In underground construction projects problematic rock mass conditions are one of the major issues causing cost overruns during the excavation phase. Before a tunneling project starts the rock mass is roughly characterized by a pre-investigation. However, in many cases, these pre-investigation does not portray the rock mass characteristics accurately and do not predict local anomalies in the subsurface. Therefore there is a need for new rock mass characterization methods that can reduce uncertainties and improve the overall tunneling process.
In the end of the 1880s, rock mass characterization based on manual drill data was investigated and rock masses were quantified using drillability. Since then, the technology has significantly changed with the introduction of hydraulic rock drills, computerized drill rigs, and advanced rock mass classification systems based on drill parameters. Nowadays, automatic drill logging systems and drilling data processing software packages are widely available and commonly used in Scandinavian tunneling projects.
This technology uses drilling parameters to characterize the rock mass. However, monitored drill parameters are influenced not only by the variations in the properties of the penetrated rock mass but also by the operator and the rig control system that continuously control the applied forces to optimize drilling and prevent jamming. In order to be useful for geomechanical purposes, the drilling data needs to be filtered, normalized and analyzed to refine the rock related response from responses caused by other influencing factors. If successful the data might be used to determine hardness, fracturing and water indicators.
Even though the technology has shown high potential in laboratory tests and field trials, it is not an obvious choice for all tunneling projects. In this paper, the background of the technology are described and the potential for the future outlined, concluding that the technique probably will be used more extensively in the future.