Hello everybody,

Using Hypoplastic model for clays, with intergranular concept parameters, I am simulating a simple oedometer test, phase by phase, using a dummy layer on the top of the specimen as the surcharge of each step. In order not to take into account the dimensions of the cluster (specimen), I am considering a weightless cluster with 1×1 m dimensions.

If i run the model using a set of parameters with Pt=1 kPa, and “e or OCR”=1,13, then the body soils collapse just in the first load phase (6 kPa); but in an extrange way, if run those first two phases using exactly the same constitutive parameters but changing Pt for 25 kPa and “e or OCR” for 11, then the model runs in a correct way, even if in the following phases (after initial conditions and first loading) I still consider the first Pt and “e or OCR” mentioned magnitudes.

It seems like the problem is solved by changing those two parameters on the two first stages, but for me it does not make any sense that the rest of the phases do run ok with the “wrong” parameters.

I hope somebody can help me with this.

Tags: hypoplastic model
Dear Daniel,

This is quite understandable, hypoplastic model has problems if stresses are close to zero.

pt shifts stresses artificially to higher values.

So, this way you pass the initial zero-stress state, in subsequent stages, when the soil is loaded already, it runs with even lower value of pt (it would run even with 0 pt, but I think some very small value is required for safety).

As for void ratio, I do not know as e=1,13 must always be evaluated together with N and mean stress. From that, model calculates OCR which is actually controlling variable. So if e=1,13 leads to OCR below 1, it would also be source of problems.

Regards David

Thank you very much Professor.