# Compaction Characteristics and Mechanical Response of Gravel-Glass-Rubber Mixtures +
## 6 Conclusions  
This paper reported on the preliminary results of a feasibility study aimed at evaluating compaction, one-dimensional compressibility, and shear strength of recycled crushed green glass bottles and recycled granulated tire rubber mixed with gravel. Dry specimens of selected gravel-glass-rubber mixtures (GGRM), prepared at 0.4 and 0.55 (i.e., 40% and 55%) rubber content by volume (\(R_{\rm B}\)) with varying glass content by volume (\(G_{\rm L}=0.15\)-0.6), were tested in the laboratory. The following main conclusions can be drawn from the study:  
* Compaction--the dry density of compacted GGRM decreases linearly with increasing \(G_{\rm L}\) and \(R_{\rm B}\). This is mainly due to the different specific gravity of the gravel (\(G_{\rm S}=2.66\)), green glass (\(G_{\rm S}=2.50\)), and rubber inclusions (\(G_{\rm S}=1.15\); lightweight). Moreover, an increase in glass breakage/crushing with increasing \(G_{\rm L}\) was observed, with the maximum amount of breakage (2.24%) obtained for \(R_{\rm B}=0.4\) (\(G_{\rm L}=0.6\)). This is associated with the higher amount of glass in the mixtures. The glass breakage, however, seems to decrease with increasing \(R_{\rm B}\) in the mixture; this is because soft rubber inclusions in the GGRM act as a cushion that absorbs part of the impact compaction energy delivered to the specimen. * One-dimensional compression--the volumetric strain of the studied GGRM significantly increased with \(G_{\rm L}\) due mainly to the glass crushability. Nevertheless, if the mixtures are subjected to unloading and reloading, their compressibility is drastically reduced, and their volumetric response becomes essentially elastic. This implies that during the unloading-reloading phase, the glass crushing and gravel grain rearrangement (mainly responsible for the plastic deformation) are minimized, and that the observed elastic response is due primarily to the rubber rebound. * Shear strength--the shear strength of GGRM evaluated by direct shear tests is comparable to that of gravel-rubber mixtures (GRM) without glass in terms of overall mechanical response, peak shear strength, and friction angle. Specifically, under normal stress up to 100 kPa, the peak friction angle is found to be between 30\({}^{\circ}\) and 45\({}^{\circ}\). * Behavioral zones--seven distinct behavioral zones can be defined for GGRM due to the simultaneous presence of soft rubber-like (S), stiff gravel-like (G), and crushable glass-like (C) materials. The investigated compacted GGRM can be considered intermediate materials; their behavior gradually transitions from stiff-soft (GS) to stiff-soft-crushable (GSC) to stiff-crushable (GC) as glass is added in the mixtures as a replacement for gravel. * Practical applications--overall, GGRM possesses adequate strength (i.e., a peak friction angle of 30\({}^{\circ}\) or greater) to be used as structural fill in eco-friendly geotechnical projects, such as lightweight backfill materials, shallow underground layers for mitigation of liquefaction phenomena, and geotechnical seismic isolation for low-to-medium-rise lightweight structures/infrastructure. While the volumetric compressibility of GGRM may be of concern in applications where high static loads are applied, this study demonstrates that it can be effectively enhanced (for example, by using preloading) and its impact minimized. Additionally, GGRM can be safely used as general fill material. While this study only dealt with the response of GGRM under static loading conditions, the authors recognize that one of the critical components of material characterization is understanding their behavior under dynamic loading conditions. Although no experimental data are yet available for GGRM, investigations on GRM by the authors [60; 61] and other researchers [62; 63; 64; 65; 66] may also provide useful information relevant to GGRM, such as small-strain stiffness and strain-dependent dynamic properties. Nevertheless, similar investigations are required to further characterize GGRM from a dynamic viewpoint.