# Use of Granulated Rubber Tyre Waste as Lightweight Backfill Material for Retaining Walls
## 4 Results and Discussion
### Design Criteria of Retaining Walls  
The case study that has been the focus of this paper compares the safety factors obtained from an L-shaped cantilever retaining wall. The backfill is a mix of type granulates with a top layer of sand as shown below (Figure 9). For geotechnical verifications, the following actions are considered: the weight of the structure itself, the lateral earth pressure, the cohesion, and a surcharge (Q) of 5 kN/m\({}^{2}\). The retaining wall calculations for samples S4, S5, and S6 are developed below, due to how they present greater workability advantages than mixtures with lower granulometry. This will make it possible to determine the degree of influence of the geotechnical parameters obtained on the structural behaviour of the scheme. Only the drained conditions have been considered for geotechnical verifications. The properties of the materials considered are summarised in Table 6.  
Design Situation and Loads  
Resistance to sliding, overturning, and bearing constitute the main geotechnical tests usually carried out on a retaining wall by means of safety factors in accordance with current regulations. For this study case, geotechnical verifications are exclusively considered for a persistent situation, which corresponds to the main condition of use. The safety factors for overturning and sliding are analysed through their evaluation in accordance with CTE-DB-SE-C (v2019) and Eurocode EC7-1 (v2015). EC7-1 addresses the geotechnical project from the definition of \"Limit States\" through the use of partial safety factors, which affects the actions, materials referred to as geotechnical parameters, and finally, ground resistance. In this regard, it reveals various \"Project Approaches\", which constitute alternative ways of quantifying the uncertainty in the calculation of the limit states by adopting the different partial safety coefficients. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Backfilling** & \(\boldsymbol{\gamma}\) **(kN/m3)** & **C\({}^{\prime}\) (kN/m2)** & \(\boldsymbol{\phi}^{\prime}\) (\({}^{\circ}\)) \\ \hline Sand & 26.76 & - & 30.00 \\ Sand + S4 & 6.36 & 15.77 & 21.21 \\ \hline Sand + S5 & 6.28 & 11.74 & 18.27 \\ \hline Sand + S6 & 6.26 & 14.25 & 21.97 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Material properties selected for the verification. Figure 9: Schema of the case study: (1) L = 5.80 m; (2) L = 2.80 m.  
The analysis of the various Project Approaches has led the Spanish National Annex to initially opt for Project Approach 2 for geotechnical actions, except for the calculation of stability and slopes, for which Project Approach 3 is selected. Project Approach 2 does not undermine geotechnical parameters and is therefore more similar to the usual practices in Spain. However, the major difference between the method used in Spain and that developed in the Spanish National Annex involves the treatment of the actions where they are increased. The disparity of the results obtained, according to Project Approach 2, has led to the development in this test of all Project Approaches with the aim of verifying the ODF factors obtained. The hypothesis developed determines the overdesign factors (ODF) obtained based on all the EC7-1 Project Approaches and in accordance with the CTE-DB-SE-C for the study of a section of the cantilever retaining wall, as shown in Table 7 and Figure 10. In addition, for a lower section, the same verifications are carried out, as shown in Table 8 and Figure 11.  
For the retaining wall of 5.80 m in height, the safety factor for overturning shows an increase between 74.80% and 78.55% in accordance with EC7-1 and 29.82% in accordance with CTE-DB-SE-C with regard to the limit value. As for sliding, a difference between 58.11% and 69.25% is observed between the average of the coefficients obtained in accordance with the design approaches and the limit value for EC7-1, while the coefficient obtained in accordance with CTE-DB-SE-C represents a difference of 25%.