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Underpinning

Underpinning

(OP)
Hi,
In typical underpinning detail where new pins are installed, say 2' wide, in sequenced construction lowering the existing basement by approximately 4'. How the lateral pressure acting on exisitng footing is transfered to new pins if no dowels are installed only shims and grout. Does code require to provide the dowels or lateral pressure from soil is transfered through friction only.

Do you always specify new basement wall to brace underpinning and existing fdn wall?

RE: Underpinning

I haven't designed underpinning, but I've been involved with it. For underpinning along shallow strip footings supporting basement walls, I would probably add dowels. For underpinning under shall footings near grade, I probably wouldn't. Check sliding since the underpinning will attract additional lateral loads.

I would assume the underpinning blocks must be doweled to the existing footing if under a basement wall. If you don't, then you are probably relying on the friction between the underpinning block and the footing to provide lateral resistance. At the depth of a basement wall, the lateral earth pressures be relatively high. If you are relying on the dead load of the structure to provide friction to resist lateral earth pressures, then you better take a nice factor of safety for sliding (2-3?) between the underpinning and footing and be thorough estimating the self-weight of the structure.

RE: Underpinning

What type of wall are you underpinning? Dowels vertically into an old brick wall or an unreinforced CMU wall would be unhelpful, at the best. Can you give a detail of the existing condition?

Around here, the soil is a stiff clay that you can cut vertically and it likely won't move for years. But, if it does, lookout. I've resupported many a basement wall and the vertical soil cut by building a mini cantilevered retaining wall of filled CMU on a new concrete footing and backfilled between the soil cut and the CMU with stone. Seems to work just fine. Of course, that means losing some of the floor area in the basement, but it's better than the wall collapsing.

If there's no real lateral load on the existing wall, then the standard underpinning detail with grout only connection seems acceptable.

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