Hydraulic discontinuity during flow analysis

Question

Good morning, I'm trying to define the global stability of an embankment supported by a gravity wall. In this case, I would like to perform an analysis in which the water level must be divided into two segments: one behind the wall at a height equal to the wall height, and one in front of the wall, 3 meters lower. We would like to perform an analysis to simulate the complete soil saturation behind the wall, while in front of this gravity wall there is a river with different water level conditions. Can someone help me to define this hydraulic discontinuity? How can I implement this in Plaxis? Thank you very much in advance

Vasileios Basas · Answer

Hi Edoardo, 
 Maybe PLAXIS 2D Tutorial #11 (Flow through an embankment) or PLAXIS 3D Tutorial #8 (Rapid drawdown analysis) will give you some helpful information or guidance for your model. It's worth checking them out as they both deal with variable hydraulic conditions. 
 Defining different water levels on each side of the wall can be done in different ways. For example, you may create a new water level with multiple line segments placed at the desired elevation (see aforementioned Tutorial examples). Note that for Groundwater flow (steady or transient), the intersection points of the water level with the active model boundaries are important. This is because the program calculates flow boundary conditions in terms of a groundwater head corresponding to the water level. The 'internal' part of the water level is not used and will be replaced by the phreatic level resulting from the groundwater flow calculation. Hence, the water level tool is just a convenient tool to create boundary conditions for a flow calculation. Alternatively, you may work by directly assigning water conditions to soil polygons (e.g. Head, User-Defined or Custom Level conditions). 
 If you still struggle with setting up your model, you can contact our Support group via the Service Request Manager: apps.bentley.com/.../ProductSupport

Faseel Khan · Answer

As my colleague mentioned that you can do a steady state or a transient groundwater flow analysis in your model. These flow analysis will determine the water table based on the specified hydraulic boundary conditions. 
 You can set up the head conditions by going to Flow conditions tab and clicking on a line segment, then assigning a head as shown in the following picture. The part above the soil is assigned a dummy soil and is assigned a Dry condition and in the phases this cluster is deactivated. This prevents any erroneous water pressures on the wall. This is explained in the following article: 
 https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45788/external-water-pressures-on-structures-above-the-soil-incorrect-in-2d-connect-edition-v20-solved

In the steady state flow analysis the Ignore Suction option can be checked to avoid any suction pressures from generating. 
 Once the calculation is completed, you can view the output heads. The results will show full saturation and submerged conditions on one side of the wall as specified above.