Hi,
I am having difficulties modelling the seepage in an earth dam enhanced with a cut off wall using Plaxis 2D. The cut off wall is made of plastic concrete and was introduced later as part of a maintenance procedure in order to prevent any potential leackage from the resevoir. The plastic concrete was defined using the soft soil creep model as a drained impermeable material. Tutorials nr.12 and nr.14 in addition to a webinar on running a groundwater analysis gave an overview of the GWF BC's and of how to use time dependent flow functions in a fully coupled flow deformation analysis and how to run a GWF analysis. I've tried to combine all these tutorials by the modeling process to obtain the most optimum seepage/flow line but still haven't succeed. The results of the analysis show that the cut off wall doesn't influence the seepage, which indicates an error in the input data or in the definition of the GWF BC's.
Could somebody please help me to solve this problem.
Thank you all in advance.
.
Dear Ahmad,
From the screenshots, it is not entirely clear what is going on.
I can see that you are using a volume column which I assume has specific horizontal and vertical permeabilities set. If the idea is to make this cut off wall impermeable you should use the interface elements and make sure the check to have them "Active in flow" is selected.
The elements that seem to "fly" can be due to low tensile strength and/or cohesion. Please make sure to use a small value, not zero, to ensure numerical stability of the analysis.
If you still have questions, I recommend reaching out to our Support team: https://apps.bentley.com/srmanager/ProductSupport
Dear Stefanos,
thank you very much for your response. The cut off wall in this model was defined as a soil/ volume element, that's why no interfaces were used hier. But I've tried though to add interfaces into the cut off wall to see their influence on the seepage during a fully coupled analysis, unfortunately there were no big changes in the seepage path. I have also changed the poissons' ratio so that the problem of the irregular elements is solved.
I still have a another question though, a flow only analysis followed by phases with a steady state groundwater flow calculation type has given the desired or the most realistic flow path inside the dam body ( picture on the right), meanwhile a plastic analysis with a steady state grounswater flow has failed to give a similar seepage pattern ( picture on the left), despite the fact that the input data/ parameter and the defined flow boundary conditions are the same for both cases. It seems like that the cut off wall in the later case doesn't have any influence on the groundwater movement inside the dam.
A flow only analysis depends on the permeability of the defined soil layers to callculate pore pressure, wheres a plastic calculation depends on the drainage type. But shouldn't both cases somehow deliver similar seepage pathes? I mean the flow in the dam body is the same whether we run a deformation or flow analysis, or?
The only difference here is the deformation analysis included. I would not expect to see big differences in the water flow field.
We need to see what exactly is defined as we have implemented a way via the interface to make them impermeable (and nowadays semi-permeable), and we have cases that demonstrate that (links below):
To clarify your last point, indeed, a flow only analysis considers the permeabilities and the boundary conditions specified. A plastic calculation itself is not the option to determine the flow behaviour. It is the option of Phreatic, Steady-state groundwater flow of the pore pressure calculation type that determines whether a material will be drained or undrained as defined in the data set (for phreatic only) or the calculation will determine that (steady-state case).
If you can still not sort this out, please submit a service request. Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport
Hi Stefanos,
Thank you for your reply. The problem was solved by deactivating the updated water pressure in the deformation control parameters menu. Unfortunately I don't have a proper explanation on how deactivating this option affects the seepage path.
Best regards
Ahmad Jazzar
The Updated water pressures option is well-explained in our Reference manual: "it is meant to take into account the effects of soil settling (partly) below a constant phreatic level."
I do not expect this to affect the seepage path. I would investigate a bit more on what is happening in the model to better understand the results.
If you need assistance, feel free to reach out via a service request: https://apps.bentley.com/srmanager/ProductSupport