Tunnel survey measurements during construction In addition to the principle of the interlaced polygon course, a surveying gyroscope affected by the earth’s rotation indicates the direction of geographic north. The surveying gyroscope supports the measurements made with modern tachymetres. Thanks to these instruments, the orientation in the tunnel can be checked and improved. The tachymeter measures a distance of one kilometre with an accuracy of two millimetres. Tachymeters send out infrared waves aimed at reflectors. Eliminating possible sources of error Due to the temperature of the rock, it is usually warmer at the tunnel wall than it is in the centre of the tunnel, so measurements are performed at the centre of the tunnel. Furthermore, it must be remembered that the shape of the Earth is not an exact sphere, due to different density ratios in the Earth’s interior. Sea water propagates along this geoid. For this reason, sea level is used as a worldwide reference surface for height measurements. Since Austria and Italy use different official height reference points, the reference point for Italy is the sea level in Genoa, for Austria the sea level at Trieste. This leads to a difference of 12.5 centimetres at Brenner. For the construction of the Brenner Base Tunnel it was agreed to use the UELN European height system with Amsterdam as reference level. Continuous monitoring During the construction phase, the Brenner Base Tunnel must be constantly surveyed because tunnel deformation may occur. To that end, convergence bolts are drilled into the rock and possible movements such as subsidence, longitudinal or transverse movements in relation to the tunnel axis, are recorded using mounted prisms. The result is shown in diagrams. They enable the geotechnical expert to evaluate the rock mass and the excavation consolidation. This representation shows that the Earth is not a perfect sphere. This must be taken into account when taking survey measurements. Geoid 26 BRENNER BASE TUNNEL
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