The math is:hello all, I am in the midst of building a concrete slab foundation for my new garage. I have dug out the site, put up my boards, and have dug the trench for my footers around ther perimeter, but what I need to know is how to backfill on the outside of the boards so that the cement will not flow out underneath them. There is a wide gap at the back corner where the ground landscape is very low in comparrison to the other side. I also need to know how much cement to order if the pad itself is going to be 6 inches thick, but i have to account for the cement which will fill the trenches around the perimeter.
Length in feet x width in feet x .5 (for 6" slab), then the answer is divided by 27 to = cubic yards.
If it is a larger slab, figure your whole "trench" (it's a footing) area this same way, right from the top to the bottom. You do it this way on a big slab (especially 6") because it is too easy to underestimate on a larger slab.
You can be more precise and calculate closer on a small slab...
If it is a small slab, figure your "trench" area only from 4" to 6" lower than the top of the slab to the bottom. In other words, don't count in most of the part that the 6" slab will be occupying above the "trench".
Backfilling: Just shovel the dirt up against the outsides of the boards, and let it go down into the gap... Stomp it in down there. Now pack it back into the gap from the inside with your hand or feet. If there is some still inside the form, shave it off neatly with a flat nosed shovel. If there are some cupped holes toward the outside, don't worry about that little bit of under flow... as long as you have a fair amount of dirt tightly banked up there to stop the concrete. The dirt, itself, will act like a form board.