Moving beyond single slope quantitative analysis: A 3D slope stability assessment at urban scale

A very large percentage of the Italian municipalities is exposed to landslides, floods, and/or coastal erosion, according to the 2021 edition of the Report on hydrogeological instability in Italy. Even the south-eastern area of the country, the Apulia Region, is affected by different geo-hydrological hazards, with a concentration of landslides in the north-western portion, named the Daunia Apennine sector. Such a significant exposure to landslide hazard imposes the need of defining reliable quantitative methodologies to assess landslide susceptibility at both the slope scale and urban scale, so that appropriate land planning policies, as well as effective prevention and mitigation measures, can be implemented according to a more rational approach. The main purpose of this study is to propose a physically-based methodology, at the urban area scale, aimed at assessing landslide susceptibility, for both shallow and deep instability processes affecting almost all the urban areas in the Daunia Apennines. The proposed methodology has been applied to the municipality of Carlantino (FG) as a test case study, using the available geological and geomorphological maps as well as the soil physical and mechanical data. A three-dimensional geotechnical model, 2.5 km2 wide, including the Carlantino urban area and the slopes surrounding the town, was created. Later on, a three-dimensional limit equilibrium analysis, taking into account the equilibrium conditions both in the slip and the transverse directions, was performed to obtain a mechanically-based map of safety factors at the urban area scale. The results of this study, obtained for three different scenarios related to the assumed depth of the groundwater table, allowed the identification of the areas more susceptible to landsliding, i.e. characterised by lower safety factor values. The proposed 3D approach represents a significant advancement with respect to the traditional 2D stability calculation methods in terms of accuracy of the representative geometrical and geological model, which is assumed to fit more effectively the complexity of the actual slope conditions. Moreover, the methodology can have significant practical applications, since the corresponding results provide a prompt overview of the slope stability conditions of an overall urban area and can be directly used for urban planning as well as risk management activities. Based on a comparison against geomorphological evidence and remote sensing data, this approach has proven to be a valuable tool to support landslide susceptibility assessments, to be promptly used for land planning policies, and is supposed to be exportable to other geological environments.