Bricks, Masonry, Asphalt and Concrete - Garage on fill

I am planning to add a garage addition in the future because I currently have only a basement garage, which is a PIA.

So anyway, the existing ground at the location where the garage would be built is 4 to 5 feet below the desired slab elevation.

I am wondering what would be the best approach. I see a few options:
1)fill the entire area with compacted soil, and install footings in the fill, pour floating slab
2)install footings to minimum depth in existing ground , build block walls, fill interior with crushed stone, and pour floating slab
3)install footings to minimum depth in existing ground, build block walls, pour a structural slab with crawlspace underneath

I don't like the idea of putting footings in fill, even with the best of compaction control. Is #2 done often with that height of fill? If I went with #2 or 3 I would still need place some fill for the driveway and some fill on the outside of the wall to provide an entrance with a minimum of steps.

All are viable solutions for your situation.

#1 is probably the cheapest, but you can expect some settlement or frost heaving. The slab should really be thicker since it is a "raft" foundation or you need thicker concrete to span between the thickened edges if you have them. Your fill slopes around the sides could go out 8' to 10'. Make sure the fill is compacts and the edges do not erode.

#2 is commonly done, but the footing will have to have soil coverage (depth or pushed up fill) over the footing to protect it from heaving. The floating slab would only have to be 4", but the fill (sand and/or stone) should be placed and worked a bit to get is compacted. Will take less fill than #1.

#3 Will be the most costly because the slab will have to be designed to support 2 vehicles and span about 24'. If you go this rout, you may want to look into going deeper to get headroom.

I don't know where you are climate-wise, soil-wise and what your site is like, so there may be a few holes in my ideas.

God luck!!

Dick

I vote for #2. In my area slate rock is the best material for fill, gravel also makes good fill but costs a lot more.

A friend of mine recently built a large garage/shop one end of which was on a steep hill. The side closest to his house has 10' walls but the opposite side had walls blocked up over 16' high. He spent a month filling it in with slate, used plenty of rebar in the slab and has only minor cracks.

My shop/barn is built completely on fill. I only have 1 bay that has concrete, suprisingly with no cracks but it did set for 7 yrs before I got that section poured.

We accomplished successfully a similar job. The job was like this. This person had a beautiful house built on a fully excavated basement and the footing was bearing on a shale / hard pan clay base. Years later they added a garage which consisted of a floating slab. That was wrong to begin with but to make matters even worse the slab was on top of 5' of white marl as the site used to be nothing more than a swamp. Marl is a white slimy clay like residue consisting of rotted plant materiel and run off silt. Perhaps you have seen it on pond bottoms. Around here, we call it Loon $h*t. Did you ever wade in a lake with that squishy bottom squeezing up between your toes? If you did, chances are it was loon $h*t you were trying to get some footing support from.

Well, to make a long story short, the garage settled and pulled away from the house over the years. The solution was of course drastic. The garage was razed and existing slab was taken out. We had the excavators dig down and remove all the marl to the footing level of the basement. Now we had something that would support an 8" X 16" OR 20" footing and 12 courses of 8 OR 10" Block stepping back to a 6" top course of blocks for the perimeter foundation. We tied into existing perimeter basement tile for exterior footing drain and once we got the walls built up to half height we summoned back the excavators to began filling. We filled o/s the walls with crushed stone and inside (beneath the floor) with sand,layer by layer mechanically compacting inside as we went up. Then we boomed in the rest of the blocks and finished laying up the walls before calling back the track hoe and more sand where we layered up the rest of the perimeter building an 8' high block wall and didn't have to use scaffold. Too bad about an extra trip for the excavators and block truck but the job was done right this time.

That was nearly 12 years ago and the attached garage has gone no where and and the concrete floor looks great.

Too bad they did not provide a decent garage foundation to begin with.

Good luck with your garage foundation.

bs5