Rock fall sources are generally well known and the protection is an important issue for the safety of infrastructures such as road in mountain areas. Among the different types of protection 2 different systems of protection come out: net fences used to contain blocs falling with limited energy galleries set up to protect roads particularly exposed This paper intends to present how modelling can be used for the analysis of the functioning and the performances of both systems and in order to help engineering in the design of efficient devices.

1.1 Introduction

Rock fall restraining nets are specific structures, located below rocky cliffs in order to protect both people and infrastructures. In most cases the metallic net sheets are made with Anti-Sub- Marin (ASM) rings (Fig. 1). Both the singular technology and the spatial array of ASM rings (whose diameter is around 0.42 m) confer outstanding mechanical properties to the net sheets and are used today in order to control rock fall trajectories. Such structures, composed of an array of metallic rings, are absolutely singular. A classic metallic net structure is composed of nodes and linear elements. For the first approximation, it can be assumed that these elements only resist tensile efforts. The main feature of such structures lies in their giving rise to large deformation in the normal direction to the sheet plane, even if strains in every linear element remain limited. But in the present case, the structural technology is more complex; instead of linear elements, there are circular rings. The behaviour of such elements is more complex too; as a macro-material, the behaviour depends on the geometrical state of the ring, as well as on the mechanical properties of its constitutive fibres. Because of the structural complexity of such a material, its behaviour is analysed on two complementary scales: - First, a ring scale is considered. The mechanical behaviour of an isolated ring, without interaction with other rings, is analysed. - Second, the spatial array of the rings is taken into consideration; at the net scale it induces rheological features. For example, a structural effect can be experimentally observed: when a group of coplanar rings is loaded in its plane in a given direction, stresses are generated in the perpendicular direction belonging to this plane. The taking into account of this information has allowed the establishment of a constitutive modelling associated with the net sheet.

1.2 Principles of the schematic spatial description of rings array

A complex structure, such as ASM nets, cannot be analysed mechanically until it has been spatially described. A simple discrete structure is substituted for the actual net. Because of large displacements of the net when a boulder hits it, it is relevant to use the Discrete Element Method, whereby every ASM ring is modelled by a node which is located at its centre (Akkaraju 1994, Hearn & Akkaraju 1995, Nicot 1999).

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