Risk analysis for the Ancona landslide—I: characterization of landslide kinematics

The Ancona landslide is a complex, deep-seated landslide displaying composite rotational-translational kinematisms and affecting a large urban area in the Ancona municipality on the Adriatic coast of central Italy. The landslide was reactivated with a large and destructive event on 13 December 1982 following a long period of precipitation and has remained active since. This paper focuses on the estimation of the landslide kinematics (more specifically, the horizontal and vertical components of average yearly velocity) for subsequent estimation of risk for a set of 39 buildings as presented in a companion paper. The study relies both on the processing of inclinometer and radar interferometer monitoring data through statistical procedures. Triggering factors are not investigated. Outputs from the two sets of monitoring data are compared quantitatively and qualitatively. The inherent limitations in available data are discussed. The validity of the quantitative results in the context of the risk estimation effort is discussed. - Introduction On the night of 13th December 1982, the city of Ancona suffered a large landslide that occurred along the coast to the north of the town, in the adjacent slopes of the Montagnolo Hill (Fig. 1). The volume of the mass movement was about 180 million m3, whereas the affected surface area was 220 ha, accounting for 11 % of the total urban area of the Ancona municipality (Cardellini and Osimani 2008). No casualties were recorded during the event. Nonetheless, the landslide caused extensive damage to structures and infrastructure, such as the University Medical Faculty and the local hospital. A total of 3,661 people (1,071 families) were evacuated from the residential districts named Posatora and Borghetto. Gas and water supplies were interrupted too, and the city remained for some days without the essential gas and water services. Two hundred eighty structures (out of a total of 865) suffered non-negligible to irreparable damage or were destroyed. Thirty-one farms, 101 SMEs, 3 industries, and 42 shops were significantly damaged, and 500 people lost their jobs. The Adriatic railway and Flaminia road were shifted laterally 10 m toward the sea. During the event, displacements started at the toe of the slope, spreading upwards. Cotecchia (2006) reported that large horizontal displacements of up to 8 m and uplifts of up to 3 m affected the lower parts of the slope. In the upper parts of the slope area, horizontal displacements of up to 5 m and large vertical settlements (up to 2.5 m) were recorded. The effects of the landslide movements were not restricted to on-shore areas; large displacements also occurred in the seabed adjacent to the main landslide. Considering the significance of the Ancona landslide event, local authorities were interested in assessing the possibility to consolidate and stabilize the affected area. A campaign of geological, lithological, geophysical, geomorphologic, and geotechnical analyses, aimed at supporting a preliminary design for remediation, was initiated. Intrusive and nonintrusive investigative techniques were used to assess the geological and geotechnical characteristics of the mass involved in the 1982 landslide, the failure mechanisms and the factors that triggered the event. Local authorities concluded that a comprehensive consolidation would have entailed very large expenses and would have brought a very severe environmental and socioeconomic impact on the area. Thus, The Ancona Municipality decided to live with the landslide while striving to ensure the safety of local residents. Nonetheless, the planned remedial scheme was partially carried out, when some stabilization works were conducted between 1999 and 2003 on the eastern part of the landslide area. A more surficial drainage system was also completed; reinforced bulkheads were built, and some parts of the landslide area were reforested. In 2002, the Regione Marche assigned the Ancona Municipality the responsibility of creating an Early Warning System and an Emergency Plan for people who are still today living in the landslide area. The Early Warning System, which is currently being improved by additional instrumentation, aims to provide an integrated and continuous control at a surficial and deep level of the entire landslide area. Details are given in Cardellini and Osimani (2008). Any decision-making process concerning human valued assets relies (implicitly or explicitly) on the concept of risk estimation and risk management. Estimated levels of risk for one or more categories of vulnerable elements are compared with acceptable/ tolerable levels. When pursued in an at least partly quantitative perspective, risk management provides a more objective and rational basis for decision-making than a purely qualitative approach. The focus of this paper is on the characterization of ground displacements in a part of the landslide area from monitoring data and qualitative inferences. Statistical methods are employed to provide in a formal framework the quantitative information necessary to perform the probabilistic risk analysis as detailed in a companion paper (Uzielli et al. 2014), in which the outputs obtained herein are used for the semiquantitative estimation of risk for a set of 39 buildings located inside the landslide area. The strengths and limitations in the implemented methods and in available datathe latter both in terms of number and qualityare discussed. Setting and features of the Ancona landslide: an overview Historical overview The coastal slopes in the Ancona area are known to have been unstable for centuries. Historical records document the occurrence of significant movements in the same slope in 1578, 1768, 1858, and 1919 (Bracci 1773; De Bosis 1859; Segr 1920; Crescenti 1986). Crescenti (1986) noted the great reduction in the actual number of the building on the Montagnolo slope, compared with a cadastral map of 1915, as an evidence of the historical activity of the mass movements in 1919. Several studies (Coltorti et al. 1985, Fig. 1 Panoramic view of the Ancona landslide area (from Cotecchia 2006) 1986; Crescenti et al. 1983, 2005; Curzi and Stefanon 1986; Dramis et al. 2002) date the onset of movements on the Montagnolo slope to 50006000 years ago, before the Flandrian Transgression, when regional uplift brought the foredeep sediments to their presentday elevation. Geological, tectonic, geotechnical, and hydrogeological setting From a structural point of view, the Ancona area lies on the external margin of the Apennines, which tectonic history is directly connected to the evolution of the Adriatic foredeep (Bally et al. 1988). Figure 2 shows a geological map of the area, together with a representative geological cross section. Underlying the recent superficial cover of elluvium, colluvium, and associated landslide debris, a succession of strata belonging to the Lower, Middle, and Upper Pliocene and the Lower Pleistocene (...truncated)