When would a retaining wall footing be required? 1

[NC-ENG]

I have a wall that landed in the landscape plans (I suspect, but do not know for a fact that structural EoR would like to have taken a look at it but was never asked to by the LA). It is holding up a fire lane and 3-4' of retained earth at its tallest point. The wall is being called a "turn down slab", but I believe it should be a considered a retaining wall with a spread footing. The geotech report has 3,000 psf as the allowable soil bearing pressure. The fire lane would be considered used only in emergency conditions (not frequently loaded).

The wall itself is 10" wide 3,000 psi concrete. Reinforcing is #4 @ 12" oc vertical and #5 @ 12" oc. horizontal (turned back into the slab). I suppose the top of wall could be considered pinned or tied into the slab condition, although it is not poured monolithic with the slab and there is a CJ between the curb/top of wall condition and the structural slab.

The wall has settled around 2" over the last 2 years. It doesn't appear to be in danger of failure, but the design is being called into question on whether a foundation should have been provided.

What would the anticipated bearing pressure be at the stem of the 10" wall at the max retained earth section of 4' (3' plus 1' of cover). To make matters worse, the curb has scuppers at 8' o.c. which allows surface water to sheet down the wall, meaning the bearing condition is almost always wet/saturated.

 

[NC-ENG]

Correct. If this was a split it 3 ways discussion, it would probably be behind us already.

 

[SWComposites]

Seems simple to me:
A “constructibility” review is to determine if the design as documented can be built for the defined cost,
It is not to determine if the design is correct.

 

[jerseyshore]

That may be, but the bearing strength and settlement is width dependent. As the width gets narrower, the soil behavior changes and the allowable bearing pressure usually falls off. That's why the geotech specify a minimum. That minimum isn't saying the structural engineer can't specify something smaller, it's saying that the allowable bearing pressure is only valid down to that width. Any narrower, and the structural engineer is on their own as the soil will start behaving in ways the geotech didn't anticipate in their calculations.

This is how it has always been explained to me by geotechnical engineers. One local geotech always tells people they don't have to follow their recommendations, but if you deviate, then you own it and take responsibility.

They provide a 2 ft minimum footing size because that's what they used to recommend 3,000 psf.

I'm just a little surprised a contractor built a concrete wall without a footing and never questioned it. I feel like any time I have something outside of the norm I get 100 questions before they even get started.

 

[zackngineer]

Retaining walls less than five feet tall are not required to have a building permit. However, retaining wall systems less than five feet in cumulative vertical relief and adjacent to a structure located closer than the vertical relief shall be designed under the responsible charge of a registered design professional and shall require a building permit and inspection.

Just because it didn’t require a permit doesn’t mean it shouldn’t have had a footing. My question is whether or not the footing detail as shown would exceed the soil bearing capacity of 3000 psf

Where I'm at, permits aren't technically required for non-structural retaining walls up to 4'. That's the official code. However, I've had to design multiple 3' landscape retaining walls because the BD says they want permits for it. I only bring this up because you're putting a blanket 5' requirement out there when it's different depending on the jurisdiction.

 

[canwesteng]

I don't see how a constructability review should have caught this - it's constructable and it's stamped by a PE. The EOR should own all the liability. A 10" is odd, but we build retaining walls out of lock blocks often and those have no footing.

 

[XR250]

If the slab is tied to the wall, the bearing pressure for this is basically straight down. I suppose a few 10k wheel loads from a pumper truck etrc. could have acted on this which I could see causing the settlement. For 10" wide and 2,000 PSF, your capacity is 1,560 plf - which ain't much. This would be further reduced by what other's have said about the narrow footing.
My guess is the Geotech screwed up though.

 

[GaStruct]

I assume the landscape plans were stamped by a design professional? They are the ones who own the slab/wall and they need to figure out if their design is adequate and defend it if so. Am I missing something?

 

[kissymoose]

$20 says it was poured on inadequately compacted fill.

 

[gte447f]

I second what others have said. Geotechnical reports that I have dealt with often include a recommended minimum width for strip footings and spread footings, and it is always more than 10 inches.

Also, in my experience, for commercial new construction design projects that I have been involved with, the structural engineer's scope of work typically does not include retaining walls, unless they are connected to the building foundation. Site work, including site retaining wall, paving, curbs, etc. would not typically be part of the structural engineer's scope unless specifically contracted. This sort of thing would usually fall under the civil engineer's scope. Whose drawings did those sections come from?

 

[NC-ENG]

$20 says it was poured on inadequately compacted fill.

All fill was witnessed by a geotech. I have numerous photos taken during placement of that fill going in with photos of the geotech in the frame. Not to say that it couldn't have some inadequate compaction (there is always a chance for things to go wrong, no matter how many eyes are supposed to be on it).