Retaining wall loads

[Enhineyero]

Hi all, We all know that the force on a retaining wall depends on the depth of the water / soil it retains, my question would be why is the Width of the soil/water being retained (the dimension perpendicular to the soil/water) is not considered? I am a bit rusty when it comes to fluid and soil mechanics, can somebody guide me on this?

Because I think the width of the soil or water to be retained has a significant on the retaining wall loads. Please see attached sketch. Would figure A resist the same force as figure B?

 

[Enhineyero]

*being retained (the dimension perpendicular to the wall)

 

[renderu]

It would be the same. Force on the wall due to water is calculated using the pressure.

 

[cliff234]

The lateral pressure exerted on a wall supporting a 1" width of water (let's say it is a tank of water the tank is only 1" wide) is the same as the lateral pressure exerted on the same height wall if it was holding back the ocean. That is not the same for soil. To some extent the width of soil being retained does influence the lateral pressure, but after a certain dimension it doesn't matter (for soil).

 

[briancpotter]

Pressure is independent of width. See

http://en.wikipedia.org/wiki/Pressure,

or

http://en.wikipedia.org/wiki/Pressure_head

Brian C Potter

http://simplesupports.wordpress.com

 

[Splitrings]

It is because soil has shear strength. Any loads outside the angle of internal friction are carried by the soil itself. Attached are a couple of ducuments.

http://www.geotechnicaldirectory.com/publications/Dm702.pdf

 

[a7x1984]

I like Splitrings mentioning of shear capacity - that is really the effect that explains the difference between an essentially granular solid and fluid. Looking at the shear capacity extremes between a solid and liquid should visually reinforce why it is what it is.

In Russia building design you!

 

[RFreund]

When you have a 'narrow wall', the soil pressure will be reduced on the wall.

Try to search Narrow wall or for MSE type walls try MSE with stable feature.
I started a post here and will come back to this and try to give a more though discussion, hopefully in the near future when I get some more free time.

As for the water case is you had a wall of water that was say less than 1' thick I believe you would have a reduced pressure as the weight of the water cube is less than 1 cuft

EIT

http://www.HowToEngineer.com

 

[briancpotter]

RFreund-

Re: water, this seems intuitively right, but is incorrect. Consider what happens if you measure in psi instead of psf. Attached is an explanation.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[Splitrings]

If the depth of the backfill perpendicular to the wall was less than the distance where phi intersects the surface, I would guess you would have to design for at rest pressures and not active pressures. These would actually be higher.

As far as water pressure behind a wall, it is independent of the depth perpendicular to the wall. A column of water exerts pressure based on gamma * H.

 

[RFreund]

Brian - good catch. Not sure why I didn't think of that. Brain fart.

EIT

http://www.HowToEngineer.com