Dig out basement (lower floor)

dhoult

I apologize if something like this has been posted before, but the search function refused to include "dig" in my search, so I couldn't find anything.

Basically, I am trying to finish my basement. Currently, the floor joists above are about 6' above the slab, and I want to lower the slab at least 2' (to make 8' ceilings). I have read a lot about what's involved, but I have yet to find someone who specifically addresses the issue of underpinning the existing footings.

So.... here is what I imagine is involved in this whole process:

Break up and get rid of current slab.
Dig to the level of the bottom of the existing footing.
Dig a hole underneath the existing footings, about 3' long.
Pour a new concrete footing (in this case I would make it the same width as the existing foundation, if not wider, and however deep is required to make my floor at the right height.)
When the concrete hardens, dig the next hole. Repeat until the entire foundation has been underpinned.
Dig out the rest of the floor.
Pour a new slab.

I have considered alternatives like raising the house, and losing several feet of floor space by making ledgers/digging inside of the footings at a 45 degree angle, etc, and I am well aware of what's required with those processes. At the moment I'm just interested in learning about this one.

My questions are: Is the process I outlined acceptable? How would I attach the existing foundation to the new one? Drill up into the old footing from beneath and put in rebar, then pour? I would attach the new pieces of foundation to one another with rebar through the sides.

Thanks for your help!

Ron6519

That's the way to do it if you have a death wish. I'm imagining you under a pile of house.
You seriously need to call professionals who do this for a living before you do harm.
Ron

dhoult

While I appreciate your concern for my life, I am trying to become more informed and educated on the subject, so simply telling me to call a professional isn't very helpful.

If you could explain why you think that's a recipe for disaster, I would be very grateful.

Thanks!

Daniel

TazinCR

What you want can be done. You need to shore the house up on each side of the current footing (with jacks and timbers) and pour the new footing then shift the weight back your new footing when it is cured (30days)
I helped my dad do this very thing 30 years ago. It is a lot of work but you can do it.
Good Luck

__________________
I may go home hungry, but not tired and hungry.

piecesurpiece

wow, just the info I am also looking for. I did glimpse this operation at one stage once, years ago, and am now looking at the same thing for my own building. Also looking for more detail, but pretty much resigned to trying to find a structural engineer to bend the ear of.

As I recall, though, the contractor had sectioned off the whole basement perimeter in 3ft increments with spray paint, labeled 1, 2, 3, 1, 2, 3 etc. When I saw it, he had already dug out and filled in with poured concrete the 1's, and was starting to dig the 2's. The sections seemed to be dug all the way under the wall, and I guess therefore the old footing as well. So I guess you dig out little sections (size determined by engineer) all around the basement at least the width of the previous footers, if not more, and fill them up, before digging the next set of sections. Therefore there are always 3ft dug up, then 6ft solid foundation, and whole thing is done evenly and incrementally. The floor had not been touched at the time I saw the work. I imagine all the central floor digging is done after the foundation is completely shored up.

The basement I saw was a 1920's 2 storey in Toronto, and it now has full height, and is dry, with no 'bench' around the basement perimeter that I can recall.

So it seems that you have part of the answer, but the idea seems to be not to go excavate then underpin adjacent sections one by one around the foundation, but rather to have multiple stages of spread out small excavated then underpinned sections, perhaps done more in the order of a star pattern...

Feedback?

caritimo

I've been researching this for a while as well - if you want to have a quick demo, check out the video on HGTV.ca's Sarah's House, Season I:
http://www.hgtv.ca/sarahshouse/video.aspx?releasePID=RydxNTbcqGviL28Wxw9qGReDbYk5 UOYS
From what I've seen already, you need to dig out portions of the foundation one by one, consolidate with concrete and then proceed to a new portion, gradually completing the basement perimeter. I think only after that is done do you then dig out the main floor and re-concrete entire surface of the basement.
I concur with Ron6519 - this is a very delicate job and I'm considering it myself for my house here in Montreal, but would NEVER attempt it on my own: playing around with the foundation of your home? Not without a full-blown warranty from a company that knows what it is doing!

Wildie

My daughters house had the basement excavated in this manner.
Looking at the signs from the forming, it appears that they under mined the footings in a 5 foot section, formed it up and poured the concrete! Then the skipped 5 feet and excavated another 5 foot section. Formed and poured again! They went around the whole perimeter under-pinning in the same manner.
I assume that once the under-pinning was complete, the excavation was completed and the missing sections finished!

Knucklez

you might try search term "excavation"

i'm thinking of similar project. but i will ask structural engineer his/her advice before i start project. also will get permit for this job too (mostly as 2nd opinion on critical reno).

remember that you loose headroom when you finish the floor & ceiling.. so you need more like 3' lower to have 8' ceiling.

please post your project & what you learn along the way!

Knucklez

Scuba_Dave

This part could be a recipe for disaster
Digging all the way around to the bottom of the footoing would be a mistake I think. If there is any outside pressure the walls could fall. I would do one section at a time

You most likely will have a ledge all the way around
The footer will stikc ina few inches past the walls:

Knucklez

scuba.. in your picture, on the top of the wall is a piece of wood flat against the top with anchor bolts. why is there no gasket?

is it right to have direct wood/concrete contact?

Knucklez

Scuba_Dave

Not my pic, just some diagram I found
PT can be in direct contact with concrete
I use foam underneath the PT

jomama45

Just for the record, adding basement depth is probably the most expensive & labor intensive square footage you can possibly add to a house. I would personally look into a first floor addition with a full basement or moving somewhere else. JMO

wildcat

I would not recommend messing with your basement. I would advise you to build up or out. Much better finished product and lighter on your pocket book.

Just for giggles though, what type of basement/foundation walls do you have? Also, where is grade in reference to the top of the basement wall or in reference to the finished first floor?

dhoult

I have concrete block walls, and grade is about 4.5 feet above the slab, and 1.5 feet below the first floor/top of basement wall.

dhoult

Actually, I have already gone up and out. I just finished adding a second story by myself, as well as a small addition off the back, so I'm certainly capable of doing a lot of the work myself to save money... now I want to go down