Based on an extensive research study of selected natural slopes on three continents, a special type of wide-spread mass movement is noted, explained and quantitatively analyzed. The paper focuses on the long-observed intensity of landslides on slopes consisting of permeable materials overlying cohesive soils. In non-arid areas, the overlying aquifer, in acting as a reservoir for infiltrated precipitation, furnishes a steady supply of water to the slope. This initiates and reactivates mass movement mainly by sustaining high pore water pressures in the slope-forming materials. Pore-pressure induced failure is not exclusive to the mass-movement scenario addressed. However, the accelerated rate of failure is. This is because the rate of failure is a direct function of the sustained supply of water from the overlying aquifer. Hydrogeologic variables are discussed in relation to other controls contributing to patterns and activity of mass movement.
New knowledge is contributed not so much by the further explanation of a relatively well-known concept, but by the presentation of a systematic methodology involving the collection of quantitative data from aerial photographs, and the processing of these data by means of computer-assisted techniques. The results obtained are consistent both with geotechnical theory and field data and have predictive value in terms of estimating the extent of mass-movement activity on slopes. As such, the methodology outlined offers an approach for researching similar problems.