## 5 Design Considerations  
GRMs are essentially lightweight materials (Figure 4). As a result, one of the benefits of using GRMs would be that the in situ overburden stresses associated with the self-weight of typical geomaterials could be greatly reduced. For instance, the reduction would be 6% for \(VRC\) = 10%, 13% for \(VRC\) = 25% and 23% for \(VRC\) = 40%. This is expected to lead to reduced design performance requirements, in terms of bearing capacity and settlement, for natural soil deposits that underlie a GRM layer installed as part of a residential structure foundation system or a geo-structure. Moreover, it would induce less settlement underneath embankments placed on compressible soil deposits or reduce the earth pressure behind retaining structures [16]. This study has demonstrated that GRMs have an adequate strength (i.e., \(\phi^{\prime}>30^{\circ}\)) irrespective of the \(VRC\) and rubber particle size, making them suitable structural fill materials for many geotechnical applications. However, the ultimate adoption of GRMs as structural fills in geotechnical application would also depend on their compressibility under sustained loads (creep). As displayed in Figure 5, it is clear that the higher the vertical effective stress applied on GRMs, the higher the vertical strain developed, and the lower the \(VRC\) that may be accepted in the mixtures to satisfy compressibility requirements. Moreover, \(AR\) also plays a key role; in fact, for any combination of \(VRC\) and vertical effective stress, it can be observed that the mixtures with smaller rubber particles (G-RS) experience greater vertical strain compared to those with large rubber particles (G-RL). Therefore, to have GRMs with reduced compressibility, gravel and rubber particles with similar grain size should be used. Permeability of GRMs can be related to the porosity of the mixes (which in turns depends on the particle size of rubber and gravel) as well as the vertical stress applied. Experimental evidence from relevant studies reported in the literature have shown that usually the hydraulic conductivity coefficient of soil-rubber mixtures with large rubber particles varies between that of typical gravelly soils at low confining stress levels and that of sandy soil at higher confining stress levels [36]. For mixtures of small rubber particles and \(VRC>10\%\), the hydraulic conductivity coefficient was similar to that of sandy soils regardless of the confining stress applied [36]. In the case of foundations, retaining walls, and embankments, this means that GRMs will function as a free-draining fill material ensuring the rapid dissipation of excess pore water pressure. Remarkably, [PERSON] et al. [30] reported that the use of GRMs (with \(VRC\) < 40%) placed on the top of liquefiable sandy soils can considerably reduce the pore water pressure generation in the sandy soil deposit, the reasons being: (1) GRMs themselves are not liquefiable and, therefore, can be conveniently used to replace the top most portion of liquefiable soil  
Figure 11: Concentration of zinc from 28-day batch leaching tests on GRMs (\(VRC\) = 40%) placed in deionized water: (**a**) G-RL; and (**b**) G-RS mixtures. layers, and (2) having an hydraulic conductivity higher than the sand, GRMs provide a preferential pathway for water to easily flow upward and quickly dissipate pore water pressure. The introduction of recycled ELT-derived materials in geotechnical applications may have benefits in terms of cost reductions and increased performance. However, it is essential to ensure that such innovations do not result in long-term negative impacts on the environment (e.g., through the leaching of toxic chemicals into the surrounding soil environment, groundwater, and surface water). The results of the leaching tests on GRMs have indicated that the leachate from smaller tire particles (RS) had a higher content of metals (e.g., Zn) compared to the large rubber one (RL), implying that particle size and surface area influence the concentration of elements in the GRM leachate.