On the influence of the angles of breakage in the pull efficiency of tunnelling by drill and blast

In tunnel excavation by drill and blast, the rate of advance per pull is the key aspect to the working site productivity, and therefore the economic profitability of the project. One defines Pull as the advance length per blast along the tunnel axis, and normally one differentiates the design pull (as the drilled length according to the blast design) and the actual pull (as the actual excavated length after the blast). Pull efficiency is defined as the ratio between the design and the actual pull. Previous studies conducted by the authors of the present work showed a lack of correlation between the pull efficiency with the most common parameters defining a tunnel blast design. The present work considers a new variable: the angle of breakage, defined as the angle of the prism of rock of competence of a single blast hole. Considering this new variable, a trend of correlation is observable, suggesting that the pull efficiency is generally lower for wider angles of breakage within the blast pattern.