Understanding the motion of particles in very dense granular flows is crucial for comprehending the dynamics of many geological phenomena, and advancing our knowledge of granular material physics. We conduct transparent ring shear experiments to directly observe the granular motion under relatively high-pressure conditions, and find that the granular velocity non-linearly decays, forming an approximately 7-particle-diameter-thick localized shear band. A fitting curve underlying non-local physics can be used to well predict velocity profile geometries that are almost independent of normal stress and shear velocity. Moreover, experimental results show monotonically decreasing granular kinetic temperature, which may be caused by energy dissipation due to more inelastic contacts under high confining pressures. The variation of granular temperature will significantly influence the local yield stress and rheological properties, which may lead to inhomogeneous fluidity of the material and thus to shear localization in very dense granular flows. Understanding how particles move under high pressure is essential for studying various geological phenomena and advancing our understanding of granular material physics. In this study, transparent ring shear experiments are conducted to observe the motion of granular particles in very dense granular flows under high normal stress. It is found that the velocity distribution progressively decays and forms a shear band with a width of approximately 7 particle diameters. We suggest an equation that can well predict the velocity profile both of the quasi-linear velocity in the fast-moving shear zone and the exponential velocity curve in the slow-motion region. Furthermore, we analyze the distribution of particle velocity fluctuation and particle density across the sample. Near the moving plate, the particle velocity fluctuation is more intense and the particle density is lower, gradually decreasing far from the moving plate. This phenomenon may be caused by energy dissipation due to inelastic contact between particles. The mechanical properties of the granular material are influenced by these variations in velocity fluctuation and particle density. Thus, this leads to an inhomogeneous shear strain rate and promotes the formation of shear zones under relatively high-pressure conditions. A nonlinearly decayed velocity profile and spontaneous shear localization are observed in plane shear granular flows under high normal stressThe monotonically decayed granular temperature may lead to inhomogeneities in very dense granular flows and thus to shear localizationThe length scale of the non-local model in very dense granular flows is estimated based on experimental observation

In this study, we develop a Smoothed Particle Hydrodynamics (SPH) 2D-model for simulating fully submerged granular flows and their arising water waves. The granular particles are characterised by a non-Newtonian flow pattern, following a Casson constitutive law, generalised by applying the infinitesimal strain theory to avoid numerical singularities inherited from the original law. The implementation of this rheological model on the weakly compressible viscous Navier-Stokes equations enables the simultaneous modelling of the motion of granular flows and their resulting water waves, establishing a monolithic representation of fluid-structure coupling. The novelty of this model lies in the numerical continuity of the generalised rheological model based mainly on the yield stress criterion, which is computed purely from the mechanical properties of granular materials, including internal friction, cohesion, and viscosity coefficients. The proposed SPH model is validated through two benchmarks available in the literature, representing a submarine landslide along an inclined plane and an immersed granular column collapse. The outcomes of our study illustrate the effectiveness of the proposed model in accurately predicting the motions of submerged granular masses and their resulting water waves, which is crucial for accurately predicting the behaviour of underwater landslides and other natural hazards.