modelling cut off wall using plaxis 2d


Hi,

 

I am having difficulties modeling 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 leakage 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 succeeded. 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 definiiton of the GWF BC's.

Could anyone please help me solving this problem.

Thank you all in advance.

Soil Mechanics / Geotechnical Analysis Education Software Groundwater Flow & Seepage

Asked 25/07/2021 12:44, updated: 27/07/2021 00:31
Ahmad Jazzar

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2 Answers

Votes: 0

Micha Van der Sloot

Hi

If you model the concrete using the soft soil creep model, and you set it to behave as a Drained material, the permeability value will still control its behavior. Or did you model it as a non-porous material?
When you want to model a screen around this concrete cutoff, you can also add interface elements around the concrete wall, and make sure that these are activated for the flow calculation. They will appear in orange in the Flow Conditions mode (PLAXIS 2D CONNECT Edition V20 and later), so then it is easy to check if these are activated.
I would advise checking the results, especially the groundwater flow arrows, and hydraulic gradients to see if the water will flow around the cut-off wall. You can see these via Stresses > Groundwater flow > |q| in the PLAXIS Output program.

For detailed answers specific to your model, and that allows us to investigate this model, please raise a Service Request here: https://apps.bentley.com/srmanager/ProductSupport 

By the way, on July 27, we also host a live Q&A, the answer hours related to water conditions.

https://communities.bentley.com/products/geotech-analysis/b/plaxis-soilvision-blog/posts/first-answer-hour-live-q-a-on-ground-water-and-pore-pressures

Another question: any reason you are using the Soft Soil Creep model for the concrete? The Concrete model in PLAXIS also has time-dependent behavior for creep.

Kind regards,

Micha van der Sloot

PLAXIS | Bentley Systems, Inc, Geotechnical Analysis

 


Votes: 0

Ahmad Jazzar

Hi,

 

thank you very much for your kindly response.

 

The cutt of wall was modelled as a soil element with low permeabilty and the drainage type was set to drained. The reason behind using the soft soil creep model to define the cut off wall material is because I am relying on a study, which has studied the creep behaviour of plastic concrete using the soft soil creep model. In addition, the available input prameters that I have are only valid for the SSC model. 

I've never known that the creep behaviour of concrete could be included in plaxis 2d, thank you for the remark. But I have an additional question for you: isn't plastic concrete by definition somewhere between concrete and soil element? Plastic concrete differs from normal concrete in composition and in compressive strength, wheres the compressive strength of the plastic concrete is almost 1/10 of the compressive strength of normal concrete, not to mention that PC is more ductile. Is it suitable to model PC as normal concrete or as soil? Which one is better in your opinion?

Introducing interfaces into the model didn't change the seepage path inside the dam that much. Seepage resulted from a flow only analysis was more realistic than that resulted from a fully coupled flow deformation analysis. Unfortunately the stresses obtained from flow only analysis aren't reliabale, are they? Isn't a fully coupled analysis supposed to deliver the same seepage patterns resulted from a flow only analysis? Is there anyway to run a plastic analysis following a flow only analysis?

 

I've already registered for the session tomorrow and I am looking forward to it. Is it possible to present a small example of a dam with a cut off wall during the session?

 

Best regards

Ahmad Jazzar