Geodetic Basis The base survey deals on the whole with the building of a stress-free special network for the transfer of the planned tunnel axis in the vicinity. This special network is also used in an increasing number of jobs as the starting point for the control surveys to be carried out during construction, as well as for geotechnical measurements. A special network in tunnelling consists in general of two portal networks. These two networks will be connected to each other in a larger measuring campaign, and joined at super-ordinate points. Such a concept has been introduced by Kahmen et al. for the staking-out of high-speed railways (Kahmen et al., 1998). The fact that the portal networks may need to remain available for staking-outs and monitoring tasks during the entire construction phase, and possibly thereafter, raises the question of long-term marking. For this purpose pillar points may be considered. This marking technique's high cost can be justified by the fact that it provides high stability and the forced-centring crucial in tunnel construction. A further advantage of a pillar network is that it offers the possibility to serve as a base for deformation analysis for longer terms and with sufficient stability. A discussion of the construction of networks for deformation analysis can be found in (Niemeier, 1985).
Figure 3: Organization of a portal-pillar network
The above described concept is most often realized today through a GPS-based survey of the special network supplemented with terrestrial measurements in the portal areas, as well as portal-connecting levelling. In most cases it is crucial not only to achieve a stress-free special network for engineering surveying tasks, but also to transport this network without large stresses to a network given by the client.
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