Hi,

Thank you for make this program available for all of us.
I have a couple of questions regarding Cam-Clay Model. When I try to use the soil material with Cam-Clay Model, it often comes back with an error message "Mean effective Pressure is negative".

So I just made a simple square with Cam-Clay soil with x-fixed boundaries at both sides and Y-fixed boundary at the bottom.  Set Gravity on with 9.81 and try to solve it. This also give me the same error. I do not understand why this is the case.

I would appreciate it if you could guide me with this problem. I assume that there may be some mistakes in setting the initial condition. Any input will be appreciated.

Thank you.

Hi

Just a heads-up?the Cam-Clay model in FLAC comes straight from its open-source constitutive model library. According to the FLAC manual (quoted below), it has a pretty specific requirement:

"The Cam-clay model in FLAC is only applicable to materials under compressive mean effective stress... The initial effective pressure (p₀) must be positive throughout the domain."

So basically, the model only works when the stress state is compressive. That means before switching to Cam-Clay, your initial conditions (either through the INITIAL command or from a previous model) need to meet that requirement.

I haven?t used this exact model myself, but here?s a workflow you could try:

1- Set Initial Stresses: Run an elastic model first to generate a proper stress state.
2- Switch Models: Then, switch to the Cam-Clay model.

Or: You can also just initialize the model with the right stress conditions upfront (make sure the stresses are compressive?so negative values).

Give that a shot and see if it helps. And if you want more info, definitely check the FLAC manual.

-Roozbeh

Hi Roozbeh,

Thank you for your prompt reply. As you commented on my reply, that was what I did.

I prepared a model with soil parameters with MC Model,
Activate the vertical gravity,
Set all deformation zero, and
Edit the target soil parameter model to be Cam-Clay model.

Since I kept stress condition unchanged, the effective stress in the model is not negative. But I am still getting the same error "mean effective stress is negative".

I also checked the esyy numbers at each node, and all esyy numbers were shown as negative stress. Does ADONIS show the contractual stress (or downward stress) with a negative sign (minus)?  I may be missing something about how to deal with the initial condition.

Please advise.

Thank you.

Yes, negative internal stress indicates compression. I?ll take a closer look at it later to figure out what?s going on. In the meantime, check FLAC?s manual and try to follow the basic example they provide?see if you can replicate it in ADONIS.