The microtunnelling works involved the installation of two pipelines each of ID 1,600 mm internal diameter. One pipeline was 1031.76 m long whilst the other was a little longer at 1138.57 m. Both pipelines were designed as straight bores which meant that no specialist guidance was required to handle in-built curves.
The drives were completed in the district of Zabbar West, between October 2015 and November 2016. Zabbar is located in the South East of Malta and it is Malta’s fourth largest town, with a population of around 17,100.
One of the major challenges for the project was that the ground comprised relatively soft rock and chalk which was quite porous at times and in places included huge cavities. This was at least in part why the project took some 13 months to complete. The difficult ground conditions meant that the Herrenknecht AVN 1600 microtunneller, fitted with a hardrock cutterhead, encountered some of these cavities which needed to be filled with concrete before tunnelling could continue. This happened once on the first drive and at least twice on the second.
For its part VMT provided the guidance system and a support team to the project. The guidance system used was the Universal Navigation System (U.N.S.) modular navigation system developed by VMT’s sister company Herrenknecht AG. VMT markets these systems for Herrenknecht in the international market.
The U.N.S. system was specially designed for use on straight bore routes of up to 200 meters in length and utilises a laser that is installed in the drive launch shaft. The laser line maintains a direct ‘visual contact’ with the guidance target unit at the rear end of the TBM that allows the machine operator to monitor horizontal and vertical deviations from the predetermined target values and make the necessary steering corrections to maintain the required drive course.
For longer drives such as those at Zabbar West, the navigation system can be extended using a reference module. To protect against refraction interference, an additional Hydraulic Water Levelling system (HWL) can be used to determine vertical deviation for longer, straight drives of up to 400 meters in length.
For even longer drives a north-seeking gyrocompass can be installed in the TBM, which does not require direct visual eye contact to the reference module in the launch shaft and makes it possible to master even longer distances, gradients and curves.
The VMT support team visited site to ensure that particularly for the first drive the machine crew were fully up to speed on the functionality of the guidance system, especially the HWL. They were also useful to the project given that whilst no particular problems occurred with the guidance system, the ground circumstances meant that drift calculations for the machine were challenging as it went through porous chalk and then just a few meters later through the harder rock.
Ultimately both drives were successfully completed to within the necessary target tolerances required by the client.