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Ingenieurgeologische und bodenmechanische Untersuchungen an Kriechhängen

  • G. Gudehus
  • M. Goldscheider
  • R. Lippomann
Conference paper

Summary

Investigations of sliding slopes have indicated that the sliding and creeping phenomena occuring in slopes consisting of overconsolidated clay cannot be explained on the basis of shear strength parameters determined in conventional laboratory tests. Thus it follows that when engineering activities take place on such slopes particular attention and improved soil mechanics research methods are required for a reliable prediction of slope stability.

The primary object of this investigation was to develop and test such a method. When developing this method procedure special emphasis was placed on geological factors like stratification, softened zones and weathering which influence clay slope stability.

Three slopes (located at the western edge of the Schwäbisch Alb; fig.1),on which failure has already taken place in form of sliding or creeping have been investigated, namely:

A creeping slope of Keuper marl (Higher Triassic) near Hechingen, a slipped slope (Middle Jurassic) near Thanheim and a creeping Keuper marl slope near Böbingen/Schwäbisch Gmünd. These slopes were mapped and explored using engineering geology methods. Soil samples for laboratory tests were taken from boreholes. Location of the shear bands were determined by inclinomter measurements performed inside boreholes (fig. 7). The groundwater levels within the sliding body were observed in additional shallow boreholes, as the exploration drillings showed a deeper groundwater table which was not significant for the slope stability.

Soil mechanics index tests were used for soil Classification and to determine the weathering categories. Specimens obtained from the presumed shear bands were tested in direct shear, ring shear, and triaxial tests. These tests were evaluated with respect to the angle of total shear strength, ϕ’r, and the angle of residual shear strength, ϕ’r. These shear parameters were considered the most suitable ones to model the shear strength of the observed softened bands in the overconsolidated clay.

In the backcalculations the sliding mass was assumed to consist of several rigid blocks moving relative to each other and the fixed subsoil along plane slip surfaces (kinematically admissible rigid body failure mechanism, fig. 8). The number, size and dimensions of these blocks were determined on the basis of field measurements and morphological conditions. The values for ϕ’s and ϕ’r obtained by backcalculations were lower than those values measured by shear tests. The difference in the case of direct shear and triaxial tests was of the order of 2 to 5 degrees, while for the ring shear test is was about 2 to 4 degrees. The measured values for ϕ’s varied from 13 to 25 degrees while those of ϕ’r were between 9 and 20 degrees.

The following conclusions have been drawn from these investigations:
  • In clay slope stability-analysis geological investigation plays an important role and should be performed in the first stage of the investigation in order to establish an efficient research programme.

  • A reliable prediction of slope stability is very dependent on the location of the shear bands being determined accurately. The location of the shear bands may be found by means of weathering categories, but when slope supporting structures are used the location of the shear bands should be determined by inclinometer measurements (fig. 9).

  • Commonly applied shear strengh parameters as ϕ’, c’ (effective shear Parameters of the overconsolidated clay) and cu (undrained cohesion), determined in conventional tests, are not suitable for slope stability analyses because there is no cohesion from the overconsolidation. The ϕ’s and ϕ’r values also seem to be a little to high and should therefore be used with a higher safety factor. If possible backcalculated values should be used.

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Copyright information

© Springer-Verlag Berlin Heidelberg 1985

Authors and Affiliations

  • G. Gudehus
    • 1
  • M. Goldscheider
    • 1
  • R. Lippomann
    • 1
  1. 1.Universität KarlsruheKarlsruheGermany

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