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Cyclic stress-strain modelling of clay

A constitutive model for cyclic and average loading in plane-strain condition is developed. The main input to the model are stress-strain contour diagrams.


 Calculated characteristic stress paths for a hypothetical bearing capacity problem where the cyclic load component is equal to the average load component (Vcy = Va)

Contour diagrams of relationships between average shear strain, cyclic shear strain, normalised average shear stress and normalised cyclic shear stress for biaxial and direct simple shear (DSS) conditions, see Fig. 1. These diagrams are for a fixed number of cycles (or equivalent number of cycles). The model assumes perfectly undrained condition. The diagrams must be valid for stress states starting at the initial stresses up to stresses (strengths) where either the average or cyclic shear strain becomes large. The model is implemented into BIFURC-2D and PLAXIS 2D.

Due to the coupling between the average and the cyclic stress-strain relationships, the calculation is divided into two separate analyses where:

  • The average stresses and strains are calculated by applying the average loads (including the weights of the soil and the structure). Input to this analysis is a file with the distribution of cyclic biaxial and DSS strains , which may be zero or calculated in the second step of the analysis. The calculated average biaxial and DSS strain distribution are then written to a file.
  • The cyclic stresses and strains are calculated by applying the cyclic loads. Input to this analysis is the average biaxial and DSS strain distribution, which may be zero or calculated in the analysis first step. The calculated cyclic biaxial and DSS strains are then written to a file.

In order to follow the non-linear behaviour of the soil, the average and cyclic loads have to be applied alternately in steps. For each step it may be necessary to run several iterations between the average loading and the cyclic loading analyses..

The model has been verified by analyses of a hypothetical bearing capacity problem and back-calculation of a cyclic model tests of the Troll platform. The calculated bearing capacity is compared with the capacity calculated using the NGI-ANI2 model. The main uncertainty in the model is found to be how to account for the coupling between normal stresses (biaxial condition) and the shear stress (DSS condition) for a stress state between these two characteristic states. The model is documented in report 20071085-1.