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In granular soils, long-term cyclically loaded structures can lead to an accumulation of irreversible strain by forming closed convective cells in the upper layer of the bedding. The size of the convective cell, its formation and grain migration inside this closed volume have been studied with reference to different stiffness of the embedded structure and different maximum force amplitudes applied at the head of the structure.
This relation was experimentally investigated by applying a cyclic lateral force to a scaled flexible vertical element embedded in a dry granular soil. The model was monitored with a camera in order to derive the displacement field by means of the PIV technique. Furthermore, the ratcheting convective cell was also simulated with DEM with the aim of extracting some micromechanical information. The main results regarded the different development, shape and size of the convection cell and the surface settlements.
The surface erosion of soil samples caused by an impinging jet can be analyzed using the jet erosion test (JET), a standard experimental test to characterize the erosion resistance of soils. This paper specifically addresses the flow characteristics of a laminar impinging jet over the irregular surface of granular beds to discuss the pertinence and relevance of commonly used empirical estimations based on a selfsimilar model of a free jet. The JET is here investigated at the microscale with a coupled fluid-particle flow numerical odel featuring the lattice Boltzmann method (LBM) for the fluid phase combined with the discrete element method (DEM) for the mechanical behavior of the solid particles. The hydrodynamics of a laminar plane free jet are confronted with the results from a parametric study of jet impingement, both on solid smooth and fixed granular surfaces, that take into account variations in particle size, distance from jet origin, and jet Reynolds number. The flow characteristics at the bed surface are here quantified, including the maximal values in tangential velocity and wall shear stress, which can be regarded as the major cause of particle detachments under hydrodynamic solicitation. It is shown that the maximal velocity at the impinged surface can be described by the free jet self-similar model, provided that a simple empirical coefficient is introduced. Further, an expression is proposed for the maximal shear stress in laminar conditions, including a Blasius-like friction coefficient that is inversely proportional to the square root of the jet Reynolds number. To conclude, finally, the JET erosion of different cohesionless granular samples is analyzed, confirming that the threshold condition at the onset of granular motion is consistent with the Shields diagram and in close agreement with previous experimental results.