Applying the concept of difficulty of tunnelling to Brazilian cases

This study applies the concept of the difficulty of tunnelling by drilling an blasting (D&B) to selected cases from the Brazilian tunnelling industry. The difficulty of tunnelling has been defined in previous work as the difference of specific drilling (SD) from industrial average trend: easier when SD is lower than average, more difficult when SD is higher than average. Results show how tunnels in Brazil tend to be unrealistically more difficult to excavate when compared to similar excavations in similar geologies elsewhere in the world. It is thoroughly discussed the reason for such anomaly (up to +90% SD compared to industrial average). One reason could be the lack of options amongst explosive products on the brazilian market. Only ANFO and explosive emulsions are available in Brazil. Lacking high VOD (Velocity of Detonation) products might result in reducing the maximum allowable burden amongst production holes, therefore increasing SD. Another reason could be found in the tunnel designers’ tendency of choosing D&B as excavation method for weak rock masses that would benefit from mechanical excavation. The reason to this tendency can be partly found in the lack of availability of mechanical excavation machines in the brazilian market. Blasting in weak rock masses requires more careful smooth blasting, hence a closer spacing amongst contour holes to avoid costly overbreak: this increases SD. A third reason could be found in the lack of skill amongst the local workforce. When dealing with unskilled workers the blast designers naturally tend to avoid highly optimized blasts that require very skilled workforce to be realized on field. The designer in this case tends to leave the blast design simpler but more robust, at the cost of higher specific drilling and specific charge. The conclusions suggest that this unrealistic difficulty when compared to world average could be a simptom of the so-called “Brazilian cost” within the tunnelling industry.