Landslide activity in Upper Palaeozoic shale sea cliffs: a case study along the western coast of the Algarve (Portugal)

摘要

A stretch of 48 km of sea cliffs up to 120 m high in the Algarve area of Portugal exposed heavily folded Palaeozoic shales. A study has been undertaken to assess the retreat of these sea cliffs. Nine sets of aerial photographs taken between 1947 and 1991 have been assessed, particularly to determine the retreat at the top of the cliffs. The time distribution of landslides shows some relationship with the average annual rainfall, and the space distribution provides a perspective of rates and patterns of cliff retreat in the different geographic locations. The landslides were predominantly planar, the larger ones having failure surfaces with a low dip consistent with long-term displacements at near residual strength. Examples are described. Back analysis indicates an average friction angle at the time of failure of between 23 and 25°, assuming negligible cohesion.