Lets talk trench footers

[sixtyfive]

Tree removal is complete and I am ready to begin work on my foundation. I am building a 26' x 43' detached garage in north eastern Iowa. I need to build 42" below ground level and plan to use a trench foundation using earth rather than forms.

Trench footer\foundations are very common in the area and my building plans using this style have been approved.

Here is what I have planned so far -
-Grade building area w/ skid
-Dig trench ~8" wide x 42" deep w/ mini ex 8" bucket
- 1/2 rerod in trench 2 horizontal rows, vertical every 4'
-Assemble 2x8 forms partially above ground partially below depending on slope of final grade for a level finish
- Pour footers ~13 yards
- Vibrate pour

Questions -
- How will I bell out the bottom of the footer trench? City mentioned they liked to see the bottom bell out an inch or so on either side. He mentioned using a shovel but I do not see that working very well at all. Is there an excavator attachment for this?
-Should I plan for my slab to end up being the same height as foundation walls or lower? I plan to put two coarse of concrete block on top of the concrete foundation to give me some extra height.
- Should there be a height gap between foundation and slab under the garage door openings? I have seen conflicting views on this.


Thanks for the help guys! I am new to concrete work but experienced with most other aspects of building.

 

[BlindViper]

Questions -
- How will I bell out the bottom of the footer trench? City mentioned they liked to see the bottom bell out an inch or so on either side. He mentioned using a shovel but I do not see that working very well at all. Is there an excavator attachment for this?

The excavator attachment is normally the labor holding the shovel.

 

[sixtyfive]

The excavator attachment is normally the labor holding the shovel.

Haha alright. If that is how it is normally done then maybe a long narrow spade would work the best.

 

[Zeke]

" Should there be a height gap between foundation and slab under the garage door openings? I have seen conflicting views on this."

I can't understand that question. AFA as the rest, you'll need some of that rebar coming up out of the foundation and/or slab if you do a monolithic pour (placement) for the block.

I recommend someone very experienced in concrete work. They will have form material and can form that in an afternoon. You might even get a slab down with no birdbaths.

 

[brownbagg]

there nothing in the code book on bell out a footer, I tell him prove it in the book or go pound sand, if its not in the book, its not legal


master icc code inspector , (structural masonry, reinforced concrete, structural steel, soils, structural welding) ( AWS certified welding inspector)

 

[NUTTSGT]

Something like this tamper/digging bar could be used to cut the walls back. Once it's cut back, use the backhoe bucket to dig out the spoils.

http://www.homedepot.com/p/70-in-Tamper-Head-Digging-Bar-GXB-441/202531238#.UZFxaaKG2og

 

[Alchymist]

When I was a kid (looong time ago!) I watched utility workers setting telephone poles by hand - and they used a spoon shovel. Long handle with a small blade set at right angles to the handle. Worked a treat for narrow but deep holes- knock stuff loose with a bar, scoop it out. Here's something similar:
http://toolmonger.com/2008/08/08/shovel-with-a-spoon/

 

[theoldwizard1]

Tree removal is complete ...

I hope you got all of the stump and roots out ! If not, they will eventually rot and your floor will crack.

...I need to build 42" below ground level and plan to use a trench foundation using earth rather than forms.

Common around here if the soil is "heavy" (clay, not sand or loam)

Here is what I have planned so far -
-Grade building area w/ skid

Usually a layer of sand or compacted gravel is required. Not in the trench but below the floor.

- Pour footers ~13 yards
- Vibrate pour

So 2 pours. One for footer and then one for the slab, or are you going to do it in one shot ? Have you hired a concrete crew ? I would !

Questions -
- How will I bell out the bottom of the footer trench? City mentioned they liked to see the bottom bell out an inch or so on either side. He mentioned using a shovel but I do not see that working very well at all. Is there an excavator attachment for this?

For some structures this is required and they do it by pouring the footing one day and the framing the the wall up to grade and pouring another day. You would need one hell of a long shovel to "bell out" the bottom.

-Should I plan for my slab to end up being the same height as foundation walls or lower? I plan to put two coarse of concrete block on top of the concrete foundation to give me some extra height.

Good question for your concrete crew boss ! Make sure to find out what the inspectors wants to attach the block and 2x4" footer to the foundation wall.

 

[sixtyfive]

" Should there be a height gap between foundation and slab under the garage door openings? I have seen conflicting views on this."

I can't understand that question. AFA as the rest, you'll need some of that rebar coming up out of the foundation and/or slab if you do a monolithic pour (placement) for the block.

I recommend someone very experienced in concrete work. They will have form material and can form that in an afternoon. You might even get a slab down with no birdbaths.

I am having someone experienced help with the slab, or hiring a contractor depending on the availability of the friend. I think I have enough experienced friends to help with the foundation.

What I was referring to, is the foundation usually steps down where the garage doors will go.

This image shows what I am talking about.

 

[sixtyfive]

I hope you got all of the stump and roots out ! If not, they will eventually rot and your floor will crack.


Common around here if the soil is "heavy" (clay, not sand or loam)


Usually a layer of sand or compacted gravel is required. Not in the trench but below the floor.


So 2 pours. One for footer and then one for the slab, or are you going to do it in one shot ? Have you hired a concrete crew ? I would !


For some structures this is required and they do it by pouring the footing one day and the framing the the wall up to grade and pouring another day. You would need one hell of a long shovel to "bell out" the bottom.


Good question for your concrete crew boss ! Make sure to find out what the inspectors wants to attach the block and 2x4" footer to the foundation wall.

I had read that all organic material larger than 2" in diameter should be removed so that is what we did. One of the stump groups was 3 stumps all together and we ended up with a hole that I could have buried my silverado in. It is farther forward on the lot than where the garage will be though.

We seen solid clay about 4ft down. Some areas it was higher than that even.

We were planning on using gravel as fill for the slab. Does the dirt need to be tamped down before laying out the gravel, or is tamping the gravel sufficient?

Planning for two pours. Foundation will be one, and then a second pour for the slab. Costs were a bit prohibitive for hiring a crew to do all of the concrete work. Quotes were between $9100 - $9900. If I could have got it done for a few grand less than that I could afford it.

I was planning to have rerod up into the sections of block that I will mortar fill. These same cavitys will be where I place the threaded rod to secure the footer. The city said this would be good, but I will double check with the inspector.

 

[sixtyfive]

Something like this tamper/digging bar could be used to cut the walls back. Once it's cut back, use the backhoe bucket to dig out the spoils.

http://www.homedepot.com/p/70-in-Tamper-Head-Digging-Bar-GXB-441/202531238#.UZFxaaKG2og

I have a long prybar with a sharp edge that may work similar to this. The weight of it would wear me out pretty quick I am thinking though.

When I was a kid (looong time ago!) I watched utility workers setting telephone poles by hand - and they used a spoon shovel. Long handle with a small blade set at right angles to the handle. Worked a treat for narrow but deep holes- knock stuff loose with a bar, scoop it out. Here's something similar:
http://toolmonger.com/2008/08/08/shovel-with-a-spoon/

I will check into these!

 

[NUTTSGT]

I have a long prybar with a sharp edge that may work similar to this. The weight of it would wear me out pretty quick I am thinking though.

It might but it will damn fine exercise.

 

[theoldwizard1]

We were planning on using gravel as fill for the slab. Does the dirt need to be tamped down before laying out the gravel, or is tamping the gravel sufficient?

Unless you have not added fill dirt you do not need to compact the dirt before the first layer of gravel. You should plan on at least 4" of compacted gravel. If you are doing more than that, compact the first 4" before adding the next "lift".

 

[Herb]

If an extra inch on either side of the footing base pleases the inspector, would it be easier for you and acceptable to them to just pour a 10 inch footer instead?

 

[csp]

What I was referring to, is the foundation usually steps down where the garage doors will go.

What you have pictured there is not a monolithic pour. Stem walls were formed up and poured first and the slab went in after the forms were removed and back filled.

As for brownbaggs comment, any local building department can define their own requirements which supercede what's in the code book.

 

[cyamaha2007]

Well it doesnt sound like your doing a monolithic pour. I would form the foundation flush with the finished floor height at the doors. I really like to have my foundation walls 6-8in above the finished floor height. This makes a great curb to detour water entry. It also keeps your pt wood sill plate happy for many years, it helps prevent wall damage from mowers and weed wackers outside, inside it keeps the drywall dry if you need to hose/mop the garage.
If your sure the soil under your garage hasnt been disturbed by tilling,farming, planting/ grading in the last 20+ years then you dont need to compact. You can google how to test the bearing strength of the existing soil. Thats the only way to know for sure. Hope it helps

 

[sixtyfive]

What you have pictured there is not a monolithic pour. Stem walls were formed up and poured first and the slab went in after the forms were removed and back filled.

As for brownbaggs comment, any local building department can define their own requirements which supercede what's in the code book.

Yes I am planning on doing two pours. Foundation in one shot and then the slab will be the second pour.

 

[sixtyfive]

Well it doesnt sound like your doing a monolithic pour. I would form the foundation flush with the finished floor height at the doors. I really like to have my foundation walls 6-8in above the finished floor height. This makes a great curb to detour water entry. It also keeps your pt wood sill plate happy for many years, it helps prevent wall damage from mowers and weed wackers outside, inside it keeps the drywall dry if you need to hose/mop the garage.
If your sure the soil under your garage hasnt been disturbed by tilling,farming, planting/ grading in the last 20+ years then you dont need to compact. You can google how to test the bearing strength of the existing soil. Thats the only way to know for sure. Hope it helps

Nope I am not planning on doing a monolithic pour. I like the idea of having the foundation walls come up 6 or so inches above the slab. I also plan to lay two courses of concrete block on top. This along with my 2x4x10's will get me closer to a 12' ceiling height for a future 2 post lift.

I am thinking I will need about a foot and a half of fill towards the back of the garage, so I will need to tamp what I add in for sure. The rest of the area is fairly well packed down from driving the excavator around the build site.

Is it common for the rerod reinforcement to rest on soil in the bottom of the trenches, or does it need to be set up on something?

 

[csp]

Well it doesnt sound like your doing a monolithic pour. I would form the foundation flush with the finished floor height at the doors.

I wouldn't. The stem wall (foundation) height at the door opening should be under the slab just like it is in the above pictures. You want any water that might hit the slab to run outside. If the stem wall is the same height as the finished floor water will get between the slab and the stem wall.

 

[sixtyfive]

I wouldn't. The stem wall (foundation) height at the door opening should be under the slab just like it is in the above pictures. You want any water that might hit the slab to run outside. If the stem wall is the same height as the finished floor water will get between the slab and the stem wall.

So I will either plan for top of the foundation in this area to be at the same height as my gravel grade, which would give me 4" or maybe a bit more.

From what I can tell there is no reason for the slab and the foundation to not touch in this area.

I am on the fence with whether I will have a layer of 2" pink XPS foam between the gravel and the slab or not. I will need about 36 4x8 sheets @ $25 a sheet. That will cost a bit over $900.

I plan to heat the garage with a gas furnace. I think the foam will keep the floor a bit warmer and eliminate slab sweat in the spring time.

 

[cyamaha2007]

Nope I am not planning on doing a monolithic pour. I like the idea of having the foundation walls come up 6 or so inches above the slab. I also plan to lay two courses of concrete block on top. This along with my 2x4x10's will get me closer to a 12' ceiling height for a future 2 post lift.

I am thinking I will need about a foot and a half of fill towards the back of the garage, so I will need to tamp what I add in for sure. The rest of the area is fairly well packed down from driving the excavator around the build site.

Is it common for the rerod reinforcement to rest on soil in the bottom of the trenches, or does it need to be set up on something?

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No, the rebar is useless at the bottom of the pour. It should be set on chairs to suspend it during the pour.

 

[sophijo]

.....and consider facing the outside of the footer with rigid insulation....top to bottom of the hole. Forget under the slab.

 

[sixtyfive]

.....and consider facing the outside of the footer with rigid insulation....top to bottom of the hole. Forget under the slab.

This would be hard to achieve with a trench footer. I only plan to dig an 8" trench and dump my footer directly into it with only the top 8" above ground formed.

 

[sixtyfive]

Is $6.60 per stick of 1/2" x 20' rebar a good price?

http://www.menards.com/main/buildin...sh/1-2-x-20-steel-rebar/p-1480919-c-12502.htm

Also, I have a guy lined up that is trenching out a basement a couple miles from my house. He will be dropping the dirt in my back yard to get me some level ground to work with. I diddnt think that I needed much fill, but I used a fill calculator and it estimated I need about 111 yards! Luckily this guy is only charging me $50 per trip since the dirt is free.

 

[csp]

.....and consider facing the outside of the footer with rigid insulation....top to bottom of the hole. Forget under the slab.

I think terminology is a problem here. The footer is the wide footprint that the stem wall will sit on after the footer is poured and cured. No need for insulating a footer. Insulation would be beneficial on the stem walls, however.

From what I can tell there is no reason for the slab and the foundation to not touch in this area.

In this area it's common for them to be not only touching, but tied together like your footer and stem walls will be. Builders leave rebar sticking up at the garage door openings which is buried in the slab as it's poured.

 

[Kevin54]

Whoever is going to lay the block will be pretty pissed if they have to work in an 8" wide trench. If you are going with a solid wall instead of block, you still want to dig it wider than 8". Most around here is 12" plus for a footer. This allows for any errors when the digging was done so you can keep things straight. If block is going to be laid, a block layer likes enough room to maneuver around in the trench.

 

[sixtyfive]

Whoever is going to lay the block will be pretty pissed if they have to work in an 8" wide trench. If you are going with a solid wall instead of block, you still want to dig it wider than 8". Most around here is 12" plus for a footer. This allows for any errors when the digging was done so you can keep things straight. If block is going to be laid, a block layer likes enough room to maneuver around in the trench.

When I refer to footer, I mean footer and stem wall, as they will be one pour. With the trench footer style I will do a single pour from 42 inches below grade to 8 inches above grade. The two course of block will be on top of the stem wall\ footer which is 8 inches above ground.

This image might help. I will not use any forms below ground, and there will be no block below ground.

 

[sixtyfive]

I think terminology is a problem here. The footer is the wide footprint that the stem wall will sit on after the footer is poured and cured. No need for insulating a footer. Insulation would be beneficial on the stem walls, however.



In this area it's common for them to be not only touching, but tied together like your footer and stem walls will be. Builders leave rebar sticking up at the garage door openings which is buried in the slab as it's poured.

I have a contractor that will be doing the slab pour finish and cut for $1500 labor cost. He requested that I not have rebar bent over from the wall into the slab. I am curious if this holds true for below the door openings as well. I will add this to my list of questions to ask him.

 

[brownbagg]

rebar here is $9 per twenty feet

code on footer is minmum 18 inch wide by foot deep

finisher does want rebar turn over, hell with finisher, when did they start designing structrual

 

[sixtyfive]

rebar here is $9 per twenty feet

code on footer is minmum 18 inch wide by foot deep

finisher does want rebar turn over, hell with finisher, when did they start designing structrual

I should have phrased that better, he does not want the slab attached to the foundation wall. Or at least that is what he suggests.

I am not up to speed on the pros and cons between the two methods.

 

[bczygan]

Something to consider. It is hard to dig an 8" wide footer. It will probably go 12" anyway. So let it. And then, if the garage ever needs a room over it, the foundation will still meet code for that. I have run into that situation, where people wanted to add a bonus room later, and couldn't.

Correction:It may be 16" wide for 2 story, as noted in the next post. Check your code.

As far as belling the bottom of footings...I don't know if testing has shown that it is of any benefit. Couldn't hurt though.

 

[ddawg16]

I would highly suggest doing it all at once....you end up with a stronger foundation since the footing, stem wall and slab are all one piece. And, it's a little cheaper since you only have to have the cement trucks out once.

Code here....`12" wide footer for single story....16" for 2 story...and 2 feet deep.

Stem wall is 8" high and 6" wide. A 2x6" PT board sits on it just fine.

Click on the Garage Build link in my signature for pics of my foundation.

NO WAY would I have a flat slab with the walls that close to the ground.

With my garage, I can wash the floor and not worry about any water getting on the drywall or wood.

 

[Kevin54]

Nope I am not planning on doing a monolithic pour. I like the idea of having the foundation walls come up 6 or so inches above the slab. I also plan to lay two courses of concrete block on top. This along with my 2x4x10's will get me closer to a 12' ceiling height for a future 2 post lift.

I am thinking I will need about a foot and a half of fill towards the back of the garage, so I will need to tamp what I add in for sure. The rest of the area is fairly well packed down from driving the excavator around the build site.

Is it common for the rerod reinforcement to rest on soil in the bottom of the trenches, or does it need to be set up on something?

This would be hard to achieve with a trench footer. I only plan to dig an 8" trench and dump my footer directly into it with only the top 8" above ground formed.

I should have phrased that better, he does not want the slab attached to the foundation wall. Or at least that is what he suggests.

I am not up to speed on the pros and cons between the two methods.

Rebar laying on the ground and concrete over the top will do no good at all. As stated above, you want the rebar on chairs. You also want rebar in the floor on chairs. That way when your slab cracks, the two pieces will still be level with one another.

As far as the slab pinned to the wall....a lot of contractors recommend that you not pin it. If you do, and the ground or stone settles underneath, which it will some, you won't end up with hollow spots underneath the slab. If the soil is virgin soil and again as stated above, you don't need to tamp it, but you do need to tamp the stone going in. Myself, I would never recommend backfilling with soil anyways. You can't get it tamped enough. There is also a lot of debate about the type of stone going in before the concrete. A lot of people recommend the small round washed stone as backfill as they state that the stone can be poured without tamping because it's round and settles as low as it can go. Others recommend crushed stone as they say the stones lock together and cannot settle. Today, a lot of people are backfilling with sand, dampened and tamped, as they say it will never settle as it's down as far as it will go. So you choose.

And as far as an 8" trench....you better be dead nuts straight in digging it. I hardly ever see a straight trench when they use a larger bucket and put up forms.

 

[91FE]

there nothing in the code book on bell out a footer, I tell him prove it in the book or go pound sand, if its not in the book, its not legal


master icc code inspector , (structural masonry, reinforced concrete, structural steel, soils, structural welding) ( AWS certified welding inspector)

With all of those credentials, you of all people should understand that the local building inspector has final say. The building code is only a guideline and it's up to each municipality to enforce it however they see fit. An attitude like yours wouldn't get anything built in my world.

91FE, AIA, LEED AP

 

[sixtyfive]

Trench foundation ended up working out for me. The biggest problem we ran into was a collapse at one corner of the trench. This was likely because there was a tree in this location previously and the soil was more wet than other areas. We dug out the collapse and ended up just running the concrete footer wide in this area. Probably cost me half a yard extra in concrete, so no big deal.

Using the trench footer was way cheaper than using forms, but we also ran into a downpour of rain while we were pouring the concrete. This caused my pour to be 6 inches out of level which I had to correct by cutting concrete block for height with a quickie saw. It was time consuming and rough on the back!

Also I had planned to form the top of the footer with 2x8's in order to have a uniform top. This was not possible as the excavator did not dig as clean cut of a trench as I had thought it would. I ended up pouring to 5 inches below grade or so. I think this is the way that trench footers are commonly poured anyways.

I have some pics up on my build thread. Check it out!
http://www.garagejournal.com/forum/showthread.php?p=3230449#post3230449