Drill Monitoring for Rock Mass Assessment in Tunnelling

Sammanfattning: Site investigations for tunnel projects are often unable to determine rock mass conditions accurately for the entire tunnel, as these investigations obtain qualitative data for only limited parts of the tunnel. This is the case at the Stockholm Bypass project in Sweden. At two ramp tunnels, the rock mass was characterised as significantly poorer during the excavation than had been determined previously in the site investigation. In this study, Measurement While Drilling (MWD) technology was employed to characterise the rock mass for grouting purposes using extraction drill monitoring data from the grout holes. In addition, MWD data were extracted from blast holes at the entrance of one access and two ramp tunnels. The MWD data included the penetration rate and feed, percussive, rotation, and water pressure measurements at less than 3cm intervals in the drill holes. The drill rig supplier’s software was able to characterise the rock mass accurately for fracture zones, as shown in a comparison of the fracturing index of grout holes, blast holes, and the mappedrock mass structures. Since the suppliers’ software packages ignored some essential features of drilling, the study developed an improved MWD data normalisation and filtering process. This new process took the effects of different rock drills and drill rod extensions on the MWD data into account. The new normalisation and filtering process showed the capability to describe the rock mass conditions more accuratelythan the current method. A holistic approach was also developed to optimise the rock support process even further. Here, the rock mass was objectively qualified using measured drilling data instead of observations. This process correlated the fracturing index to rock mass quality and rock support requirements. Lastly, a correlation was made between the rock mass quality and the grout requirements per grout umbrella;these requirements were linked to the rock mass conditions determined by the MWD parameters. Based on these relations, a conceptual grout decision-making model was developed. The study found all-in-all tunnel practices could be improved with the implementation of MWD for rock mass characterisation and as part of a rock mass quality control plan. The MWD technology provides additional information on the rock mass and could be incorporated into the observational method for rock mass excavation.