Assessment of grouted rock-bolts behaviour subjected to axial loads

In mountain environments an important aspect, both from an economical and from a technical point of view, is the protection against gravitational hazards. Rock-falling in fact can cause tragic events not only to engineering and architectural works but also to human lives. These are some among the reasons that make the mitigation measures and protection structures, like rock-bolts and cable nets for example, extremely important . It becomes then very critical for us, as geotechnical and civil engineers, to understand properly the behaviour of these structures, and of the elements they are composed by. It is not a case, in fact, that the testing campaigns that will be presented in this work were commissioned by SNCF (Société Nationale des Chemins de fer Français), which acknowledges that a better understanding of these structures can lead to an improvement in the design approach, which can translate in a signification gain, not only economical but also in the safety of human lives. The aim of this thesis is to study the foundations of rockfall nets, which are usually made by grouted rock-bolts. Studying the behaviour of these elements is not an easy task, in fact, they are not only influenced by the characteristics of the steel bar and the grout, but also by the ones of the surrounding rock, and by its state. As it will be presented in this work, in fact, the state of the rock massif can change the reaction of these elements: a fractured rock and a non-fractured one react differently. To accomplish this objective and study how these elements act once loaded, the rock-bolts were monitored with the use of fibre optic and sensors in the head of the anchorage that measured the displacements and the loading. During this work the applied loads were axial ones, which were applied with the use of a hydraulic jack on a three meters bar, which had only the last meter grouted. This was done on three different sites : Seyssel, Bourg-d’Oisans and Mongalgon. The behaviour of these elements has been studied also with some numerical tools. With the use of the software Abaqus, in fact, an axisymmetric model that represents a pull-out test, with the same geometry as the one realized for the in situ tests, was developed. This model has enabled us to compare its results to the ones obtained from the campaign tests.