Delayed failure of quarry slopes in stiff clays: the case of the Lucera landslide

Abstract

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 characterised 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.

Two and three-dimensional numerical analysis of the progressive failure that occurred in an excavation-induced landslide

“…Analyses were also performed assuming a different thickness for this layer (50 and 100 cm), but the results from these analyses were consistent with the conclusions of the present study. Considering the uncertainties for defining the geologic history of the site and the lack of specific geotechnical data (Lollino et al, 2011), the initial stress state of the slope (i.e., before excavation) is reproduced by using the gravity loading procedure, under the assumption that the soil behaves as an elastic-perfectly plastic medium with Mohr-Coulomb failure criterion. At the end of the gravity loading, the associated displacements and strains are reset to zero.…”

Effect of negative excess pore-water pressure on the stability of excavated slopes

“…Generation of either pore-water pressures or pore-water tension plays a key role in the stability of slopes (Ridley et al, 2004, Conte and Troncone, 2012, Zhang et al, 2011, Lollino et al, 2011, Yu-qi et al, 2005, Vaughan and Walbancke, 1973, Eigenbrod, 1975, Cai and Ugai, 2004. When saturated soil is under compression in undrained condition, the system resists being compressed by generating excess pore-water pressure which leads to a reduction of effective stresses.…”

“…It follows that the stability of the formed slope may vary after excavation until reaching steady-state conditions at the rate at which swelling occurs (James, 1970b, Griffiths and Li, 1993, Vaughan and Walbancke, 1973. From the literature it is a well-known fact that several slopes have failed after the equilibration of the generated NEPWP (Vaughan and Walbancke, 1973, Hughes et al, 2007, Potts et al, 2009, Kovacevic et al, 2011, Kovacevic et al, 2004, Lollino et al, 2011.…”

Analysis of the Slope Response to an Increase in Pore Water Pressure Using the Material Point Method

“…Generally, stability of such slopes is assessed by analyzing only the deformation processes that occur in the pre-failure and failure stages. Simplified approaches [6][7][8][9][10][11] and numerical methods based on the assumption of small strains [12][13][14][15][16][17][18][19][20][21] are often used for this aim. On the contrary, the post-failure stage is usually ignored owing to the unsuitability of the above-mentioned methods to deal with problems involving large deformations.…”