Delayed failure of quarry slopes in a stiff clay deposit: the case of the Lucera landslide

This paper concerns the landslide process occurring on a cut slope in stiff clays located at the north of the town of Lucera in southern Italy. This unstable slope lies between a hospital at the top and an abandoned quarry at the toe: the quarry was active until the end of the 1970s. A first landslide occurred around 1980, and was characterized by a subsequent retrogressive evolution. This paper presents an interpretation of the slope failure mechanism based on the results of geomorphological studies, field monitoring, laboratory testing, and both limit equilibrium and coupled numerical analyses, these latter carried out with FLAC2D. In particular, the numerical analyses have been performed in order to interpret the evolution of the slope movements with time, and to assess the influence of the quarrying-induced excess pore pressures on the development of the failure process. The numerical results show that during the excavation stages the negative excess pore water pressures due to undrained unloading allowed for a temporary stability of the slope. The analyses also indicate that the process of pore pressure equalisation triggered the 1980 failure, and initiated the retrogressive evolution of the landslide.