Landslides (s.s.) occur on slopes with gradients between 10 and 40°. In their appearance they are manifold (size, depth, shape of slide plane) and the movements vary depending on the underground structure, the soil or rock conditions and the availability of water.
There are two types of landslides: rotational and translational slides. A landslide is triggered when the ratio of the shear strength to the driving forces is slowly or spontaneously reaching a critical value. The shear strength of a slope is altered mainly by internal conditions (e.g. weathering) and trough increased water pressure (continuous rainfall, downpour, or snow melt). Natural or anthropogenic external impact (e.g. the surcharge of a slope or the cutting at the toe of a slope) increases the probability for movements.
Outcome of landslides
The movements develop tension cracks and often open scars along the main scarp and the lateral border. The surface of the sliding mass can be broken up into individual blocks when the movements are fast or superficial, or when there are secondary slide planes. The surface becomes hummocky.
Such phenomena occur within very short periods of time and with related high velocities (m/s) or in very long time scales (mm per year or even mm per century). Extended deep-seated landslides generally move with a few millimetres or centimetres per year, hardly recognizable over decades. However, they may occasionally be reactivated / accelerated as it was the case for the Falli Hölli Landslide (Canton Fribourg, 1994). This landslide travelled with a speed of 6 m per day involving a volume of about 33 million cubic metres.
The threat of landslides is not only determined by the speed and volume of the mass but by the differential movements of individual earth blocks. Such movements can dramatically reduce the stability of buildings. This causes cracks in walls, the subsiding or even toppling of buildings (types of impact). In the worst case a complete collapse of the structure results. Landslides can cause damage to roads and pipes. Flash floods or debris flows can occur when a landslide travels into a watercourse and temporally dams the water.
The danger of landslide is estimated using the volume of the mass and the average velocity. A better and more detailed consideration of the potential for reactivation and of differential movements is presently discussed in Switzerland.