TUM researchers report on their work in the Gotthard Base Tunnel in Switzerland

Precise navigation and safe tracks

Geodesy: Reliable navigation underground

TUM: Prof. Wunderlich, how was your department involved in the construction?

Wunderlich: We developed a new measurement method enabling precise orientation on the shaft floor. Huge tunnel boring machines or blasting operations and diggers are used to build a tunnel; experts often speak of boring the tunnel. The core skill of an engineer geodesist is to ensure that two opposing drilling operations, which start at different points, join up precisely to within a few centimetres even after several kilometres. Extremely high precision is required, especially when you are talking about a railway tunnel for high-speed trains. Even if calculations are out by just a few decimetres, the tracks can no longer be laid as planned. As a result, the trains would have to travel at much lower speeds.

TUM: How many drilling operations took place when constructing the tunnel?

Wunderlich: Had the tunnel only been bored from the two end points, it would still be a long way from completion. That is why there are two additional horizontal accesses to the planned tunnel axis. Here artificial cavities have been created from which drilling can take place on two sides. This enables a number of machines to drill short distances at the same time. Geodesists are tasked with ensuring that the machines navigate reliably. The challenge we faced in the Gotthard Base Tunnel involved dealing with the vertical access in Sedrun through a shaft 800 metres deep. Imagine eight Cathedrals of Our Lady one on top of the other; now that is pretty big. The problem here is: how can you transfer the geometric reference points from the surface to the underground area with enough accuracy to ensure that the machine is pointing in the right direction?

TUM: How did you solve this problem?

Wunderlich: You can use GPS satellites to define reference coordinates above the tunnel. Transferring the coordinates to the underground area can be done via mechanical and optical plumbing. The Swiss were brilliant at solving this problem. The second task involved finding the initial orientation for drilling from the shaft floor. A gyro-theodolite is normally used to determine directions and can also be used to establish a reference to the magnetic North, even under the special conditions in the tunnel. However, if the underground temperature exceeds 40 degrees, the equipment slips outside its calibration range and becomes less accurate. That is why we joined forces with the ETH Zürich and developed an independent new method.

TUM: According to which principle does the method work?

Wunderlich: In the cage that is used to transport materials through the shaft, we added special sensor technology - the so-called inertial measurement system. We can use this platform to measure how much the cage twists if it travels downwards from above. A geodesist first transfers the orientation of the network on the surface, which we received via GPS, to a special mirror system on the platform. The cage now travels downwards. Below we transfer the orientation to the underground network using the same method. The correction variable from top to bottom is the twisting angle measured from the inertial platform.

TUM: What other challenges did you face when working in the Gotthard Base Tunnel?

Wunderlich: The overall scientific effort was actually reasonable compared with the logistical effort required to be able to use the cage for the measurements in the shaft for eight hours on a given day. You could compare the shaft with the aorta - if geodesists were going to block it for a day, this meant strict planning a year in advance. However, this also meant that it simply had to work. To this end, we trained in the elevator at the Olympic Tower in Munich for days on end to ensure that everything ran smoothly.

Road, railway and airfield construction: The best material composition for the non-ballasted track

TUM: Prof. Freudenstein, you were involved in the Gotthard Base Tunnel with your Chair and Institute of Road, Railway and Airfield Construction ...

Freudenstein: Yes, we made a significant contribution to the design of the track superstructure. In the Gotthard Base Tunnel, the rails and sleepers are not in the ballast; instead, a non-ballasted track was assembled. The non-ballasted track system - low vibration track (LVT) which is common in Switzerland - was used. Here concrete blocks - elastically embedded into a rubber boot - are moulded into the surrounding supporting plate concrete. Since the tunnel sits below more than two kilometres of rock, the temperature is very high due to the underground rock pressure that is present. It can get up to 40 degrees inside the tunnel. These temperatures affect the track. The elastic components of a non-ballasted track behave differently at higher temperatures.

TUM: How did you examine these changes?

Freudenstein: We set up a section of the track at our testing facility and also simulated the climatic conditions. We then checked to see whether it had retained its elasticity having endured the strain of countless pulsating test runs. We examined various material compositions to find the best one. In addition, we were present at the so-called high speed trials to check the sinking behaviour of the track when subjected to dynamic stress. At several cross sections inside the tunnel, we implemented our measurement technology and were able to perform control operations from afar using remote data transmission that we developed. A German ICE train completed the high speed trials at speeds of up to 275 km/h.

TUM: What stood out for you when working in the Gotthard Base Tunnel?

Freudenstein: The commute is unusual. The tunnel is some 57 kilometres in length. Which means that the average distance travelled to reach your place of work is 28.5 kilometres. At 40 km/h, it can take up to an hour to reach your destination. However, the work itself is very safe. For example, everyone must wear a registered backpack and you can always be geolocated.