# Compaction Characteristics and Mechanical Response of Gravel-Glass-Rubber Mixtures +
## 2 Materials  
The materials assessed in this study consist of pea gravel (specific gravity, \(G_{\rm S}\) = 2.66; mean diameter \(D_{50}\approx\) 5.5 mm), granulated recycled rubber (\(G_{\rm S}\) = 1.15; \(D_{50}\approx\) 5.2 mm), and crushed green glass (\(G_{\rm S}\) = 2.50; \(D_{50}\approx\) 5.5 mm). The gravel and rubber were both commercially sourced. The gravel was washed and dried before any testing began to remove any trace of fine material. The rubber is free from steel wires and textile reinforcement. The crushed green glass consisted of a range of angular particle shapes and sizes, with a few long, slender pieces. It was obtained from wine bottles that were stripped of their labels and cleaned before crushing.  
Previous studies on GRM have shown that the use of smaller rubber inclusions produces materials with lower friction angles and high compressibility [38]. Additionally, other studies reported in the literature have demonstrated that sand mixed with large rubber mixtures also produces highly compressible materials [29]. Thus, the authors have recommended the use of gravel particles and rubber inclusions of the same \(D_{50}\) or aspect ratio = 1 to form GRM [38]. In the case of crushed glass fragments, preliminary investigations, propoedeutic to the GGRM reported in this study, have indicated that the use of larger glass results in greater crushability of the GGRM. Therefore, to minimize the crushability of large glass fragments and the compressibility of large rubber inclusions, rubber and glass having the same size as the gravel were used in this study to form GGRM.  
Representative photos of the various materials are shown in Figure 1a, along with their particle size distribution (PSD) curves (Figure 1b). As schematically shown in Figure 1b, the gravel has rounded to subrounded grains with aspect ratios (\(H/L\)) ranging from 1 to 1.6, the glass fragments have a plate-like shape of 2.8 mm thickness on average and aspect ratios varying between 1 and 1.8, and the rubber inclusions have a cube-like shape with aspect ratios in the range of 1 to 1.5 and an average thickness of 5 mm.  
As shown in Figure 2, different GGRM mixtures were formed by keeping the \(R_{\text{B}}\) constant at 0.4 and 0.55 (i.e., 40% and 55%) but changing the proportion by volume of glass (\(G_{\text{L}}\)) and gravel (\(G_{\text{R}}\)) in the mixtures.  
Figure 1: Materials assessed in this study: (**a**) photographic images and (**b**) particle size distribution curves and schematic illustration of idealized particle shapes.  
Figure 2: Summary of GGRM tested in this study and GRM previously tested by [PERSON] et al. [38].  
The volumetric fraction of each material was calculated using Equations (1)-(3):  
\[G_{\rm L}=V_{\rm Glass}/(V_{\rm Glass}+V_{\rm Gravel}+V_{\rm Rubber}) \tag{1}\]  
\[R_{\rm B}=V_{\rm Rubber}/(V_{\rm Glass}+V_{\rm Gravel}+V_{\rm Rubber}) \tag{2}\]  
\[G_{\rm R}=V_{\rm Gravel}/(V_{\rm Glass}+V_{\rm Gravel}+V_{\rm Rubber}) \tag{3}\]  
where \(V_{\rm Glass}\), \(V_{\rm Rubber}\), and \(V_{\rm Gravel}\) are the volumes of the glass, rubber and gravel in the mixture, respectively.