Simulating granular materials interacting with solids, such as granular avalanches impacting protection structures or soil-wheel interaction, requires accurately capturing their complex trans-phasic rheology. While extensive research in civil and geotechnical engineering has focused on modeling granular materials in their solid and liquid-like phases within a continuum framework, limited efforts have been made to coherently capture their gas-like behavior and the transitions between these three phases. These transitions, particularly during separation (solid-liquid-gas) and reconstitution (gas-liquid-solid), are critical for accurately modeling granular dispersal, reconsolidation, and soil-structure interactions in low-confinement regions. Previous studies have primarily focused on modeling dry, frictional grains within a continuum mechanics framework, whereas research on wet granular flows, where cohesive particle-to-particle interactions play a significant role, remains limited. Modeling cohesive grains is particularly challenging due to the hysteretic nature of interparticle attractive forces. In this study, we propose a constitutive theory and rheology for cohesive-frictional granular materials, incorporating shear-induced dilation and hysteretic granular separation, for granular assemblies with stiff grains. This constitutive model is implemented within the Material Point Method (MPM) framework to ensure numerical stability and efficiency. This work will present the theoretical formulation, numerical implementation, and simulation results demonstrating the model’s capabilities.