Factors Affecting Blasting Accuracy and Influence of Rock Type on the Difficulty of Tunnels Excavation by Drilling and Blasting

Drilling and Blasting is a common technique of tunnel driving, either for mining or civil purposes, at least for lengths shorter than 3 - 4 km. The technology is surely “mature”, hence it offers a huge variety of cases to be analyzed. The report deals with the subject of the compliance of the actual blasting results with the design requirements (contour and pull). Three factors are obviously involved: explosive, rock and drilling; the main objective of the report is to asses in some simple cases the relative importance of each of them, and to describe a simple methodology to do that. A number of rounds in mine drifting operations, covering a variety of rock-drilling system-explosive assortments have been carefully surveyed; rock shows important differences from one site to another with respect to the quality class; drilling is performed with manually steered jumbo in some drifts, with computerized jumbo in other drifts; the general blasting pattern is not different from one site to another, but at places pipe charges are used in contour holes, at places fully coupled charges. The evaluation of the influence of the rock on the excavation difficulty is another objective of the study. Difficulty can be inferred from specific drilling work consumption, specific explosive consumption or from both. Drilling consumption, which means the relative fineness of explosive distribution in the rock-mass required in order to obtain a satisfactory blast, has been assumed to be a reliable criterion on the basis of the narrower dispersion of the data, with respect to explosive consumption. Being the study aimed to classify by relative difficulty the rock types, rather than the operations, the effect of the cross section (whose influence on specific consumption by far exceeds the one of rock type) has to be accounted for by normalizing consumptions, i.e. by defining a normal cross section v/ specific consumption correlation, to be used as a reference line. Rock classes are related to difficulty classes; a wide dispersion is still observable, which indicates that the rock designation criterion based on lithological type should be improved by adding at least a second descriptive element. Anyhow, the simple lithological designation provides a first approximation ranking of the rocks to the intended purpose. The sensitivity of the pattern performances to drilling precision is analyzed in the same way. Tolerance standards for drilling machinery related to drilling pattern features are proposed. Data collected to establish the quality of the results of the blasts are: the overbreak value, the efficiency (actual pull/design pull ratio), the half cast factor of the contour holes. The relationships linking these indicators of quality to the features of the rock, of the drilling system and of the charging are analyzed and discussed. The statistical basis of the study is provided by data collected from a large number of cases, obtained from technical literature and from on site inspections. Analysis has been carried out on rounds of variable cross sections (from few to 160 m3), with either parallel or inclined hole cuts, blasted using a wide range of explosives and ignition systems. A general methodology to establish the main reasons of observed inaccuracies and to foresee the consequences of changes in the drilling and blasting practice is presented in the conclusions.