Connecting Slab to Top of Tall Concrete Stem Wall 3

[PEVT]

I am designing a foundation in a flood hazard area for which FEMA recommends back-filled stem walls as a resilient type of system for this application. See attached sketch from FEMA Technical Bulletin 1. I have planned a 7+ft tall reinforced concrete wall capped with a slab as shown on the attached sketch. I would like to tie into the top of the wall to create an efficient structure. With this detail I can calculate the load at the top of the wall and carry it through the slab to the opposite wall with reinforcement in the slab. Thus avoiding the need for a large retaining wall type footing. Then I consult ACI 360 while designing the slab and it seems to go on and on about disconnecting the slab from the walls due to shrinkage, cracking, warping, curling, etc. The interior will be filled with a well compacted structural fill or crushed rock product, for which I can specify temporary fill on the outside of the wall to brace it prior to slab installation. The site soil is very sound sand and gravel, relatively free of any clay. The interior of the slab is relatively lightly loaded as an assembly space.
My questions are:
1) Is it reasonable to pursue the connection of the slab to the top of the wall?
2) Should I be particularly concerned with the long term deflection in the center of the 34ft slab span given the depth of the interior fill?

 

[JAE]

Your "cap" slab can be tied into the surrounding walls. ACI 360 is essentially dealing with large interior floor slabs and their performance as a non-structural surface. If you tie in the slab to the walls, you do risk having issues with settlements of the underlying fill. This could be dealt with by tight specifications on backfill/compaction as well as reinforcing the slab to bridge over areas loosing support below. This would especially be prevalent directly adjacent to the walls where the slab transitions from being supported on the wall to on the backfill.

 

[Teguci]

Because you are transferring structural loads through the slab, the slab must be designed to the requirements of ACI 318. Otherwise, this design is pretty typical. So in response to your questions:
1) Yes, this connection is typical and is commonly referred to as a "Basement" type wall where the connection at the top of the wall resists the overturning. This is contrasted with a "Cantlevered" wall where all the moment is resisted by the footing. The geotech should give you a higher lateral soil pressure to be resisted because a "basement" type wall doesn't allow the backfill to settle onto its friction angle. I also recommend that you provide a "Recommended Sequence of Construction" for the contractor to follow. This should include bracing, backfilling and curing issues (ex. place soil in front and back of wall equally to lower elevation, brace wall with "", Cast slab and cure, remove brace).
2) I don't think so. Deflection down (settlement) in the middle of your slab would pull the top of the wall into the retained soil even more in a catenary action (make sure your top bar is developed properly, ldh, into the wall). Of course if your slab deflected that much down, I'd be very upset with you.

 

[hokie66]

It is wise to design the slab with a thicker section, reinforced in the bottom, adjacent to the wall. This allows the slab to span across an area which settles with time.