plaxis 3D to 2D

A 2D analysis assumes plane strain conditions. A 3D analysis does not make that assumption but if your 3D model is just an extension of a 2D model (I am calling it a 3DE model) in the out-of-plane direction, then you would mostly get similar results. Mostly similar, because it would depend on the type of analysis and also on the dimensions of the model. For example for settlement and seepage models, you should get similar results between 2D and 3DE models. However, for a slope stability model, a 3DE model will generally give higher Factor of Safety than a corresponding 2D model. 

If, however, you create a very thin slice of a 3D model as a 3DE model or in other words you extend the 2D model in the out-of-plane direction by only a tiny bit to create the 3DE model, then the differences between the 2D and 3DE models would be negligible even for the slope stability model.

Hope this helps.

A 2D analysis assumes plane strain conditions. A 3D analysis does not make that assumption but if your 3D model is just an extension of a 2D model (I am calling it a 3DE model) in the out-of-plane direction, then you would mostly get similar results. Mostly similar, because it would depend on the type of analysis and also on the dimensions of the model. For example for settlement and seepage models, you should get similar results between 2D and 3DE models. However, for a slope stability model, a 3DE model will generally give higher Factor of Safety than a corresponding 2D model. 

If, however, you create a very thin slice of a 3D model as a 3DE model or in other words you extend the 2D model in the out-of-plane direction by only a tiny bit to create the 3DE model, then the differences between the 2D and 3DE models would be negligible even for the slope stability model.

Hope this helps.