New Retaining Wall 1

[RM87]

Hello all,

I was asked sometime ago to design a new retaining wall on one side of a garage, and the other side would keep the existing concrete wall. During demolition, the contractor discovered that the existing 12" concrete wall that was intended to remain actually had 6" of unreinforced glory to it. The solution the owner and the contractor decided was to place a new wall in front of the existing wall to avoid the necessity of temporary shoring plans, etc.

Question is: The new retaining walls will now have two separately poured slabs, both with their independent keys to combat sliding. They will be abutting one another.
Should I dowel one retaining wall into the other?

If so, how deep would you suggest I dowel the rebar? A tension splice would seem to call out for something like 26" but that seems unnecessary (#5 bars @ 12" O.C. top and bottom at the toe of both retaining walls, if my memory serves me...)​

Should I provide a joint instead?
Is there a specific calculation that comes to mind that you would recommend I consider?

Additional notes to consider: There's an 8" concrete slab specified to sit above the garage - the connection between the stem of the retaining walls and the slabs will have some fixity. Plus, there will be a significant amount of concrete dead load applied to those stems as well, which have been accounted for, but as an FYI. The water in the soil is a matter of high concern.

Thanks,
Rod

 

[steellion]

Are you trying to create a composite section out of the new and existing wall? You could simply and conservatively design the new wall to take all the soil load, and use the old wall as formwork (and it provides a good surface to place a waterproofing membrane if you need it. I see no need to dowel unless you want to rely on it for strength.

 

[RM87]

@steellion, The latter is exactly what I've been proposing. I'm just wondering if there's any need for doweling? And if so, what? The numbers seem to be acceptable without them, but I'm wondering if I'm being oblivious to something significant.

 

[KootK]

Could you post a cross section RMassoudi? I don't see a need for dowels but I'm having difficulty visualizing the situation. If the new retaining wall footings oppose one another, perhaps putting them in contact such that compression could be transferred would eliminate the need for the keys.

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.

 

[RM87]

@ Kootk, That's exactly what's going on - two keys against one another where the compression would stave off any sliding. The only reason I'm calling out for a key is to in part, satisfy the building inspector, and to beef up their independent sliding requirements. I was wondering if there's any need to dowel between the two retaining walls? I'm not entirely convinced it's needed, but it might help mitigate some differential settlement/deflection in the retaining walls through the shear in the dowels... That was my thinking.

 

[KootK]

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Is this six inch wall or it's footing sandwiched between the two retaining wall slabs that you mean to dowel together? Would you be doweling through those elements. Are the structures on either side contiguous occupancy's such that it would be important to minimize differential vertical settlement?

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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.

 

[RM87]

Hahaha! Okay. I'll upload something visually shameful - my apologies.

And the surrounding structures are strictly residential.

 

[KootK]

Why Rodney, how thoughtful of you to provide a sketch! That really wasn't necessary...

1) If you keep both keys, I would add some dowels to limit differential vertical settlement. I'm not sure how one would calculate the demand on those dowels without getting ridiculously fancy. I'd just throw in two rows of 15M@300 along the 18" face of the new slabs.

2) If the "original retaining wall" currently exists in the field I'd definitely forgo the key on the new retaining wall footing. Same dowel requirements as #1.

3) If neither retaining wall has been constructed, I would consider switching to just old fashioned, top restrained basement walls rather than cantilevered retaining walls. No keys, no 18" footing, and much less rebar. Just a slab on grade restraining the bottoms of the two walls laterally. Who knows though, maybe you've chosen the path that you have because you're tanking the structure as a result of the water issues that you mentioned above.

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.

 

[RM87]

15M@300? Could you explain that shorthand?

Thanks for the other recommendations.

 

[KootK]

#5 @ 12".

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.