Soil collapse

Question

Hi, I am trying to construct a retaining wall with geogrids but I can't stop the soil from failing at phase 1. I have attached a photo of the output data but I don't understand why so if anyone can tell me if there are any obvious mistakes? I am new to Plaxis and can't seem to find anything in the manuals that will help or similar to what I am constructing. any help would be great, thank you!

Dennis Waterman · Answer

Hi, 
 The geogrid elements do not have a bending stiffness, so when a load is applied perpendicular to the geogrid, the geogrid has no effect as a perpendicular load will not induce any axial force in the geogrid. The geogrid will start working as reinforcement if it deforms so that the load of the fill material on top is no longer perpendicular to the geogrid. However, this is a second-order effect of deformation that is not by default taken into account by PLAXIS. 
 So there are 2 things you can do here: 
 1) Use a plate element with low bending stiffness rather than a geogrid 2) Tell PLAXIS to take into account second order effects by activating "Updated Mesh" (Phase settings -> Deformation control parameters). Solution 2) is generally better, but also numerically harder to calculate. So generally we first try 2) and if we really can't get that to work then 1) is the backup plan. 
 With kind regards, 
 Dennis Waterman