Shear parameters and slope stability

Influence of different test parameters on the shear strength and stability of a typical slope situation in the Lower Austrian road network.

The shear parameters (friction angle and cohesion) have a significant influence on the evaluation of the stability of slopes and embankments. These shear parameters are determined by means of laboratory tests (shear tests), whereby the test procedure is regulated by a standard, but this standard allows a certain range with regard to the choice of some test configurations and test parameters. Often the choice of test configuration and test parameters is done by the laboratory. This can have an impact on the test results and, as a result, the parameters determined (friction angle and cohesion) can fluctuate within a certain range. This ultimately has an impact on the assessment of the stability.

This will now be investigated in more detail using a concrete case study. For this purpose, a typical situation in the area of the Lower Austrian road network will be selected and first the in-situ situation will be recorded by means of underground investigations and field measurements (geodetic measurements, inclinometer measurements). On the basis of soil samples, laboratory tests are to be carried out in order to be able to assess the stability in-situ. In particular, it will be investigated how the choice of different test configurations or different test parameters affect the test results (friction angle and cohesion) and thus influence the stability assessment.

The following test variants, among others, are envisaged:

  • Direct shear test with constant pore volume ("Vienna routine shear test")
  • Direct shear test (3-point shear test):
    • Test with fixed upper shear frame as well as with floating and weight-compensated upper shear frame.
    • Test with and without water
    • Test with different shear rates
    • Test with different shear gaps
    • Test with disturbed/undisturbed soil samples
  • Comparison with results of triaxial tests

The shear parameters determined in this way are to be compared with each other and the influence they have on the utilization factor of the slope is to be investigated on the basis of stability calculations. The results of the inclinometer measurements will also be taken into account. Furthermore, the conditions during a shear test are to be analyzed with the help of numerical simulations.

4 technical drawings with different device configurations.

© Stoewahse, Blümel; 2001

Principle sketches of the mechanical function of different direct shear devices. [Stoewahse C., Blümel W.: Einfluss der Rahmenlagerung auf die Messwerte der Scherfestigkeit im Rahmenscherversuch; 1. Siegener Symposium „Messtechnik im Erd- und Grundbau“; 2001]

A shear frame with a built-in soil sample hangs from a crane; 2 people hold the frame.

© TU Wien, Institute of Geotechnics

Installation of a soil sample in the large shear test device.

There are 3 open shear frames on a table, one behind the other, showing the soil after the shear test.

© TU Wien, Institute of Geotechnics

Sheared cohesive soil samples of the small shear test device.

View of a mechanical shearing device.

© TU Wien, Institute of Geotechnics

Mechanical shearing device for the Vienna routine shearing test.