Underpinning Concern 2

[JohnTerry4]

We have a townhouse building that is being underpinned to lower the cellar slab by approximately 4 feet.

The existing footing is a stone footing that protrudes into the building by 12-inches. Therefore, Owner is concerned that underpinning will reduce cellar width by 12-inches on each side, so 2 feet total.

Does anyone have experience underpinning only under the foundation walls (and NOT under the footing) so that the footing protrusion can be saw cut after all underpinning is complete? I have seen other plans call this out but I want to know if I should be comfortable with this. Stone footing appears to be in decent shape.

TIA.

 

[HouseBoy]

IF the stone foundation wall is at a basement (and therefore no lateral load on it) THEN it seems like the concrete underpinning would have sufficient strength for bearing on top of it.
The underpinning components would have to be proportioned for the unbalanced condition but (if the above assumption is correct) the existing stone wall might be fine that way.
How thick is the existing stone wall?
How old is the existing foundation?
There may be a different problem regarding the soil bearing - the possible eccentricity of the load from the stone wall onto the new underpinning/footing. The farther back you extend the underpin (such as to counteract unbalanced soils) the more "eccentric" the existing stone wall becomes on it. Might be chasing your tail a bit on that one. I extend the underpins much deeper what that happens and use the new basement slab to help resist overturning.
Much will depend on the heights and soil capacity (you know...)

 

[PEinc]

It is not uncommon to underpin the thickness of the wall and then cut off the toe of the existing wall footing. Just be thorough with your design. Check sliding, overturning, and bearing for all components (existing foundation wall, wall with underpinning, and underpinning alone).

http://www.PeirceEngineering.com

 

[dik]

I've never removed the 'toe' of a stone foundation wall. The owner has to live with the reduced floor space. With old stonework this can be real problematic to remove it, if I'm understanding the question properly.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[BAretired]

The wall has to retain 4' more than it is retaining now, so how will the vertical wall reinforcement be made continuous in order to resist the moment?

BA

 

[XR250]

The wall has to retain 4' more than it is retaining now, so how will the vertical wall reinforcement be made continuous in order to resist the moment?

I usually design the lower stem as a cantilevered retaining wall - sometimes using a thickened floor slab as the footing. The sequencing is a bitch though.

 

[PEinc]

I agree with dik. Trying to cut off, remove, or shave back a portion of a stone foundation wall or a stone wall footing can be a big problem. The wall footings and column footings that I have cut back have been concrete, not stone.

EDIT: For underpinning jobs that need the footing toe trimmed off, I recommend that the toe of the footing be cut off when each individual underpinning pit is being excavated. Do not cut off the entire length of the wall footing toe before doing the underpinning. You don't want the building to settle or tilt before you start underpinning.

http://www.PeirceEngineering.com

 

[SleeplessEngineer]

@PEinc Sorry for digressing from this post. You posted a pic of your underpinning project- 1706_Rittenhouse_Square_Street in another post; the post was closed for comment. I am curious about the sequencing of the work.

Thank you for reading my post! I appreciate your time.

 

[PEinc]

SleeplessEngineer, usually you excavate along the wall to within 1 foot of the bottom of the foundation to be underpinned. Make sure the area is sloped away from the building so rain water does not accumulate along the wall being underpinned. Mark out on the wall where the underpinning piers need to be excavated. Along the wall, mark out the piers as A, B, C, D, A, B, C, D, etc. Excavate, concrete, and drypack the A piers leaving the B and C piers unexcavated. Then repeat for the B piers, then the C piers, then the D piers. If there are column footings to be underpinned, do not work on any 2 adjacent piers concurrently. Deeper underpinning piers need to be excavated and poured before shallower, adjacent piers. Unless the underpinning excavation is less than about 4 or 5 feet AND the soil is very competent, stiff cohesive soil, and the concrete will be placed as soon as the pit has been excavated; underpinning excavation is almost always hand-dug by a worker inside the pit, installing shoring boards as the pit is being excavated. If there is ground water above the bottom of the underpinning piers, it needs to be dewatered before underpinning is started.

An underpinning pier is supposed to be excavated all the way to its bottom elevation so that all of the concrete can be placed in one pour from bottom to top of pier. For example, if underpinning piers need to be 15' deep, you dig AND SHORE a 15' deep pit. You should not dig and pour 3 successive, stacked, 5' deep piers.

Warning: There are a lot of (most?) YouTube videos that show improper underpinning procedures. Get a book on underpinning (there are not many) rather than surf YouTube.

http://www.PeirceEngineering.com

 

[0.125pLL]

Get a book on underpinning (there are not many)

Can you please recommend a book on underpinning?

 

[SleeplessEngineer]

@PEinc Thanks for the detailed explanation. Apologies for my vague question. I meant to ask specifically about the 40' underpinning. I have designed couple of underpinning projects, but never that deep. I was not sure if the piers were cast in one lift or if not, how was that sequenced.

An underpinning pier is supposed to be excavated all the way to its bottom elevation so that all of the concrete can be placed in one pour from bottom to top of pier.

But you covered that as well. Thanks!

Thank you for reading my post! I appreciate your time.