use of recycled ELTs, the disposal of waste tyres has been reduced to 20% or less (Pehlken and Essadiqi, 2005; Torretta et al., 2015; Ministry for the Environment, 2015).

Recycling of ELTs as granulated tyre rubber

Tyres have a mixed composition of carbon black, elastomer compounds, steel wire, in addition to several other organic and inorganic components. From a civil engineering viewpoint, ELTs represent a great source of low-cost, environmentally friendly and sustainable construction material having excellent engineering properties.

The current use of scrap tyre derived materials (in the form of chips, crumbs, granules, and shreds) mixed with granular soil (mainly sand) in civil/geotechnical applications includes light backfill material for retaining walls and bridge abutments, subgrade fills and embankments, drainage systems, slope stabilisation, landfill liners and caps (Mashiri et al., 2015). More recently, laboratory investigations on the dynamic properties of soil-rubber mixtures highlighted their potential use as seismic-isolation materials for foundation design (Senetakis et al., 2012). Numerical modelling conducted by Tsang et al. (2012) indicated that by the inclusion of a soil-rubber layer around the foundation of medium-rise buildings, the maximum horizontal acceleration at the roof and footing under earthquake loading could be reduced by 40% or more. Similarly, Brunet et al. (2016) reported that a layer (2-3 m) of soil-rubber mixture placed underneath a two-story building could decrease peak acceleration at the base by 54%. However, despite promising experimental and numerical results demonstrating the potential use of soil-rubber mixtures to enhance seismic isolation foundation systems with energy dissipation, to date there has not been any practical application and/or field-based validation.

The “Eco-rubber seismic-isolation foundation systems” project

Aimed at addressing the impelling problem of ELTs disposal and finding a sustainable way of recycling waste tyres in New Zealand, the Ministry for Business, Innovation and Employment (MBIE) funded a multi-disciplinary research project jointly put forward by researchers of the University of Canterbury and the Institute of Environmental Science and Research Limited (ESR). The main objective of this collaborative project is to recycle ELTs (in the form of granulated tyre rubber – GTR) mixed with gravelly soils and concrete to develop cost-effective seismic-isolation (with energy dissipation) foundation systems for low-rise residential buildings. As schematically shown in Figure 2, these proposed systems consist of two key elements:

i. a seismic-dissipative filter made of gravel-tyre rubber mixtures placed underneath the foundation structure, and;

ii. a flexible raft foundation made of fibre-reinforced rubberised concrete to accommodate differential ground displacements induced by liquefaction and lateral spreading.

The material composition (e.g. percentage and size of granulated rubber; amount of steel fibres in the rubberised concrete) and geometry (e.g. thickness of gravel-rubber layer; shape of rubberised concrete foundation) of these two elements may significantly change, depending on the assumed overburden pressures, seismic demand and building performance. Therefore, to properly design ideal foundation systems, complete material characterisation and criteria for system optimisation are required. This is achievable by combining:

a. Geotechnical and environmental engineering investigations to identify optimum gravel-GTR mixtures, having adequate mechanical properties and minimal leaching attributes;

b. Structural engineering tests to design flexible, fibre-reinforced, rubberised-concrete raft foundations with satisfactory structural performance;

c. Numerical and physical models to prove the concept, evaluate the seismic performance of the entire foundation system, and quantify the level of reduction in the seismic response of prototype buildings.

Although these systems are conceptually similar to conventional discrete elastomeric seismic isolation on rubber bearings (i.e. base-isolation; Skinner et al., 2000), they differ in that the proposed systems are continuously distributed along the contact surface separating the building or series of multi-storey/multi-dwelling complexes from the ground. As a result, it is expected that the accelerations and consequent seismic inertial forces affecting the superstructure to be drastically reduced (Tsang, 2008; Brunet et al., 2016).

There is a growing market in New Zealand in terms of the development of residential areas. Entire areas in Christchurch, Auckland, Wellington and other major towns are going to be subjected to residential development by 2023 (Redstall, 2016). Past earthquakes have shown that such buildings are extremely vulnerable to earthquake damage. In this context, the proposed seismic-resilient foundation systems would be potentially implemented across New Zealand providing widespread environmental and socio-economic benefits, as well as reduction in earthquake damage and economic loss. Moreover, the reuse of waste tyres currently disposed of in stockpiles would provide valuable land to be used for development.