Finite element modelling for foundation

[avnei]

I am designing a foundation of size 16'X 11'.Depth is 2' for the left 6' and 4' for the remaining length of the foundation. It rests partly on a existing foundation (2'ft depth portion) and rest of the length(with depth 4') is on soil. I will use modulus of subgrade of soil for the soil portion. I was wondering what should I use for the portiuon under concrete.
What is the best sizing for the finite elements?. Should I use plates or Solids?. Thanks for the consideration.

 

[VAD]

Any particular reason why you would want to use the finite element approach for this problem. Is the foundation (existing)a spread footing? What loads will be transferred from new to existing foundation.

 

[avnei]

Two pairs of couple forces act on the foundation. Existing is a spread footing on pile foundation. I need to control the settlement. Hence using Finite element. The loads are from a pipe support structure. Thanks.