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

A wide area of the Assisi town, in Umbria (central Italy), is interested since its first edification (1950-1960) by an extensive landslide. The rate of motion is very slow (around one cm/year) but the deformations accumulated during almost 60 years have produced considerable damage to many private and public buildings, including the civic hospital and an important Franciscan monastery. The University of Perugia established in 1995 a geodetic monitoring network over the Assisi landslide, connecting a number of control points inside the moving area to an external reference network with markers placed on stable geological formations. The monitoring has been based since the beginning on GPS satellite positioning with static observations, aiming to obtain a three-dimensional accuracy of about one centimeter level. From 2001 on, GPS+GLONASS receivers have been used for all measures, and 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 been set up over the landslide area, in order to improve the accuracy of the height component, increasing its monitoring sensitivity to a few millimeter level. During the years, observation campaigns have been repeatedly performed on both networks, accumulating a consistent and increasing quantity of data. Such database permits to carry out a series of analyses (coordinate time series, annual and accumulated displacement vectors, deformation velocity, deformation field) leading to a better comprehension of the landslide phenomenon and its evolution, helpful for the design of technical interventions.