Drain: Factors depend Groundwater head drawdown

Question

Hi all, 
 I modeled a drain with a normal behavior (starting from y=15m to y=12m). I provided the groundwater head of the drain as h=12m. I did the steady state flow analysis and thought the groundwater would be drawdown to h=12m from h=15m. But in the output result the reduction of head is only 2.456m instead of 3m as shown below (since groundwater flow analysis is very much sensitive to mesh, I locally refined the mesh around the drain).

Can anyone tell the reason behind this and the factors depend on these drain element in Plaxis? 
 
 *** For the below-highlighted part, in consolidation analysis pore water pressure calculation type should be phreatic (if steady-state flow then with the reduction of the excess pore water pressure, the head also decreases), for confirmation.

Thanks.

Faseel Khan · Answer

From the first screenshot, it seems as if the flow arrow results at the stress points are quite far apart meaning that the mesh is not as dense as it should be. Try using the coarseness factor for the drain as 0.03 and re-run the analysis.

Stefanos Papavasileiou · Answer

Dear Nitharshan, 
 I cannot tell precisely what happens in your model via the Forum, as we handle these questions usually via a Service Request. 
 I would suggest checking the input parameters of the material to see if the soil can manage (i.e. permeabilities) to transfer the water and achieve such a lowering of the water table. 
 On the second part, I am not entirely sure I follow what you describe: in a consolidation analysis the main purpose is the build and dissipation of excess pore pressure. 
 The way to compute the pore pressure distribution ( psteady , pexcess ) depends on the pore pressure calculation type you select. A Phreatic will base the psteady on the water line you draw (or WaterConditions defined), but no flow is run. A Steady state groundwater flow is a flow analysis in which the boundary conditions and permeabilities are considered to give you the exact pore pressure distribution (psteady again). However, the developed pexcess are not part of this. 
 When activating a drain element in a Consolidation analysis, it will simply ignore any value you specify as head (it won't matter if you set it to 12m or 13m) as it will act as a boundary in which the excess pore pressure is zero, thus acting as a physical drain and a way out of the excess pore pressures. Since the purpose of the is the build and dissipation of excess pore pressure, the drain here will help dissipate the excess pore pressures directly. 
 Regardless, as my colleague mentioned, by refining you can probably achieve better accuracy in the results.

Dennis Waterman · Answer

Dear Nitharshan, 
 The flow velocity is larger closer to the drain. Since flow velocity is calculated in the stress points, and for a fine mesh the stress points are closer to the drain than for a coarse mesh. Hence, when using a fine mesh the calculated maximum flow velocity is larger. 
 So both results are fine, they just show the flow velocity at different distances from the drain or well. 
 With kind regards, 
 Dennis Waterman

Dennis Waterman · Answer

Dear Nitharshan, 
 If you refine the drain, you also automatically the soil around the drain. The (finite) elements of the drain must match the finite elements of the surrounding soil - so smaller elements of the drain must lead to smaller elements in the adjacent soil. 
 You can simply see that yourself by looking at the finite element mesh around the drain. 
 With kind regards, 
 Dennis Waterman