Foundation Question

[pwhaakej]

I know there are a bunch of foundation threads out there already, but I'd like a little input from those who have more experience than me and am wondering if anyone has done something similar to what I'm planning. I'm in upstate NY where the frost line is at about 42". I had an existing area off of a brick walk where some stone was laid down, where I will put my oven. It has about 2" of fine-ish gravel compacted down on top of about another 6" of coarser gravel/stone.

I considered digging down the 3.5 ft to put in footers and then build up from there, or using piers in sonotube to support a slab, but the stone that was in this area was still absolutely flat and I'd prefer to avoid digging that deep, so I'm leaning toward extending the stone/gravel another 6 inches or so after finding this quote in another foundation thread (quote is credited as coming from Canadian Building Digest):

Where a detached building is located on a concrete slab on grade, protection will be provided by placing the slab on a mat of coarse granular material, which will act as a buffer against any movement of the soil under the mat. A mat 12 to 18 in. in thickness is usually adequate.

My questions are mainly about what grade of crushed stone or gravel should I be using? I would assume starting fairly coarse at the bottom so water drains out of it readily, then having a finer layer or layers closer to the surface. Pictures of what I currently have to work with are below. Is crusher run stone suitable for a base that will drain well or am I better off going with clean crushed stone that has no particulate in it? I'm guessing the dirt/sand that's in the top 8 inches already down won't be a problem since any excess water should drain through the larger crushed stone below.

[purplehaze]

I am not a foundation expert but since no one else has answered I can tell you how I built mine. My frost line is about 4 feet also and I didn't dig down very far at all (around 12"). I mostly just removed the top soil. As far as a drainage base I used broken clay bricks from an old chimney and cement sand. It was pitched to allow drainage out the back of the base. My slab is 4-5" thick with rebar. The slab will move a bit but it will move as a whole and not matter much. If mine has moved I have not noticed.

Hope that helps

[rwiegand]

Where we are (48" frost line) you just need to dig out any soil with a significant organic component, backfill with gravel (3/4" aggregate usually), compress in 3-4" lifts with a compactor, then pour your slab on top. I'd use at least a foot of gravel. If your drainage is really lousy Id be tempted to do something more to keep water from pooling beneath and heaving your slab when it freezes, like installing a perimeter drain that runs to daylight if possible. We're on top of an 80 foot sand bar left here several 10's of millions of years ago, so drainage is not an issue at our house.

If you live someplace with expansive soils I'm sure the process is different-- see what the local standard is for slab construction.

A 6" thick slab the size of a pizza oven with rebar seems really unlikely to break, so at worst your oven could get put a bit off level by soil movement if it moves as a unit.

[pwhaakej]

My concerns are mostly with the slab moving and the whole structure getting out of level, not really concerned with the slab breaking. Really just want to avoid the crooked picture frame on a wall sort of look.

I think I'm just gonna dig up what I have and put down half a yard of clean crushed stone, then put the gravel I dug up back, compacting as I go.

Worth renting a power compactor or are the hand tampers capable of doing the job correctly? I'd have to go with the jumping jack probably since itd be tough to get the corners with the vibratory ones.

[rwiegand]

Do 2" lifts with a hand tamper. Won't take but a few minutes.