Bracing Options - Double Underpinning

[ahypek]

Hello,

I'm dealing with an unfamiliar problem here and wondering if anyone has any experience with this:

I have designed an existing structure to support 2 additional stories and a new exterior wall.

My structure and the adjacent structure are on top of typical 18"-20" NYC rubble walls (independent) with a 2 inch gap between them. The property line sits directly in the middle of the gap (1" from each face).

The bottom of my excavation is nearly -17', and the existing walls are both -7'-4".

I am trying to figure out an efficient way to brace the newly poured adjacent underpin to my building's pin in order to prevent stability issues. I have already designed my shoring system but now I need to transfer that support to the wall behind it.

My pins will be poured in two separate stages.

Maybe I'm overthinking this one but I'd love to hear some people who have dealt with this.

 

[bookowski]

Not my area of expertise but since no one else is biting:
- A sketch would help to clarify your question
- If I understand you are: doing about +/-10ft of vertical underpinning, need to first underpin the neighboring rubble wall and then your wall. Pretty serious stuff potentially.

in nyc:
- For double lift pins I typically see soil anchors or rakers for lateral stability, and vertical reinforcement through the joint. For single lift usually nothing is provided (< ~6ft)
- You would want to maintain a bond break between the two properties. Underpinning + adding stories, you want to minimize dragging them down with your settlement
- Are you adding new basement walls in your building? I assume so and if so then you just need temporary bracing of the pins and the permanent wall will then brace. If not then you are brave - rubble walls + 2 story of underpinning is not very stable
- If you are adding 2 stories I assume this triggered the seismic upgrade per nyc requirements?

You might want to enlist a geotech/soe engineer, this can be scary stuff and rubble walls are sometimes very unstable.

The nyc dob has some good presentations on underpinning failure and some basic examples of calcs for lateral stability. Check out their presentations for some good photos of failures. Here's an example from dob website attached.

 

[ahypek]

I'll try to get a sketch later in the day when I can take a seat but hopefully I can explain it in more detail.

My new top of slab will be less than 3' below the bottom of the rubble wall. (-10'-0") so most of the pins are simple. My issue is at the elevator bay right next to the adjacent building, where my B.O.F. will be at -16'-8".

This area of underpinning will only be a total width of 12' wide along the wall and I'm stepping them down an additional ~8' on each side in ~2' increments.

I have designed a a raker/waler system for this area and as you probably know, soil anchors here are practically never permitted.

Yes, our building will be self-underpinned with our own basement walls.

My primary concern here is means and methods of how I'm going to brace the neighbor's pin past the first 4' stage of excavation. My wall is supported with anchors and rakers but the adjacent building's pin is not and in agreement with you, I can not/do not want to connect them to each other.