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Progressive failure in clay

The purpose of the project was to develop a simplified computer program for analyses of progressive failure in sensitive clay. The computer program may be used to back-analyze existing slides, checking the stability of existing slides and estimate the change in the stability during geotechnical construction and stabilizing treatments of critical slopes.

The program is based on the following main features:

  • Progressive development of a shear band along a pre-defined critical failure plane. The shear band is modelled by curved interface elements with a non-linear strain hardening/softening stress-displacement relationship. The input to the material curve is the peak and residual shear strengths and the corresponding relative displacements at these stresses.
  • The zone above the critical failure plane is modelled by 1D displacement elements with varying height (thickness) along the elements. The normal stress in these elements is described by a non-linear stress-strain curve with a peak active and passive (earth pressure) strength. The transversal shear stress along the elements is also described by a non-linear stress-strain curve with a peak shear strength.
  • The material models take into account the effect of strain rate effects. The strengths are scaled by a factor that depends on the strain rate. The scaling factor as function of strain rate is given as input.
  • The geometry is updated during the analyses. The lowering of the terrain in the back and the heave in the front (see the figure below) during the development of the progressive  failure may be stabilising effects that is necessary to include in order to describe the large extension of this failure mechanism in very gently inclined terrain.
  • The inertia effects should also be included. When the progressive failure mechanism starts to develop the propagation speed may be governed by the acceleration of the mass above the shear band. The final extension of the slide may also be governed by the retardation of the moving mass.

The program is verified by back-calculation of one of the examples in Stig Bernader's thesis (1982).


Illustration of a calculation model for progressive failure in a gently inclined slope proposed Stig Bernander in his thesis (1982).