The stability of slopes in the Ardennes region

Abstract

A number of slope studies in the Ardennes region have revealed that the mode of slope angle frequency distribution for the steep slopes is about 30°. It has been suggested by several authors that this angle corresponds to 'the angle of rest' of the regolith materials on these slopes. Recently, however, new concepts of slope development under mass failure have been elaborated, which show that there may exist more than one critical slope angle value. Steep straight slope segments were measured and the stability properties of regoliths were determined in a part of the Ardennes region. Stability analysis performed with the obtained strength parameters, revealed that the slopes can develop by mass failure towards two so called ultimate threshold values, one of about 42° in case no pore water pressure develops in the regolith, and one of about 21° in case the regoliths are completely saturated with ground water that runs parallel to the slope. It turns out that the population of the measured steep slope segments is enclosed between these two threshold values. The mode of the measured slope angle values does not differ between the different lithological units. It is suggested that in periglacial times the slopes became unstable, due to a blockage of the groundwater by the permafrost in the subsurface. At that time the slope population changed from the upper threshold value for dry regoliths to the lower one for completely saturated regoliths via a flattening of the slopes.