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Beobachtungen und Berechnungen zur Böschungsentwicklung bei der Talbildung im wechselhaften veränderlich festen Gestein

  • H. J. Baumann
Conference paper

Summary

Since 1970 landslides in the Isar river valley south of Munich have been observed by the Geological Survey. One landslide spreads over 700 m along the eastern riverside near Gruenwald (fig. 1). The failures usually take place in Miocene marls consisting of silt (siltstone), sand (sandstone) and fissured clay of high plasticity. The marls are overlain by gravel, which is 30 to 40 m thick (fig. 2). The river is deeply cut into these alternating layers by erosion. At a critical depth of the cut the failures start by sinking and spreading and continue as retrogressive slides (fig. 3).

This mechanism can be observed also in similar geological formations in many Bavarian river Valleys north of the Alps. The investigations by the Geological Survey will increase knowledge about the failures and will allow to predict them and their development. The applied methods of measuring, their accuracy and the type of results are described in this paper (tab. 1).

The displacements are compared with the Variation of the overburden, caused by fluctuations in the river or in the groundwater, by precipitation and by displacement of the center of gravity in the sliding mass. The distribution of the horizontal displacements along the eastern riverside (fig. 1) gives a good impression of the gradual expansion of the sliding movement.

The displacement vectors evaluated in several cross sections enable the determination of the failure mechanism, which must be adopted for the calculations. Figure 4 presents several mechnisms of failure and their hodographs, which agree or seem to agree with the observed displacement vectors. They all show a vertical sinking movement in the upper parts of the slope and a horizontal movement at its foot. But there are many cross sections showing very different states of sliding, which influence each other and which must be considered when the mobilized angles of internal friction are computed.

In order to simplify the computation each cross section is divided into three parts (fig. 5). Table 2 shows the results of computation at several stages of sliding and different water tables. At one stage (April3 0th, 19 76, low water table) the distribution of the computed driving and breaking forces is plotted in fig. 6.

At the beginning of a new failure, the mobilized angles of internal friction were computed between 17° and 21° (tab. 3). The division of the cross sections is shown in fig. 7. But it is not possible to predict the beginning failure exactly enough. It is easier to determine the safety of the slopes by investigating the structures of the sliding mass in the neighbourhood. The best way to find out when a new failure will begin is to measure the movements at the foot of an undisturbed slope.

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Literatur

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Copyright information

© Springer-Verlag Berlin Heidelberg 1985

Authors and Affiliations

  • H. J. Baumann
    • 1
  1. 1.Bayerisches Geologisches LandesamtMünchen 22Germany

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