Prior to an underground excavation a site investigation is carried out. This includes reviewing and analysing existing data, field data collected through outcrop mapping, drill core logging and geophysical investigations. These data sources are combined and used to characterise, quantify and classify the rock mass for the tunnel design process and excavation method selection.

Despite the best approaches used in a site investigation, it cannot reveal the required level of detail. Such gaps in information might become significant during the actual construction stage. This can lead to; for example, over-break due to unfavourable geological conditions. Even more so, an underestimation of the rock mass properties can lead to unplanned stoppages and tunnel rehabilitation. On-the-other-hand, the excavation method itself, in this case, drill and blast, can also cause severe damage to the rock mass. This can result in over-break and reduction of the strength and quality of the remaining rock mass. Both of these attributes pose risks for the tunnel during excavation and after project delivery.

Blast damage encompasses over-break and the Excavation Damage Zone (EDZ). In the latter irreversible changes occur within the remaining rock mass inside this zone, which are physically manifested as blast fractures. In this thesis, a number of methods to determine blast damage have been investigated in two ramp tunnels of the Stockholm bypass. Herein, a comparison between the most common methods for blast damage investigation employed nowadays is performed. This comparison can be used to select the most suitable methods for blast damage investigation in tunnelling, based on the environment and the available resources. In this thesis Ground Penetrating Radar, core logging (for fractures) and P-wave velocity measurements were applied to determine the extent of the blast damage.

Furthermore, the study of the two tunnels in the Stockholm bypass shows a significant overestimation of the actual rock mass quality during the site investigation. In order to gain a more accurate picture of the rock mass quality, Measurement While Drilling (MWD) technology was applied. The technology was investigated for rock mass quality prediction, quantifying the extent of blast damage, as well as to investigate the potential to forecast the required rock support. MWD data was collected from both grout and blast holes. These data sets were used to determine rock quality indices e.g. Fracture Indication and Hardness Indicator calculated by the MWD parameters. The Fracture Index was then compared with the installed rock support at the measurement location.

Lastly, the extent of the damage is investigated by evaluating if the MWD parameters could forecast the extent of the EDZ. The study clearly shows the capability of MWD data to predict the rock mass characteristics, e.g. fractures and other zones of weakness. This study demonstrated that there is a correlation between the Fracture Index (MWD) and the Q-value, a parameter widely used to determine the required rock support. The study also shows a correlation between the extent of the blast damage zone, MWD data, design and excavation parameters (for example tunnel cross section and charge concentration).

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 mapped

rock 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 accurately

than 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.