# Investigation of Geotechnical Seismic Isolation Systems Based on Recycled Tire Rubber-Sand Mixtures
## 2 Materials and Methods
This outcome can be anticipated by comparing the stiffness of rubber particles to that of soil grains. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Granular Rubber** & **Moist Unit** & **Dry Unit Weight** & & **Relative** \\
**Content (\%)** & **Weight (kN/m\({}^{3}\))** & **(kN/m\({}^{3}\))** & **Void Ratio** & **Density Dr (\%)** \\ \hline
0.0 & 22.0 & 20.64 & 0.259 & 91.47 \\
5.0 & 20.84 & 19.42 & 0.253 & 91.96 \\
10.0 & 19.47 & 17.96 & 0.266 & 91.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of the prepared samples. Figure 1: Changes in damping ratio, shear stress amplitude, and confining pressures for 0.0%, 8%, 10% and 14% granulated rubber obtained from the three-axis compression test based on from [12]. Addedly, replacing soil grains with granular rubber softens the mixture, resulting in a decrease in shear modulus. In this study, the type of rubber considered is granulated tire rubber derived from waste tires that were mechanically chopped through multiple chopping steps. The granulated rubber particles used in the experiments have nonspherical shapes and dimensions ranging from 0.15 to 9.5 mm. The specific gravity of the granulated rubber was found to be 1.1 at a temperature of 20 \({}^{\circ}\)C, following ASTM standards. Damping ratio-shear strain amplitude curves for soil-granular rubber mixtures are presented in Figure 3 for confining pressures of 50 and 100 kPa, with granular rubber additions of 0.0%, 5%, and 10%. The results show that as the amount of rubber increases, the damping ratio decreases for both 50 and 100 kPa confining pressures. This difference can be attributed to the high elastic deformation capacity of rubber particles, leading to increased elastic deformation and a subsequent decline in damping ratio at lower confining pressures (50 and 100 kPa). Damping ratio-shear strain amplitude curves for soil-granular rubber mixtures are presented in Figure 3 for granular rubber contents of 0.0%, 5%, and 10% and confining pressures of 50 and 100 kPa. In soils without rubber addition, an increase in confining pressure results in a decline in the damping ratio. This behavior can be explained by the increase in interparticle friction as confining pressure rises, leading to reduced plastic deformation. As a consequence, the width of the hysteresis loop decreases, causing a reduction in the damping ratio. Figure 3 also shows that in soils with granular rubber, the damping ratio increases with higher confining pressure, while the opposite trend is observed in soils without rubber. This difference can be attributed to the compression of rubber particles under higher confining pressure, which reduces their elasticity; as a result, the sliding of soil particles over each other increases when deviator stress is applied. Consequently, plastic deformation increases, leading to a rise in the damping ratio. Additionally, the results signify that with 5% and 10% rubber inclusions, not only does the damping ratio increase with confining pressure, but the effect of confining pressure on the damping ratio also intensifies with higher rubber content. This phenomenon can be explained by the increased plastic deformation under high confining pressures and greater  
Figure 2: Evaluation of the variation in shear modulus with shear strain amplitude under 50 and 100 kPa confining pressures for different percentages of granular rubber. elastic deformation at low confining pressures when higher proportions of rubber are used. Therefore, increasing rubber content leads to greater differences in damping ratios under different confining pressures. Figures 4-6 depict the G/Gmax values, represented by curves, obtained from laboratory tests on soils selected for the isolation layer. The results suggest that for a given percentage of rubber, G/Gmax values increase as confining pressure rises, indicating an increase in soil stiffness with higher confining pressure.