A strategy for the monitoring of tall structures in urban area using GNSS technology

GNSS technology has become widely used for monitoring purposes. The high precisions nowadays available have made the technique suitable also for the monitoring of structures that are usually affected by very small displacements. In this work we investigate the issue concerning the monitoring of a tall structure such as the Garisenda tower, which lay in the Bologna city centre, by using GNSS data gathered by a permanent station placed on its top. We consider the need to investigate the variations in the leaning of the structure, therefore also the position of the ground at the bottom of the structure should be known. Unfortunately it is not possible to place a GNSS receiver under a tall structure in urban context because of the too poor sky visibility. A solution would be to choose another permanent station located as close as possible assuming its behaviour coherent with the ground under the monitored structure. This hypothesis has proven not to be verified in the analysed case, where four permanent stations located within few kilometres far from the Garisenda tower were available. Therefore a strategy to combine data from the five permanent stations using a uniform strain model was developed in order to define a reference to which compare the positions given by the GNSS sensor placed on the top of the tower. The impact of such strategy will be shown and discussed in terms of mean variation of the leaning of the tower over a period of about four years.