Concrete foundation advice

Boondox · Aug 3, 2003

I've got the Big Dig finished and managed to get it four feet deep EXCEPT near the driveway, where it matters most. There is a large section of granite ledge sticking so far up that the top of it will be just four inches below the level of the finished floor. It's big and solid and stable; it just sticks up too far. Can I safely fasten the slab to this outcropping with rebar, or must I resort to some sort of extreme measure and cut/chip/blow it down to size? Photo attached

Pete

Egon · Aug 4, 2003

If the granite ledge is solid You probably will not have any problems.

The biggest problem I can think of would be settlement back from the granite but good compaction should solve that.

Egon

WVBill · Aug 4, 2003

It probably depends on whether or not you need to have the building inspector out.

I had a similar problem when I build a deck on my house in Fairfax County VA. I hit ledge about 24" down and prepared it by drilling and setting re-bar in it with epoxy. The inspector wouldn't sign off. He simply said, "Four feet means four feet, sir." So I ended up hiring a guy with a full-size jackhammer and two days later passed the inspection then promptly filled the hole in the ledge back up with concrete. /forums/images/graemlins/confused.gif

BB_TX · Aug 4, 2003

The granite is probably not going to move. But I would be concerned about settling on the rest of it. With only 4", I would be afraid that the deeper part would settle more and eventually cause cracking at the thin part. That would be a nightmare trying to fix after the fact.

tc35dforme · Aug 4, 2003

Is that in the middle of the slab or where you will pour a footing and frost wall ???

In the middle, I wouldn't worry. It won't move and you can span over and around it with some re-bar to strengthen the slab just in case. If your worried, you could probably pop its top of a bit with a good electric hammer ( rental ).

If its where the frost wall will be, that may be a different story. You can use bedrock as a foundation support, but you need to be sure its bedrock ( not a boulder ) and that it is stable ( not cracked ).

Boondox · Aug 6, 2003

Advice taken, and another Q

Okay guys, in keeping with the better-safe-than-sorry theme of TBN, I've taken your advice regarding the big honkin' rock in the middle of where I wanted the footing to go. After briefly considering making the new shop shorter than planned, I figured I'd be stuck in that little shop for the rest of my life, so...

I rented a Hilti demolition hammer and attacked the stone. It turned out to be a mix of granite and shale. The shale broke up easily. The granite broke, but grudgingly. Bottom line is the stone is now two feet shorter and no longer in my way.

Now for my next question. I called our local gravel pit to get some 3/4 inch gravel. I planned on leveling the dig with the gravel, compact it, and build my forms on top of that...then using compacted sand within the form to bring the level up so my slab wouldn't have to be two feet thick. /forums/images/graemlins/shocked.gif

But the fellow at the gravel pit suggested using all sand, no gravel at all. Said it's more stable and cheaper to boot. I'd never heard of that. Comments?

Pete

yooperdave · Aug 6, 2003

Re: Advice taken, and another Q

I like your idea better.

A layer of base course with a layer of sand or granular fill above all properly compacted should give you a good result.

Though I like your idea better, his suggestion should also work.

Good luck

Yooper Dave

markct · Aug 6, 2003

Re: Advice taken, and another Q

yea but the problem is over time the sand will work down into the spaces between the stone and they ya will have a problem. go with either all sand or all stone

Egon · Aug 7, 2003

Re: Advice taken, and another Q

Pete:

For compaction you want a well graded material [ small to large sizes makes it more dense] that has many fracture surfaces that will bind against each other.

I'd go with the straight crushed gravel compacted in four inch lifts and forget the sand. Sand looks like it is very well compacted but usually its consolidated. A thin lift for the final grading is okay.

Egon

Currmudgeon · Aug 7, 2003

Re: Advice taken, and another Q

</font><font color="blue" class="small">( I'd go with the straight crushed gravel compacted in four inch lifts and forget the sand. Sand looks like it is very well compacted but usually its consolidated. A thin lift for the final grading is okay. )</font>

I concur with this. Order the stuff the State DOT uses for road base course. It has a different name in every state, but most of the gravel pits produce it, and they do so to consistent specs to satisfy the state.

"Sand" is sometimes reject material they've got too much of, which is why it is cheap.