I PLAN ON CONSTRUCTING A 24'X24' DETACHED GARAGE IN MY BACKYARD. THE GARAGE WILL BE ON A SLOPE, CONSTRUCTED OF FILL. THE HOUSE WAS BUILT 30 YEARS AGO. I PLAN ON USING 24" WIDE CONTINUOS FOOTERS. MY QUESTION IS WILL THE FILL HAVE STABILIZED AFTER 30 YEARS. THERE LOOKS LIKE 8' - 12' OF FILL. THE FOOTING WILL BE 44" BELOW FINISHED GRADE FOR FROST PROTECTION.
I'm gonna assume that your site once had a slope, was filled in 30 years ago and is level, or close to it. Am I right?
Seems to me that it should be stable by now, but there are a lot of variables, like where you live (I know it's north of me!), what was used for the fill, was it compacted, etc.
As far as the footings go, they sound like sturdy dimensions to me, (how many stories up are ya gonna build?) sbut I'm not your building inspector. Check with your building dept. as to specs and requirements. Your concrete person would be able to help you out a little I'm sure. Gonna use rebar? Anyway, just throwing some info at ya to consider.
24" wide foundation is more than a bit overkill. Definitely check your local building dept. Our local (No. CA) guys actually take questions over e-mail and respond within a few days. I'm in the process of building my shop (24x28). I had the foundation & slab done for me, but I'm doing everything else myself. The floor was poored in two runs. The first was 8" wide foundation 24" - 36" deep (on a bit of a slop). Reinforced with rebar throughout (per county code). An intigral slab was then poored on top of the foundation in a second poor. Foundation bolts spaced every 3' (accept for doors).
BTW, I did run into an issue for during my rough framing inspection. The local code requires a bolt no more than 12" from any break in the bottom sill. Of course I didn't know this so I had to drill and epoxy in an addition 8 new bolts after framing. Oh yeh, the foundation was inspected and apporved before I started framing. 1st inspector just missed it. To make it work with the original bolts, I would have had to have 28' long lumber for bottom sills.
Jim is a bit east of my earthquake region (San Andreas, et al) :) so my level of concern might be a bit more critical whenever I build on any slope, including a flat one.
The belt and suspender solution would be to have a soils/siesmic engineer review your site. My guess is the cost would be a minimum of $1,500 to get the drill rig out there and a signed report.
Alternatively, I would opt for a pier and grade beam foundation. I could get more specific for you, but at a minimum, and assuming you have a reasonable idea of the depth of the fill, I would go 10' beyond the fill into competent soil or bedrock, whichever comes first.
For a single story shop/garage, 12 to 18" end bearing piers at 5' o.c. with a 8" x 16" grade beam should work. I can send you a CAD example of what I am describing if you want.
I know these things depend on where you live, but I'll agree it sounds like massive overkill. I poured a monolithic slab in Md, 'footers' had to be 24" below finish grade and 12" wide. I think the 12" wide was a mistake on the part of the inspector, but it was his opnion that since the sides of the slab were the same as the footer, 12" was requried. Call your local juristiction to check, and remeber that concrete gets expensive fast.
Fellas - thanks for your responses. I'm hoping the extra wide footings will produce less lb/ft on the soil since I'm not sure what the compaction is. In spreading the load I'm mitigating the potential of settlement. I like the pier and grade beam design. One of the contractors actually bid it like that.