Spanning slabs over backfill 1

[manstrom]

Long time lurker, first time poster

I have been in the residential construction field for over 10 years, I am a Structural P.E. and routinely I question some of the things that I have held as fact.

Where I have a large foundation step in a building - say a garage next to a basement in a single family home. I typically design the upper slab to span over the backfill next to the lower retaining wall (1:1 slope). For a residential garage, this typically entails grade beams and a structural slab. To my knowledge, this is typically done because the backfill is not compacted well enough behind the retaining wall. Regardless, we still ask the builder to compact the fill behind the retaining wall AND provide a structural slab. This is a belt and suspenders approach, but I have held to it. Keep in mind, this is mostly for residential builders, not skyscrapers.

These questions have arisen:

1. If I compact the fill behind the retaining wall to 95% compaction per the geotech report, do I need a structural slab? (I would then have a slab on grade next to a wall designed for the surcharge)

2. Can I keep the structural slab and not compact the soil at all behind the retaining wall? The structural slab is designed to span this distance. So why even compact the soil?

3. What if I backfill the wall with gravel or something that is self-consolidating. Would I need a structural slab still?

 

[ztengguy]

So you are designing a structural slab parallel to the basement wall, outwards at 1:1, then a grade beam, then slab on grade after that?

 

[manstrom]

The grade beam would span perpendicular to the retaining wall. The structural slab would span to the grade beam(s).

Alternatively, I would run a thicker structural slab from the top of the retaining wall spanning perpendicular.

 

[ztengguy]

What do you suppor the grade beam onto, the basement wall?

 

[manstrom]

The grade beam would be supported on the basement wall on the near side and span to good soil on the far side.

 

[JAE]

1. If I compact the fill behind the retaining wall to 95% compaction per the geotech report, do I need a structural slab? (I would then have a slab on grade next to a wall designed for the surcharge)
Even with 95% standard proctor compaction you still can get settlements. The issue is whether the slab-on-grade settlement can be managed with the use of proper details, expansion joints, etc. to allow a moderate amount of settlement without creating problems in building use or distressed conditions.

2. Can I keep the structural slab and not compact the soil at all behind the retaining wall? The structural slab is designed to span this distance. So why even compact the soil?
Yes - the soil will simply settle downward leaving a small void space below the structural slab.

3. What if I backfill the wall with gravel or something that is self-consolidating. Would I need a structural slab still?
Gravel backfill may have less net settlement than re-using the excavated soils. This is certainly an option to consider. This would also require proper detailing and management of slab settlements, even though they might be smaller.

I've always questioned just how well residential contractors can compact soils anyway - I rarely see them with soil testing firms out there. Usually the maximum you might see is the backhoe occasionally pounding the soil with the bucket or a small vibration plate compactor running over it a few times without any testing.

 

[manstrom]

Thanks, JAE

 

[cvg]

gravel with no compaction vs soil with slight compaction, not sure one is superior to the other. Gravel is not self-consolidating but most contractors will argue that it is. The cost of the gravel is certainly higher and most contractors will not do it unless forced.

backfilling walls with gravel will promote drainage towards the wall which needs to be considered in the basement design.

 

[msquared48]

Idea 1. Contractors hate messing with their forms with holes for rebar. This could affect the placement of any stub bars to the slab from the wall unless it is grade 40 material and field bent into place after the wall is poured.

Fact 2: All slabs on grade will settle some over the years, regardless of any compaction, eventually affecting the wall interface connection with a vertical gap if the wall is placed on top of the stem wall.

Idea 3: If you use the filled in area next to the wall as a "structural slab", and span it to the wall with a 2" bearing width for the slab at the top of the wall with the rebar tie mentioned in #1 above, then, depending on the wall height, minimum slab reinforcing could work.

Idea 4: Each and every contractor has his own way of doing things, personal nuances where he can do it cheaper. If you are dealing with a standard 8 to 9 foor high basement retaining wall for the garage, it would probably be cheaper overall not to backfill behind the wall, and use standard wood floor to frame to the top plate on the top of the wall. The wall reinforcing and footing size would be less, and the backfill cost elimilnated. You might want to consider using a crawlspace at that point over a slab-on-grade for the first story. Better long term mechanical utility and maintenance too.

Just some points for discussion...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[rowingengineer]

How far can a slab span without reo? Say 4' lower grade. I would base this on long term loads.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[JAE]

Not sure of any code (or engineer) that would span concrete without reinforcement.

 

[rowingengineer]

In residential construction waffle pods is a common method of construction, in this case the slabs are reinforced with only wire (in the top of the slab) and they span 1m in each direction. So clearly there is some distance a slab can span without reo as the reo in these slabs is generally in the top half of the concrete as would be the case for many residential slabs connected to deep edge footings.

While I am not suggesting that you should design suspended slabs as un-reinforced, I am questioning the conservationism when considering the settlement of back fill and the slab strength requirements as a structural slab. In the case of the OP above it is possible that the slab will not act as a suspended slab until it has reached full strength, thus can we consider some slab strength from the concrete. Also what are the consequences of failure of the slab in bending? are these critical enough to consider the slab as a structural element or is it just a SOG?

Should we be treating it as having the same tensile and flexural strengths as a SOG?

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[XR250]

You can easily cause your retaining wall or basement wall to fail by trying to compact the fill behind it. They are not designed for that kind of force.
Most contractors around here use the gravel method and have reasonably good luck with it. I typically go the structural slab and grade beam route.
We also sometimes just put a crawlspace or basement under the garage and design a suspended slab. Gives you a very cheap storm shelter to boot.

 

[rowingengineer]

Normally the retaining wall would be propped if you were going to compact the back fill. However I have viewed an un-propped wall with a 300x300 footing rotated out after compaction, no other signs of damage.

In honesty I change over my designs to a structural slab over 1500mm (from foundation level to top of slab), under this I generally prop and back fill compact. This is the detail recommended in our national standard AS2870 in Australia so it is common practice.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."