# Compaction Characteristics and Mechanical Response of Gravel-Glass-Rubber Mixtures +
## 1 Introduction  
Traditionally, freshly quarried aggregates and dredged soils have been the primary source of construction materials in geotechnical applications. However, nowadays, the recycling of waste materials (i.e., industrial by-products, commercial waste, and construction and demolition rubble) is becoming a priority. This shift towards secondary usage of waste materials has numerous benefits, such as reducing the need for fresh primary materials, minimizing waste disposal, benefiting the environment, enhancing sustainability, reducing carbon emissions, and promoting a circular economy [1, 2, 3, 4, 5, 6]. In New Zealand, each year, over 6.3 million waste tires are produced [7]; additionally, 120,000 tons of glass and 380,000 tons of plastic are sent to landfills [8]. The amount of waste sent to landfills increased by 47% from 2009-10 to 2018-19 [8]. The large volume of discarded materials highlights the need to reuse and recycle waste materials [2], which is a crucial strategic approach for meeting the UN Sustainable Development Goals (SDGs) [9] and becoming a more sustainable country. From a geotechnical engineering perspective, recycled crushed glass [10, 11, 12, 13, 14, 15, 16, 17, 18, 19] and recycled tire rubber inclusions [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32] have excellent mechanical properties and durability. These waste materials are readily available and cost-effective. The benefits of using such recycled materials are significant, especially if they are used as a replacement for fresh construction materials made from non-renewable resources, e.g., quarried gravel and sand. The engineering characteristics of discardedgranular materials can be utilized in a beneficial way to promote sustainability in the geotechnical construction sector [6]. The reuse of granulated rubber derived from waste on its own or mixed with granular soils has been found suitable in applications such as lightweight embankment fill, drainage layers, conventional fill, and retaining wall backfill [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. Direct shear strength parameters reported in the literature for pure rubber, sand-rubber mixtures (SRM), and gravel-rubber mixtures (GRM) have been comprehensively summarized by [PERSON] et al. [33]. Although the friction angle (15-35\({}^{\circ}\)) of pure rubber is generally lower than pure soils, there is a range of volumetric rubber content (\(R_{\rm B}\)) (i.e., 20% \(<R_{\rm B}<\) 50%) within which the friction angle of SRM exceeds that of the pure sand. This means that the shear strength of pure sand can be enhanced by mixing it with rubber. For these mixtures, the friction angle varies between 26\({}^{\circ}\) and 62\({}^{\circ}\). Compared with SRM, tests on GRM are still very limited. Yet, the available data suggest that the friction angle of GRM generally decreases with increasing \(R_{\rm B}\), so the addition of rubber does not improve the strength of gravel soils. Yet, it can be debated that the addition of gravel can enhance the mechanical properties of highly compressible and low-strength rubber as compared with sand. In the New Zealand context, the recycling of waste tire-derived granulated rubber mixed with gravel for use in eco-friendly geotechnical projects has been investigated by [PERSON] et al. [34; 35; 36], [PERSON] et al. [33; 37; 38; 39], and [PERSON] et al. [40; 41]. It has been found that GRM with \(R_{\rm B}\leq\) 40% have adequate strength, low compressibility, and high energy dissipation properties; hence, they can be used as structural fill materials. GRM with 40% \(<R_{\rm B}\leq\) 55% may also be appropriate as fill materials if their compressibility meets performance requirements. The use of gravel-size rubber inclusions in GRM has also been recommended. While different rubber sizes show minimal to no effect on the peak shear strength, the larger rubber inclusions (similar in size to the gravel grains) show less compressibility [33; 34; 35; 36; 37; 38; 39] and the least leaching of toxic chemicals [40; 41].