Geomembrane / liner in a 3D model

Hi everyone,

I was wondering how to deal with very thin geomembrane/liners in rather large 3D models (eg.: a geomembrane underneath mining tailings) ? Here is an approach, but I would definitely appreciate sharing thoughts with you on this one...

1. Slices
+ using a couple slices to represent the liner (eg.: be 3 nodal slices / 2 elemental layers)
+ assign thickness to individual slice much greater than the actual thickness of the liner (may be on the order of 0.5m per layer) - I believe this is much more stable numerically than trying to implement millimeter-thick elemental layer
2. Hydraulic conductivity (K) value of the liner
+ determine K of the liner using Bonaparte and Giroud and considering typical liner defects density and size --> what else do you use to determine the equivalent K of liners?
+ scaling the hydraulic conductivity of the liner on the elements using thickness weighted geometric mean of K liner and soil above the liner
+ performing a sensitivity analysis performed for poor vs. good liner installation cases

Do you have any suggestion to improve this?

Best regards,
Thomas

Hello Thomas,
I agree with your way of setting the problem. Have you previous experiencies of instability by using very thin layers? Otherwise, I'd start giving the layers representing the liner a thickness comparable to the real one, enlarging it (should be easy if the layer is flat) as it shows instability. By this way, you may avoid to introduce an equivalent K.

Bye,

Giovanni

Thank you Giovanni,
you've probably experienced numerical instability in one of your model. ;)
It seems like many factors can create instability in a numerical model. For layer thickness, I can't confirm. This might be just one of several things I started  believe with time in my modelling career... there is a lot of controls I'm using in Feflow thinking it will have the models running smooth.
I might give it a try anyway.
Thanks again,
Thomas

You can bet layer thickness is among the factors potentially giving instability, but I always start optimistic with my models! The end is another story...  :)

Feflow is less sensitive to layer thickness variation than modflow, so using thin layers does not always result in much instability.  However, I recommend a layer thicker than the geomembrane with a thickness or volume averaged Kzz.  I've also put 1d vertical pipe elements in to mimic holes in the membrane and used the K of the pristine membrane as an alternate to an averaged K with assumed holes.

Pete

Hi Pete,
Thanks for the discrete feature suggestion Pete.Never thought of it.  Much appreciated.
Thomas

I wouldn't go through 1D discrete elements: means adding parameters (holes spatial density, cross-section area) and making an uncertainty assessment more difficult to accomplish. It looks better on simple problems.