gravity displacement, mass movement — English

The down-slope falling, sliding or creeping of lose weathering products or debris (see “erosion”). There are numerous ways and reasons for this to happen, but the most important causative factors are gravity, exceeding the angle of repose, loading and water saturation. A pile of debris – which might be sorted material, as in the case of mine dumps, or unsorted materials as in the case of a mass of glacial debris will heap-up and come to rest with the side-slopes of the heap forming a certain angle to the horizontal. This angle is the angle of repose. For natural materials (such as weathering debris) the angle of repose is normally less than 35˚. Should the sides slope at a steeper angle, the pile will collapse and come to rest with the sides sloping at a lesser degree. Water saturation of the material is, of course, crucial. Compare a sand heap built with relatively dry sand on the beach, to one built with wet sand – the former would be a steeper heap than the latter. A small child would probably be able to sit on the heap of dry sand without causing it to collapse, but the same child will cause the collapse of the heap built with the wet sand. The child is the “load”, that is the mass (weight) that the heap can withstand without collapsing. A larger child will cause the collapse of the heap of dry sand. Water poured on the heap of dry sand will also cause the heap to collapse into a much flatter heap, proving that water saturation is an extremely important factor in slope stability. While gravity remains the same, a change in either the level of saturation or the load placed upon the material, will cause a change in the angle of repose of the pile resulting in the pile of material to slip, creep or instantaneously collapse. Rock falls, landslides and mud flows are dramatic, sudden occurrences which often cause lives to be lost and property to be irreversibly altered. In our modern, engineered and economical form of life, the relentlessness of the natural processes of mass movement is often disregarded, and always to the detriment of the ordinary people. Probably the most well-known example of mass movement that caused a terrible loss of life is the Aberfan incident in Wales, Great Britain, when a mine dump collapsed in 1966 and a mass of oversaturated mine waste simply covered a nearby school burying all the pupils and teachers in one fell swoop. South Africa too has its examples of dramatic, costly and often tragic anthropogenic (human-induced) mass movement. One of the most well-known scenic roads in the world is Chapman’s Peak Drive around the peninsula just south of Cape Town. The road was built by undercutting the steep slopes of the peninsula, thus disturbing the natural stability of the slopes and consequently rock falls are common occurrences all along the route. Enormous amounts have recently been spent to prevent rocks from tumbling onto the road, and after heavy rains the road is often closed for a few days in order to avoid calamities and remove debris from the road surface. An ever-present form of mass wasting is soil creep which is the slow, continual creeping of soil downwards over a shallow slope of as low as three degrees (3˚). This is a completely natural process, but human activities such as animal husbandry and crop production might accelerate the process and render vast tracts of fertile land unproductive.

Afrikaans     Sesotho sa Leboa     isiZulu    

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