Excavating for footing/foundation

[cturboaddict]

Slowly but surely progessing on the details/financials of my detached garage project.

I put preliminary drawings together and had a sit down with the building inspector. He shot down my mono pour slab on grade unless I went down to 30" frost depth.

So I figured, why not do a traditional footing/block foundation or footing/poured foundation. (Haven't decided yet on that direction) And then pour slab afterwards.

I got pricing to bring in dirt for where needed. But was curious about digging for the footing/foundation.

Would it better to excavate the entire area to the footing depth and then backfill the middle. Or just dig a trench as deep and wide as needed?
Also, after footing/foundation is done. Wouldn't it be better to pour the slab before building the walls?

I'll be using a forklift to set trusses sometime later.

 

[readhead]

Don't give up on the mono pour just yet. If the trench can be dug neatly it will still be the least expensive way to go. Consider over excavating, form and pour footing, strip footing, form and pour walls, strip forms, backfill and compact and then pour slab. Which do you think will be less expensive?

 

[DougWil]

You probably want to scrape off the soil under where the slab will be and put down a 8" bed of compacted gravel. Then a 5" or thicker slab.

 

[cturboaddict]

Don't give up on the mono pour just yet. If the trench can be dug neatly it will still be the least expensive way to go. Consider over excavating, form and pour footing, strip footing, form and pour walls, strip forms, backfill and compact and then pour slab. Which do you think will be less expensive?

Oh, He said I could do a monopour. But my sides had to go down to frost depth of 30". Is that still common? I'd rather do it in one shot vs 3 phases. I guess the only downside to that would be that I have to have the monopour at a higher elevation than the grade when finished.

 

[cturboaddict]

You probably want to scrape off the soil under where the slab will be and put down a 8" bed of compacted gravel. Then a 5" or thicker slab.

Correct. I have a large sloped area on my side yard where I want to place the garage. I'm having 30 trucks of dirt brought in to bring it to level. They are going to rock the entire thing as well.

Then dig for foundation afterwards.

 

[readhead]

What do you mean by higher than grade when finished?

 

[cturboaddict]

What do you mean by higher than grade when finished?

IE: Slab Top Surface is 6" Higher than grade outside of garage.

Correct me if I'm wrong.
But when building a foundation, doesn't the top of foundation end up at approximately 6" higher than grade. (To keep water out, and to keep siding out of the dirt)

To accomplish this with monopour, Id have to have the top of surface of the pad area higher than grade.

I just revised my concrete section view to the 30" depth with the monopour. Maybe I'll run it past him to see if he has any issues.

 

[lakeroadster]

Are you acting as the contractor on the project?

I've done footer-block-slab myself, and had built a mono, footer-concrete-slab and (2) pole buildings.

Monolithic pours put all your eggs in one basket. If you've got an excavator / concrete guy, one that does both, that you really trust and have used before then I can see the advantage.

If not the concrete guy will be pointing fingers at the excavator, and you'll end up paying for concrete overages.

And getting the sub grade compacted properly for the slab, with the open trench right there.. takes a skilled task force that may not be readily available.

I've always liked a poured footer, a poured wall and then a poured slab. Breaks the project up into more manageable pieces.

One thing I noticed on your drawing, did you want a 6" curb wall? That's always nice on a garage. What about a sloped slab, have you though about that?

 

[cturboaddict]

Are you acting as the contractor on the project?

I've done footer-block-slab myself, and had built a mono, footer-concrete-slab and (2) pole buildings.

Monolithic pours put all your eggs in one basket. If you've got an excavator / concrete guy, one that does both, that you really trust and have used before then I can see the advantage.

If not the concrete guy will be pointing fingers at the excavator, and you'll end up paying for concrete overages.

And getting the sub grade compacted properly for the slab, with the open trench right there.. takes a skilled task force that may not be readily available.

I've always liked a poured footer, a poured wall and then a poured slab. Breaks the project up into more manageable pieces.

Yes, I'm acting as contractor.
I plan to build it all myself but sub out the excavating/concrete/electrical main hookup.
I have one excavators numbers, but that's just bringing in dirt in lifts, packing it and rocking the entire area at the moment. I just want all of my direction planned before doing anything.
I do not have a concrete guy at the moment.
I'll have to do it in phases due to finances. But if I could get the dirt done and concrete this winter, then by summer I could be putting up walls.

 

[Chaz]

In most places you can do a monoslab with foam insulation board around the perimeter... a combination of 4 feet (2 ft down, 2 ft out) or ( 1 ft down 3 ft out) etc. I'd check with the inspector on this option. He SHOULD know about this.

 

[Orionrising]

Yeah what is wrong with a monopour are you insulating the floor anyway? Frost free monolithic slab then.

Sent from my Moto G (4) using Tapatalk

 

[readhead]

If you are not going to do a mono pour then there is no point in bringing in dirt now. You need to commit to a plan. Mono pours will consume more concrete but the extra cost for mud will be far less than all the labor for all the other steps. Either system will work fine but you need to decide pretty soon which way you want to go if you are trying to beat the weather.

 

[cturboaddict]

If you are not going to do a mono pour then there is no point in bringing in dirt now. You need to commit to a plan. Mono pours will consume more concrete but the extra cost for mud will be far less than all the labor for all the other steps. Either system will work fine but you need to decide pretty soon which way you want to go if you are trying to beat the weather.

Because the ground is no where near the level it needs to be at. I would have to build an 7'-8' stem wall.
I just need to run past the building inspector again and ask if it's okay that the monopour is good at the frost depth.

If it has to wait until spring, that's fine. I can still bring dirt in during the winter.

 

[Radix2]

You will never get the edges of the interior compacted with that deep of a perimeter.

Either go with a traditional two step process or revise your plan to a frost protected shallow foundation - search and research that exact thing and see if your building department will accept it. It will also require much less perimeter concrete.

I would recommend not doing the dropped garage door lip thing either. A better set up is to simply slope the area under the door by a 1/2 in or so. It will keep all the water from running inside, look better and avoid the bump. Also less likely to freeze up in the north.

 

[Radix2]

Because the ground is no where near the level it needs to be at. I would have to build an 7'-8' stem wall.
I just need to run past the building inspector again and ask if it's okay that the monopour is good at the frost depth.

If it has to wait until spring, that's fine. I can still bring dirt in during the winter.

If you are going to build your footer on fill ( not run your stem walls all the way Down to virgin soil) then I would get the fill in an compacted as soon as possible the longer it sits and weathers the better.

 

[readhead]

7'-8' kind of changes the story a bit. Is the building department aware of this fact? Is an engineered foundation design in your future? If you are going to build on the fill you might be talking structural fill and grade beams in the concrete. Sounds like you might not be ready to proceed.

 

[cturboaddict]

7'-8' kind of changes the story a bit. Is the building department aware of this fact? Is an engineered foundation design in your future? If you are going to build on the fill you might be talking structural fill and grade beams in the concrete. Sounds like you might not be ready to proceed.

Building inspector is very aware. He's been to my house with me and looked it all over. It's common in my area.
No, not an engineered foundation.
Yes, going to build on the fill. Excavator (local 30+ years) said I could dig as soon as they pack the fill in for foundation.
Building inspector also recommended the excavator.

 

[cturboaddict]

Right at the edge of the gravel driveway I will have the detached. Approx 13' from house.
Walk out 26' from that mark (garage intent is 26'x26') and your at eye level with the gravel. (I'm 5'8")

 

[readhead]

I can see several ways where building a garage on an 8' pile of dirt can go wrong but it sounds like you have a plan once you and the inspector agree on the foundation plan. I would at least ask the excavator for soil tests for compaction. Hope all goes well.

 

[cturboaddict]

I can see several ways where building a garage on an 8' pile of dirt can go wrong but it sounds like you have a plan once you and the inspector agree on the foundation plan. I would at least ask the excavator for soil tests for compaction. Hope all goes well.

Agreed, but thee pile of dirt will be spread way beyond the garage.
Appreciate all the insight today too, fyi.

Sometimes I get to far into different scenarios that I have to be reeled back into reality.

Don't know if I really have a direction after this thread though. HAHA
Monopour or footing/foundation.

 

[lakeroadster]

Wow, that's going to be a lot of fill.

So how far away from the new building are you planning to have a slight slope, and then how far out are you planning to fill?

Captain Obvious would say "Compaction is going to make or break this project."

 

[bullnerd]

What type of foundation was used on your house? Block, poured, slab?

 

[readhead]

Considering that 100% compaction is unobtainable and seeing the picture I would be inclined to run stem walls down to natural and either backfill and do a structural slab or use the space underneath and do a suspended slab.

 

[cturboaddict]

Wow, that's going to be a lot of fill.

So how far away from the new building are you planning to have a slight slope, and then how far out are you planning to fill?

Captain Obvious would say "Compaction is going to make or break this project."

Approx 30 trucks of fill dirt will be brought in.
6' from building will be level, and every 3' out will drop 1' after that.
Captain Obvious is correct. Pushed in 6" lifts and packed. Lather, Rinse, Repeat

 

[cturboaddict]

What type of foundation was used on your house? Block, poured, slab?

House was built in the early 1920s. Block
Walkout Basement

 

[cturboaddict]

Considering that 100% compaction is unobtainable and seeing the picture I would be inclined to run stem walls down to natural and either backfill and do a structural slab or use the space underneath and do a suspended slab.

That was my original thought, with a stepped foundation on the front and back.
I about fell over when the shady concrete guy game to my house and told me it was going to cost 40k for the concrete.

 

[lakeroadster]

Nice lot layout, thanks for posting.

If a fella had the money... that would make an awesome bank barn style building location.... with a big walk out shop underneath.

 

[bullnerd]

Dig the trench, build your own forms, pour the footings, build a block wall so you can do the block yourself. Backfill inside and out evenly and pour slab when done. You can do it as you do it, even through the winter. Block is cheaper than concrete(I think?), you can do it your self and not have all that nice concrete and money buried under ground.

And....it worked for your house all these years. JMO, that's what id do. I'm a machinist and have built block foundation additions to my house, very easy to do, and DIY friendly.

Good luck, keep us posted on how it goes.

 

[cturboaddict]

Dig the trench, build your own forms, pour the footings, build a block wall so you can do the block yourself. Backfill inside and out evenly and pour slab when done. You can do it as you do it, even through the winter. Block is cheaper than concrete(I think?), you can do it your self and not have all that nice concrete and money buried under ground.

And....it worked for your house all these years. JMO, that's what id do. I'm a machinist and have built block foundation additions to my house, very easy to do, and DIY friendly.

Good luck, keep us posted on how it goes.

Thanks for the direction. My machinist said the same thing. LOL
(I work in a metal shop, project manager now)

 

[wssix99]

I about fell over when the shady concrete guy game to my house and told me it was going to cost 40k for the concrete.

This is not shocking, for what you are envisioning.

Approx 30 trucks of fill dirt will be brought in.

This is really expensive.

my mono pour slab on grade unless I went down to 30" frost depth.

This is really expensive.

I got pricing to bring in dirt for where needed.

This is not the worst part. It's all the machines and machine time you'll have on site to move the dirt around and compact it.

Oh, He said I could do a monopour. But my sides had to go down to frost depth of 30". Is that still common? I'd rather do it in one shot vs 3 phases. I guess the only downside to that would be that I have to have the monopour at a higher elevation than the grade when finished.

If you pour the foundation and floor all in one shot, it will need to be engineered and reinforced. That could be an extra $5-10K, right there. On top of that, if you don't heat the space, the ground under the garage will heave in the winter and will try to rip the floor from the walls. So, you'd have to add more cost and engineering to deal with this or heat the floor.

Pouring the floor separately from the walls is a much more durable, maintainable, and cost effective way to go for your situation. It's an extra day for your concrete company to be on-site, but it makes the whole universe surrounding your structure easier to deal with and will be a much lower cost.

Would it better to excavate the entire area to the footing depth and then backfill the middle. Or just dig a trench as deep and wide as needed?

The earthwork you have planned is significant. I would expect that the cost to do your foundation and slab will eclipse the rest of the building unless you make some design changes.

Do you really need the garage at the same level as the house? (Building the structure to accommodate the earth is much more economical than re-designing what mother nature worked centuries to make.)

If you need to be at the same level, you might also look at not filling the space and having an elevated structural floor with a crawl space underneath. This is also expensive, but may give you more space/utility for a similar cost do do the fill.

If you want to stay with the fill plan, I'd build the structure/walls first and then fill inside before you pour the slab. You don't need dirt for this - you can use something like recycled concrete to lower the cost and make compaction easier.

Also, after footing/foundation is done. Wouldn't it be better to pour the slab before building the walls?

No. You just chute the concrete in through the garage doors. Having the structure over the curing slab is nice because it makes it easier to control the cure. If you build before the slab pour, you also don't need to worry about messing up the slab.

 

[cturboaddict]

30 trucks with grading/compacting etc and rocking the entire things is $4800.

While that may seem a bit crazy when you think about it. 5 grand to have flat ground.....

That's a far stretch from 40k.

 

[cturboaddict]

Also, I've really scratched my head with my property to find a suitable location for the garage that would also be functional and economical.
Currently, without some sort of exotic buried structure this is the best place.

 

[forAK]

Do you really need the garage at the same level as the house? (Building the structure to accommodate the earth is much more economical than re-designing what mother nature worked centuries to make.)

This makes way more sense and the way I would approach it. Putting a retaining wall along side the current drive and building down the new garage. This would require way less money out with equipment and fill. Plus you can use what you dig out to spread out towards the property line with flat usable ground. You could also go with a bigger garage. JMO

 

[cturboaddict]

This makes way more sense and the way I would approach it. Putting a retaining wall along side the current drive and building down the new garage. This would require way less money out with equipment and fill. Plus you can use what you dig out to spread out towards the property line with flat usable ground. You could also go with a bigger garage. JMO

Agreed.
My issue would be that it would be way low. (Feel free to input your opinion, as I don't mind) The driveway already slopes backwards towards the rear of the house. It would already be lower than street level if I place the garage at the elevation with the current driveway. See snapshot of elevation.
The slope would be too drastic to enter the garage.

The only alternate option that I can think of is to turn the thing 90 degrees and face the neighbors house. Have a driveway that went around and wrapped in. But still, would have a steep slope down to it's level. And I don't really want my garage door facing my neighbor's house.

 

[forAK]

Looking at the lot layout , would it be possible to put in a second drive closer to your property line (-4'6") going uphill and still keep the same orientation to the road? I too would hate to have my garage pointed at my neighbors.

 

[cturboaddict]

Looking at the lot layout , would it be possible to put in a second drive closer to your property line (-4'6") going uphill and still keep the same orientation to the road? I too would hate to have my garage pointed at my neighbors.

Hmmm. That's a thought. I'll stare at it today when I get home.
I know the city doesn't take kindly to modifying the sidewalks and what not. But would it be $4800 less? I dunno.
I have to factor in the extra concrete/forms.

But, putting it deeper would force me to have higher ceilings. And enough room for a lift. But could make for more parking space on the lower level for the boat and landscape trailer.

 

[wssix99]

The only alternate option that I can think of is to turn the thing 90 degrees and face the neighbors house. Have a driveway that went around and wrapped in.

Yes, the meandering driveway is how you pull off this elevation change. You could even do a hybrid solution. The less you have to fill and raise, the more you'll save and the more simplified things will be.

30 trucks with grading/compacting etc and rocking the entire things is $4800.

While that may seem a bit crazy when you think about it. 5 grand to have flat ground.....

That's a far stretch from 40k.

Will this company guarantee the bearing pressures for this compacted soil? If not, you may still have to dig your foundation walls to the bottom of the fill.

$5 K for all that fill is a good price - if you don't need a retaining structure to keep it in place and if it is strong enough to support a structure on top of it.

 

[cturboaddict]

Yes, the meandering driveway is how you pull off this elevation change. You could even do a hybrid solution. The less you have to fill and raise, the more you'll save and the more simplified things will be.



Will this company guarantee the bearing pressures for this compacted soil? If not, you may still have to dig your foundation walls to the bottom of the fill.

$5 K for all that fill is a good price - if you don't need a retaining structure to keep it in place and if it is strong enough to support a structure on top of it.

Didn't chat about warranting bearing pressure. But the building inspector said it would be fine to fill and then dig down 30" from top of new fill. They are actually doing this right down the street. (Another excavating company)

 

[DougWil]

Drilled and cast piers or helical piers and grade beams.

Building on that depth of fill and varying depth of fill, you are going to get settlement and differential settlement.
No matter what a excavator guy says.

 

[brownbagg]

30 inch not nothing, less than three feet, just do a mono and be done in one day.

prep the pad, then get a backhoe to dig 30 inch footer, throw some rebar in and pour.

somebody mention, five inch of rock, why, just fill up with concrete and be done.