## 4 Experimental Results
### Shear Strength of GRMs  
Alike conventional soils, shear strength is one of the most important characteristics contributing to the performance for GRMs. The shear strength is the result of friction and interlocking between particles and likely bonding at particle contacts. Due to interlocking, GRMs may contract or expand in volume as they are subject to shear strains.  
The stress-strain-volumetric behavior of G-RL and G-RS attained by direct shear tests at 60 kPa normal stress (\(\sigma_{\text{n}}\)) is compared in Figure 6. It can be seen that, as _VRC_ increased, the GRM response progressively changed from dilative with a clear peak shear state (gravel-like behavior) to contractive without a peak shear state (rubber-like behavior). Similar tendencies were observed at \(\sigma_{\text{n}}=30\) and 100 kPa.  
Figure 5: Compressibility of GRMs in 1-D compression tests with 2-h creep: (**a**) G-RL and (**b**) G-RS mixtures.  
The summary plot reported in Figure 7a shows the change in maximum shear stress (\(\tau_{\max}\) that was evaluated at peak state for dilative materials or at large deformation for contractive materials) with increasing \(VRC\) for all mixtures sheared at \(\sigma_{\rm n}\) = 30, 60, and 100 kPa. It can be seen that the trends were very similar for both G-RL and G-RS, indicating that the effect of \(AR\) on the strength of tested GRMs was almost insignificant. Alternatively, for any given \(\sigma_{\rm n}\) value, \(\tau_{\max}\) decreased considerably with increasing \(VRC\). Moreover, such decay was more significant at higher \(\sigma_{\rm n}\). For example, at \(\sigma_{\rm n}\) = 100 kPa, \(\tau_{\max}\) was approximately 137 kPa at \(VRC\) = 0% (gravel) and 73 kPa at \(VRC\) = 40%, corresponding to a reduction of 1.9 times. At \(\sigma_{\rm n}\) = 30 kPa, \(\tau_{\max}\) is approximately 45 kPa at \(VRC\) = 0% and 28 kPa at \(VRC\) = 40%, corresponding to a reduction of 1.6 times.  
Figure 7b reports the values of the Mohr-Coulomb effective friction angle (\(\phi^{\prime}\)) for all mixtures. Essentially, \(\phi^{\prime}\) decreased significantly with increasing \(VRC\) (and only slightly with \(AR\)) from about 54\({}^{\circ}\) (gravel) to 29\({}^{\circ}\) (graduct rubber). Notably, excluding G-RS with \(VRC\) > 85% and G-RL with \(VRC\) > 95%, most of the GRMs had a high strength (i.e., \(\phi^{\prime}\) > 30\({}^{\circ}\)) regardless of the \(VRC\) and rubber particle size, making them potentially suitable structural fill materials for many geotechnical applications [17; 19].  
Figure 6: Direct shear behavior of GRMs at 60 kPa normal stress: (**a**) G-RL; and (**b**) G-RS mixtures.  
The packing properties of GRMs described earlier (Section 4.1) may provide valuable information on their mechanical behavior (refer to Figures 5-7). Nonetheless, the load-transfer mechanism within the mixtures depends predominantly on the skeleton material [39; 40; 41], which is formed when particles of the same material are in contact with each other and are able to transfer loads. The material forming the skeleton becomes the matrix material that governs the general mechanical behavior of a mix. As shown by Figure 8, DEM-based micro-mechanical strong-force network analyses conducted by [PERSON] et al. [43] have indicated that two matrix materials can be expected for GRMs: (i) a gravel matrix, leading to a stiffer gravel-like response of GRMs (\(VRC\leq 30\%\)); and (ii) a rubber matrix, producing a softer rubber-like behavior of GRMs (\(VRC\geq 60\%\)). In between, an intermediate gravel-rubber behavior will likely take place (\(30\%<VRC<60\%\)). Such distinct behavioral zones provide a framework explaining the gradual change in behavior from stiff gravel-like to soft rubber-like, as observed in the direct shear tests.  
Figure 8: Behavioral zones for GRMs in direct shear tests evaluated by DEM strong-force network analysis.  
Figure 7: Strength characteristics of GRMs: (**a**) variation of maximum shear stress with normal stress and \(VRC\); and (**b**) variation of effective friction angle with \(VRC\).