Underpinning a rowhouse - need to underpin at neighbor front & rear wall? 2

[Ben29]

The title says it all. I have a rowhouse that will get deep underpinning - see plan and detail below. We are going from a crawl space to a full basement.

Logic tells me that I should underpin the neighbor's front and rear wall (highlighted yellow in plan). Perhaps this can be done with helical piles? I am not sure if I am allowed to show that on permit drawings without getting special permission from the neighbor. PS, I am obviously not finished with the sequencing of the plan, so no need to point that out.

 

[kipfoot]

EDIT: I accept the correction about whether the neighbor's prior agreement is necessary. It seems that (in the US) the property easement related to the party wall means that each owner has the right of use of the entire wall.

I'd say yes. because I don't like supporting adjacent foundations at different levels

Also, I'd expect that you need the neighbor's agreement to do any work you show along the shared foundation wall. It looks like what you've shown is over the property line and would be of interest to the neighbor, who, it seems, shares ownership of the foundation wall.

The contractor for this work would want to have an agreement in place to repair cracks after this work is done, as well. What happens when the excavation isn't as vertical as you've drawn it and the soil falls out from the adjacent crawlspace? I don't see any of this happening going smoothly without the neighbor's consent.

 

[PEinc]

If the wall truly is a party wall, where the property line is in the center of the wall, both parties own the wall equally and you usually do not need the neighbor's permission to underpin the wall. However, if you place any underpinning beyond the neighbor's inside face of the party wall, you would need permission.

As I mentioned in another of your underpinning threads, your underpinning details are atypical, expensive, and difficult to build. I have never seen a book on underpinning that has the details you show. The details you show are common inventions of structural engineers who knows little about how underpinning is designed and constructed (sorry). Underpinning pits very rarely have reinforcing steel and are supposed to be excavated in hand-dig pits that are shored on all four sides with horizontal lagging boards as the excavation progresses in vertical cuts of about 3 to 4 feet. Doing so provides safety for the man in the pit and prevents (or at least minimizes) building settlement caused by loss of ground adjacent to the pit and inside the neighbor's crawlspace. In addition, there would be an OSHA violation if a worker is in an unshored pit excavation that is over 5 feet deep. Look at my web site for a presentation titled Philadelphia’s Recent Building Collapses – Causes and Prevention[/

EDIT: Rarely are perpendicular walls underpinned (i.e., the neighbor's front and rear walls). If you did underpin them, it would need to be done totally on their property or from inside the neighbor's basement and good luck with that.

http://www.PeirceEngineering.com

 

[Enable]

There is no need to underpin the perpendicular walls. You are not imparting any load in their area of influence. Rather, you need to design your new underpins for the surcharge load of the perpendicular walls.

PEinc: I'll admit the configuration is a tad strange but Ben29 is absolutely correct in designing the underpin (or what we are calling one) with reinforcing steel. The depth is simply too great and lateral loads too high to go with plain concrete. Your safety concerns are valid and Ben29 will have to figure out a way to stage the process so that stability and safety can be maintained. Depending on the soil conditions benching might be the best approach.

I'm unsure if doweling into the existing is a good idea. If the wall deflects and induces movement in the existing wall at the dowelled connection you might observe cracking throughout the main floor drywall. I would consider designing the lower portion as a very stiff cantilever to minimize deflection and maintain a grout connection only.

EDIT - Agree with kipfoot in that there may be some damage to the adjoining property. You will want to instruct the contractor to take precondition photos of the neighbor's interior. If you don't then they can claim all manner of cracks were the fault of your project and you'd have no idea if it was BS or legit; even if you suspected BS you couldn't prove it either.

 

[milkshakelake]

Where I practice (NYC), I've done hundreds of projects and never seen anyone underpin a perpendicular wall. The logistics of it is crazy. Here, you need a legal agreement with the neighbor to underpin the wall, even if it's a party wall.

I'm not experienced with this deep underpinning. I've heard arguments on both sides about whether to break it up into two tiers of underpinning or not. I generally do two tiers and use rakers to support the first tier. If you do such deep underpinning, it's worth calculating the support for lateral pressure.

 

[PEinc]

Enable, the reinforcing steel is needed in Ben29's "underpinning" because the concrete is too thin. The underpinning piers should be approximately 3' thick, not 1' thick. How does anyone expect to build, in short width sections, a footing and then a thin wall stem? The many excavated pits would need to be shored and there would need to be hundreds of rebar splicers linking the piers. There is a sliding problem (a greater problem than the overturning problem) but the sliding problem would be reduced if Ben29's "underpinning" did not need such a wide and deep footing. There really is no good way to do what they want to do for this project without thicker piers that encroach into either the neighbor's space or into the proposed, deepened basement.

milkshakelake, stacked underpinning is not recommended (although I have seen it detailed many times). In fact, I believe NYC does not allow it. Also, nobody would want permanent raker braces projecting into their new basement. The floor joists will not properly brace the underpinning and dowelling into the bottom of a brick wall could be problematic, especially if the brick wall is old and without a footing.

The logical, but undesirable, solution to this problem is to use thicker, unreinforced, concrete piers that project into the new basement and which would, unfortunately, reduce the floor area in the new basement. The rear face of the underpinning would/could/should extend toward the neighbor's property only to the far edge of the existing wall or wall footing, if there is a footing. Brick walls generally have a concrete footing, especially when the brick wall is only 12" wide, if as detailed.

http://www.PeirceEngineering.com