Oddball Underpinning Condition 7

[KootK]

Attached, and shown below, is an atypical underpinning situation that I need to sort out for a one story light frame wood building. Some questions that I have include:

1) Can the system that I've shown be installed incrementally as regular underpinning would be?

2) Is there any way to hide the tie back anchors within the thickness of the wall somehow?

3) Is there some better way to do this? Maybe an L-shaped retaining wall installed as underpinning?

4) Should I do anything around the corners to address potential soil sloughing etc?

It's worth noting that the existing building will be demolished to the ground floor level before being rebuilt. Rebuilding the wall altogether is an option, albeit not preferred by the client.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RFreund]

#2-> If you are doing this in 4' segments, I don't see what the need for the lateral brace is, but I may be missing something.
#3 -> I suppose it could, PE Inc will know.
#4 -> Usually underpins are unreinforced however in this case, if you are using the vertical well to resist lateral forces then you most likely need the vertical bars. You could probably omit the horizontal bars.

I'd be inclined to extend the underpin underneath the existing slab until sliding/overturning is eliminated. Then you don't need the vertical wall. Typically you would be worried about undermining the existing slab, but in this case, who cares? It's going to be demo'd if I understand correctly.



EIT

http://www.HowToEngineer.com

 

[PEinc]

I don't see the need for the temporary brace. However, a brace or some other type of lateral support could be needed for the final condition. Reread my original response.

http://www.PeirceEngineering.com

 

[KootK]

Sorry it took me a while to get back to this. Thanks again for all the help. This is great.

I have to confess that, in the past, I haven't given any explicit consideration to the lateral stability of underpinning. My bad. The sketch below shows my understanding of the statics of the situation based on the posts above. Some additional questions:

1) Are we sure about this cohesion business? It's great but a little scary.

2) Is cohesion something that can be calculated from other soil properties? Or conservatively estimated? Or do I need to seek geotechnical help?

3) FOS the same as for retaining walls? 1.5 on OT and 2.0 on Sliding?

Several folks have inquired as to why I think that I need lateral bracing in the temporary condition. My plan was to do the underpinning in 4' segments but pour the upper wall element as one continuous pour after all of the underpinning is done. I thought that would be more construction friendly. However, it would yield a condition where all of the underpinning would be in place but none of the upper wall would be. Working under the perhaps incorrect assumption that the underpinning would not be stable on its own, the "hinge" between the underpinning and the existing footing would require bracing. If I go that route, I'll query the contractor as to whether or not they'd prefer to do the upper wall in 4' segments and forgo the lateral bracing.

Although since you are demo'ing most of the building you may not need much pre-loading so might be possible to count on the head to to fill in all voids and full bearing.

I'm not sure that I follow. What is pre-loading in this context and how does it make it easier to count on the head to fill in the voids?

I always have the contractor drive rebar into the soil each side. You are not going to get a full Codified lap splice, but you get continuity.

So this is horizontal rebar dowels driven into the dirt on each side of a segment that ultimately becomes a pseudo-lap for the horizontal reinforcing in adjacent segments? Clever.

I don't see the need for the temporary brace. However, a brace or some other type of lateral support could be needed for the final condition. Reread my original response.

Trust me, I read it carefully. I know that you have considerable expertise in this arena. I've been unable to get the unbranded statics to work so far. However, it's getting closer with each iteration!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RFreund]

Soils information does help, but it's seemingly difficult to get for residential jobs. You are correct to have caution when using cohesion. I would neglect it. Typically around my area and in general a typical soil design assumption is phi = 28 degrees, cohesion = 0psf and soil unit weight = 120 pcf. This usually yields a Ka of 0.35 or an EFP of 42-45 psf/ft for a flat backslope. Now I'm not saying this is completely correct, but I see it often and is typically conservative (sometimes overly).

You're forces/statics are on the right track. Solve for moments about the toe, check sliding and bearing pressure (I see bearing pressure get neglected often for some reason). Use dead load from above, dead load of soil on top of your concrete mass and your concrete mass as resisting vs your lateral pressure.

As for factor of safety, typically I see 1.5 for OT and sliding.

Place underpin in 4' segment. Pour to within 3" of existing. Set overnight. Drypack void, tamp into place.



EIT

http://www.HowToEngineer.com

 

[PEinc]

Try belling out the front of the underpinning piers to extend under the proposed floor slab. This will help overturning but not sliding (which I suspect is your main problem).
The attached photo is exactly what you are trying to do except that intermittent underpinning piers were used instead of continuous piers. The other difference may be that my building is heavier than your building (see my original response). Going deeper with your underpinning probably will not help because the sliding will get worse.
Try using Coulomb's active earth pressure coefficient instead of Rankine. Use reasonable soil properties, not overly conservative values. High unit weight usually means higher phi angle for granular soils. Therefore a high unit weight with a low phi angle may be too conservative for a granular soil. While I also usually ignore cohesion, 50 psf is next to nothing. If the soil has cohesion, perhaps more that 50 psf is reasonable?

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