underpinning stone foundation

[sonnyson]

I am working on a 150 year old building with perimeter stone rubble foundation walls supporting wall and roof load and first floor framing over a crawl space. The owner wants to add a full basement. I am worried about the stone rubble foundation falling apart during underpinning procedure. I am thinking about using a chemical grout to strengthen and stabilize the soil below the existing foundation. Then concrete underpinning can be installed in alternating 4' panels down to new basement level. Once completed, I would pour a cantilever retaining wall on the exterior face ("L" footing) to carry lateral soil loads. Any thoughts on this? Does this seem feasible?

 

[PEinc]

Sounds expensive and redundant. Sounds like you need to talk to some who knows something about underpinning.

 

[PSlem]

Typically you excavate and pour a concrete slab under the footing, then dig the underpinning pit. The consequences of a rock falling on someone in the pit would be bad. Your chemically grouted soil would not address this problem as the first dig would remove it. This procedure is too expensive for residential unless it's a very high end neighborhood. A smaller basement with walls inside the influence line from the existing ones may be a better choice. It also allows a good waterproofing, something hard to do on existing rubble foundation.

 

[sonnyson]

How can you pour a slab under the wall without disturbing the stone rubble foundation? That is why I thought the chemical grout could be used to stabilize and support the wall in order to allow the undepinning to proceed

 

[Ginger]

Why not look at grout-stabilising the stone wall itself at foundation level? By doing so you will have created a structure that is capable of spanning across a 4' gap while you carry out the underpinning work. The height of the arch of the grouted masonry will be approx 0.6 x the span of the opening you make so you would need to make sure that your wall can arch across this distance.

Grouting would be specialist contractor work.

I would consider constructing the walls of the basement inside the line of the existing rubble walls as suggested by PSlem. Could use a secant pile approach to the walls using a low-headroom drill rig.

 

[PEinc]

Your problem is a common one for buildings with rubble stone foundations. You need an experienced underpinning contractor.

Your bigger problem is that most experienced underpinning contractors are not interested in working on homes.

 

[DRC1]

I would suggest shoring the framing in the crawl space, remove the stone, cast a grade beam in its place and then remove shoring and underpin conventinally. Then should be significantly cheaper and more reliable than chemical grouting. If your underpinning is properly designed you can use it for the strucural wall of your basement.