Landslide risk management in large, slow mass movements: an example in the Northern Apennines (Italy)

After a long period of quiescence, in mid-November 1994 a large ancient slope movement, over 3000 m long, 1000 m wide and up to 120 m deep, classified as a slow, intermittent complex-type landslide resumed its activity, striking the village of Corniglio in the Parma Apennines (Northern Italy). The movement developed within arenaceous, calcareous and clayey geological formations and consisted of multiple roto-translational slides in the upper and middle portion and translational slides in the toe portion associated with earth flows. The causes of the landslide are ascribable to decrease of geomechanical parameters, owing to weathering and seismic activity, and increase of porewater pressures, after periods of intense rainfall. In 1995, during a phase of quiescence, the following works were carried out: thousands of trees displaced by the landslide were cut down, new topographic mapping of the landslide body was performed, the runoff network was re-established and draining trenches were dug to a depth of 12 m. Early in 1996, after a 4.2 magnitude seismic shock hit the area, large new detachments occurred along rotational surfaces of rupture. In February 1996, the formation of tension cracks in the ground and fissures in several buildings and streets of the village followed mass re-mobilisation of the ancient landslide body. Considering the landslide size and complexity, effective remedial measures seem to be rather uncertain and, in any case, very expensive. Nevertheless, monitoring equipment will be further improved with the setting up of fixed inclinometers and electric piezometers with automatic data acquisition to safeguard the most important built-up area.