Looking for some help on sizing of footing for a residential ICF build. 

Existing soils at footing depth consist of silty-clay. 

With the addition of subbase material such as crushed non compactable stone can the the size of the footing be reduced?

When adding subbase below footing what depth would be appropriate.  12'' ???  

The home will have a full basement and 1st floor.

Exterior Walls will be poured 6'' walls to the eaves. 

Backfill will be placed around the basement to a depth of 7 feet

The area is northern New York with frost depth to 4 fbg.

Seismically the area falls within Zone C of NY seismic Map.

Anyhelp on subbase material, footing size and reinforcemnt would be helpful


Thanks In Advance

4000PSI

You really need to have an engineer in your area size the footings and spec the reinforcement.  It may even be something you need to submit to the building official for approval as well.

I'm in upstate NY an I generally use 20" footings with a 6" subbase. The only time I change this set up is if we find wet conditions when the site is excavated and you only know that is when you dig

On reinforcing the" presciptive method" is the place to determine the reinforcing needed. You didn't say how high the foundation will be.

Silty clay covers a lot of variables. Depending on where you are, you can have a great suitable base, while other places, not at all. And any time you're doing that much backfill, I'd have an engineer look at it. It's worth the investment.

I was told by my engineer that without soil borings he had to assume 2000psi, and size for that. In one hand cost of extra concrete, other cost of soil borings.

Supposition will still be a guess as to effectiveness. It's better to go on solid information. you might still have to pay for the concrete, but it will be worth it. Peace of mind for all.
I am frequently confronted with the reluctance to do a soil report because of cost. About $1500 here. About 0.6% of the cost, on average. Then they'll spend that much on the tile in the entry way. I will use very conservative numbers as I don't want that liability, and will most assuredly cost more. Remember the foundation supports it all. An inadequate foundation, and that tile is destroyed.

Dear 4000psi....

change your name to something less confusing please ... i thought you figured out your soil to be 4000psi... it took me a few seconds...

Here's what IRC 2009 calls for... Maybe someone around here can confirm my interpretation...

4000 psi soil? Pretty hefty! :o) I think 4000 psf. I thought it was his concrete strength, but figured it out. Thought it was a cool moniker.
Anyway, those classifications still need to be determined by someone who knows, but local building people may know. It still doesn't change that a soil report will likely increase it. The people writing the codes are as conservative as engineers.