ABSTRACT
Today satellite survey techniques (GNSS) allow to design and construct any kind of engineering infrastructures with very high accuracy and relatively simple measurement operations. Such capabilities cannot be exploited during the construction of long railway tunnels, when very high accuracy is required. It is fundamental that the bearings of the excavations, usually driven from two or more entrances, are determined with the highest possible accuracy, because once the work is completed any axis correction would result in significant extra-costs. Conventional surveying instruments and methods are not sufficient for such goals, while a consistent aid can be provided by precision gyroscopic devices that allow independent checking of the azimuthal directions, provided that gathered data are correctly processed. This report deals with measurements, data processing techniques, relevant problems and specific solutions adopted to overcome and limit errors and their propagation in the construction of “Terzo Valico” tunnel.
