If I understand this, you are saying that your pipe is restrained from sideways motion and vertical motion due to the hole in the dike being about the same size as the pipe that penetrates it. So why not use a lateral and vertical support at two points, one where the pipe enters the dike wall and one at the exit of the dike wall. The stiffness of these supports would be based on the dike wall stiffness at a compaction of 90%. A gap is only necessary if you are sure the hole will open up wider and provide some play. I suspect the dike wall will never act as an anchor unless it undergoes a significant collapse around the pipe.