An integrated approach for monitoring soil settlements at the VIrgo site

The Virgo detector, currently in its 2nd generation configuration Advanced Virgo (AdV), is a Michelson interferometer aimed at the gravitational waves research and at opening a new window on the study of the Universe. It is made of two orthogonal arms being each 3 kilometers long and is located at the site of the European Gravitational Observatory (EGO), in the countryside near Pisa, Italy. After the construction of the Virgo facilities completed in 2002, over the years a steady subsidence process has been observed as a consequence of the building and embankment overloads. In consideration of the subsoil characteristics, whose surface portion is mainly formed by a 25÷60 m thickness layer of clay with limited thin layers of sands, the evolution of settlements was expected and properly considered for the design of the civil engineering infrastructures, so that the vacuum tubes can be readjusted to keep the original alignment. However, along 15 years of time life, the initial estimates of the expected displacements were continuously compared with the observed effects. The measured settlements have been regularly monitored and adopted for implementing the necessary realignment activities. This paper reports the monitoring activities conducted over the years, mainly consisting of regular high accuracy levelling surveys, periodically integrated by GPS and classical theodolite measurements. These sets of measurement were adopted to perform the Virgo realignment procedure needed to keep the interferometer rigidly tied in a 3x3km plane. In order to improve the knowledge on the trend of the settlements affecting the Virgo infrastructures, an analysis based on differential interferometry using satellite Synthetic Aperture Radar (SAR) data has been performed and compared with the outcome from in-situ data.