Guidance the key for accurate cable installations 04 Jul 2019

A combination of a Gyro MWDII and a Paratrack 2 magnetic probe system was developed and used by VMT for the first time to guide six small diameter cable ducts on their 506m radius curve alignments. Installed by the E-Power Pipe® installation system developed by Herrenknecht, the project, in Bacharach, Germany, comprised the boring of six 688m long x 505mm diameter ducts for high voltage power cable installations. Guidance of the small diameter Herrenknecht AVNS350XB microtunneller at the excavation end of the E-Power Pipe® system on the long distance drives was vital to the success of the curved drives.

E-Power Pipe installation of six power cable ducts

To meet the challenge, the contractor, Implenia, called on VMT to design a special guidance system to provide the necessary alignment accuracy. The system developed, combined two proven techniques, both a Gyro MWDII and a Paratrack 2 magnetic probe system.

The challenge for VMT was to provide a guidance system that would not only ensure accuracy of the individual bores but also handle the challenging environment which included high voltage power lines that crossed the construction site and which, as the project proceeded, led to erroneous readings from the Paratrack 2 System.

The project was something of a pilot project in that the systems had not been used in this combination before. The heart of the solution was that the comparison of the sensor readings from both navigation devices would provide sufficient data to show if the navigation system could be trusted. Manual measurements with a TCA showed that the vast majority of the data from the magnetic probe was very useable. Where any minor discrepancies from the magnetic probe did occur this could be adjusted for by using the data from the gyro, so that at no time was there any potential for the microtunneller to operate without confidence in the steering.

Successful breakthrough of the final curved drive

The fact that the personnel involved had no experience of such a system did not deter the VMT staff and with the new enhancements to the VMT guidance software, prior to the project, a system was developed that was believed would provide the accuracy required.

Ultimately, on the successful completion of the bores, the VMT system utilised by the E-Power Pipe® system proved to be a suitable guidance solution for TBMs without a line of sight available and where no conventional control measurements were possible as the drives advanced to their target.

According to the VMT crew onsite: “We learned a lot about the system during the project and we were able to gradually reduce the service on site and transfer more automation in the process as the project progressed.”