Retaining wall on expansive soil

[vato]

I got a soils report, after the foundation was designed for 2000 psf soil bearing and footing forms in place, and we have "slightly" expansive soils.
Now I am redesigning for 800psf min dead load and 3000psf maximum soil bearing. So far I have been able to use 6 square foot pads
spaced 8'-0" o.c. with a 4' deep grade beam. Most of the forms on the ground can be reused. Frost depth is 48" here with 100psf snow load, 10,000 ft elev.
The building is a one story wood framed structure. Not much dead load to work with.

Here is where I am scratching my head....
The east side of the building foundation was originally designed as a 5' tall retaining wall with 4' wide footer.
I can remove portions of the footer, similar to the rest of the foundation, and reinforce the wall as a grade beam between pads.
This will ensure the 800psf dead load but I am concerned about still retaining the soil with the voids required between footings using this approach.
Feels like I'm removing the concrete at the point of highest lateral pressure if I do this.

I am leaning toward voiding portions of the retaining wall footing, leaving an edge/soil contact on each side of the footer with a contact strip under the wall as well. My retaining wall would still work as a cantilever and allow some expansion beneath it, kind of like a waffle slab. Seems like someone has done this by now. It's not imperative that I reuse the forms on the ground, but I should be able to work it out.

Thanks in advance for your thoughts on this.
ps the foundation will have a perimeter drain as well as a drainage plane below the floating slab, and we will be spending more time and money on the geotech, before the forms are set next time.

 

[TXStructural]

I guess I'm missing something. Why would you not leave the wall intact if it is capable of supporting the structure. Cutting away the footing will NOT increase the load capacity, and doing so will destabilize the wall.

 

[vato]

I'm trying to increase the load to the ground. I need to provide 800psf of dead load where the footing contacts the soil to counteract the expansive soil swelling potential. Since the structure is light, the dead load needs to come mainly from the footing/fndn wall. Right now I'm beefing up the retaining wall and footer thickness to reach the 800psf dead load minimum.
Should I apply this minimum to the heel pressure result for the retaining wall and keep the heel at 800psf?

 

[hokie66]

I think you are probably overthinking this. With slightly expansive soils requiring only 800 psf to resist the expansion, you are dealing with guesses. I would design the superstructure with enough articulation to accommodate the movement, and leave it at that.

 

[vato]

Thanks hokie66, I specialize in "overthinking", no sarcasm intended, I'm good at it and I sincerely appreciate your observation on that.
What I have found is that soil pressures, hydrostatic loads and dead loads are all in chapter 3 of asce7.
Seems like I do need to keep my heel pressure above 800psf. I have moved away from a waffle slab type footing, but I can still
see where that might be a solution option. 800psf seems low, but this is a one story wood framed structure so extra dead load is not easy to
come by.

"I'd be a contractor if I didn't overthink." There's some sarcasm.

 

[emmgjld]

I have designed a number of wide footings for basement walls and true free-standing retaining walls on expansive clays. In all situations, I have placed form-void material beneath the footing, having the proportional amounts in the toe & heel area. For your low expansive pressure of 800 psf, I would apply a minimum S.F of 1.5, designing for a dead load of 1200 psf, AT A MINIMUM.

 

[eRok]

vato, I live in the midwest, where we have some pretty expansive soils sometimes, and I've never heard of anyone trying to maintain a minimum footing load to try to counteract heave due to soil expansion (maybe I'm too green?). There is a book by Dr. Richard Handy called "The Day the House Fell" that addresses different soil problems with homes and some ways to address them, with some special attention given to expansive clays. In every instance of expansive clays I've been exposed to, the first solution is always to mitigate changes in soil moisture content by using good drainage techniques (clean fill under the footing, drain tile to sump, clean backfill, soils that slope away from the structure and good guttering practices); if the soil moisture stays relatively constant, it doesn't matter how expansive the soils are. If that is not an acceptable solution, the next step we would take is usually some kind of piering system.

However I certainly don't know everything and would be happy to be corrected - love learning new stuff .