Underpinning

 
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Old 07-20-2007, 08:13 PM   #1
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Underpinning


Here is my latest project. Making a walkout basement in a home. Need to do some underpinning of the footing along the new grade. Any tips on digging it out other than by hand?

Also, does anyone see any problem with pouring a new footing 42" below and blocking up to the existing footing with 10" block?
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Old 07-20-2007, 08:26 PM   #2
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Re: Underpinning


Yep! Looks like all hand work, keep us posted.

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Old 07-20-2007, 08:37 PM   #3
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Re: Underpinning


Using a jackhammer drive 2" pipe along the existing footing,about 6' apart. drive to refusal ,than bolt into footing. This will hold the load until the new Masonary sets up.. It's all easy for Me ,I'm also a welder and can fab a drive shoe . Please take some pix of the process,whatever You decide . This is an interesting project . Suitable for fine homebuilding or the journal of light Const. Good luck
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Old 07-20-2007, 08:44 PM   #4
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Re: Underpinning


Make that 3 ft. on center on pipes. This came from an Eng. site.
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Old 07-20-2007, 09:40 PM   #5
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Re: Underpinning


Did some under pinning a few months ago. It was designed to be poured in 3ft sections.
Personally if it were me I wouldn't undermine large sections.
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Old 07-21-2007, 09:17 AM   #6
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Re: Underpinning


I don't know of anyother way besides hand digging. How long of a section are you underpinning? and how deep do you have to go? Why do you want to come up with block rather then just pour concrete? Do you have to preload the underpinning?

One problem I potentially see with block is that there will end up being a void between the soil face and the underpinning. This could cause settlement in the slab of the existing building.
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Old 07-21-2007, 11:37 AM   #7
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Re: Underpinning


I would usually do it in 5' sections @ a time and pour concrete as opposed to CMU's. Since this is a walkout you stated 42", does this include frost protection as well, if needed?
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Old 07-22-2007, 07:09 AM   #8
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Re: Underpinning


I am digging down 42" below finished grade for frost protection. I guess it would be much easier to form it some how and pour it, but I can't figure out how to pour up to the bottom of the existing footing without leaving a large enough opening that the cement would pump out of.

If you have some tricks, I am open for suggestions. This is my first underpinning, though I have excavated many of them for others in the past. Always dug the way I did this and left. When I came back to back fill it was done so never learned the critical part.

And if you are wondering, I didn't dig thsi without having a plan. This part of the dig was to relocate a water line 48" below grade and a gas line to make room for the walkout.I made sure to put those on the outside of the trench too
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Old 07-22-2007, 09:32 AM   #9
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Re: Underpinning


digger,
Your supposed to have a space between the underpinning and footing. When the underpinning cures then you can dry pack the space. The main thing is to only undermine small portions of the footing at a time.
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Old 07-22-2007, 03:34 PM   #10
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Re: Underpinning


Looks like hand work from here.
We have done some underpinning with block walls we just have the mason put a solid block on his top course, and back fill behind the wall with stone to avoid the settling issue brought up earlier.
The biggest problem we have with underpinning work is water proofing the "other" side of the wall.

I just had a thought...could you clamp (bolt, weld) a piece of cutting edge to your excavator bucket and use it as a scraper to loosen the soil below the existing footing?
Have it extend out the distance you need, and plow it out. Might have to do it incrementally but it sure would cut the time down.
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Old 07-22-2007, 08:12 PM   #11
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Re: Underpinning


Quote:
Originally Posted by tgeb View Post

I just had a thought...could you clamp (bolt, weld) a piece of cutting edge to your excavator bucket and use it as a scraper to loosen the soil below the existing footing?
Have it extend out the distance you need, and plow it out. Might have to do it incrementally but it sure would cut the time down.
This was an idea I had years ago when I did a number of digs like this. The masons involved in those projects didnt want me to. Now I know why. A small section at a time.

Chompie:

So we don't need a continous footing under the underpinning? I should still run the rebar from one section to another though, correct?
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Old 07-22-2007, 11:25 PM   #12
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Re: Underpinning


correct, a continous footing is bad as it will cause you to undermine building. Often underpinning does not have any rebar as it is completley in compression. If you need to use it, tie the rebars together in the alternating underpinning pits. But be carefull you don't want to disturb the soil in one pit too much while excavating the other.
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Old 07-23-2007, 02:06 AM   #13
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Re: Underpinning


The under pinning I worked on. We doweled into the exisiting ran one hor bar at the bottom with hook dowels that tied into the slab.

As for waterproofing valclay panel were attached to the dirt and we poured right to it.

Never had to dry pack the top of the pour except for some small voids.
Built a birds mouth higher than the existing.
Poured stung it poured and stung some more till the birds mouth was filled to the top. Let the mud set up and knocked the birds mouth off and troweled.
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Old 08-08-2007, 05:41 PM   #14
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Re: Underpinning


Wondering how you made out with this project Digger. I was working on some underpinning last week and was thinking of this thread. Heres a pic.
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Old 08-12-2007, 09:14 PM   #15
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Re: Underpinning


I have not started it as of yet. But with all this advice and now pictures, I am comfortable with going forward.

Was the jack hammer used to remove the dirt or is that a drill? I am guessing the rebar is used to tie the existing footing into the new footing. Wondering how necessary that will be if I am just supporting a single story home and I make the footing 24" thick? I was originally thinking of just going 12" thick with a 24" thick x 12" high foot, but that would reqire more effort in the construction of the form.

Do you have any other pictures for this job?
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Old 08-12-2007, 10:46 PM   #16
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Re: Underpinning


Digger,
I don't have any other pics of this job. But this is a four story building in manhattan, and no rebar was used in the underpinning. The jackhammer and splitter are being used to remove some large pieces of concrete that are on the site, but attached to the building being underpinned. These underpinning piers were constructed by digging out a pit under the basement wall 1 ft thick, 5ft wide, and down to bedrock. Forms were only placed on one side (the one you can see in the pic) the rest if formed by the soil. The first pit was cured for more than 7 days before the second was excavated.
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Old 08-12-2007, 11:17 PM   #17
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Re: Underpinning


I guess because you are pouring onto bedrock and not soil, the foot print is fine at 12" thick, but as I am pouring onto a clay soil, I think I will stick to 24" thick but I do like the idea for just forming the front and maybe sides though a rougher side will help "lock the individual piers together. FYI: The Building Inspector is flying blind on this one. He has never seen it done before so he is lettign me run with what ever.

I am thinking of doing them 4' wide and use 3/4 ply as my form. I will also dig out ever other 4 foot section. So I can do two to three pours at once. I also want to pout my retaining wall oppisite this at the same time to increase the CY and get a better concrete price.

I am figuring on 3 total pours:
1/ 3 piers and the retaining wall footing
2/ remaining piers and the retaining wall
3/ pad and steps.
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Old 08-12-2007, 11:27 PM   #18
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Re: Underpinning


sounds like a solid plan to me. The only thing I would add, is instead of 2' thick, make them as thick as the existing footings. If your existing is 18" and you make the underpinning 2', there is no harm. But if your exising is 3' your underpinning should be 3'. Lets see some pics when you get under way.
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Old 08-13-2007, 10:45 AM   #19
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Re: Underpinning


The underpinning i did this year was in Toronto. The homeowner wanted the pins to be flush with the existing foundation walls to maximize space. We did 4' sections but chipped off the existing footings first (one section at a time)so the form could be flush to the foundation wall and we could angle the top of the form up and away from the wall and above the existing footings. When we pour the concrete into the form we make sure to vibrate the cement to minImize air pockets and make sure the cement meets the underside of the existing footings. This eliminates dry packing NS grout. When underpinning sections you should make sure that there is a min. of 8' of undisturbed soil between the pins. After all the sections are complete use Delta MS Waterproofing membrane on the walls making sure the top of the membrane is above ground level and the bottom of membrane is below the new concrete floor level but is above gravel base. You can add weeping tile on perimeter of newly poured walls and attach the weeper to a sump pit.
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Old 08-13-2007, 12:31 PM   #20
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Re: Underpinning


Quote:
Originally Posted by cmesweat75 View Post
When we pour the concrete into the form we make sure to vibrate the cement to minImize air pockets and make sure the cement meets the underside of the existing footings. This eliminates dry packing NS grout. When underpinning sections you should make sure that there is a min. of 8' of undisturbed soil between the pins.
I always do the dry pack of the NS grout, though I have never actually seen anyone have an issue with wet pouring concrete up to the existing footing, I would have it haunt me knowing that all concrete does shrink a little as it cures. I can't risk a cracked foundation due to even the slightest movement. Vibration is good too.

I, like you, will dig a 4' section then skip two sections and do another 4' section, esentially breaking the process up into sections A, sections B, and sections C. Unless the structural engineer specifies otherwise.

It's funny that this thread came back to the top, I just looked at a job that will have a lot of underpinning, I have to get the bid out this week.

Anyone want to price it for me?

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