Greetings,
I have a simple model of a levee and am trying to conduct fully coupled flow-deformation analyses. However, the preliminary steady state GW flow analysis to determine p_steady cannot converge. I have thus tried to make a steady state flow only analysis using the same materials and same "final" boundary conditions, and indeed it does not converge. If the solver runs through all iterations the error code is #34, Ultimate state not reached in groundwater flow analysis. However, if I just change the permeability of the berms to a much lower value, the calculation does converge. All of the conductivities are within the expected values for such materials:
Berms: ky = 1e-3 m/s, kx = 1,7e-3 m/s (does not work even if the anisotropy is removed, work when both are changed to a maximum of 1e-6 m/s)
Levee body: ky = 8e-7 m/s, kx = 2.4e-6 m/s
Core: ky = 1e-9 m/s, kx = 3e-9 m/s
Topsoil layer (purple): ky = 4.6e-8 m/s, kx = 1.4e-7 m/s
Foundation soil (green): ky = 4.6e-7 m/s, kx = 1.4e-6 m/s
All the materials are set to use the Hypres data set parameters for the Van Genuchten model, but this should not be relevant for steady state flow.
The flow is either defined by boundary conditions, or by global water levels, but both behave equally. I have tried with different mesh sizes, with relative element size factor from 2 to 0.17 (lower than very fine), with local refinements, but no success. I have also exported the geometry and copied the materials to try in different software (only steady state analysis in Geostudio), and convergence is achieved.
What else might be causing convergence issues in simple steady state analyses which are required for the fully coupled flow-def. analyses? Transient flow-only analyses with the "problematic" parameters work (converge) just fine.
Dear Nicola,
In general numerical solutions do not really "like" large ratios, e.g., stiffness, permeabilities, above or lower 1E6.
Indeed, looking at the numbers provided you see that the Berm vs. the Core has a significant difference in permeability. 1E-3 m/s is 86.4 m/day, which enormous amount of water, while on the other hand, the Core is 3E-9 m/s or 0.00026 m/day.
Is this huge gradient required to get the geotechnical behaviour you need for this project? That is a question you need to ask yourself in such case.
In any case, as this concerns a specific project we can handle this via a Service Request. Please submit a service request.
Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport
Alright, I will submit a service request. However, 86.4 m/day is not at all enormous, it is in fact around the permeabilities suggested by USACE, CIRIA and others for sand and free-draning berms, while 0.000086 m/day is also perfectly normal for an "impervious" core within a levee. I cannot just decide to change these parameters. Besides, the ratio as it is is 1e-6, and it doesn't work even at 1e-5 (when the berm permeabilities are reduced by 10, to 8.64 and 14.4), and even at 1e-4 (when reduced to 0.864 and 1.44) - with and without anisotropy.
Yes, that is exactly we need to check the specifics of the project as just reading numbers is most likely not showing the full picture.