Underpinning 9" brick walls 1

[Jbran]

I am working on a 1920, 2-story balloon-framed addition with 9" thick brick foundation that is 30" deep. I want to pour in 60" high underpinning sections for a total wall height of 7'6". There will be 6' of dirt cover outside. The sections will be 3' wide and 20" deep. The sections will also have a footer that is 12" high and 30" front to back. The footer will be poured monolithic with the wall sections. No rebar. Each section will be formed with a "bird mouth" ramp set higher than the brick-to-concrete connection so I can vibrate the concrete to conform perfectly to the brick. I am wondering if all that sounds OK and what I should change. Engineer says I need a bunch of vertical and horizontal rebar, but that the walls are plenty thick.

 

[phamENG]

If you have an engineer on the project, you should do what they indicated. If you think they're wrong, hire another engineer to take over the project and provide a different design.

 

[Jbran]

The engineer is actually just a friend who gave some advice. He hasn't actually been involved in a basement underpinning before. He is just working off what I mentioned above.

 

[SwinnyGG]

Wellll... if you don't actually hire an engineer who provides stamped drawings, you are 100% liable for this structure from the time you place the concrete until the end of time. You're also very likely operating outside of the law with regard to permitting. Unless you are in some extremely rural jurisdiction, the type of work you are describing almost certainly requires a permit, and to get a permit you need drawings stamped by an engineer who has assumed liability by taking on the responsibility of being the Engineer of Record.

If your eyeball engineering results in injury or death due to something collapsing, you will very likely lose whatever assets you have. You won't need those assets because you'll be in prison for a while.

In short, hire an engineer.

 

[PEinc]

Please post a sketch of the proposed underpinning and existing structure.

http://www.PeirceEngineering.com

 

[Jbran]

I attached a drawing of the wall and new underpinning. The longest wall is 24 feet long.

 

[SwinnyGG]

I attached a drawing of the wall and new underpinning. The longest wall is 24 feet long.

Your sketch confirms the mental image from your description.

This is not a minor undertaking. This is major structural work. To properly design this foundation requires a whole bunch of information which you haven't included about the building itself, the soil conditions, etc etc. Based on your posts so far, I think the assumption that you don't have any of this information available is a safe one.

Hire. A. Competent. Structural. Engineer. Period.

What you're doing is exceedingly dangerous to not only the building, but to you personally.

Frankly I'm surprised that any of the professional engineers on this forum are even entertaining the idea of giving you feedback on a back-of-the-napkin sketch.

 

[PEinc]

Jbran,
[ol 1]
[li]I would not use the bird mouth ramp you describe. I would pour the underpinning concrete to within 2.5 to 3 inches of the bottom of the existing foundation wall and then dry pack the remaining space.[/li]
[li]Your underpinning excavation will be about 7.33 feet deep. In order to protect the building and the person excavating the pit, the excavation should be shored with wood lagging boards as the pit is being excavated. E.G. dig about 3 feet deep and install the shoring boards. Then repeat until the pit is fully excavated[/li]
[li]If you excavate the pit without shoring boards, you will probably expose or undermine too much of the existing foundation wall and damage or settle the building. It is illegal and unsafe to work in an unshored pit deeper than 4 or 5 feet. Your pit will be over 7 feet deep. In the US that is an OSHA violation.[/li]
[li]I would not widen the bottom of the underpinning to place the new slab on top of the underpinning.[/li]
[li]Also for underpinning that deep and narrow, with a light building above; you should have stability problems with overturning and sliding of the underpinning.[/li]
[li]This underpinning should be designed by an engineer who is very experienced in underpinning design and construction.[/li]
[li]There is a Presentation on my web site about underpinning and building collapses due to improper underpinning design and construction.[/li]
[li]There are many ET threads concerning underpinning. Do a search.[/li]
[/ol]

http://www.PeirceEngineering.com

 

[Jbran]

Thanks a lot for the info, I'll do that.

 

[Jbran]

Thank you again for your reply, I have a couple other engineers comming to discuss the project, but it is difficult to find someone who is very experienced with underpinning. I have read a lot of posts on here and PEinc's experience seems much more expansive than the average engineer and really appreciate the feedback.

I can see why the 20" Wall is narrow as 24" is more recommended. I was wondering if the extra footer size would compensate, but I guess the slab may put extra tilting pressure on the underpins?

As for the bird mouth, I guess there is not a great way to know if there is a complete bearing connection, and a dry pack is more certain. I was thinking, maybe with only a 13" footer base of the old wall, it would be a pretty certain bearing connection with vibrating concrete.

I see what you mean about a light structure and the risk of overturn and slipping. Would extending the underpinning into the room 4" and bringing it up 4" above the base of the existing footer better solve this? It would make the walls a full 24", but I don't see how dry packing could work with this amd I would be back to vibrating the connection.

I know I am giving limited info on soil structure and the building, but I would you have a very general recommendation for underpinning thickness with no footer, as I proposed, and such a light structure?

 

[PEinc]

but it is difficult to find someone who is very experienced with underpinning

Unfortunately, it is not difficult to find an almost unlimited number of engineers who claim to be underpinning experts. I've seen too many crazy, dangerous underpinning designs. They are usually loaded with unnecessary, expensive details and lack lateral stability for overturning and sliding. There is usually more to underpinning than just supporting vertical building loads.

http://www.PeirceEngineering.com

 

[PEinc]

You said: I can see why the 20" Wall is narrow as 24" is more recommended. I was wondering if the extra footer size would compensate, but I guess the slab may put extra tilting pressure on the underpins?
Response: Underpinning pits are usually hand-dug and shored with wood. If a pit excavation is deeper than about 4 feet, someone needs to get inside the pit. The usual minimum sized pit is 4 feet wide along the wall by 3 feet front to back. Any smaller, there is insufficient room for the worker. Look at my presentation.
You said: As for the bird mouth, I guess there is not a great way to know if there is a complete bearing connection, and a dry pack is more certain. I was thinking, maybe with only a 13" footer base of the old wall, it would be a pretty certain bearing connection with vibrating concrete.
Response: It is hard to believe that a building's brick foundation wall has no concrete footing that is about 2 feet wide. 13" is awfully narrow. Be careful excavating under a brick wall if there is no concrete footing. You could easily lose bricks, especially if you are excavating more than 3 or 4 feet wide. If there is no concrete footing, I would not undermine as much as 4 feet. My maximum pit/pier width might be only 3 feet maximum. That means the pit needs to be about 4 feet perpendicular to the existing wall and you will need to bulkhead inside the shored pit to use less concrete.
You said: I see what you mean about a light structure and the risk of overturn and slipping. Would extending the underpinning into the room 4" and bringing it up 4" above the base of the existing footer better solve this? It would make the walls a full 24", but I don't see how dry packing could work with this and I would be back to vibrating the connection.
Response: No. Belling out the basement side of the underpinning pier, below the floor slab, will help with overturning but not with sliding. Without using reinforcing steel in the belled pier "toe," the toe would need to be at least as thick as it projects from the front of the pier.
You said: I know I am giving limited info on soil structure and the building, but I would you have a very general recommendation for underpinning thickness with no footer, as I proposed, and such a light structure?
Response: Deeper underpinning with lighter structures is often problematic. There is not enough building and pier dead load to resist overturning and/or sliding, especially if the building's foundation wall is sitting at the front face of the underpinning pier, as your sketch shows.

http://www.PeirceEngineering.com

 

[Jbran]

Any recommendations for a very experienced underpinning engineer in the Richmond, Va area would be much appreciated.

 

[Jbran]

I saw your presentation on hand-digging the pits and shoring with wood. That makes a lot of sense and I see how it directs a lot of the strategies you recommend. I have seen you stress this in a lot of posts so I am sure it is the way to go. It would seem a more thick underpinning would help with the overturning, but the sliding is a different matter. I can assure you the foundation is only 13" thick of brick (2 layers of brick for the wall, 3 layers of brick for the foundation). We dug a small section out to see what we were working with. The brick foundation is strong with no loose bricks. The dirt is very hard clay that is difficult to dig. Could only make progress with an SDS Max.

To prevent slipping, would it help to pour concrete up the outside of the existing foundation to hold back the dirt load on that existing foundation? I attached a new drawing of what I am talking about.

 

[Jbran]

Sorry, I meant to say 36" thick pit/underpinning for room for the worker.

 

[jerseyshore]

This thread is giving me anxiety.

Even with hard soil, the risk of foundation failure goes up exponentially when excavating for underpinning or even just cutting out the old slab. As PE said, loosing bricks into the pits when they have such crappy footings is common with these old walls. And of course the sliding concerns remain until the new slab is poured and cured.

Not only do you absolutely need a SE to design this, but I would hire a foundation specialty contractor that has experience in underpinning as a sub if you don't haven't done it before/ a lot.

 

[PEinc]

I would not try to support the top of the underpinning against the bottom of the brick foundation wall. The brick wall has very, very little to no bending strength. It seem that you would need to make the underpinning a lot thicker (to provide more resisting dead load) or else tie it back with tie rods and deadmen (not too practical). Having the underpinning extend into the basement helps overturning (sliding, not as much) but greatly reduces the useable basement area (usually not an option).

You could hire a design-build underpinning contractor to take care of the entire problem. Try

https://www.schnabel.com/

or

https://steelefoundationllc.com/

or

https://www.berkelandcompany.com/

http://www.PeirceEngineering.com

 

[Jbran]

Thank you so much for those design/build suggestions. I left messages with all three and I also have someone from Kefficient comming to look at the basement.

I am curious, if there is a massive underpinning, such as 4 or 5 feet thick, wouldn't an extension of the underpinning up behind the brick foundation act as a dirt hold back rather than put pressure on the foundation? Seems like a good way to prevent slippage, which is a big concern here.

 

[PEinc]

Jbran, having the underpinning run up the outside of the brick foundation wall will increase the dead load of the underpinning piers but that is all. It will probably have minimal effect on increasing the safety factors for overturning and sliding. Draw a section of each underpinning scheme and calculate the earth and surcharge pressures and the dead loads of the building and underpinning. Then check overturning and sliding with only the dead loads of the building and the underpinning as the resisting forces. Building live load affects bearing but should not be used to check overturning and sliding as it is a stabilizing force.

http://www.PeirceEngineering.com

 

[jerseyshore]

I looked into a very similar underpinning detail a few weeks ago and discussed it with everyone on here and a full underpinning that adequately resists sliding and overturning with the appropriate safety factors is usually way larger than feasible.

You'd be better off just lifting the house and building a new foundation wall. These old brick walls get dicey once disturbed.