Effect of raft thickness on rotation in piled raft foundations

Question

Hello, 
 I have modeled a piled raft foundation using Plaxis 3D and increased the thickness of the raft (plate element) to investigate its effect on the foundation behavior. However, the results showed that increasing the thickness of the raft caused rotation in the foundation. I am unsure why this rotation is occurring and would appreciate any explanation or insight. 
 (Attached are the parameters used in the modeling and images of the results.) 
 Thank you,

Faseel Khan · Accepted Answer

If the raft has a unit weight assigned to it then increasing the thickness of the raft will result in higher pressure on the underlying soil. If the mesh is perfectly symmetric then you should get symmetric stress/deformations results. In your model the mesh is not symmetric and may lead to non-symmetric results. Depending on the soil properties assigned to the underlying soil (plastic points development), and a non-symmetric mesh, it is possible that some part of the raft experiences more stresses/strains compared to the other zones Try to compare results when you have a perfectly symmetric mesh and no plasticity developing under the foundation. If stresses are high then increase mesh density for more accurate results.

Faseel Khan · Answer

In Structures mode, you can add surfaces in the shape of a plus sign that extend all the way to the mid points of all the sides of the raft. Furthermore, the elements that connect the edges of the raft also need to be symmetric in order to create a symmetric mesh within the raft. To do that you can create a bounding box slightly bigger than the raft. Then create surfaces from mid points of each surface of the bounding box that will pass through the center of that bounding box and extend it to the mid point of the surface at the opposite end. You can also try diagonal lines, Idea is to force element generation along symmetric objects to create a symmetric mesh. The more the guiding surfaces the more symmetric mesh you will get. However, adding too many surfaces may not be ideal.

Faseel Khan · Answer

The mesh looks much more symmetric now. We will have to investigate more inputs and results to come up with the reason for this behavior. To assist you further please submit a service request. Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport

Effect of raft thickness on rotation in piled raft foundations

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