Slab On Grade Fill Options?

James Ellis

Hello,

I'm starting a 2 story home in San Diego designed as slab on grade foundation. Problem is that after going to site and marking out footprint, I'm finding a 3 ft difference in elevation from the highest corner to the lowest. Obviously I need to fill a large section of this to keep my 6" minimum above finish grade, so I'm looking for a solution that will allow me to still pour it all at once. (Rather than form a foundation wall and then pour the slab later.) I swear I've seen pictures of a project that had some kind of bagged fill (appeared to be gravel) set in place before the pour. I don't know if it would work, but I could see using a system like this to fill the area completely without spilling over into the footing trenches. Using the bagged fill to create a wall that would hold back the lose fill. Am I Crazy?

mdshunk

I'm curious my own self. I wouldn't think you'd get the compaction numbers you'd need to pour a slab using fill in bags.

tgeb

Are you talking about a monolithic pour?

In this type of condition why not do a foundation, then slab.

You could then just fill the 3 feet with stone. That will be cheaper than fill and compaction to get the sub grade up where you need it.

__________________
Tom

www.gebcon.com

James Ellis

As designed it is a monolithic pour. I'm figuring I'll have to pour foundation first with the additional forms and then fill, but to avoid the second inside form and follow the "approved" plans, it is supposed to be one pour.

Tscarborough

It is pretty common around here, though the prefered method would be to cut and fill, then dig the footings and use the sandbags to form the inside of the footers if needed. That said, they also use post tensioned slabs almost exclusively for this type of footing/slab layout.

ch0mpie

The company I do inspections for recommends "a well graded material with no particals larger than 1.5" and maximum of 10% retained on a #200 sieve compacted to 95% of the opt. dry density" for use as structural fill.

James Ellis

This statement makes me feel really dumb.

ch0mpie

Sorry James, I was in technical mode for a second

Basically what they recommend is a material that is about half sand and half gravel with varying sized particals, very little clay or 'dust', and no large boulders. An example would be DGA or even recycled concrete agregate. Then you would need place the material in 8 to 12 inch lifts and put a viboratory roller on it a few times depending on how much moisture the material contains.

Also, what are your soil conditions? By filling in half of a site and leaving the other at natural grade, you could get uneven settlements.

ch0mpie

Another thought. I have no experience with fill that is left in a bag, but I would think that if it was used under a footing or slab that there would be spaces between the bags. Over time the bags would break open and the material would spill out. The spilled material would fill the spaces between the bags and cause settlement of the slab. Not 100% on that, just what I would think would happen if bags were used under a slab.

leiffearn

I'm in ventura county Ca, How we normally decide on a dual-pour as opposed to a monolithic pour is based on the size of the slab and also the time line. Dual pour is much easier and cleaner. It allows a bobcat to get in and fill. Sucky part is you have to dig for plumbing again sometimes. I normally stab my slab steel into the footing after the pour and then fill, vapor barrier, sand, then bend the steel into the job and tie all the remaining rebar.
But for the monolithic, I've done it the way that you asked. Except not with bagged stuff but rather broken pieces of slab from demolishion earlier in the job. Or pavers or block. Then that creates a nice wall. You end up having to place them back from the footing six inches to a foot so they don't fall in! But its hard cause workers are constantly caving in the barrier walls and digging the loose crap out of the holes over and over...especially in the corners. Anyway it works and its nice to have the ready mix and the pumper there for only one morning then youre done!!

James Ellis

Thanks for all the input guys. It looks like my best bet is going to be two pours. I think my S.D. inspectors will more likely go for that. They tend to like simple standard methods over ones that have to be explained. I like the idea of stacking demo'd concrete, but I'm sure they'll give me hell if I try that.