The Cathedral of Saint Andrea and its Bell Tower in Portogruaro, near Venice, were built in 1839. Today it is the third most leaning tower in Italy and it continues to move slowly towards northeast.
Since 2002 the tower is monitored with many different sensors and since January 2017 it hosts fiber optic deformation sensors. SOFO sensor is based on white light interferometry and thanks to the customizable lengths, high accuracy, low interference and excellent long-term stability is the right gauge for this application. The 18 SOFO sensors have been installed in three different periods: 8 in 2017, 4 in 2019 and the last 4 in 2020. Two independent but synchronized SOFO Lite reading units are measuring the 1.5m to 2m sensors continuously.
Six sensors have been installed inside the wall of bricks and are measuring the horizontal deformations at different heights, see figure 1.