Difficulty with staged embankment construction using Modified Cam Clay in FLAC3D (yield envelope error)

FLAC3D
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1

I am attempting to simulate a compacted highway embankment in FLAC3D following the general framework for unsaturated hydro-mechanical behavior of compacted fill. My workflow involves staged construction of the embankment in lifts and eventual coupling with unsaturated flow.

During lift activation (especially when assigning MCC or shortly after), I repeatedly encounter: “Cam-clay: Yield envelope cannot be reached”

Is this a known limitation or sensitivity of the built-in Modified Cam Clay model in FLAC3D during staged construction?
Is there a recommended way to initialize MCC zones during lift placement to avoid yield surface incompatibility?

I would appreciate any guidance or examples of stable workflows for staged embankment construction using MCC (especially with later unsaturated coupling).

Additionally, if a more appropriate approach is to implement a fully coupled unsaturated constitutive model using FISH, I would greatly appreciate guidance or examples on how to structure such a zone-by-zone constitutive update within FLAC3D.

Error Message
Error Message1096×684 159 KB

2

It is hard to comment without datafile duplicating your issue. The error is usually due to loading step is large or the effective mean stress is numerically negative.

Be caution of the unsaturated flow using MCC. Once it causes negative pore pressure, it will lead to negative effective mean pressure (p’ < 0) , which cannot be reached to the ellipse yield surface of MCC since the ellipse is on the right half plane of p’ >= 0.

Steps could be (1) set up a cut-off non-negative pore pressure; (2) from a reasonable initial p’ > 0 when assigning MCC (could be calculated from other simpler constitutive models, e.g., MC); (3) sufficiently small load steps.

Please send your datafile and specify which FLAC3D version and subversion to flac3dsupport@oneitasca.com.