Cantilever retaining wall with narrow site cut 3

[bhiggins]

Hi all!

I've got a project here where a basement retaining wall is being loaded with only 2-3 feet width of backfill. The site is on hard limestone so the cut is nearly vertical. The geo report provides active and at rest EFP. Since this is not a standard retaining condition, I'm wondering if these EFP can be reduced based on my situation.

If so, is it appropriate to use a different angle of internal friction or some other rational method? Can this be done without the geo recalculating the EFP for me? I'm just wondering for the future if this can be done without geotechnical intervention.

My guesses are that either the EFP is lower, or the EFP remains the same and the load resembles a triangular distribution that is "capped" off at some point, resulting in a triangular load to a uniform load.

 

[Okiryu]

Can you share the silos related equations?

 

[KootK]

To clarify the specific situation being addressed here, it is my understanding that:

- The truly hydrostatic lateral load on the wall arising from ground water is independent of the horizontal extent of the retained mass. Just like fluids class.

- The equivalent fluid pressure load on the wall arising from soil shear failure is very much dependent on the horizontal extent of the retained mass. Less horizontal extent means less load in a way that may be complicated by friction against the wall, increasing shear strength with overburden depth etc. The stuff Okiryu pointed us too leaves little room for debate on this in my opinion.

- The behavior of a shear-weak material like soil is not the same as the behavior of a shear-less fluid like water. The equivalent fluid pressure business is just a convenient device used for calculation, not an endorsement for soil masses being characterized as fluids.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

I agree with KootK. The key difference between soil and water is that soil does have a shear strength while water does not. For 99% of structural applications this can be ignored and the soil idealized as a equivalent hydrostatic pressure.

I had a similar wall project just last week involving a wall near to ledge and spent a lot of time pondering this situation of a narrow strip of soil behind the wall and the loads imparted on it. My understanding is the failure plane leading to the wall will be greatly changed by the presence of the unmoving vertical surface. Thus, the soil will slide not along it's typical failure shear plane but some different plane, which reduces the load it puts on the wall.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[SlideRuleEra]

The Equivalent Fluid Pressure (EFP) of soil:

1. Depends on two, and only two variables, the unit weight of the soil and the soil's angle of repose.

2. The one, and only one correct way to use this EFP is to consider the soil to be an ideal liquid that performs as an ideal liquid.

3. Based on Item 2 (above), when using EFP the horizontal dimension of the soil is not a factor.

Don't believe it... here are three authoritative sources from past and present to confirm it.

From: "Highway Bridge Superstructure Engineering: LRFD Approach to Design & Analysis", Taly, Published 2014, Page 313

From: "Handbook of Building Construction", Hool and Johnson, Published 1920

From: "US Steel Sheet Piling Design Extracts", Published c. 1932

Thanks to jayrod12 and hokie66 for the vote of confidence. This subject is very important, the load from the soil is real (not a statistical probability, like wind loads). Making erroneous assumptions increases risk.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Settingsun]

Two different questions are being asked and answered here:

1. How to use equivalent fluid pressure?
If I were being unkind, I'd say throw it and anything that mentions it in the bin. As SlideRuleEra's source says in section 3.22.4: "The equivalent-fluid method may be used where the Rankine earth pressure is applicable." Rankine coefficients are so simple to derive and use there is no reason to start talking about mythical fluids. If, however, you choose to use EFP and don't know how the EFP numbers were derived, best stick to what SlideRuleEra says.

2. Is EFP conservative in this case?
Maybe so. Use one of the other methods mentioned in this thread or linked threads and compare to Rankine. I don't think you'll end up with a hydrostatic lateral pressure distribution though, so it won't back-convert to an EFP.

 

[appot]

Hydrostatic Paradox (youtube)
Historical Hydrostatic Paradox Experiment


There comes a point when other forces will no longer be negligible compared to gravity, as stated above:

In that case, it's not the theory or the equations that are wrong, it's just that other issues override them.

When the diameter of your water column approaches its limit (single molecules of water stacked vertically) then atomic and molecular forces, surface tension, and capillary rise will likely play a large role, and can not be ignored. Luckily this is outside the scope of civil engineering.

 

[Yt.]

some of the assumptions for earth pressure is that the substrate is considered uniform and infinite. That's why that factor isn't reflected in equations.

I'm far for being able to give a proper answer, because i'd never researched to deep on something like this. But i'll give you the following images, just to note that there are several ways to estimate the backfill pressure depending on differents assumptions and also the coloumb scheme (with infinite width substrate) which match the equivalent "water-pressure" (only when the substrate is width enough).