Rainfall-induced shallow landslides in Northern Tuscany (Italy): geotechnical characterization and rainfall thresholds

In this preface we introduce the PhD Thesis on rainfall-induced landslides entitled “Rainfall-induced shallow landslides in Northern Tuscany (Italy): geotechnical characterization and rainfall thresholds”. Rainfall-induced landslides deserved a large interest in international literature in the last decades. The literature on this matter shows a markedly interdisciplinary approach, with contributions from different fields, such as engineering geology, soil mechanics, hydrology and geomorphology. The subject is of high interest for many practical and scientific reasons. In fact, rainfall is the most relevant factor for triggering shallow landslides, and rainfall analysis is the most frequently adopted approach in forecasting the occurrence of such phenomena. Moreover, the estimation of the properties of materials mainly involved by shallow landslides is fundamental in understanding their triggering mechanism. With the aim of contributing in studying the rainfall-induced shallow landslides in Northern Tuscany, different topics were considered: the analysis of the rainstorms of 2009-2010 period and of their consequences, the geotechnical characterization of the source areas of shallow landslides, and the determination of critical rainfall thresholds for triggering such phenomena. For these reasons, the manuscript was structured in three different but complementary papers. The papers are briefly introduced, examining their contribution in understanding of the rainfall-induced shallow landslides, as follows. The first paper “The 2009 and 2010 meteorological events in Northern Tuscany (Italy): Characteristics and effects on slope stability” describes characteristics and main effects on slope stability of the rainstorms which hit the Northern Tuscany in December 2009 (north-western Tuscany), June 2010 (Serchio River Valley) and October 2010 (Massa-Carrara province). The rainfall events were analyzed in terms of rainfall amount, intensity and duration. Moreover, the antecedent rainfall related to such events was analyzed, focusing on its role in causing instability conditions of slopes. The paper highlights again that the shallow landslides commonly occurred in peculiar geologic and geomorphologic environments: colluvium/debris thin slope cover, semi-permeable or impermeable bedrock, hollow shaped slope, high slope gradient. Moreover, it highlights that the shallow landslides in Northern Tuscany mainly involve the soils covering the arenaceous formations, such as the Macigno Fm. In this context, the second paper “Geotechnical characterization of source areas of shallow landslides by dynamic penetration tests in Northern Tuscany (Italy): first results and perspectives” aims at contributing to the characterization of the typical source areas of the shallow landslides in the Serchio River Valley, by means of dynamic penetration tests (Dynamic Probing, DP). In fact, these tools are particularly suitable to obtain geotechnical properties of the soils in difficult access slopes. Original data, coming from inspections of existing databases or expressly performed tests, are presented and discussed. Relative density and friction angle were determined processing the results of DP tests by means of empirical methods. Moreover, the soil properties obtained by DP tests were compared with those obtained by Standard Penetration Tests (SPT) and direct shear tests. The comparison suggests that the DP tests can be an effective tools in geotechnical characterization of potentially unstable soil slope covers. Finally, the last paper “Critical rainfall thresholds for triggering shallow landslides in the Serchio River Valley (Tuscany, Italy)” proposes the critical rainfall thresholds for triggering shallow landslides in Middle Serchio River Valley. The rainfall data recorded by three rain gauges in the 1935-2010 period were analyzed and compared with the occurrence of shallow landslides. The rainfall thresholds were defined in terms of mean intensity, rainfall duration and normalized using the mean annual precipitation. Some attempts were also carried out to analyze the role of rainfall prior to the damaging events. Moreover, the rainfall thresholds obtained for the study area were compared with the local, regional and global thresholds proposed by various authors. The results of these analysis suggest that in the Middle Serchio River Valley, and in general in Northern Tuscany, landslides activity initiation requires a higher amount of rainfall and greater intensity than elsewhere. At present, further research is necessary in order to reach a zonation of the shallow landslide hazard in the study area. The methodological approach used for the characterization of the source areas of the shallow landslides may be improved in several aspects (e.g. performing of direct shear tests on undisturbed soils, calibration of DP tests results and performing of empirical relations specifically calibrated on the considered soils). Other important parameters, such as slope gradient and soil thickness, may be considered. The critical rainfall thresholds obtained in this work may be tested and compared with those obtained by statistical or deterministic approaches. The evolution of this research is towards performing of susceptibility maps and comparison with rainfall thresholds for triggering shallow landslides. This is crucial for the preparation of landslide hazard maps and scenarios for different rainfall amounts in an area particularly prone to shallow landslides