# Use of Granulated Rubber Tyre Waste as Lightweight Backfill Material for Retaining Walls
Nevertheless, it has been demonstrated that the substitution of recycled aggregates for conventional materials leads to reductions in CO\({}_{2}\) emissions. In large-scale projects, reductions can reach more than 50%, obtaining environmental benefits [5]. Nowadays, the USA, the European Union, Japan, and India produce almost 88% of End-of-Life Tyres (ELTs) worldwide [6]. End-of-Life Tyres account for approximately 6.7 million tonnes of waste in the European Union (EU) annually. France, the United Kingdom, Italy, and Spain are responsible for approximately 80% of the European waste production generated by ELTs [7; 8]. Considering that 70% of the tyre is rubber [8], it can be stated that around 4.7 million tonnes of rubber granulate is generated annually in the European Union. This makes it important to develop new applications and new markets for this waste. The ELT Directive (2000/53/EC) [9] is in charge of the management of ELTs in the European Union. Within its regulations, it indicates the selective collection of tyres for their proper management. The directive encourages recycling and, in recent years, has made drastic changes in order to render the management of tyre waste more sustainable. The problems of over-accumulation of industrial waste have become a global problem, especially regarding waste that takes up such large storage spaces as ELTs do [10]. The major issue related to the large areas of tyre storage involves their high flammability and potential risks for the environment and human health [11; 12; 13]. Disease-carrying mosquitoes can find a breeding ground [14] in the storage of tyres. In order to find solutions to the environmental problems and health threats caused by tyre waste, those countries with the largest industries have a legal framework for its management. Regulations to date have focused on the general characteristics that ELTs must have in the treatment plants. Over the last decades, there has been a marked increase regarding the interest shown in international research into the properties and potential uses for ELTs in sustainable applications [15]. In this sense, ELTs present attractive characteristics in the implementation of civil engineering work and their application has been studied for more than 40 years. It is well known that the use of industrial waste in geotechnical work produces benefits to the environment, along with the benefits of the particle characteristics and optimal use of each type of waste [6; 16; 17; 18]. Many types of industrial waste are available in large quantities; whether they are used or not generally depends on the lack of proper testing methodology and standards to ensure good performance. Furthermore, there is little experience and diffusion of the implementation and also a lack of financial means, technical equipment, time to develop research work, adequate training, and legislation. Within this line of work, the use of ELTs has favoured the appearance of research focused on their application as lightweight filling of retaining walls [19; 20; 21; 22]. For such applications, the behaviour of the retaining wall is conditioned by the lateral earth pressure, which is defined by the characteristics of the material (angle of internal friction and unit weight). [PERSON] et al. [16] used only tyre chips (12 and 50 mm) as a lightweight retaining wall fill and achieved higher safety levels against sliding and overturning compared to those from using sand as a filler. [PERSON] et al. [23] observed that the horizontal pressure on a retaining wall is lower when it is filled with a mixture of tyre shreds (50 and 305 mm) and sand than with a gravel backfill. However, these proposals used ELTs waste containing large size particles. The following research proposes a new revaluation of rubber granulate in an application that allows a greater impact per cubic metre used, carried out with a detailed range of particle sizes smaller than 12 mm. This approach aims to provide a solution to the problem of over-storage of this particle size in treatment plants.