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Every solid or liquid mass on Earth is influenced by gravity. A mass of soil or rock remains stable if the gravity force is counterbalanced by the reaction forces exerted by the adjacent bodies and the terrain. Rock masses and soils on the surface of the Earth appear steady at first sight. However, this impression is often deceiving, as the masses may slowly creep, terminating with a sudden collapse. Natural buttressing of a potential landslide may be removed of weakened, causing portions of the mass to fall. Change in stability conditions may be consequent to a variety of causes such as river undercutting or ice melting. Earthquakes can instantly change the local force equilibrium, anticipating the fall. The process of mountain building continuously overloads rock masses with renewed stress throughout time scales of several million years. Newly produced deposits may also become unstable. For example, volcanic eruptions deposit enormous amounts of pyroclastic materials, which may subsequently be mobilized by rain.

A landslide starts as consequence of terrain instability, and for this reason it is important in geotechnical practice to ascertain the stability conditions of soils or rocks. Owing to the significance in the prevention of disasters, slope stability has been the subject of much effort. There exist numerous numerical models, textbooks, and computer programs for assessing the stability on different kinds of terrain. Here the problems of instability and the initiation phase of a landslide are very briefly considered, limiting ourselves to only a few basic concepts.

document: friction, cohesion, slope stability