Sagging

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In geophysics and geology, subsidence is understood to be a type of settlement in alpine valleys and on steep slopes that is independent of the pressure-bearing layers . In the upper part of the sagging, terrain subsidence predominantly takes place, whereas in the lower part, terrain elevation occurs.

Geomorphologically, a subsidence has similarities to a landslide , but its dynamics are clearly different: on the one hand, it runs much more slowly (cm ... dm per year), on the other hand, part of the rock mass undergoes an overturning, i.e. H. Rotation around approximately horizontal axes. Sagging is not limited to loose rock , but has also been observed in solidified rock, e.g. B. in the rock sags on the slopes of the Aletsch Glacier .

The physical causes of such mass movements have only been partially researched. They can express themselves as a slow flow when e.g. B. the cohesion (soil mechanics) of the soil particles is reduced by adhesive water . Due to the resilience of a soaked soil and the loose rock near the surface , there is a steepening and sometimes also wrinkles in the lower third of the slope, which can lead to the valley thrust . A change in the water table or in the distribution of tensile stresses can also trigger subsidence , for example when a steep valley flank loses the ice as an abutment when glaciers recede.

The details and the course over time depend heavily on the grain sizes of the rocks or soils close to the surface and the cohesiveness of their mix. Measures for this can u. a. be the angle of friction , the clay content , the water content of the pores or the fine material content in the rock ( grain size distribution ). Evidence of an ongoing sagging process are cracks in the slope or fissures between its boulders.

The sagging phenomena run faster, the less the material is already compacted , i.e. H. is compacted by overlying layers. The qualitative, soil mechanical investigation of such mountain slopes in the Eastern Alps was first investigated in the 1970s in projects by the Swiss-Austrian geophysicist Adrian Scheidegger .

Sacking is also sometimes used for recultivation and for plant substrates in horticulture , although this is a settlement. In planting soil -Schüttstoffmischungen it is between about 10 and 15 percent, wherein the dewatered peat soils at 1 cm / year.

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