How much fill dirt inside a foundation wall?

[KentT]

I'm getting ready to build a 2 car garage on a compound slope, and will be using a cinder block foundation, with the slab poured inside. Due to the way the ground slopes in two directions, if I cut the grade down to where the whole garage footprint is perfectly level, it will require a lot of grading (three feet or perhaps more in one corner) and change the whole landscape in that part of the lot in order to get proper drainage away from the foundation...

Can't I just built the foundation walls a bit higher, and fill inside them in the back and on one corner? How deep can I safely fill (and compact) within the foundation walls before pouring the slab, without worrying about "blowing out" the wall?

Any experiences? Opinions?

 

[dmccarty]

KentT,

We have a concrete slab floor in our house. Our house is built on a slope such that the slab on the low side of the house is about a a foot or so off of grade. The high side of the house is a good 5 feet above grade. Our builder who is a PE put in 67 stone as fill. He said that 67 stone, which has no fines, will automagically compact to 95% with out having to use mechanical compaction. The 4 inch concrete slab is pour on top of the stone. Well, to be accurate the slab is on two inches of rigid insulation but the stone is a good 5 feet deep on one side. The wall is CMU with brick facing. The slab has been poured for 18 months and we have been in the house for a year with no problems.

Later,
Dan

 

[KentT]

Thanks, Dan. That gives me a bit more confidence. I'll pay for the higher foundation walls to avoid cutting into the slope as much as I'd have to, otherwise...

 

[inveresk]

KentT, unless the foundation wall has been designed as a retaining wall, it's unwise to subject it to permanent loads of any significance on one side only. You can get away with a small amount of infill higher on one side of the wall than the other but I think you're asking for trouble by building the wall high and backfilling without a corresponding and balancing load on the other side.

I'm not entirely clear on the geometry of your site and the building but if it's possible to step the foundation to overcome the problem, it;s likely to be safer. Or design the foundation wall as a retaining wall.

 

[KentT]

</font><font color="blue" class="small">( I'm not entirely clear on the geometry of your site and the building but if it's possible to step the foundation to overcome the problem, it;s likely to be safer. Or design the foundation wall as a retaining wall. )</font>

Here's a picture showing the approximate location staked out to determine the amount of clearing needed. As you can see, it slopes gently down to the lake, straight ahead, and steeply down to a stream on the left. Some excavation will absolutely be necessary -- but I want to minimize it as much as possible.

Garage location

 

[HGM]

Kent, I agree with inveresk... Be carefull. I would recomend speaking with someone in the bussiness, but I would think since your talking about 3'(if I understood you correctly) you should be able to get by with building you wall foundation, the creating a tie beam across the top. Basicly you would fill some of the cells in the blocks with concrete to add collumns to the foundation and pouring a solid concrete(w/rebar) around all the top perimiter. This is how most home construction is done in south Florida and proves to be strong, but under different circumstances. Your local building code might shed some light on it too.. Just my thoughts..Hope it helps..