In Sollentuna-Edsberg north of Stockholm (Sweden) for a short but highly complex section of the sewage network expansion required specifically adapted tunnelling technology.
The construction of a sewer tunnel with a diameter of 1,110 millimeters over this distance, presented a number of unusual challenges.
The location of the jobsite caused headaches to begin with. Because the rococo Edsberg Castle is right next door. Vibrations and dust formation caused by conventional construction methods would have severely impacted the valuable building. The preliminary geological investigations predicted extreme ground conditions: predominantly hard and abrasive granite and gneiss with a uniaxial compressive strength (UCS) of up to 248 MPa.
Accordingly, this called for a hard rock machine specifically designed for the small diameter, which had to be equipped with particularly wear-resistant cutting tools. Because the small machine diameter meant any wear-related cutting tool changes that might be necessary could not be carried out from inside the machine.
With the final breakthrough, the new sewer opens directly into an existing sewer tunnel. Slightly sloping with a longitudinal gradient of 5.9 ‰ and a cover of 5 to 15 m, it runs in a straight line to the so-called Käppala Tunnel, into which it opens directly. A target shaft for recovering the machine cannot be sunk here. And because of the local conditions, construction of an intermediate shaft was also out of the question. It was therefore imperative that the machine could be pulled back to the launch shaft: on the one hand to change worn tools during the drive and, on the other, to recover the machine once the drive was completed.
Deployed was an AVN 800 HR microtunnel boring machine adapted for Edsberg with a shield diameter of 1,110 millimeters. The AVN 800 HR – HR for hard rock – is specifically designed for boring in hard rock: wear resistant cutting tools with tungsten carbide inserts (TCI). For hard rock tunnelling, the remote-controlled microtunnelling boring machine (MTBM) is designed for a maximum contact force of 890 kilonewtons (89 t), around twice that of the standard AVN 800 A version. Herrenknecht installed a main bearing optimized accordingly for these extremely high contact forces. Taken together, these and other hard rock adaptations significantly extend the possible drive length without changing tools – up to 200 meters, depending on the rock characteristics.
Another key element of the overall solution in Edsberg: flangeable steel pipes. Temporary, bolted pipes, instead of the concrete jacking pipes commonly used in pipe jacking, allow the machine to be retracted for tool changes and machine recovery.
This requires a specially designed jacking frame. It pushes the tunnelling machine and the pipes forward and can pull the machine back out of the tunnel by the pipes when required.
In fact, the machine was only pulled back to the launch shaft three times: after 20 meters, the cutting tools were inspected – with no significant wear. The one and only replacement of the TCI roller bits was only necessary after 72 meters.
In November 2023, the machine was able to be pulled back the entire 163 meters after the final breakthrough. With this, a world premiere of a retractable small diameter hard rock machine was successfully completed.
