Quasistatic response of loose cohesive granular materials

DEM-simulated model cohesive assemblies of spherical grains of diameter d, with contact tensile strength F₀, once prepared in loose states, are quasistatically subjected to growing isotropic pressure P, and then to triaxial compression, maintaining lateral stresses σ₂ = σ₃ = P while increasing axial stress σ₁ = P + q and strain ε₁. Reduced pressure P^* = d²P/F₀ varies from 0.1 (cohesion dominated case, for which systems typically equilibrate with solid fraction Φ ' 0.35), to large values for which the cohesionless behavior is retrieved. In triaxial compression, while the moderate strain response (ε₁ ~ 0.1) is influenced by initial coordination numbers and mesoscale heterogeneities, the approach to the critical state, as both q (deviator) and Φ steadily increase, gets slower for smaller P^*. Critical ratio q/P strongly increases for decreasing P^*, as roughly predicted in an “effective stress” scheme. Anomalously small elastic moduli are observed in the gel-like structures. While extensive geometric rearrangements take place, no shear banding is observed. Loose cohesive granular assemblies are thus capable of large quasistatic stable plastic strains and ductile rupture.