Combined Wall/Footing 1

[XR250]

I'm designing an addition on some sketchy soil. The Geo-tech is recommending to excavate down 4 – 6 ft. to get to some decent bearing. He is against filling the trench with washed stone. The contractor wants to dig with a 12” wide bucket and fill the trench with concrete and use that as the footing. Any issues with that? The bearing capacity numbers work out. The “footing” is about 20 ft. long. Is rebar necessary? If so, how much is recommended. Flowable fill is not available, however, can that be used in this situation?

Thanks

 

[SteelPE]

I understand that bearing pressures may work out, but whenever I use a narrow footing like you are proposing I will be using a reduce bearing pressure. This is in relation to every geotechnical report I have ever received that gives allowable bearing pressures for footings (minimum 3'x3') and continuous foorints (2'-0" wide). The reports go on to say if these minimum dimensions are not used then the allowable bearing shall be reduced by some equation. So even when using bearing values from chapter 18 of the code I will take the reduction.

In regards to the footing , ACI requires a minimum 0.0018 bh for footings. You might want to comply with this requirement.

 

[phamENG]

Why is the geotech against stone? If the soils are bad enough, I'd imagine it's probably a stability concern within the stone "wall". If so, that doesn't necessarily go away with unreinforced concrete. If the soils aren't strong enough to stabilize it, you'll have to design it to be strong enough to "stand" without the lateral support of the surrounding soil.

12" is pretty narrow - confirm it with the geotech as their settlement/bearing calcs were probably based on a minimum width of 18" or 24" for a continuous strip footing (that's what I'm used to seeing out here on the coast).

 

[KootK]

In principle I think the concept is viable. In some parts of the world, and in some parts of the US, these "trench footings" are the normal practice. Granted, it's at depths closer to 2' than 6' but, still... I'd probably just toss a couple of #5 top and bottom to:

1) Get some grade beam action in case you need it and:

2) Minimize cracks from telegraphing from the blob up into the wall elements that folks might see.

0.0018 tends to feel like a lot of rebar in these situations where your concrete is basically just soil replacement.

Flowable fill is not available, however, can that be used in this situation?

Sure, with due consideration to reasonable proportions and stability per the other guys.

 

[XR250]

Sounds like I need to confirm with the Geotech. I might have to run some vertical rebar out of this blob to engage the three courses of masonry/diaphragm above to eliminate a hinge as Pham eluded to. Ironically, this is first time I have encountered a Geotech who was against filling a hole with washed stone. None in my memory seem to think the stone needs some sort of confinement.

Thanks for the advice!

 

[XR250]

Geotech says I am good with a 12" wide footing. Should probably have 3000 PSF bearing capacity and only 2000+/- PSF demand.

Thanks

 

[phamENG]

3000psf!? Man...must be nice to live where there's rocks...

 

[KootK]

Seriously? I've had 1,500 PSF on swamps. Good numbers in WI/AB are more like 5,000psf - 12,000 psf.

 

[jayrod12]

I got one the other day from a geotech, maximum allwable bearing capacity 1000 PSF. I responded saying I feel like I can get 1000 PSF in quicksand and would like him to revisit the testing results to be sure.

 

[phamENG]

1500 is the norm here, some of the slightly higher and drier areas get 2000. But then we also have areas where you just can't do shallow footings - period. Did a one story house next to the river that had to be on piles.

 

[strucbells]

I've reviewed a couple of geotech reports recently where they recommended 1000 psf allowable bearing pressure. Generally occurs on sites with undocumented fill where the owner doesn't want to overex down to native.

It's looking like we're going to do some rammed aggregate piers on one project and I'm pushing for new borings on the other to verify the old report.

 

[emmgjld]

Interesting discussion!
Hmm, 6 feet of washed rock, not properly compacted. What is missing is a description of the actual character of the washed rock. Is it crushed? Is it angular,subangular or subrounded? The use of crushed, angular rock vs subangular to subrounded river rock is day vs night.
In my part of the country, with crushed river sand, gravel & small cobble, I usually see subangular to subrounded particles, usually poorly to very poorly graded. In a thicker fill (poorly compacted), this could be a definite candidate for settlement (actually collapse) up to 1/2" per foot and probably little or variable side support for the fill if the upper soils require such deep penetration.
A properly placed & compacted fill or extended concrete footing seems a reasonable answer.

 

[Lomarandil]

If you need any more justification than just the gut feel, I'd definitely be comfortable finding the minimum grade beam depth required for a beam-on-elastic-foundation calc (assuming your load isn't entirely uniform), reinforcing per that area * 0.0018 for some basic cracking and shrinkage restraint, then calling any further depth of concrete unreinforced soil replacement.

As for bearing pressure... depends on the soil composition. Where I practiced in the mountain US, anywhere you could drive a passenger vehicle or light truck without making significant indentation was good for 3-4ksf (without rocks). But I'm currently in an alluvial basin with crazy silt/clay content where 1000-1500 psf isn't uncommon.

Of course, sometimes it's just the geotech not being paid enough to stick their neck out farther than IBC will cover.

----
just call me Lo.

 

[oldestguy]

Filling the deeper excavation with washed stone is asking for trouble unless the trench is sufficiently wide to provide lateral support.

 

[MTNClimber]

The crushed stone method proposed isn't a road worth going down. I would never allow that to happen on any of my projects.

The concrete method could also be an issue but that depends on many things that we don't have access to. I would follow the guidance typically used for pit underpinning for a good starting point and then talk to the project's geotechnical engineer.

 

[Ron]

XR250....Your building code specifies that for masonry walls, the footing has to be at least 8" wider than the masonry, so if you are using standard unit masonry, your footing width has to be at least 16 inches.

 

[cristiano.ronaldo]

Fill excavation with plain concrete.

 

[XR250]

XR250....Your building code specifies that for masonry walls, the footing has to be at least 8" wider than the masonry, so if you are using standard unit masonry, your footing width has to be at least 16 inches.

Here is what the code says.....

Regardless, I don't feel it is required from an engineering perspective in this situation.

 

[r13]

To me it is a practice call if soil bearing is not a problem. However, would it stable for gravity load with a small eccentricity? Remediation of a wall footing is difficult, why risk the trouble only to save a few dollars? Do provide longitudinal reinforcement with transverse spacer bars to bridge over weak spots.

 

[Ron]

XR250...my bad. You are correct. That is a change from the previous code versions that I quoted.

My only caveat is that the code is minimum and you need to stick to the prevailing standard of care for your area. Think beyond the numbers!