Abstract
During construction of the Šentvid tunnel a unique opportunity arose to measure the 3D displacements ahead of the motorway tunnel excavation face, since the exploratory tunnel was already constructed in the axis of the main tunnel. According to reviewed literature such measurements had not been performed yet and several problems regarding equipment and complete scheme of the experiment needed to be overcome. The paper gives a brief description of the Šentvid tunnel project, presents significant factors that affected the choice of the geodetic equipment and describes the scheme of the experiment. A special attention is focused on the problems relating to the operation of the instrument in demanding environmental conditions (water, dust).
Highlights
Construction of a tunnel or any other underground structure depends strongly on an observational method, especially if the structure is constructed under difficult geological conditions [1]
Geodetic methods like terrestrial laser scanning and 3D total station measurements are used for monitoring the convergence of the circumference of the underground structure in absolute coordinates, while the geotechnical methods enable recording of the relative displacements in the surrounding rock [3]
The 3D displacement measurements ahead of the tunnel face can only be obtained by geodetic monitoring, when a small diameter tunnel exists within the alignment of the future tunnel with considerably larger cross section
Summary
Construction of a tunnel or any other underground structure depends strongly on an observational method, especially if the structure is constructed under difficult geological conditions [1]. In comparison to the laser scanning the 3D geodetic measurements of the optical reflector targets using total station with integrated distance measurement give information on the underground structure response in selected points only General accuracy of this method is rather higher if compared to the laser scanning and is in the range of a few millimeters [2], depending on the accuracy of the applied geodetic instrument, the type of the reflectors and the distance to the reflectors [4] and the conditions on the site (presence of dust in the area, the size of opening, the length of tunnel). The 3D displacement measurements ahead of the tunnel face can only be obtained by geodetic monitoring, when a small diameter tunnel exists within the alignment of the future tunnel with considerably larger cross section Such opportunity arose during the construction of the Šentvid tunnel. A brief presentation of the Šentvid tunnel project and the details on the execution of the experiment and equipment are given in the sequel
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