Quantifying the difficulty of tunnelling by drilling and blasting

This study deals with industrial trends in tunnelling by drill and blast (D&B). We perform a statistical analysis of accumulated experience from the 1950s to the modern day to provide advice for proper project management in tunnel driving. The basis of the study is a wide database of tunnel blast schemes. This database is made of excavation parameters, and considers two main families of blasts: with parallel hole cuts and with inclined hole cuts. Such parameters are analysed by means of statistical regression. Correlations are shown. We present a general curve of correlation between tunnel sections and specific drilling and specific explosive consumption. We show how pull efficiency cannot be correlated to a single parameter, and how tunnelling by D&B needs to be treated as a complex system. Finally, we propose a method for quantifying and classifying the difficulty of tunnelling. The deviation of specific drilling (SD) from industrial average trend is used as an indicator of difficulty: easier when SD is lower than average, and more difficult when SD is higher than average. We show how such deviation can be preliminarily associated with lithotypes. This provides to designers and cost estimators a tool of a first approximation for D&B cost prediction at the pre-feasability and feasibility stages of a tunnelling project.