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.