Wall with unbalanced soil 1

[DTS419]

Considering the attached sketch, how would you apply the soil forces to this wall? In other words, how does the soil on the right respond to the soil on the left...how much can the resultant of the soil on the left be reduced by the soil on the right?

 

[jayrod12]

Is there a situation where the soil is excavated on one side but not the other? If so, probably best to design around that. Perhaps for each direction. Then you don't really care.

 

[DTS419]

jayrod, the goal is to take advantage of the opposing soil loads so that the wall doesn't have to be designed for unbalanced fill which would be too conservative in this case.

 

[jayrod12]

How will you enforce that they don't backfill one side and not the other, or how will you ensure in the future that one side is not excavated while the other remains backfilled?

You give no indication of actual height of everything. Is that a slab on the right side of the picture? If so, then this entire conversation is moot anyway.

I'm usually one to ignore contributions, in terms of support, when it's something that can be easily removed. I'd anticipate that at some point in the future they remove the backfill on the outside for whatever reason, waterproofing failure etc. Often this type of work gets completed without involving an engineer.

However, with the presence of the slab on the inside face, I'd be using that as a point of support. If in the future that needed to be removed for some reason, they'd better damn well be getting an engineer to look at it. So I'd be comfortable designing as if that is permanently in place. Then the contribution of the soil below the slab, from either side, is a relative non-issue design wise.

But maybe I'm missing something.

 

[DTS419]

Don't worry about all the things that might happen during the life of the structure, etc. This is really a question of soil behavior. How to account for the active/at-rest/passive pressures that are opposing each other.

 

[jayrod12]

Well you do you.

My opinion on the soil pressures is you should be using at rest for both sides.

 

[Celt83]

ASCE 7:

 

[CuriousBearcat]

DTS419 The wall is inclined to rotate clockwise for your case. Let's start with that assumption. There will be active pressure from the left. And the resisting passive pressure from right. You need to check for sliding (forces) and overturning (moment). That slab on the right will provide some restraint too. If it is a solid slab or grade beam connected to the main structure, I would have treated it as a pin/hinge support if I was doing this project and checked for bearing at that location. If it is something like paver blocks or loose slab on grade, I would not have considered it at all in providing resistance if I was doing this project.

-Just a curious engineer

 

[DTS419]

For the passive pressure to be engaged, there must be ample movement. How do we know that will happen?

 

[Celt83]

The pressure starts at the at-rest value and increases based on the amount of movement into the passive side soil mass.

 

[CuriousBearcat]

DTS419 That is true. It depends on how much restraint is provided by that slab/support against movement. If based on your engineering judgement, you have a solid restraint, I would have used At-rest pressures rather than passive pressure.


-Just a curious engineer

 

[DTS419]

Celt, so then would you say the pressure on the right contributes it's at-rest pressure to oppose the soil on the left?

 

[Celt83]

If you can guarantee the soil will always be there then I would include it as an at-rest pressure with the appropriate load factors. If stability becomes a concern with at-rest pressure and movement can be tolerated the I'll increase the resisting soil to whatever it takes to satisfy stability up to but not exceeding the passive pressure defined by the geotech.

For retaining walls IBC chapter 18 I believe has it's own stability checks where the load factors are all 1.0 and you need to meet the min. safety factors for overturning and sliding, again though here I would start with at-rest pressure to see if the checks are satisfied only increasing to full passive resistance if movement can be tolerated.

 

[Greenalleycat]

I agree with Jayrod. I don't think it's appropriate to design for the soil resistance in the majority of cases.
For a typical private client you can almost guarantee that your design assumption will get lost in the wind.
No contractor I've ever met is going to dig up your design docs to check whether they can dig the soil up on the lower side without the wall falling over

I would perhaps do it for a Government or similar client who I had confidence could document and manage the excavation risk in exchange for the upfront material savings.

Edit: even if you're certain that the soil will exist, careful consideration needs to be given to how much resistance to take
Soil passive pressures typically require significantly more movement to mobilise when compared to the active pressures that drive the wall movement
I imagine that you'll need to dig some of the lower soil out for your wall installation, how will you QA and manange this risk during the filling works etc?
Is it really worth it?

 

[DTS419]

The fun part about this is there is no geotech report. It's residential, so nothing but assumed values for unit weight, friction angle, etc.

Using ASD for the design of wood framing, but I agree the 0.9 factor on the resisting soil is appropriate.

 

[DTS419]

Greenalleycat, I general agree except this resisting soil will be under a basement slab. I'd say it's not likely to be removed during the life of the structure.

 

[Celt83]

Using ASD for the design of wood framing, but I agree the 0.9 factor on the resisting soil is appropriate.

Under ASD the load factor for the resisting side is 0.6, refer to the footnotes for the ASD combos in ASCE 7.

 

[Greenalleycat]

DTS, in that case I agree with Jayrod again. Fuck the soil, use the slab.
You can design a reliable tie into the slab that will do more for you than the soil ever will
If a contractor wants to dig out a basement slab without considering whether it's part of the wall foundation then that's on them

I don't know if water pressures are a significant concern in your area (thicker slab to resist buoyancy) but I would recommend upping your slab thickness in this case
Or at least doing a thickening near the base of the wall before it tapers out into the slab
I've made a super scientific sketch of my thoughts below

 

[DTS419]

I didn't mention this before, but I am using the slab as support. What I ultimately need to do is design the wood beam that spans across the top of the wall for the reactions based on the soil loads. Think of it as a simply supported beam that overhangs one of the supports.

 

[jayrod12]

If you are considering the slab, then I don't actually see how the soil below the slab helps in any way. If your footing is one support, and your slab is another, then the load between the two below the slab doesn't help. If anything it hinders and makes the top of wall move more. So therefore I go back to my original recommendation of ignoring that soil.