I want to pour some piers as a foundation for two 40' shipping containers. The containers weigh 8800 lbs each, and will be placed side-by-side, but offset 12', so that the central "room" can be 28 x 28 when the interior walls are cut out. There will be additinal weight from drywall and wood stud walls, spray foam insulation, furniture and fixtures,a flat steel wood-framed roof, etc. The soil is very red clay, with some light sand and small pebbles mixed in. This clay layer is about 3-6' deep, covering great slabs of solid rock (typical Missouri Ozarks setup). The land is gently sloped, so some of the uphill dirt has been pushed downhill to level the lot, maybe 3+ feet of fill at the end. It seems to be packed fairly well from the bulldozer, rain, my truck, etc. My question is, how many piers would I need per container (40' x 8') to hold these up, and could I just dig down to undisturbed soil ( which is nearly as hard as rock) or should I continue down to bedrock. If down to bedrock, would I have to drill pins into the rock to keep the piers from sliding off the rock? Any and all comments would be appreciated.
It appears you are trying to build some kind of an office or home and possibly want to avoid permitting.
If you are not going to occupy it and use it for storage of the items you listed, that many a little different story until you go for electrical.
For general contruction, you will have to go down below the frost level (if your have cold enough weather).
Don't build on a combination of fill and natural soil unless you want to end up with twisted containers. Based on what you say, the undisturbed soil should be adequate for your loads and you would not have to go to rock. Containers are meant to be handled, but can definitely rack and twist.
Trucks and rain do not compact clay, but only make it look compacted when drive on it or kick it. Settling concentrated loads on clay fill is a different story.
The loads you have given are just peanuts and not realistic in comparison to what you might get. If that is all you really will have, you can set it on concrete block like a trailer.