# Use of Granulated Rubber Tyre Waste as Lightweight Backfill Material for Retaining Walls
## 5 Conclusions  
This paper presents studies on the use of granulated rubber mixtures for their applications in retaining walls. Experimental work is performed in order to determine the geotechnical properties of granulated rubber as a lightweight backfill soil; these properties are subsequently verified in two retaining walls of different dimensions. From this study, the following conclusions can be drawn based on the results obtained:  
* Research concerning the use of ELTs as a material in geotechnical works has focused on tyre shreds in a wide range of sizes, ranging from 76 mm to 12 mm. This document provides specific information on the characteristic values of the geotechnical parameters of a detailed range of particle sizes smaller than 12 mm. * The results show uniformity coefficients lower than three, even in the combinations of S5: 90% 2.0-7.0 + 10% 0.6-2.0 mm and S6: 50% 2.0-7.0 + 50% 0.6-2.0 mm. These are described as uniform particle sizes. They represent samples where the particle size trend tends to equalise. * The density of the samples tested ranged from 9.79 kN/m\({}^{3}\) to 10.32 kN/m\({}^{3}\). Figure 11: ODF sliding and overturning versus schemas of the case study in the 2.80 m retaining wall. * The Proctor Compaction Test was performed on the rubber granulate samples and the maximum dry unit weight obtained was 6.03 kN/m\({}^{3}\), which is less than one third of the maximum dry unit weight of a conventional soil. * The characteristic values of the geotechnical parameters have been estimated according to _Eurocode 7_. The friction angle results range from 18.27 to 23.21 degrees and the cohesion results are wide-ranging, with values from 9.35 to 17.83 kPa. According to the values provided by the Table 2, the results obtained are in agreement with the results of the literature reviewed, with values of 3 to 57 degrees and 7.6 to 39 kPa, respectively. * The walls of lower height show a greater increase in partial safety coefficients regarding the limit values in accordance with EC7-Part 1, mainly due to the decrease in lateral earth pressure at the back, as a result of the low unit weight and lack of cohesion of the tyre granulates. * The comparison between the coefficients obtained in accordance with the various Project Approaches and with the CTE-DB-SE-C shows that the executed procedures fail to provide similar results and that, therefore, the effect that cohesion exerts on the safety coefficients obtained in the calculation should be evaluated, such that they are not so obviously oversized as in the case of EC7-1. * No effective differences are observed in the ODF obtained based on the different performing mixes analysed for both the elevation of 5.80 m and 2.80 m in length. Therefore, the choice of performing mix should not be made on the basis of the granulometry. * The constructive solutions using granular rubber as filling in the back of the containment walls allow for the optimization of the section of the concrete wall, and therefore, a greater saving in the execution. For the sand type filling, the sliding safety coefficients do not exceed the limit values. Conceptualization, E.C.-M., M.A.-G., E.M.A.-G. and E.J.M.-A. ; Formal analysis, E.J.M.-A. and R.R.-H.; Investigation, E.C.-M. and M.A.-G.; Methodology, E.C.-M., M.A.-G., E.J.M.-A. and R.R.-H.; Project administration, A.J.-M.; Resources, A.J.-M., E.J.M.-A. and C.S.-C.; Supervision, E.M.A.-G. and A.J.-M.; Validation, E.J.M.-A., R.R.-H. and C.S.-C.; Writing--original draft, E.C.-M., M.A.-G., E.M.A.-G., E.J.M.-A., R.R.-H. and C.S.-C.; Writing--review & editing, E.J.M.-A., R.R.-H. and C.S.-C. All authors have read and agreed to the published version of the manuscript. This research has been carried out within the framework of the specific collaboration agreement between the University of Seville and the company SIGNUS.