Developing tools for the prediction of catastrophic coastal cliff collapse

International audienceThe erosion of coastal cliffs is inevitable. The cliff collapses are a hazard, a problem for coastal land use planners and limit the use of the coastline as an amenity. Techniques to increase our knowledge of coastal cliff failure and provide pre-cursors to impending collapse are presented. The rock mass cracks before a collapse and the cracking generates high frequency seismic signais. An increase in emitted seismic energy has been recorded on accelerometers, in boreholes within a cliff, during the fifteen hours before a cliff fall. Comparison of the seismic signals with those generated in the laboratory has identified a number of collapse mechanisms that occur during different phases of the fall. A second technique assumes that in cliffs composed of a highly fractured rock, any sub-vertical fractures will gradually dilate with time before a collapse. The electrical resistance of the rock varies with azimuth reflecting the dominant fracture orientation. A factor of anisotropy, calculated from the measurements, was found to change dramatically due to a cliff fall. Large values of anisotropy prior to the fall were interpreted as evidence of dilating fractures. In addition, the research has demonstrated the existence of a cliff parallel fracture adjacent to the cliff edge. It is likely that cracking within this fracture set is responsible for the seismicity and the variations in the measured anisotropy. Laboratory analysis has identified a significant weakening of the rock due to sait crystals within the matrix. The integration and implications of these results are discussed