Why does the initial state analysis diverge for simulations with Modified Cam-Clay model
Setting the initial stresses σ0 super element for Modified Cam-Clay model is required
The Modified Cam-Clay model (MCC) requires an initial guess for the initial effective stress state to be set by the user. In the initial state computation hardening is deactivated and MCC behaves as classical elasto-plastic model with deviatoric flow rule.
Notice that once the unloading branch, that we assume does not produce any plastic strains, is linear in e-ln(p) axes the soil stiffness must depend on the effective stresses. The bulk modulus is expressed as K = (1 + eo)/p.
So every computation with Modified Cam-Clay model (also HSs) will require to set up the initial guess for the initial effective stress. No matter whether the initial state driver is active or not.
How to define initial stresses σ0 super element
How can we guess the initial stresses in that case?
Setting the initial effective stresses can be made exclusively at the FE level through the option in right menu:
- FE Model/Initial conditions/Initial stresses/On 4 nodes/points (for 2D cases)
- FE Model/Initial conditions/Initial stresses/On 8 nodes/points (for 3D cases)
To define the initial stresses in quadrilateral regions (covering our FE mesh) we can set all stress components at each vertex separately or we can use a simplified method where we fix the vertical stress at the top and we define the in situ Ko coefficient and soil unit weight that will produce correct σo .
Note that by running the Initial State driver and assuming Ko coeficient locally at the material level we do not need to be very precise when we guess the initial in situ stress state. The driver will correct it anyway.