There were large sections of the very abrasive rock formations with high rock strengths of up to 250MPa and extremely fissured
A breakthrough by a Herrenknecht tunnel boring machine (TBM) signaled that eight years of tunneling through a Swedish mountain was complete, part of an ambitious project to remove a major bottleneck in the rail network. Two 5.5km sections of tunnel were bored, beginning in 2005.
The tricky project had utilized a specially adapated multi-mode TBM, with a diameter of 10.5 metres, designed to work in both the closed slurry mode with hydraulic removal of excavated material and the open hard rock mode with belt conveyor removal.
Due to its geology, the project occupies a top position on the list of tunnel projects with extremely complex ground conditions, said Herrenknecht. There were large sections of the very abrasive rock formations with high rock strengths of up to 250MPa and extremely fissured. In addition, the tunnel was exposed to extreme groundwater pressures of more than 10bar on large parts of the route.
The conditions in the mountain meant that at times the TBM was operating in hard rock conditions, while at other times it had to cope with high pressure ground water, necessitating the use of grout.
Permanently installed drilling and injection tools ensured water inflow could be controlled by grout injection when needed. As part of comprehensive test series, the sealing system of the machine was designed to withstand a groundwater pressure of up to 13bar.
Work on the project began in 2005, and after a breakthrough in 2008, the highly worn cutterhead was replaced by a new one with larger disc cutters (19″ instead of 17″).
Jobsite reports showed regular progress, although the time-consuming cement grouting and the required service intervals meant limited speed. Nevertheless, man and machine were able to force some hundred meters of tunnel per month from the mountain.