Coupling analysis for pile supported embankment- Please share your insights

Hello @itascan ,
I am running a coupled analysis for pile supported embankment.
When I activate the drained condition, initially the grid -point pore pressure drops and then gets constant at some higher values than the initial value before loading and drops very slowly. And the fluid time-total seems to increase very slowly. Is this normal ? It would take so long to reach to equillibrium and I could not understand why would there be higher gridpoint pressure in the middle layer than the bottom? Can you kindly refer to attached photographs and share your opinion?

Further, in the console pane, the mechanical substep seems to run but the fluid substep seems to stay still ?

Hello @Newuser,
The initial drop in pore pressure when changing to a drained condition maybe okay depending on where you pore pressure history is being recorded. Also, be sure that prior to the drained analysis when you add the embankment that your model is in equilibrium.
Looking at your small timestep I would imagine it would take a long time to reach a steady state. You may need to investigate options of reducing the fluid bulk modulus or using fast-flow. As for the mechanical substep I suspect the mechanical substep 50 is too small. See No Flow - Mechanical Generation of Pore Pressure and Coupled Flow and Mechanical Calculations for more information.

Hi Newuser. Is your fluid analysis to establish a phreatic surface or simply the hydrostatic conditions, or what?
I ditto on what Itascan Blanksma said and would add that I typically initialize with a lower bulk modulus of water (say 2.2e9Pa/10000) to get things going, establish mechanical and fluid equilibrium and then turn both on with the actual Kw to finish off. This might do the trick.

Actually I applied Kw 1e4 to bring the model to equilibrium initially and then applied the undrained condition with model fluid active off (fluid modulus 2e8) when I simulate the staged construction of embankment and then after the completion of embankment construction, I turned the fluid flow on with the mechanical follower on (coupling) as well to facilitate the consolidation process?
Is there any guidelines to select the fluid sub step or mechanical sub step? Or it is a hit and trial approach?