The main part, 18.5 km, of the 20 km long twin-tube tunnel at the Follo Line project were excavated by four TBMs operating from one centrally located rig-area. During construction of the TBM-tunnels, pre-grouting was performed in areas where leakages from fracture-zones ahead of the TBMs exceeded specific trigger values. The potential water-leakages were on a daily basis detected by probe-drilling from the machines. The pregrouting stopped or limited the leakages into the tunnel during excavation. Leakages could result in drop of pore pressure in sediments, mainly clay, within a large influence area along the tunnel section, and by that give settlements on buildings on the surface. Such leakages could also create problems to achieve a good quality of the backfill grouting behind the lining.
In a few occasions, leakages occurred and resulted in a drop of the pore pressure in observation wells in rock almost one kilometre from the tunnel alignment. Thus, probe drilling in front of the TBM with a sufficient number of probe holes, customised for the geological conditions and the sensitivity zone classification, are essential to detect fractures with high permeability and to avoid situations like this. It became clear that performing only one probe hole located at top centre position and drilled approximately 30 m in front of TBM cutter head was not enough in areas with large identified fracture zones. A revised strategy for the probe holes was implemented to adapt to the encountered geology. The new concept involved minimum two holes, located at top and invert of the tunnel. The leakage criteria for when to start pre-grouting from the TBMs were also revised. Trigger values at the Follo Line Project were after some experience set to 40 l/min total from minimum two probe holes in moderate to sensitive areas. Experience has shown that increasing the number of probe holes to four provides more reliable information of the conditions ahead of the TBMs. In areas with small sensitivity around 80 l/min was experienced as a maximum value for water ingress to avoid problems with the back-fill grout. In sensitive areas the number of probe holes are set to six and the trigger level reduced to 25 l/min in total, whilst mandatory pre-grouting was required for some of the areas classified as highly sensitive.