Draw water table in Plaxis 3D

Question

Hi all, 
 Can anyone tell how to draw the water table for this following cases using points? 
 
 Thanks.

Stefanos Papavasileiou · Answer

Hello Nitharshan, 
 I am assuming this is a top view of your water table. 
 How can it be physically possible to have a gap in a water table? Usually, this is because of an external way to remove water from the system (pumps, wells, etc.). 
 Therefore, in PLAXIS, to simulate this you can use (created) different water tables for the excavation area or assign different WaterConditions to the volumes that belong to this excavation. 
 Note that depending on what you are doing in the Staged construction you will need to choose from Phreatic or Steady state groundwater flow pore pressure calculation type or Fully coupled flow-deformation analysis to properly simulate the pore pressure distribution. 
 In general, you should not try to draw a water table with a gap as it would incorrectly define the boundary conditions in any model.

Stefanos Papavasileiou · Answer

Hello again, 
 A water level in PLAXIS 3D is defined in Flow conditions mode exactly like any surface in Structures mode. 
 Especially to make it easier in PLAXIS 3D, for a water level using the GUI (mouse) you can simply click on any elevation and you will get a horizontal water level generated. 
 The water level is determined by the project model boundaries, which also define the side of the borehole soil or rock volume. 
 
 As you see above, a single set of coordinates can create a horizontal water level in PLAXIS 3D: waterlevel 0 2 78 
 If you want more complex shapes you can provide a set of (x, y, z) coordinates, just like a surface.

Stefanos Papavasileiou · Answer

Hello Nitharshan, 
 I am not sure if there is an issue other than visualisation accuracy. 
 Have you checked the pore pressure distribution or the groundwater head value? This should help you identify if everything is okay. 
 By making the excavation Dry, you force the water level condition inside the excavation to ignore what you have drawn, which makes sense. 
 By the way, I am not sure if you are using a Phreatic pore pressure calculation type, but this is what we call a " z-shaped " water level which is to be avoided. 
 We have mentioned this in our various webinars, which can help you with your PLAXIS model, too: 
 
 Modelling of Excavation and Influence of Groundwater: An Introduction 
 Geotechnical Analysis PLAXIS + PlaxFlow &ndash; Running a Groundwater Analysis 
 PLAXIS - Ground Water and Pore Pressures

Stefanos Papavasileiou · Answer

Hello again, 
 The interpolate pore pressures work as you show, but it should be used when it fits the soil conditions and behaviour. I can see that you extend the zone of interpolation of pore pressures up to the exact depth of the walls, but then you would have an odd transition at the tip of the wall. That should also not be the case as this may cause inaccurate results for the structure. 
 In all cases, try to consider what would happen in reality, if you try to have a different water condition inside the excavation, how far do you expect this to extend deeper? Then, you should be able to understand the impact of the extent of the interpolate pore pressures zone and when the global condition should be used. 
 For the particular case where there is no embedded depth, I would say that this goes into the design area, which I cannot really comment. I would consider that if the difference in pore pressure is that high, probably you need to do something about it (counter support?). Again consider how it would be in reality; if you remove the water from the excavation (how do you do that? wells?) wouldn't the soil exactly at the excavation bottom also show a reduction of pore pressures (as the head reaches the same elevation)? 
 In any case, I leave that up to the designer as this is a situation that you have to evaluate and act accordingly. Note that PLAXIS gives the tools, but cannot do the design for you. There are decisions to be made by the engineer on how to prevent unwanted situations, provided that the simulation is done in a manner that models reality as closely as possible.