Underpin A Crawl Space Wall For A Small Addition


Old 01-20-14, 05:44 PM
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Underpin A Crawl Space Wall For A Small Addition

I am doing a small addition on the back of my house, approx 13' x 13'. With this addition, I would like to get to a full 8' deep finished as the rest of my house sits on a block foundation over approx 3' - 4' of crawl space.

One wall (the wall with two windows) sits on a block foundation with about 4' of crawl space behind it. I will be squaring off the back of my garage with the back of my house (notice the survey I sketched quickly to get an idea of what I'm doing).

With about 1' of the foundation exposed and approx 3' of it currently buried, I will have to dig about 5' beneath the existing block to get to a finished 8'.

I understand the underpinning process of digging and pouring in sections. If I can avoid that and simply shore (build temp wall) up the house from the crawl space (floor joists run left to right not front to back) as it is only a 13' run, could I avoid the typical underpin process and simply dig, form and pour that wall all at once?

Any advice is greatly appreciated. I've spoken to a few architects all of whom have different opinions. I'm looking for a site guy who does work like this on a regular basis and looking for insight prior to hiring my excavator.
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Old 01-21-14, 12:14 PM
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Bridgeman might be the only one here to give you the go ahead on a temp wall, from the crawl space floor. It sounds too shaky to me. Wait for his opinion.
Old 01-21-14, 07:00 PM
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Thanx for the kind words, Pulpo, but unfortunately I'm not licensed to practice engineering in NY. Not to mention the difficulty in making informed site evaluations from 3000 miles away.

I don't think there's a "quick-and-easy" way to build the addition the way the OP suggested. Even if the dead load of the house above the stemwalls would be removed (by constructing temporary support walls in the adjacent crawl space areas), completely excavating with a vertical 5' cut immediately adjacent to the footings runs the risk of subgrade failure directly under them. Depending on soil type and moisture content, just the weight of the two walls and their footings bearing on the exposed, vertical edge could be enough to cause failure, by outward rotation. In my mind, a preferable alternative plan would be to first install a system of deep helical piers along both exterior walls, driving them to bearing well below the proposed basement excavation elevation, and then catching the lip of the existing footings at 3' or 4' intervals with the piers attachment bracket assembly. Such a method would eliminate the need for any temporary walls, as helical piers are designed to withstand complete house loads. The piers could be embedded in the basement's full-height, CIP concrete interior foundation walls.

Since the local building department will probably require a stamped set of project plans when the permit is applied for, my advice to the OP is to contact his local building department ASAP. They will be able to refer him to a local engineering firm familiar with soil types and foundations in his area, who can proceed with plan development while working with local subcontractor sources for the helical pier installation.
Old 01-21-14, 08:06 PM
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Bridgeman45, you presented it far more eloquently that I could have but my thoughts were very close to that. There is no way that I would take the chance of temp walls supporting that weight.
Old 01-22-14, 07:18 AM
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In addition to what Bridgeman45 has already said... In my area the inspectors will generally allow at most digging at a 45 degree angle out from the base of the footer. Anything closer to the footer or steeper side angles and they require Professional Engineering. Work around existing footers is really something that needs to be done properly and is not the best for a DIY project as the stakes can be quite high.

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