Light at the end of the tunnel

Across the globe, more and more tunnels (and longer tunnels) are being built. Currently, the longest tunnel in the world is the 57 km (35 mi) long Gotthard Base Tunnel in Switzerland, but this could change in the decades to come with the planned 123 km (76 mi) under-sea tunnel to be built between the Chinese cities of Dalian and Yantai. Every tunnel project is a multi-million dollar investment, and the level of accuracy required for tunnel measurement increases continually. When trains are expected to travel through at a speed of up to 300 kph (186 mph), the planned tunnel's axis has to be maintained with maximum precision. In the case of tunnel construction in ground water such as with the Elb Tunnel in Hamburg, the giant tunnel boring machine has to be driven into a special watersealed target construction with centimetre precision when finished. The smallest directional error in the heading can lead to considerable technical problems and financial risks when working on critical projects of this magnitude.

The tunnel surveyor plays a crucial role in making sure that the breakthrough of the tunnel occurs precisely at the specified target point. The challenge is to guide both sides of the tunnel in the right direction. The measurements for directional transmission occur using elongated traverse lines, which can only be connected to a control network of a known point at the tunnel's entrance. There is no way to check the directional accuracy of the advancing opposite end of the tunnel. As tunnel length increases, configuring both ends for the correctness of proper tunnel direction result in considerable risks and uncertainties.

Surveying under difficult conditions
Many tunnel tubes have entry starting shafts. From these starting shafts, fixed-point coordinates are transfered down to the tunnel's level so that the tunnel can be bored correctly and navigated toward its target, this being the other end of the advancing tunnel. This process, known as plumbing, always involves an element of risk, when transferring fixed reference points in such small and narrow shafts. If the measured data is so much as a millimetre inaccurate, this inaccuracy compounds itself and leads to considerable deviations in the lateral traversing line of the tunnel's many curves and its direction.

The measuring risks in the tunnel itself occur when the line of sight is diverted and subject to refractive influences such as temperature differences, humidity or dust. These make measuring angles and reliable measurements difficult and errors unavoidable. This applies even more due to the fact that in most tunnels the surveying points cannot be situated in the centre of the tunnel for logistical reasons and must therefore be located at the tunnel walls. Targeting close to the wall increases the risk of refraction even further. Tunnel courses with numerous (and tight) curves also require maximum accuracy.

As the tunnel length increases, errors from plumbing and refraction can add up to as much as several meters, making breakthrough at a desired position impossible. A considerable amount of additional work is then often required in such cases.

The solution is a “toy”
Previously, miners and tunnel builders solved this problem using compasses. In the modern tunnels of today, however, this is not possible due to the considerable amount of iron and steel used. Initial developments in solving this problem using gyroscopes came about in the early 1950s.

Just about everyone is familiar with gyroscopes from childhood, when playing with a spinning top. We are constantly using the underlying physical principle of precession in our daily lives, for example, when we take our hands off a bicycle's handlebars while riding and continue going straight as if by magic.

Precession is the directional change of the axis of a rotating body (a gyroscope) when external forces apply torque to it. If such a gyroscope is built into a measuring device which is positioned somewhere on the Earth for a certain period of time, the Earth's gravity will act on this gyroscope as the external force during this time. The gyroscope tries to counteract this external force and to remain in its original position. If it then manages to measure these values, such a gyroscope can be used to determine the direction to the Earth's axis (cartographic north).

Written by Norbert Benecke, Volker Schäpe and Volker Schultheiss

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