Foundation Replacement 1

[Wasib Jamil]

Hello All.
I am working on a project where client wants to lower the basement. Since, the existing foundation is in shambles they want to replace the whole foundation with a new one. I am trying to calculate the length of segments in which foundation needs to be replaced (sequencing) and was hoping if I can get a general direction to do the calcs for it (sketch attached).I will be grateful if can be nudged in the right direction.

Thanks

 

[XR250]

Hire a house mover to support the structure and replace it all at once.

 

[Wasib Jamil]

I just realized that I posted the wrong sketch. My apologies. Please find the right one attached:

 

[phamENG]

What XR said. I've been involved in several of these, though the goal of those was to elevate the house out of the flood plane so lifting it was the only option. You could do it in segments but it's not going to save any money. Everyone will have to mobilize and de-mobilize at each segment, making trade coordination a very costly headache. Shoring is likely to be much less robust and more informal, which could lead to problems. You'll also have durability issues at joints, extra work making those joints work.

 

[jerseyshore]

Agreed above x2. I do this all the time around here and lifting the house and building a new foundation below is the way to go. Trying to dig out in stages for a full basement can get dangerous quickly and doesn't pay off.

 

[Eng16080]

I imagine you would want each segment as long as possible so the foundation can be completed in as few steps steps as possible (less mobilizing and demobilizing, as noted above).

I've had projects where we discussed replacing the foundation in this manner, but as everyone else has already said, we almost always ended up temporarily supporting/lifting the upper structure, demolishing the foundation, pouring a new foundation, and re-connecting the upper structure. I bet your foundation cost is 3 or 4 times more with the segmented approach, and I wouldn't be surprised if you couldn't even find a contractor willing to do it. If you're set on this approach, I would talk to some contractors first before getting too far into the analysis.

 

[milkshakelake]

I think what they mean above is to use helical piles at something like 4'-0" on center to make the house "floating" and build a new foundation wall/footing under it.

What OP described sounds more like underpinning. I usually use 4'-0" long underpinning pits. You will need lateral support (something like diagonal wood or steel rakers, and probably walers) for the lateral support. Calculating the waler/raker loads is just taking lateral earth pressure + surcharge and applying as axial loads on rakers.

1. The reason the underpinning pits are 4'-0" long is based on industry experience. For a real calculation, since the bearing wall is CMU, you can calculate the CMU wall to span 4'-0" over air and be supported on both sides, most likely simply supported. Masonry does have arching action which allows this to happen.

2. The lateral support is for earth pressure and incidental loads like wind, seismic, and vibrations that could destabilize the structure.

3. If you're removing the wall in 4'-0" sections, the quality of concrete work, formwork, and rebar splices will be extremely low. Very few contractors can do it properly to make an entire foundation wall and footings. The floating idea sounds better because they won't have to do it in sections.

4. If you were thinking of something like 8'-0" or 12'-0" sections, I think it'll be quite dangerous because CMU can't be unsupported for that span.

5. If you don't want to use helical piles or there is no access for them, something like needle beams with columns might work. It just might cost way too much to be feasible, but it's an older method of underpinning. I've never actually seen it used in modern day. Underpinning is more common in that case.

6. Make sure there is a good bond between the upper structure and the new foundations. I think some people use non-shrink grout, but I prefer to pack it with repair mortar. You might get some pushback. Also consider hammering in steel wedges every 2'-0" or 3'-0" to ensure that the new foundations are really taking the required loading.

7. If the building gets supported on helical piles, their expected settlement is quite low. In this case, your footing at the bottom will not be doing much. So you might have some leeway in designing those footings. I still wouldn't just go footing-less, just in case.

8. With helical piles, some do use a combination of end bearing on the helices and also skin friction. So make sure that it's actually designed for the skin friction it will be receiving, i.e. starting below your new foundations.

9. I might be way off about the helical pile thing. Maybe the ones above are talking about a different system.

 

[phamENG]

milkshakelake - no helicals. House lifter knocks big holes in the foundation wall (or digs pits and tunnels under the slab) and slides big steel beams through. Sets up cribbing using 8"x8" wood timbers, and attaches hydraulic jacks with hoses leading back to a big, central and synchronized manifold. Then nice and slowly they raise the house, adding more cribbing and resetting the jacks every 8 or 9 inches. It's a slow and laborious process, but it gets the house up in the air.

Here's a good example: [URL unfurl="true"]https://www.youtube.com/watch?v=VrYmBx9Upmc[/url]

 

[milkshakelake]

@phamENG That video is really cool. Anyway, I stand corrected. I agree that it's the right approach instead of something like helical piles, so OP can skip that wall of text. Though the point about load transfer and shims could still be useful.

 

[phamENG]

One of the GCs I worked with had in house carpenters that would handle the shimming. They'd build the foundation wall up nice and level, and they'd take measurements every 6" or so along the foundation wall. They'd put one full thickness sill down and then cut a sill to match the profile of the old house. It was painstaking. But they got it nearly perfect every time. They'd come back and fix minor inconsistencies with treated shims. The trick is to get the bearing as near to exactly the same shape as it was before or all the wood that has crept into that shape will be forced to creep into a new shape, causing buckled flooring and cracked walls.