2D Groundwater flow - Global water level influence

Question

Hi 
 I am modelling a dewatering of an excavation, using 2D Plaxis V21 in the "steady state ground water flow" analysis. 
 For me it is not clear how I should set the global water level or what kind of influence this water level does have. 
 I made three cases to compare. Independent where the global water level is, the result of the calculation is the same. Does it mean, that the global water level only sets the starting point of the calculation assumptions? But where is Plaxis taking the hydraulic potential from, only from the cluster (dry, interpolate, global water level) or from the previous stage? The Case 3 is the only different case, where the total groundwater flow is lower compared to case 1 and case 2 
 thanks in advance for any feedback and recommendations with your experience. 
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Stefanos Papavasileiou · Accepted Answer

Dear Manuel, 
 Modelling water in a steady state groundwater flow means that PLAXIS will consider any external boundary conditions and the soil permeabilities for the calculation. 
 I would strongly advise you to follow the relevant webinar titled: PLAXIS CONNECT Edition: Excavation Dewatering and Common Pitfalls : learn.bentley.com/.../LinkToIndividualCourse 
 Regardless, I can see that you are comparing three different cases and I can explain why the end result is as shown (assuming a couple of things without having the projects), however, the most important question is what do you want to simulate? What is your situation? And what do you want to achieve? 
 Now going to the three cases: 
 Case 1: the water level seems to either be short or going vertical up at the level of the wall. For a groundwater flow analysis, a water level drawn inside the soil will not be considered as only the external boundaries are prescribing a condition. Since your left-hand side is a closed boundary that means practically in the excavation there is no water. That is also confirmed by having dry clusters as water conditions. On the right-hand side I cannot see but I am assuming a high water level which is why you see it being fully saturated (I guess that is the first plot). Having a high water level on the right-hand side means that water will try to go towards the excavation which is why you start seeing some water rising under the excavation bottom. 
 Case 2: the water level is above the excavation bottom but is overwritten by the dry water condition specified. Naturally, you cannot have a high water level in the excavation and have all the excavation dry at the same time. In this case, the Dry water condition wins over the global phreatic level. The horizontal water level is correct for the right-hand side and as described in Case 1, the result would be the same as on the excavation side practically there is no water condition defined. 
 Case 3: the water level inside the soil, also set to dry is ignored. However, this situation is different to Case 1 and Case 2 as you have set the clusters of the excavation bottom as dry meaning that no water can be present there. The rest is as described for the other cases.