Underpinning a poured concrete foundation/footing

[Awilcox1821]

thread256-54204

Hi,
I have read the post referenced above, all great information I still have a few questions.

1) should I include rebar in the new pour? To tie each successive section together?

2) The existing wall is poured concrete with a stone footing-if you want to call it that!, the wall is 10" thick, if I dig out a 2' long section of dirt, to the required depth, would it be sufficient to have an overall finished footing 16" in thickness/depth underneath the 10" thick/deep wall?

3) the soil is very very stiff clay, if I do not disturb the soil at the bottom of the pit do I need gravel or sand? Or can I just pour onto the dirt?

4)lf cost isn't an issue can I use a non shrink grout? If so how big should the gap be between the new and old footing?


Many thanks Anthony

 

[Ron]

Perhaps you could provide a sketch or photos.

 

[PEinc]

1. Rarely is reinforcing steel needed in concrete underpinning. The piers are big. The bending stresses are minor and may be offset by the compressive load in the piers. This assumes shallow underpinning. Very deep, braced or tiedback underpinning may required positioning of the support points so that the concrete bending stresses are not too high for the unreinforced concrete. Allowing some tension in the unreinforced underpinning concrete is common.
2. If the soil under the existing footing is the same as that at the new lower bearing level, you will need underpinning that provides at least the same amount of bearing area as the original footing.
3. Pour on native, virgin, undisturbed soil.
4. Non-shrink grout is expensive and un-necessary. Use a damp sand-Portland cement mixture at about a 2:1 ratio. The drypack should be about 2.5 to 3 inches thick. There is so little water in it that any calculated shrinkage will be insignificant.

There is more to consider about underpinning but you need to read some references first. For instance, underpinning pits are almost always hand excavated and shored on all four sides with wood lagging boards. If you do not know what I'm talking about, you probably need help.

http://www.PeirceEngineering.com

 

[oldestguy]

A couple of points. Stiff clay may be that due to low moisture, but in the process is likely full of shrinkage cracks. It may be hard digging also. However I once was almost buried in such a clay situation. Those hard pieces are not clued together. They are separate pieces. That advice on shoring is something you do not want to skip.

To make the final wall a single unit, at least some re-bars are needed between units. You get that by driving the rods into adjacent undug areas. It means probably damaging some of the shoring. By using smaller units, not all dug full depth, things may be easier. Digging around those protruding bars ain't easy either.

 

[Awilcox1821]

All very good and appreciated advice...

I can sketch some stuff out and post it...

The overall height of my new footings/wall will be approx. 2 feet

And if I understand the responses correctly my new footing would be more than sufficient at 16" in thickness?

Just for clarification, I can envision a 4 sided snug fitting shoring box that also doubles as my form, for each section of pour, and then I will lose the back side of the form/shoring to mother earth when I rip off the two ends and the face of that four sided form.... Does that sound right?

Also I had envisioned leaving rebar hanging out the ends of my four sided shoring box/form,through some sealed holes, and then having to sawzall the two captured/trapped sides of the form to get it off the rebar, and then when I progress to the next pour my new concrete will act as the side for my next pour.... Thus making each form/shoring box after that a 3 sided box...?Does that make sense...?

Thank you all for the quick responses! Anthony

 

[Awilcox1821]

Another question(s) regarding the same project, what is your opinion on the following?

1) gravel and/or sand underneath the concrete footings for the steel support columns? Or does the same hold true about pouring on undisturbed soil? These footings are deep, about 3 feet below the original floor level.... So I know this is unmolested soil (clay)

2) gravel and/or sand underneath a 4" slab (basement floor)

3) rebar or mesh? Noting at all in that same 4" slab?

4)please cment in the use of a plastic vapor barrier underneath that slab?

 

[PEinc]

"I can envision a 4 sided snug fitting shoring box that also doubles as my form, for each section of pour, and then I will lose the back side of the form/shoring to mother earth when I rip off the two ends and the face of that four sided form.... Does that sound right?"

Correct.

http://www.PeirceEngineering.com

 

[Awilcox1821]

Thanks for the quick response...

Any insight into the other details regarding the floor slab and such?

Anthony