Impact of dry granular masses on rigid barriers

This work concerns the impact of dry granular masses on rigid artificial obstacles. The authors approached the problem by performing an extensive campaign of numerical analyses with a commercial code based on the discrete element theory. The standard approaches employed to design sheltering structures are exclusively based on the assessment of the Maximum Impact Force (MIF) exerted by the soil mass on the obstacle, and the sheltering structure is usually designed according to simplified pseudo-static approaches. In a previous paper the authors considered the dependence of MIF on the Froude number and on a large series of both geometrical and mechanical parameters. Indeed, the impulsive nature of the force exerted by the soil onto the structure has to be considered in order to optimize the design of this type of structures. For this reason in this paper the evolution with time of the impact force and the mechanics of the phenomenon are investigated

Dry Granular Flows Impacts on Rigid Obstacles: DEM Evaluation of a Design Formula for the Impact Force

AbstractIn the design of sheltering structures or embankments for the mitigation of the risk due to rapid and long spreading landslides, a crucial role is played by the evaluation of the impact force exerted by the flowing mass on the artificial obstacle. This paper is focused on this issue and in particular on the evaluation of the maximum impact force on the basis of the results obtained by performing an extensive numerical campaign by means of a 3D discrete element code (PFC3D), in which a dry granular mass is represented as a random distribution of rigid spherical particles. The analyses regard the impact process only, while triggering and the propagation phase of the flow are not considered. For this reason, in the model the granular mass is generated just in front of the obstacle; its initial volume, velocity distribution, height, length and porosity are assigned as initial conditions. The initial conditions are varied to take into consideration a large variety of geometrical/mechanical factors, such as the initial front inclination, its height, the initial void ratio, the length of the impacting mass and the inter-particle friction angle. A design formula is also proposed on the base of the obtained results and critically compared with the literature data and previous formulations based on hydrodynamic models

Debris flow impact forces on rigid barriers: existing practice versus DEM numerical results

As is well known, fast landslide risk mitigation is a very crucial issue in the territorial planning of mountain regions. In this framework the design of mitigation measures like sheltering structures plays an important role. In this perspective, in this paper the problem of the assessment of the maximum impact forces transmitted by dry granular (non-cohesive) masses (MIF) to rigid barriers is critically tackled by using a formula recently proposed by the authors, which has been derived from the interpretation of DEM numerical results. The formula defines a quite simple non-linear dependence of the maximum impact force on the Froude number and defines a change in the regime (from linear to quadratic) according to a suitable measure of the dynamic material stiffness, this evaluated by starting from the assessment of the compression wave propagation velocity within the impacting soil mass. The approach proposed by the authors is thus employed to (i) discuss the reliability of the existing practice procedures, (ii) highlight the reasons of the scatter among the values obtained by using the approaches proposed in the literature, based on hydrostatic, hydrodynamic and boulder impact theories, (iii) underline that the empirical formulas available in the literature do not take into consideration a series of factors, like, for instance, the front inclination and the void ratio, that in contrast severely affect the maximum impact force value

3D scan line method for identifying void fabric of granular materials

Among other processes measuring the void phase of porous or fractured media, scan line approach is a simplified “graphical” method, mainly used in image processing related procedures. In soil mechanics, the application of scan line method is related to the soil fabric, which is important in characterizing the anisotropic mechanical response of soils. Void fabric is of particular interest, since graphical approaches are well defined experimentally and most of them can also be easily used in numerical experiments, like the scan line method. This is in contrast to the definition of fabric based on contact normal vectors that are extremely difficult to determine, especially considering physical experiments. The scan line method has been proposed by Oda et al [1] and implemented again by Ghedia and O’Sullivan [2]. A modified method based on DEM analysis instead of image measurements of fabric has been previously proposed and implemented by the authors in a 2D scheme [3-4]. In this work, a 3D extension of the modified scan line definition is presented using PFC 3D®. The results show clearly similar trends with the 2D case and the same behaviour of fabric anisotropy is presented

3D scan line method for identifying void fabric of granular materials

Among other processes measuring the void phase of porous or fractured media, scan line approach is a simplified “graphical” method, mainly used in image processing related procedures. In soil mechanics, the application of scan line method is related to the soil fabric, which is important in characterizing the anisotropic mechanical response of soils. Void fabric is of particular interest, since graphical approaches are well defined experimentally and most of them can also be easily used in numerical experiments, like the scan line method. This is in contrast to the definition of fabric based on contact normal vectors that are extremely difficult to determine, especially considering physical experiments. The scan line method has been proposed by Oda et al [1] and implemented again by Ghedia and O’Sullivan [2]. A modified method based on DEM analysis instead of image measurements of fabric has been previously proposed and implemented by the authors in a 2D scheme [3-4]. In this work, a 3D extension of the modified scan line definition is presented using PFC 3D®. The results show clearly similar trends with the 2D case and the same behaviour of fabric anisotropy is presented

DEM assessment of impact forces of dry granular masses on rigid barriers

In the design of sheltering structures/embankments for the mitigation of the risk due to rapid and long spreading landslides, a crucial role is generally played by the assessment of the impact force exerted by the flowing mass on the artificial obstacle. This paper is focused on this issue and in particular on the evaluation of the maximum impact force on the basis of the results obtained by performing an extensive numerical campaign by means of a 3D discrete element code, in which a dry granular mass is schematised as a random distribution of rigid spherical particles. The granular mass is generated just in front of the obstacle: its initial volume, velocity distribution, height, length and porosity are arbitrarily assigned, and the impact process is exclusively analysed. The initial conditions are varied to take a large variety of geometrical/mechanical factors, such as the initial front inclination, its height, the initial void ratio, the length of the impacting mass and the inter-particle friction angle, into consideration. A design formula is also proposed on the base of the obtained results and critically compared with the literature data