Bounding Cam Clay Model

[Lei Lei]

Check your selfweight, as you put it is 15 MPa, are you sure?
Then use adaptive time step

Thanks， reducing the self-weight and using adaptive steps really make sense.

But, why does no plastic deformation occur under confined compression?
Moreover, the shear cycle cannot be completed at 75kpa compression as the paper shows.

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[STKO Team]

I don't understand what you are trying to do in this example, however
1) you can see plastic deformation in your cycle (when sigma_12 unloads to zero the deformation is not zero).
2) Then you material fails in the 3rd stage where you are removing (?) the vertical compression (probably it makes sense, it cannot bear anymore those shear values?)

[Lei Lei]

2) Then you material fails in the 3rd stage where you are removing (?) the vertical compression (probably it makes sense, it cannot bear anymore those shear values?)

In fact, the shear behavior in my third shear stage consists of four pushover processes. However, the model fails to converge at the beginning of the third pushover stage, and it has not yet progressed to the fourth stage of unloading. Why can't I complete the entire shear process as described in the article? The shear stress level at failure has not yet reached its historical maximum.

In addition, I also conducted a biaxial loading and unloading unit test. During the biaxial compression process, no plastic deformation occurred in the plastic stage. However, during the unloading process, the soil was stretched before the load was completely unloaded, and the calculation did not converge. It is so weird.

[Lei Lei]

Are there other soil models that exhibit volumetric hardening? Most other FE and FD package have a model for clay with a volumetric cap. For example, Plaxis has the Hardening Soil Model and FLAC has a soft-soil model.

Hi, do you find other soil materials with a volumetric cap(both sand and clay)?

[STKO Team]

The results seem weird because when you do the loading, you assume an elastic behavior and the elastic moduli do not depend on the confining pressure.
When you unload, you switch to plastic and the unloading modulus change with the reduction of compression. Please verify this

[Lei Lei]

The results seem weird because when you do the loading, you assume an elastic behavior and the elastic moduli do not depend on the confining pressure.
When you unload, you switch to plastic and the unloading modulus change with the reduction of compression. Please verify this

For constitutive models with a plastic stage, I think that when analyzing the initial stress state, the load is applied before the elastic stage. After activating the plastic phase, Opensees will automatically update the elastic calculation results to plastic results. Is it necessary to divide the entire load into two parts and apply them to the elastic phase and the plastic phase respectively?

[STKO Team]

Try this and you will see what I mean.

BoundingCamCly-K0.zip

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If you look at the source code of the CamClay you will see that, one you switch to plastic (during unloading in your simulation), even if the material does not reach the yield surface, its elastic behavior depends on the mean pressure (nonlinear elastic)

[Lei Lei]

If you look at the source code of the CamClay you will see that, one you switch to plastic (during unloading in your simulation), even if the material does not reach the yield surface, its elastic behavior depends on the mean pressure (nonlinear elastic)

I understand that soil exhibits nonlinearity after plasticity activation.
However, I have a question: when should plasticity be activated?
For PIMY and PDMY, once plasticity is activated, the soil's deformation updates based on elastic analysis, leading to plastic deformation. In contrast, the BCC model begins to show nonlinearity without this update. Is it necessary to activate plasticity at the start of the initial stress analysis for BCC? If so, should a load be applied during the elastic stage, or is it better to apply a very small value? Additionally, how should plasticity be activated in other constitutive models that include plasticity activation?

[STKO Team]

First, your model did not go into plastic regime.
However, you see an elastic nonlinear behavior due to the dependence of the elastic constants on the pressure (when you activated the nonlinear stage)

I understand that this concept of elastic/plastic stage is messy, but it's the only way they have to make these soil model work, otherwise, they will show a singular matrix if the pressure is 0.
Probably this is acceptable in soils since they will have a pressure anyway due to self-weight.

The problem is in your simulation. Why are you doing a loading-unloading biaxial compression?
At least, add the self-weight only during the elastic analysis -> switch to plastic (nonlinear) -> run the cyclic biaxial compression using the additional load.
In this way the self weight during the elastic stage will be used to define the initial stage for a non-zero initial pressure

[Lei Lei]

In this way the self weight during the elastic stage will be used to define the initial stage for a non-zero initial pressure

Thanks!
Therefore, the elastic analysis stage is only to obtain the initial ground stress. In this process, the performance of the soil does not change, because the parameters set by the soil are determined under this stress condition. The properties of soil will only change under the action of additional loads. In order to reflect the influence of external loads, it is necessary to activate the plastic phase. I don't know if I can interpret it that way.

The problem is in your simulation. Why are you doing a loading-unloading biaxial compression?

And I did that because the weight of the soil mass is negligible for the unit test.