alumpkin, so you dont have to calculate the walls of the silo? is a`premaufactured silo and you only have to design the foundation? is this alright?. I think that overturning could be critical when the silo is empty, and seismic when the silo is totally filled. Also i think that a mat foundation will have more probalilities of differential settlements than a ring with piles.In my opinion the influence of the loaded silo supported by a mat will be important until a depth equals to two times the silo´s diameter, so your soil must be very good and non compresible to avoid important settlements. There are good examples of silos foundation design in the Ravennet book.
( As always,I apologize for my english)
LG