Machine name: NLUTSRSP03

Although Geocon continually refines its techniques, the positioning of tunnel units is usually performed by means of a standard procedure. Intersections made with optical instruments are used to measure points on a tunnel unit after the position of the instruments has been determined – a system that is easy for the human eye to follow. During and after sinking, the mast and shaft on the tunnel unit remain above the water. The mast and duct have measuring points, whose relationships to the unit are known.

Geocon is involved in the construction of the longest sunken traffic tunnel in the world, in South Korea. This project demands a radically different approach. The tunnel must be sunk to such a great depth that the mast and shaft would need disproportionate dimensions if they were to remain above water. GPS would seem a logical alternative, but this is not possible, as GPS signals cannot be received underwater. Geocon therefore opted for relative positioning. After all, the matter in hand is not where in the world the unit is, but where it is in relation to the previous unit.

Before the units are sunk, a pin and nipper system is added. This guide ensures that the unit being sunk is aligned with the previous unit. Determining the position of the pin and nipper in advance in comparison to measuring points in the tunnel enables the location of the nipper to be determined from the inside after sinking. Using the pin in the next unit, small but essential adjustments can be made to align the units along the theoretical axis.

On account of redundancy it was decided that two independent systems that operate in accordance with this principle would be used for this giant traffic tunnel project, namely the Tautwire (literally, a taut wire) and the SSBL or Super Short Base Line system.

Once it has been established where the Tautwire is attached on the unit that is to be sunk and the previous unit, the position is determined based on the length and angle of the wire. The 3 mm wire is not insensitive to currents, but their influence decreases in proportion to the distance between the units, while the accuracy increases as the unit nears its ultimate position.

The same basically applies to SSBL, although this system utilises acoustic signals as opposed to a wire. Two transponders are mounted in predetermined positions on the previous sunken unit. A transducer that transmits acoustic signals and receives their reflection from the transponders is mounted on the unit that is to be sunk. The distance from and route to the two transponders can be determined based on the signal travelling time and the processed phase differences.

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