Back-filling my raised slab?

[SardonicSmile]

Image of my foundation:


I have my footers and my block up.. it is 3 blocks high. Every 5' I filled the block with mortar and rebar.

My uncle says that I need to pack dirt around the outside before I can back-fill (#57 rock) or pour the slab. I hope not, because that will set me back another 2 weeks. No easy access to heavy equipment.

Will the block collapse if I put my back-filling in there? What do people usually do with raised slabs?

[JRR]

There is a risk.  Both the rock "pour" and the wet concrete will put outward forces against the block wall.  Of course, careful placement of the rock and easy placement of the concrete ... initially pouring mostly away from the wall, in the middle, ... and then carefully pushing/pulling the wet concrete toward the wall ... will go a long way to prevent rupture.

Backfilling will help support the wall against the internal forces, but it also presents a risk of pushing the wall inward.  The big difference is that the wall is repairable in this case ... before the ready mix arrives.  Again, "care" is the order of the day.

I have also seen folks just brace-up the wall by using timbers/boards and angle braces driven into the soil.  This is most popular.  The temporary bracing is removed after the concrete cures.

You can strengthen the wall by installing a reinforced concrete "cap" around the perimeter of the wall ... this would connect those vertical "posts" you already have included in the wall.  You could also apply surface bonding cement on both sides ... this would be my choice.

Choices, choices .... gambles.

[Redoverfarm]

Are you going to run a perimeter drain line?  If so that needs placed in either on top of the footer or directly beside.  A layer of filter and then the stone on top and the sides.  I normally use a layer of plastic up against the wall, lying under the drain line and back up against the earth side. It doesn't appear that you have enough heigth to hurt you either on the outside.  Normally you would wait for the building to be set on the foundation but that is for a highwall (basement).    

The advantages of a crawlspace and backfilling on the exterior before pouring the slab is that it will provide some stability to the wall from "outward forces",  I am not sure but it appears that you will have approximately 21-24" of fill.  That's alot of fill even more so with stone alone.  I think if it was mine I would do the backfill in 6-8" lifts.  The biggest problem is if the fill is not compacted or tight it will leave way for the slab to crack.  That's what's nice about 57 stone is it has about the best compaction rate in the raw pour and by compacting you are just bringing it up to "tight" .  I wouold go easy on the perimeter(inside) compaction.

As JRR stated a few braces on each wall would be extra security.

[ScottA]

I've seen alot of raised slabs and I've never seen one have dirt packed around it before it was filled. Just be careful not to push on the walls with the tractor. I'd fill it with sand or crushed limestone screenings. I'd also dig the plumbing stubouts before you fill it.

[ScottA]

One question: I don't see any rebar sticking out of the walls? You should have rebar in the walls down to the footing and the bars should be bent over at the top so they tie into the slab.

[SardonicSmile]

One question: I don't see any rebar sticking out of the walls? You should have rebar in the walls down to the footing and the bars should be bent over at the top so they tie into the slab.

There is rebar/mortar every 4 to 5 feet (not connected to the footings), but I don't have any that are sticking up. There are a lot of people around here that do raised slabs with no rebar/filling in the blocks. Think I should fill in the rest?

I also have rebar in the foundation, sticking up about 6" every 4 feet. Then, I filled those blocks with mortar and added another piece of rebar. The block layers and an inspector both told me this was overkill.. Confused 

[psammy]

I don't see any anchor bolts...

Backfilling....I would begin backfilling soil in 6-8" lifts inside and out, compacting (plate compactor) and repeating until you are at 8"-10" below finished slab height (4" gravel, vapor barrier, 2" xps insulation?, 4" slab.)  First talk to your plumber about where he wants to fit into this process.

psammy

[SardonicSmile]

I don't see any anchor bolts...

Backfilling....I would begin backfilling soil in 6-8" lifts inside and out, compacting (plate compactor) and repeating until you are at 8"-10" below finished slab height (4" gravel, vapor barrier, 2" xps insulation?, 4" slab.)  First talk to your plumber about where he wants to fit into this process.

psammy

We are using anchor "straps" instead. With the top L-blocks I have, it would make sense to add them after the slab is poured. The slab concrete is going to flow into the unfilled blocks.. see my house thread to see an example. Anchor straps don't have to be perfectly lined up.. bolts are a different story.

[ScottA]

When you put your steel in are you going to turn a few pieces down into the blocks to tie the walls to the slab? I would. Around here they usualy leave 4' or so sticking up out of the footing that they set the blocks over. The extra rebar is bent over at the top and tied into the floor rebar.

[cbc58]

you need to be asking the person who is going to pour your slab and/or the person who did the walls.  with age comes wisdom and you should likely listed to your grandfather.  someone with a machine can backfill that in an hour.  you indicated in another thread that you were not going to put in a perimeter drain but i think you should ask the experts.  the last thing you want to do is worry about the foundation of your house and slab since the slab is going to act as your floor.  just my 2 cents.

[SardonicSmile]

you need to be asking the person who is going to pour your slab and/or the person who did the walls.  with age comes wisdom and you should likely listed to your grandfather.  someone with a machine can backfill that in an hour.  you indicated in another thread that you were not going to put in a perimeter drain but i think you should ask the experts.  the last thing you want to do is worry about the foundation of your house and slab since the slab is going to act as your floor.  just my 2 cents.

I can put in a perimeter drain any time, right? I may consider doing it after researching more.

When you put your steel in are you going to turn a few pieces down into the blocks to tie the walls to the slab? I would. Around here they usualy leave 4' or so sticking up out of the footing that they set the blocks over. The extra rebar is bent over at the top and tied into the floor rebar.

I'm beginning to think that our two locations are largely different when it comes to building standards.. People around here only use wire mesh for the slab, and usually no filling or rebar at all in these type blocks. I DO want my house to be better than the minimum, so I'll probably add more rebar everywhere. All of the blocks will end up filled with concrete when I pour the slab anyway, though. I'll have to take a better picture to show you what i mean.

Thanks to both of you for the input!

[JRR]

.....
When you put your steel in are you going to turn a few pieces down into the blocks to tie the walls to the slab? I would.....

I think this is a very good idea.  The "hook" that sticks down into the block could be mortared in place early ... to hold it tight to the block.  The other end could be wire-tied to the steel net ... or another rebar reaching the opposite wall.  Enought of this ... and the wall is going no where!

[diyfrank]

You don't have enough wall there to be worried about backfilling first. Whether you backfill the inside or outside first, it puts some pressure on your wall. If it was taller you could backfill some but you should keep it balanced. 2' on the outside followed by 2' on the inside, etc. Careful goes a long way, just like carelessness can really mess things up.

I would agree using rebar to tie the slab and walls would be good insurance. Around here it's common to use wire mesh or nothing in  residential slabs.
Do you have a good place to drain your footing to? A perimeter drain only works if you can get rid of the water. Otherwise you just invite water to stand around the footing. Sometimes it is best not to use a perf line. It's more a case by case on drains. Tight lines for your gutters should be installed before the backfill if your plan is to use gutters.