Stepping Down a Foundation

[TRAK.Structural]

I've got a residential design project with a sloping piece of property that drops a couple feet from front to back. Contractor wants a turndown edge slab foundation, which is fine, but I'm wondering how to this gets built where the grade drops. Making the bottom of the concrete step down as required is straight forward enough but it feels awkward and potentially difficult to construct what would be a 3 to 4 foot deep turndown at the low point in the rear of the home. Should I transition to a strip type footing with some type of stem wall when the grade drops to more than say 1 foot below the FFE? What kind of detailing would you use in this scenario?

 

[Tomfh]

We typically switch to reinforced blockwork on a strip footing at around 1000 mm (3–4 feet), though it often comes down to contractor preference. If it's just a small zone of 3-4 foot I'd think they'd just prefer to form and pour it, rather than build a small retaining wall first. But both ways work.

 

[milkshakelake]

I think you need a small retaining wall, possibly as part of the slab. It would need at least some kind of haunch footing. I don't see a way around it. You can try to engage the upper slab for some overturning restraint, but I think that's more trouble than it's worth; you'd be getting into development lengths, concrete-soil coefficient of friction, zone of exclusion, and annoying things like that.

 

[sdz]

I would agree with Tomfh. Put in a footing, then reinforced blockwork to slab level, then backfill to underside of slab.
Otherwise you need to consider the slab support both short and long term.

 

[XR250]

2 ft. is usually my limit on turned down slabs.

 

[TRAK.Structural]

We typically switch to reinforced blockwork on a strip footing at around 1000 mm (3–4 feet), though it often comes down to contractor preference. If it's just a small zone of 3-4 foot I'd think they'd just prefer to form and pour it, rather than build a small retaining wall first. But both ways work.

Are you saying you would use a combination of the systems and transition from a turndown slab to a retaining wall, or just use a block wall on the whole foundation with varying heights as needed?

I would agree with Tomfh. Put in a footing, then reinforced blockwork to slab level, then backfill to underside of slab.
Otherwise you need to consider the slab support both short and long term.

In this scenario, pouring the slab over the top of the wall and providing some bars from the slab to the wall at regular intervals makes the retaining part easy, but I am on the fence about rebar for residential work in my area. A floating slab that stops at the inside face of the block wall eliminates this but means a larger concrete footing for the wall. What's your preference?

Maybe I need to just push for a crawl space, or grading to make a flat area for a slab foundation.

 

[Tomfh]

Are you saying you would use a combination of the systems and transition from a turndown slab to a retaining wall, or just use a block wall on the whole foundation with varying heights as needed?

I meant a transition from downturn slab where there is no real soil to retain, to a retaining wall, at a certain point.

A floating slab that stops at the inside face of the block wall eliminates this but means a larger concrete footing for the wall. What's your preference?

I was meaning the slab sits on the wall. A reinforced concrete block wall, with reinforced footing. Slab tied to the wall, such that the block wall and footing functions the same as an integral concrete downturn beam. I wouldn't want the slab floating on soil inside the wall.

We also design the last few meters of slab to be spanning across to the wall, as you don't tend to get good sub-base behind the wall.

 

[KootK]

Maybe I need to just push for a crawl space, or grading to make a flat area for a slab foundation.

That's where my head goes right out of the gate. I feel that would be a pretty natural solution in many markets.

...construct what would be a 3 to 4 foot deep turndown at the low point in the rear of the home.

To be clear, this means that, where the turn down would be large, the grading under the SOG will be built up in the interior, right?

A tall turndown unsettles me a bit, as it does others, but playing Devil's advocate, what would be the problem with it in the sketch below? Just gobs of concrete? The likely SOG crack location moving around the plan? More rebar to get to that crack location?

Perhaps this is one of those things where, once the contractor understands the cost implications of a version of the turndown that would work for you, they would wind up preferring something else.

 

[milkshakelake]

@KootK Would that need restraint at the bottom to avoid sliding along the soil slope when it's loaded? I'd say make it 5' to at least have a passive-pressure-soil-pin at the bottom. Anyway I did a quick calc, that's about 1.5 trucks of concrete (assuming 20' long), so it's quite doable. However, it will still need formwork at the end, so it doesn't save on materials or labor compared to a retaining wall.

 

[KootK]

@KootK Would that need restraint at the bottom to avoid sliding along the soil slope when it's loaded?

I feel that it wouldn't but this is a new thing for me. My expectation would be that the soil slope down from the SOG would be chosen specifically to eliminate lateral earth pressure on the inside of the blob. I'm not entirely sure if that's a reasonable expectation or not. Some amount of earth pressure could probably be dragged back into the SOG, I suppose, so long as there was enough weight on the perimeter to rectify the implied twist.

This is sort of what I was imagining in plan. Maybe an abrupt transition would be simpler / cheaper.

 

[TRAK.Structural]

I meant a transition from downturn slab where there is no real soil to retain, to a retaining wall, at a certain point.


I was meaning the slab sits on the wall. A reinforced concrete block wall, with reinforced footing. Slab tied to the wall, such that the block wall and footing functions the same as an integral concrete downturn beam. I wouldn't want the slab floating on soil inside the wall.

We also design the last few meters of slab to be spanning across to the wall, as you don't tend to get good sub-base behind the wall.

How does that transition look? Presumably the turndown is somewhere in the range of 16"w x 20"d before you get to the wall, then when you pour the slab over the wall you may have a small thickened area say 8"w x 8"d. Making that transition in the concrete cross section and getting the rebar right seems difficult for residential work. See elevation sketch below, is this how that looks?

 

[TRAK.Structural]

That's where my head goes right out of the gate. I feel that would be a pretty natural solution in many markets.



To be clear, this means that, where the turn down would be large, the grading under the SOG will be built up in the interior, right?

A tall turndown unsettles me a bit, as it does others, but playing Devil's advocate, what would be the problem with it in the sketch below? Just gobs of concrete? The likely SOG crack location moving around the plan? More rebar to get to that crack location?

Perhaps this is one of those things where, once the contractor understands the cost implications of a version of the turndown that would work for you, they would wind up preferring something else.

View attachment 15823

Yes, there would have to be some amount of cut/fill to fully support the slab in the middle, no intention of having an elevated slab. The large mass of conc at the low side is the only way I see a turndown working over the entire footprint. I would imagine with that much concrete I could justify enough rebar to drag any incidental sliding load to the two side turndowns and effectively engage the whole thing in friction.

Solving this whole exercise with grading is simplest, at least for me.

 

[KootK]

Solving this whole exercise with grading is simplest, at least for me.

How would one solve this with grading? Extend the subgrade buildup beyond the building footprint?

 

[milkshakelake]

@TRAK.Structural Would you be putting structural foundations at the edge of the slope next to the grading?

 

[TRAK.Structural]

How would one solve this with grading? Extend the subgrade buildup beyond the building footprint?

The fill option would bring in some dirt and extend the flat area far enough beyond the footprint.

The cut option would drop grade at the front, put in a swale to divert surface water around the home, and again have it mostly flat for a slab.

Or some optimized combination of cut/fill.

@TRAK.Structural Would you be putting structural foundations at the edge of the slope next to the grading?

Not sure I understand your question.