ABSTRACT
Every slope experiences a gravity-controlled shear stress, the magnitude of which increases with slope inclination and unit weight of slope-forming material. In general, at-a-site slope instability depends on the balance of opposing forces of shear stress and shear resistance of the slope-forming earth materials. The slope becomes unstable and deforms when the shear stress due to external causes, internal causes, or both exceeds the shear resistance of the earth materials (Terzaghi, 1950; Hansen, 1984). External controls of shear stress refer to a variety of natural causes and anthropogenic effects as land cover and land use change that increase the magnitude of stress in the earth materials due to changes in slope geometry, moisture conditions, pore pressure, and loading and unloading of the earth materials in periods of seismic stress. Internal controls of shear stress like weathering, seepage erosion, and progressive failure are inherent to the nature of the earth materials. The external and internal variables of shear stress provide a framework for the classification of mass movement activity into types. However, possible permutations and combinations increasing the magnitude of shear stress on slope-forming earth materials are so numerous that the application of even a selected few among them yields complicated mass movement classifications of little practical use.
