Fully grouted rockbolts are used to anchor protection structures against natural gravitational hazards. In recent years, contractors have observed a premature degradation of the steel bars of these rockbolts. In the dimensioning of a rockbolt, the choice of the steel bar is currently made only on its pure tensile strength. The objective of this study is to identify whether the degradations observed in situ are related to the metallurgical nature of the bar steel and/or the load conditions. 16 damaged rockbolts were taken from four different sites in the Alpine area. They were subjected to a metallurgical analysis and a supplementary analysis of their environment. In conclusion, it was underlined that the damages were due to a lack of knowledge of the effective loads transmitted by the snow-filled structures, but also to a lack of consideration of the deformation behaviour of the foundation. The loads applied to these rockbolt bars generated a bending depending on the nature of the surrounding ground. It has been shown that these loads can occur in successive jolts and with modification of their orientation in relation to the number of links between the rockbolt and the protection structure.
Fully grouted rockbolts have been widely used for several decades to attach, to stable rock masses, protection structures against natural gravitational hazards (wire mesh attenuators, catch fences, anchored wire meshes, snow nets: [1], [2]). The principle of making a passive rockbolt is to drill into the rock mass to seal in a steel bar. A fully grouted rockbolt is thus composed of four principal elements [3], which are steel bar, grouting material, borehole within rock and external fixture to the borehole wall (figure 1). At the head of the bar, a steel plate tightened by a nut allows the anchoring of guy wires, nets or supports of the protection structure to the rock mass (figure 2).
In recent years, various contractors have noted in the field a premature damage of the steel bars of this type of anchors with the risk of causing a deficiency of the protection structures of which they ensure the foundation and thus contribute to the resistance to the loads (figures 3a and 3b). Premature failures have been observed for a long time in the mining field [4] and are therefore the subject of specific studies. The latter have highlighted in particular the problems of load combination and stress corrosion cracking [5].
