# little help with a mono pour



## BoulderBuilder (Nov 15, 2009)

I have an interesting retaining wall job in a crawl space. It was a typical crawl space with the dirt floor being at about the bottom of the footings which are about 30" below grade. But someone dug out half the space to make it about 7' of head room. So there is a danger of the dirt collapsing under the foundation since this hole was dug within 6-12" of the footer. Now we are coming in and building a concrete retaining wall around this hole. There are two engineering plans. One is to bring in structural fill and fill it back in with a maximum 45* slope toward the middle and 8" lifts. The other is this 8" reinforced retaining wall and slab idea. 
For the fill it would require about 45 yards. The fill is a PITA because we would have to man handle it by wheelbarow from the front of the house and then dump it in the crawl space (about 75') where a couple guys shovel it to where we need it. Then we would obviously have to get a stamper down there and the space would of course get tighter and tighter the more we filled. Not to mention this slope. So there is of course alot of man hours involved with this. 
The retaining wall seems a better solution as it would actually yield usable space. But the preliminary estimate is going to be about $6K where the prelim on the fill is about $5K. 
Anyway, getting off topic. With the retaining wall we need to pour it in one shot, wall and slab. I've done plenty of concrete work before but never tried to pour a wall and slab at the same time. We would build the forms leaving them up 14" at the bottom for the tie in point and rebar. We can figure that out by bracing each wall off eachother with 2x4's accross the whole span. They would be high enough to allow us to finish the slab when needed. The thinking is that the concrete would be poured in the walls, which are about 52" high from dirt to top, then move right into pouring the slab. The thinking is with a 3 or 4 slump that the concrete would stay in the wall for the most part and not just pour out the bottom 14" gap into the slab area. Of course it will come out some, especially at first, but once it starts filling it would be ok. So. What do you think? Will this work or is this a disaster waiting to happen? I supose worst case is that it failed and the concrete would then be moved and formed for the slab. Let it cure and then do the walls. The rebar would all still be good it just would not technically be a mono pour which the engineer would like. Sorry for the long post.


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## lukachuki (Feb 11, 2005)

BoulderBuilder said:


> Pouring concrete retaining wall slab in one shot.


My quick take if you go the concrete route.

Personally I would talk to the engineer about the extreme difficulty and expense of doing it all at once for very little if any improvement in strength. Of course engineers don't like to hear that, so appeal to his pride and encourage him to figure out a structurally superior way of doing it in 2 pours.....something that is feasible.

I would bet that you could do it faster, cheaper, safer, and just as strong in 2 pours if I understand the situation correctly. I'm sure there is a way to do a reverse monolithic pour but it would take some fancy forms. It is easy to pour the wall first and tie the slab in with rebar. I'm sure i'm missing something.


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## ScipioAfricanus (Sep 13, 2008)

> But someone dug out half the space to make it about 7' of head room. So there is a danger of the dirt collapsing under the foundation since this hole was dug within 6-12" of the footer


That sounds like some scary sh!t right there.

Andy.


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## lukachuki (Feb 11, 2005)

In rereading i just saw the 52" needed to retain the dirt. 

I can think of no way not to do it in 2 pours. (not that i'm the final authority around here on the subject)

You will need some serious reinforcement for a wall that tall, and you will only have one side to brace it from, which further confirms in my mind the need for a 2 step process.

BTW if you break that thing into paragraphs it is much easier to read and you may get more advice that way....


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## DJ9222 (Apr 28, 2009)

This project might be easier if you pour the footing and slab together with rebar placed where the wall goes ,and then just do a cement block 8" or 12" wall filled solid

There should not be much pressure on the wall since the dirt is holding now.


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## Chris Johnson (Apr 19, 2007)

If you use a stay in place form (i.e. and ICF) it can be done...if you have the right crew with the right experience.

I would recommend pouring the footing and slab and come back the next day and do the wall, why create a headache if you don't need one


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## CJKarl (Nov 21, 2006)

Did the exact same thing about 20 years ago. Just like you explained. It all worked....plus we had the wonderful option of water running in one side of the basement and out the other.


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## cdkyle (Jul 12, 2009)

Three pours: Footer, wall, and then slab.


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## Sar-Con (Jun 23, 2010)

cdkyle said:


> Three pours: Footer, wall, and then slab.


 
Excactly. This is how new construction goes together, so this job shouldn't be any different. Key your footing, throw some dowels in there and have at it.

The head pressure on 6' of conrete is pretty high, I wouldn't think of doing a mono pour.


And inquire about renting a concrete pump and get a mix that will self-consolidate. Sounds like the wall will be hard to vibrate. The extra cost for the pump should be offset by the savings in labour, and minimize demurage from the ready mix truck.


Do you have any pics? Sounds like a nice, challenging job. Good luck!


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## lukachuki (Feb 11, 2005)

cdkyle said:


> Three pours: Footer, wall, and then slab.


yup....somehow in my prev posts i forgot about the footer. Good thing I just work at McDonalds.

Actually on fifth thought for a non livable space such as this, could you form a mono pad, key it, rebar it and then form your wall, lay your block or do your ICF and still be strong enough?


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## ricksault (Mar 25, 2011)

*slab and wall in one shot*



BoulderBuilder said:


> I have an interesting retaining wall job in a crawl space. It was a typical crawl space with the dirt floor being at about the bottom of the footings which are about 30" below grade. But someone dug out half the space to make it about 7' of head room. So there is a danger of the dirt collapsing under the foundation since this hole was dug within 6-12" of the footer. Now we are coming in and building a concrete retaining wall around this hole. There are two engineering plans. One is to bring in structural fill and fill it back in with a maximum 45* slope toward the middle and 8" lifts. The other is this 8" reinforced retaining wall and slab idea.
> For the fill it would require about 45 yards. The fill is a PITA because we would have to man handle it by wheelbarow from the front of the house and then dump it in the crawl space (about 75') where a couple guys shovel it to where we need it. Then we would obviously have to get a stamper down there and the space would of course get tighter and tighter the more we filled. Not to mention this slope. So there is of course alot of man hours involved with this.
> The retaining wall seems a better solution as it would actually yield usable space. But the preliminary estimate is going to be about $6K where the prelim on the fill is about $5K.
> Anyway, getting off topic. With the retaining wall we need to pour it in one shot, wall and slab. I've done plenty of concrete work before but never tried to pour a wall and slab at the same time. We would build the forms leaving them up 14" at the bottom for the tie in point and rebar. We can figure that out by bracing each wall off eachother with 2x4's accross the whole span. They would be high enough to allow us to finish the slab when needed. The thinking is that the concrete would be poured in the walls, which are about 52" high from dirt to top, then move right into pouring the slab. The thinking is with a 3 or 4 slump that the concrete would stay in the wall for the most part and not just pour out the bottom 14" gap into the slab area. Of course it will come out some, especially at first, but once it starts filling it would be ok. So. What do you think? Will this work or is this a disaster waiting to happen? I supose worst case is that it failed and the concrete would then be moved and formed for the slab. Let it cure and then do the walls. The rebar would all still be good it just would not technically be a mono pour which the engineer would like. Sorry for the long post.


 Mono pour save money....do all your steel work once one side of verticle form work in place. Tie uprights into slab grid. form up other side of verticle forms. pour slab to bottom of wall while filling bottom of wall at same time. rod and float slab. this will give enough time for mud to set a bit. fill wall vibrating throughout pour. Use 4" line pump. Let ready mix know mix engineer specifies and let them know you plan to pump it. they will blend mix for 4" line pump. The key is setting your wall forms. 8" is a standard size for stemwalls so use either 2x10 or 2x12 with simpsom form ties (wedges) or use snap ties and 1 1/8 ply. I've done it recently for a foundation for a massive 25' x 8'diameter water tank. It worked great. I too was concerned about mud not stacking....It stacks...


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## lukachuki (Feb 11, 2005)

ricksault said:


> Mono pour save money....do all your steel work once one side of verticle form work in place. Tie uprights into slab grid. form up other side of verticle forms. pour slab to bottom of wall while filling bottom of wall at same time. rod and float slab. this will give enough time for mud to set a bit. fill wall vibrating throughout pour. Use 4" line pump. Let ready mix know mix engineer specifies and let them know you plan to pump it. they will blend mix for 4" line pump. The key is setting your wall forms. 8" is a standard size for stemwalls so use either 2x10 or 2x12 with simpsom form ties (wedges) or use snap ties and 1 1/8 ply. I've done it recently for a foundation for a massive 25' x 8'diameter water tank. It worked great. I too was concerned about mud not stacking....It stacks...


Seems to me one of the problems here is the fact that the dirt is probably NOT cut away exactly vertically. In order for complete stabilization to take place I'd think that concrete would need to be pumped against the dirt....or possibly a plastic barrier pinned to the dirt. The other alternative would be to cut the dirt exactly vertical which has its own risks considering the situation. I may be in left field here, and I haven't actually seen the project but this is what jumps into my head.


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