Assessment of the drill-and-blast related overbreak in the BBT system

Drill-and-blast is the common conventional tunnelling method in hard rock conditions. This paper analyses the characteristics of the blasting-induced overbreak in some of the tunnels of the Brenner Base Tunnel (BBT) system, in order to evaluate its correlation with the main geometrical and technical parameters of the excavation as well as the geomechanical properties of the rock mass. In particular, starting from Terrestrial Laser Scanning measurements and direct overbreak observations, it was possible to distinguish so-called geological overbreaks (due to the pre-existing discontinuities in the rock mass) from technical ones (related to the appropriate performance of the excavation technique) on the basis of the after-blasting presence of half-casts. Data were used to develop a multiparametric regression model based on the RMR index and length of the blast holes, recognized respectively as the main factors influencing the two mentioned overbreak types. Model parameters were initially calibrated over a portion of the Mules access tunnel excavated in micaschists and subsequently validated against experimental evidence collected along tunnels of different geometries driven through granite and paragneiss rock-masses. The overbreak model proved to successfully predict the correlation between overbreaks (especially that fraction related to the geomechanical characteristics of the rock-mass), RMR and length of the blast holes. As such, its preliminary application can provide a reliable first-assessment of extra costs and time during the design stage.