Ground surface deformations induced by tunneling under deep-seated landslides in the Northern Apennines of Italy imaged using advanced InSAR techniques

SAR interferometry has proven capable of measuring surface displacements in various situations including ground-settlement, landslides and excavations. We present a case study that consists of a tunnel excavation under a slope where dormant deep seated landslides are recognized. We take advantage of the high resolution images of the COSMO SkyMed Satellite system, that has a high sampling frequency, to measure the distribution of displacements in space and time. Our results are compared to ground-based InSAR data, surface geophysical techniques, GPS and inclinometer measurements. The data were acquired between 2011 and 2015 during the construction phase of the highway that will connect Bologna with Florence across the Northern Apennines. The tunnel passes under the small villages of Ripoli and Santa Maria Maddalena. The geological conditions are characterized by highly sheared flysch sandstones that belong to the LigurianMonghidoro Formation, a unit that is well known for its poor geomechanical properties and high landslide susceptibility. The work on the tunnel started in 2011 and, with the advance of the tunnel front, severe surface displacements were induced, causing damage to houses and infrastructure. The deformation slowed down after the tunnel was completed in November 2014, and low displacement rates were registered during the spring of 2015. The data illustrate the evolution of ground deformations as the excavation proceeds. In particular the InSAR results obtained with the Stanford Method of Persistent Scatterers (StaMPS) for the X-Band data of COSMO SkyMed give interesting insights into the nature of observed surface deformations that include clear downslope movements. 55 SAR scenes were analyzed that had been acquired in ascending and descending orbits, between December 2012 and February 2015. Together with the new findings from combined HVSR-MASW techniques and a detailed remapping of the area, a new conceptual geological model of the slope is developed.