Underground cavity investigation within the city of Rome (Italy): A multi-disciplinary approach combining geological and geophysical data

Abstract

Large areas of Rome (Italy) are prone to sudden ground collapse because of the degradation of a cavity network related to the long-lasting underground quarrying activity of volcanic Pleistocene deposits. In recent years, the municipality authorities have promoted scientific research in the field of engineering geology to locate these underground voids and to establish a standard methodology to assess the associated risk affecting the overlying structures. In this paper, a multidisciplinary – geophysical and geological – method oriented to cavity location is described, tested and validated. Within this investigation, Electrical Resistivity Tomography (ERT) is selected as the primary geophysical survey method. The potential for underground cavity mapping is assessed using synthetic simulations and observations at a test site where the position of underground voids is known. Consequently, the ERT diagnostic capability is quantitatively evaluated in the geological scenario of interest and possible interpretation pitfalls when interpreting resistivity anomalies in a geological sense are pointed out. On this basis, an integrated investigation program encompassing ERT measurements and borehole drilling is carried out at a site where the cavity network is unknown.The physical reconstruction offered by ERT, when supported by a detailed geological model, was found very useful in defining the geometry of the subsoil and consequently increasing the percentage of success in cavity location when compared to the one obtained by drilling alone.Two test sites are described: one is characterized by a well-known cavity network, and the other has an unknown tunnel arrangement. The corresponding geological models, resulting from in situ investigation, are discussed. When assisted by a detailed conceptual geological model, ERT is proven to be a very useful tool to detect unknown galleries and consequently to optimize the drilling investigation program. In addition to that, the extensive in-situ testing of the near-surface volcanic deposits of the Rome urban area, permitted to characterize extensively both the electrical resistivity and the seismic (P- and S-waves) velocities of these formations, whose in situ properties have never been studied to this extent in previous literature.