The Assisi landslide monitoring: report on a multi-year activity based on geomatic techniques

A wide area of Assisi town, in Umbria (central Italy), is interested since its first edification (period 1950-1960) by a large landslide. The rate of motion is very slow (around one cm/year) but the deformations accumulated during 50 years have produced important damage to many private and public buildings insisting on that area, including the civic hospital and an important Franciscan monastery. The University of Perugia established in 1995 a geodetic monitoring network over the landslide, connecting a number of control points inside the moving area to an external fiducial network whose markers have been placed on stable geological formations. The monitoring technique has been based since the beginning on GNSS static observations, aiming to reach a three-dimensional accuracy of a centimetric level. More control points have been added in 2006, for a better description of the field of movements. Since 1999, a high precision levelling network has also been set up over the landslide area, in order to improve the accuracy of the height component, increasing the monitoring sensitivity to a few millimeter level. During the years, observation campaigns have been repeated many times on both networks, accumulating a consistent and increasing quantity of data. Such database permits now to carry out a series of analyses (coordinate time series, annual and accumulated displacement vectors, deformation velocity, spatial analysis) yielding to a better comprehension of the landslide dynamics and its evolution, helpful for the design of technical interventions.