The guidance of a tunnel boring machine (TBM) is comparable with the navigation of a super-tanker: it takes a long time before the effect of a course correction is visible.

Skilled machine drivers know the performance of their machine in different geological conditions. Therefore a precise and reliable determination of the TBM’s position is the most important information for steering control. Although the TBM moves slowly it is possible that the machine bears away from the planned course and exceeds the requested precision of 100mm radius from the given axis. It is a real challenge to eliminate these deviations in a tunnel drive as long as those in the Gotthard Base Tunnel, which can only be met with perfect co-ordination between surveying and machine driving.

All geodetic information must be carried out by a polygonal process from outside the tunnel right up to the machine’s cutter head. For this purpose there is normally a dedicated clear area or laser window available throughout the complete area of the machine, along the tunnel wall and through trailing gear.

One special characteristic of the Gotthard project was that various tunnelling activities needed to be done concurrently with the advance and consequently the machine and trailer assembly was designed accordingly. Therefore the navigation system has to be adapted to this situation too.

System demands
A guidance system is equivalent to a navigation system. It provides information for starting a directional control or course correction. Therefore it is indispensible that the actual position of the TBM in relation to the planned tunnel axis is continuously present and displayed. As a 98 per cent availability of the TBM position is required, continuous measurement of the TBM position is necessary as well as the pitch and roll values of the TBM, which are all collected and displayed. A status indication of all relevant sensor components of the guidance system is requested as well as automated directional control.

Normally the well-known navigation systems are working with GPS, but in a tunnel there is no satellite reception. So, the determination of position is carried out in the classic (back sight) method with the help of motorised measuring instruments.

Gotthard system
In the Gotthard tunnels two different trailing gear concepts were in use for the north and south drives (from Amsteg and Bodio respectively). To cope with these circumstances different navigation systems had to be designed but using the same hardware components, including motorized total stations, inclinometers and also software-controlled shuttered prisms marking the key machine measurement points. Additionally geometric machine data (including ram extensions) from the TBM’s programmable logic controller (plc) were stored and used in the calculation of the actual position. Concurrently with the advance, various works needed to be done: initial shotcreting, erection of wire mesh, arch mounting and rock bolt drilling and insertion, which caused heavy interference with the line of sight to the machine measurement points. Therefore a standard measuring method couldn’t be applied. Extreme vibrations would also affect hardware components from the total station and computer up to the shuttered prisms.

Gotthard North – Amsteg
In the North Section the first three trailing sections were pulled together continuously along rails during the advance. The following units hung on roller-brackets mounted on the segmental lining and were pulled only after the advance. The area around the last trailing units was stable for a short time and could be used for measuring the cutterhead position, albeit only in the lower laser window. The coordinates and orientation for the automatic total station in the lower section had to be determined again after each advance. In this section it has been accomplished by carrying forward key machine measuring points in the invert area. For measuring the tracks on which the first trailer section was pulled forward these points had to be pegged out anyway. They were also used for the automatic measurement of a ‘free chainage’ of the total station (see figure 1).

After each advance this trailing gear area was pulled forward, whereby the co-ordinates and the orientation of the total station changed. After the grippers were engaged again, a signal was sent to the control computer which started the automatic measurement of the total station with the new key machine measurement points. If the co-ordinates and the orientation of the total station were known, the TBM position could be measured by using the automatic machine prism at the cutterhead (see figure 2). The total station was mounted at a self-levelling tribrach (AD-12) which compensated for any roll and pitch of the trailer and automatically levelled the total station.

Gotthard South – Bodio
In the southern section the trailing gear was advanced using the two grippers that were secured during the advance. After the advance these grippers were contracted and moved forward. Here the grippers could be assumed to be of temporary stable construction (see figure 3).

Four motorised, shuttered prisms (machine prisms) were mounted on the machine frame (see figure 4) and measured on the machine axis. This ‘local’ co-ordinate system was incorporated into the computer calculations. During the advance the machine framework moves forward.

The machine station (amotorised total station on an automatic tribrach AD-12) was mounted on a divert frame which was connected to the gripper and so independent from the trailing gear (see figure 5).

During the advance the gripper does not move, only being pulled forward after the advance. With the temporarily stable machine station, the motorised prisms were measured and the global co-ordinates calculated during the measurement cycle. The TBM’s position was determined by a special transformation (see figure 6). As the TBM stays in advance mode within these measuring cycles, a track correction is added to the measurements of the motor prisms (dynamic transformation).

As with the Gotthard North system, the co-ordinates and orientation of the machine station were only stable for a short time. This means they changed with each advance.

When an advance was made, the grippers were contracted, moved forward and afterwards extended again on the tunnel wall. A signal was then given by the TBM to the control computer, which started the measuring of the machine station from the wall-station mounted in the rear. The measuring operation took about two minutes.

Afterwards the shotcrete work in the backup area could continue.

Display of TBM position
On the monitor (see figure 6) all the data relevant for control are displayed for the machine driver.

Besides the deviations from the planned axis (horizontal and vertical), the roll and pitch are also shown. Indication of the operational status of the connected sensor system is displayed as well as the station and the advance number. From the same display the directional control and a display of the last (historical) shield drive can be activated. The latter acts primarily for identification of the TBM performance directly influencing the control.

Conclusion
The adaptation of the navigation system to the special drive operations was doubtless a big technical challenge. The components and materials used were subject to very problematic conditions such as vibration, dust and heat.

The operating mode of the navigation system has to be fully orientated to these drive operations as the driving process should be in no way affected. Several times during the advance not only were geometric system adaptations necessary but also changes in some hardware components. For example the controller unit (data conversion and network) had to be cooled with compressed air as well as conforming to the IP62 protection category as a minimum.

The use of similar hardware components and a modular software system in all systems was beneficial as all the necessary adaptations could be done with relatively low effort.

Things didn’t always flow smoothly – therefore sincere thanks are given to all parties involved, for their patience and understanding during the set up of the system and the necessary revisionary phases.

All things considered, this project has accounted for the enhancements of the technique and methods in many respects from which many future projects will benefit.

Acknowledgement
This article is adapted by T&TI from a translation of the original German version
supplied by VMT.


Figure 1 – The shielded total station used in Gotthard North TBM guidance Figure 2 – Installed shutter prisms (closed) Figure 3 – Section (top) and plan of the TBM train assembly and guidance instrumentation concept on south Gotthard TBMs from Bodio] Figure 4 – Shuttered prisms on machine frame Figure 5 – Divert frame with machine station Figure 6 – Operator screen showing TBM position in relation to the planned tunnel axis calculated by transformation