Slope instability mechanisms in dipping conglomerates over weathered marls: Bol landslide,Croatia

In February 2005, a landslide of significant dimensions occurred at Bol on the Island of Brač, in Croatia. The location of the landslide was in a geological structure of weakly bounded conglomerates lying above a flysch, in an area where numerous slides have been previously reported. In the paper the results of a geological engineering investigation of the Bol landslide are presented. The investigation results show that the landslide was triggered by a combination of improperly started earthworks, an increase in air temperature in days preceding landslide that caused snow melt, and abundant rainfall, but the primary cause of the landslide was weathering of the flysch. In other words, despite the fact that contact between the flysch and the conglomerates slopes is in approximate correspondence with the slope of the hillside, the landslide surface was not in contact between the conglomerates and marl, but deeper in the flysch, provoked by the penetration of the weathering front into the flysch. Numerical modelling results were able to reproduce this mechanism by incorporating strength degradation into the weathered zone, determined by field investigation. The results of the research can be used to predict potential slides not only in the area under study.