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Underpinning and increment connections

Underpinning and increment connections

Underpinning and increment connections

I have never done this before so it may sound pretty basic. When we do underpinning, how do we want to connect the rebar of each increment? Specifically, we are looking at giving more depth to the walls of a rowhouse so the basements extends to 7.5ft from the existing 5ft depth. We consider doing that in increments of 4ft, with 1ft thickness and the depth of 2.5 ft. I am not sure how the rebar from each segment of concrete shall connect to the next segment. We consider having rebar extend its development length into the next segment. Should that be the appropriate way to connect the segments?

RE: Underpinning and increment connections

If you need the rebar to act as a single, continuous piece of tension reinforcement across a joint, you need to provide the appropriate splice length of the bar or use a mechanical bar splice. If you are anchoring (terminating) the bar, you need to provide the development length of the bar.

RE: Underpinning and increment connections

If these are older row houses, I'd suspect there is no re-bar. Otherwise you drive the bar from the excavated to the non-excavated zone and later dig around that bar and lap or secure to the next bar as with cable clamps..

RE: Underpinning and increment connections

I agree with oldestguy. Drive rebar into unexcavated segments. We would typically use #6 rebar x 4'-0" long and lap 24" (2 top and bottom). I suggest making the underpinning 24" to 30" thick minimum with no footing.

RE: Underpinning and increment connections

While I agree with the others regarding extending rebar into the adjacent soil and then digging around and lap splicing.... I question whether or not any horizontal reinforcement is needed (i.e What job is it doing?). Not saying it is not needed, just trying to understand what you are attempting to accomplish.
My understanding of the detail is that the new concrete is placed underneath the existing 5 ft tall basement walls.
I usually find that vertical bending is the way that such a foundation wall will behave. Size and placement of the underpin is critical to dealing with bending behavior whatever direction it is occurring in. Arguably there is a significant horizontal reaction at the base of the existing 5 ft wall (and therefore the top of the new underpins) and so horizontal bending would need to be addressed. I would not expect that a 2 ft lap of a #6 bar would be adequate but much would depend on the size and positioning of the underpins, composition and thickness of the existing walls etc..

RE: Underpinning and increment connections

I suppose we need to know if this is atypical row house, with one "house" after another with common dividing walls. In such a case, no reinforcing is needed and a wider than the upper wall would affect usage. Only at front and back would one have earth to retain. Even then, I'd opt for horizontal reinforcing, since the floor provides little restraint at the top, with joists spanning the short dimension. One added item. Maybe think about casting the sections with a few inches clearance. Follow a few days later with a low moisture mix vigorously tamped in to pre-load the section a little.

RE: Underpinning and increment connections

It is a typical row house. I was considering that I could assess these additions/extensions that will deepen the basement as footings under the load-bearing walls. What concerns me more actually is the fact that these segments are not going to be connected and settlement will be different on each 4' segment, causing issues to the walls above.
For the front, I was considering using some vertical rebars and make the segments thicker, but that was just a thought out of the box.

RE: Underpinning and increment connections

I have had some of these jobs and no settlement issues have come to my attention, but that is only my view. I suggest that you also post the question in the geotech room for "foundations". The "dry"-packing does a lot to make things more secure in my view. One item I've never called for that may help on preventing settlement. Tamping the grade at the bottom of the excavation before placing concrete is another step you may wish to do providing the vibration does not affect nearby footings in some sandy sites. I've seen sand flow out from under footings when nearby areas are vibrated for compaction.

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