Deformation monitoring with robotic total stations. Pushing the limits

[EN] Deformation monitoring using high accuracy surveying methods are techniques widely demanded and used. Even in these conditions, there are several misunderstandings and misconceptions regarding several topics, such as differences in accuracy between the instrument in laboratory conditions and the system running in real conditions, and variations of accuracy in time and space, among others. The paper provides a case study focused on the construction of a new building above an underground transport hub where a deformation monitoring plan was set up to monitor the existing tunnels and structural elements. The monitoring plan included manual precise levelling for monitoring settlement of several pillars in both the building and the transport hub. It also included three robotic total stations (one in each existing tunnel) to monitor displacements and convergences, specifically in sections previously modelled by the finite elements method. According to several as-built uncertainties of the existing tunnel and transport hub and the fact that the new building loads would be transferred via a shared slab, the threshold limits defined for the convergence control were within the limit of the theoretical accuracy of the system, at ±1mm.The paper describes the monitoring system setup, the checks for monitoring performance of the system and an evaluation of changes in accuracy of the systems related to environmental conditions, station-target distance and acquisition geometry. Finally, the paper concludes with an analysis of the resulting challenges faced when dealing with low level thresholds.