Underpinning Rubble Foundation

[Geoluk]

Hello,

I have a job where we have to underpinning a wall that is resting on a single layer of rocks that are 1.5 - 2 feet in diameter. Looks like they just lined up the rocks next to each other, put concrete on top to create a level surface, and began laying bricks. It's only one layer of stones.

My idea is to do some concrete underpinning under the rocks themselves and potentially require that grout be added first if the stones are found to be loose (which it seems they are). Also will probably have to keep the width of the underpinning sections to a minimum since the stones can't quite bridge the underpinning pit.

My questions is, does this reasonable? Is there anything else that I should be aware of?

I considered removing those stones if loose but who know what that would do to the wall above it.

 

[PEinc]

I would not consider that a rubble stone wall. What is the condition of the brick wall that is sitting on the single layer of big stones? How thick is the concrete on top of the stones? I have had a couple of projects where the rubble stone wall was in such bad shape, we removed it in sections as we installed each underpinning pier. We dry packed the wall to the top of the pier.
How deep is the underpinning? You do not want anyone inside an underpinning pit if there are loose stones.
Are you hand digging and shoring the pits in lifts as the the excavation proceeds?

http://www.PeirceEngineering.com

 

[Geoluk]

Yeah it's not exactly a rubble wall but I'm not sure what else to call it.

The brick wall seems ok, but not great. It is about 100 years old. The concrete on top of the rocks is about 2-3 inches thick (between the rock and brick).

Ok so removing the "rubble" is a possible option then. How thick can the drypack be? Sounds like a lot from what you said.

The underpinnings are 5 feet deep at most. Agreed, no one inside the pits.

Have you ever done an sort of double underpinning? See image below.

 

[LittleInch]

This sounds very much like something which clearly has stood the test of time but as soon as you need with it it will literally fall apart in front of your eyes.

So 1) why underpin now after 100 yrs?
2) what's the wall holding up?
3? Any sketches or photos of this?
4) can you support the base of the wall while you excavate underneath?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Geoluk]

1) Building needs to be almost fully rebuilt but some walls are "historic" and need to be underpinned for lowering the basement.
2) Wall is holding up a couple of floors.
3) No, sorry.
4) How? What do you mean? If I excavate enough to support it then I might as well build the underpinning, right? I can't support it from above if that's what you mean.

 

[Greenalleycat]

I'm assuming that there is no seismic involved in this and that wind lateral loads aren't a meaningful concern - because I don't see how you can put lateral loads through the existing or proposed arrangement

If you're underpinning to create a basement then does your building code require you to bring this up to code?
You're creating a full new space - how can you make a basement work when you have bricks on 3" of concrete on random stones on underpinning?

Rubble foundations and common here - typical construction is ~2-3" concrete outside, a stack of bricks in the middle, then some big riverstones and nominal concrete on the inside
So not quite of the form of yours, but rubble nonetheless
Underpinning is also very common here as half our city is built on drained swamp and performed terribly historically and in the earthquakes...
Turns out it's very hard to underpin our type of rubble (let alone yours) - they have basically no spanning capacity so they're very difficult to lift
They commonly fracture into a pile of fragments and get rebuilt

Your underpinning also needs to be continuous - it's a classic to see shit engineers and shit contractors underpinning at 2m crs
The foundation then has high risk of differential performance, as typically the originally foundation is in poor soil (topsoil) and the underpinning goes to a competent layer
Sounds like that isn't so much of a risk with your situation though?

 

[PEinc]

Geoluk, underpinning should not be done in multiple lifts (i.e., underpinning the underpinning). Underpinning in multiple lifts compounds the possibility of problems and settlement ( 2 lifts, double risk, 3 lifts, triple risk, etc.). Underpinning should be performed using hand excavation, not a backhoe. If the pits are excavated too wide, you risk wall and/or foundation damage. At 5 feet deep, the excavated pits should be shored with 2x10 dimensional softwood lumber in lifts of about 4 feet or less. Check OSHA shoring requirements for excavations and confined space entry. There are plenty of threads on ET concerning underpinning of buildings and rubble stone wall. My web site has a presentation on building collapse and underpinning, including details. In addition to needing sufficient bearing, underpinning piers also need to be checked for sliding and overturning due to lateral soil loads and any existing or possible surcharges inside the building being underpinned.

http://www.PeirceEngineering.com

 

[LittleInch]

Geoluk,

What I meant was something like this?

https://www.brickbrace.com/product/prop-wise/

So you may need to install a temporary strong point or beam and essentially hold the wall up whilst the rubble wall is removed and replaced.

Just an idea.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Geoluk]

@Greenalleycat
Somehow it all works out. I just have to do the underpinning.

@PEinc
Thanks for your response. Somehow it slipped my mind that you'd double the settlement risk with multilevel underpinning. And I agree with all the other points you made.

@LittleInch
Wow! I have not see that before. Thanks a lot. I will look into it in detail but I think I might go with that. Very helpful.

 

[LittleInch]

They usually use it when they're knocking a new entrance or window or something like that to carry the weight of the structure above. for your case you would need to create something strong to sit the acrows on but it does allow a way to hold up the wall whilst you build something decent underneath.

bit scary to see your whole house resting on them whilst they insert the new beam but it works.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Geoluk]

My holes will be on the narrower side so I probably will only need one to make sure the bricks don't fall in.
Thanks.