Special condition retaining wall 2

[Dad2one]

I'm looking for advice on the design pressures for a wall in a condition shown in the attachment. The wall is being cut into over-excavated rock so that waterproofing can be applied to the back side of the wall. The rock produces no lateral pressure and there is only a limited width of soil behind the wall that does produce pressure (to minimize the excavation). Does the fact that there is only a few feet of soil behind the wall result in a lower total horizontal soil load on the wall (that is, a lower overturning moment)? Or must the wall be designed as any other wall retaining that height of soil? That is, will the overturning moment be the same whether there is 1 foot of soil or 50 feet of soil (width, not height) behind the wall?

Thanks.

Dad2one

 

[PEinc]

This question has been asked several times. Did you search the forums yet?

http://www.PeirceEngineering.com

 

[Dad2one]

Yes, I did. But apparently not deep enough.

 

[BigH]

Look for silo pressures . . .

 

[aeoliantexan]

The lateral soil pressure will be lower because friction between the soil and the wall and the rock will reduce the vertical pressure, especially at the greater depths.

Handy presents a discussion and formula for the pressure on page 572 of Soil Engineering, Fourth Edition, by Spangler and Handy.

I would be cautious about taking advantage of this theoretical condition. My mentor used to say "Don't design any wall that can't hold back water." If a water line breaks and water fills the backfill, the pressure will be something greater than 62 pcf. If the wall can't withstand this pressure at ultimate, it will collapse. And the opposing attorney will ask "If you were sure there would never be water there, why did you apply waterproofing?"

 

[Dad2one]

BigH- thanks for pointing me in the right direction. I don't know why I did not think to search for 'silo' pressures. For future reference 'narrow backfill' is also a good search term.

I've read many posts and many theories on this question and most are saying to use a silo analogy and take advantage of arching. But I am not comfortable with that approach because a cantilever retaining wall (mine is 18 feet tall) will be much less rigid than a rock face and will rotate, both immediately on loading and over time with creep. That is going to disturb any arching and approach a more conventional active pressure condition. A silo does not have any such concerns.

It's an interesting condition. I'm surprised there is not more research on it.

 

[RFreund]

I would be very interested it what you find as I have had this same question myself.

Here are a couple articles which I haven't fully gone through:

http://www.vulcanhammer.net/geotechnical/FHWA-NHI-10-024.pdf

(See chapter 6)

http://www.ce.utexas.edu/prof/zornberg/pdfs/CP/Kniss_Yang_Wright_Zornberg_2007.pdf

I'd be interested to read this:

http://www.nrcresearchpress.com/doi/abs/10.1139/t01-063

Can you use a segmental wall? You could extend grid from the face of the wall back around a steel pipe then back to the face. The steel pipe is then anchored into the stone. We have done this in the past (although its usually to an existing concrete wall).

EIT

http://www.HowToEngineer.com

 

[Dad2one]

RFreund,

Thanks for the references.

I came across a paper that was very helpful. See the attachment.

The authors use the method for silo pressures as a starting point but expand on it because it is not appropriate for direct use since it does not account for movement of a relatively flexible cantilever retaining wall. There is a method for modifying the active pressure coefficient as the height of retained soil increases. I am calculating moments that are roughly 2/3 of the full Rankine-based loading. It also counts on friction between the backfill and the rock face. There is a parameter 'delta' which is the angle of friction between the walls and the fill. If anyone can give me a reasonable value for use I would appreciate it. Assume a 57 stone backfill and a rock face.

D2O

 

[Dad2one]

The second attachment did not show up in my previous post.

 

[Dad2one]

RFreund,

The utexas (UT) paper is relevant to our discussion but the vulcanhammer one is not. However, the UT paper specifically applies to NON-yielding walls. Such a condition between a rigid rock face and a stiff retaining wall is similar to a silo condition and the resulting experimental agreement is not surprising. This method does NOT apply to more flexible (e.g. tall cantilever) walls. In fact, the paper states that work on narrow backfill and yielding walls is currently underway. This paper is dated '07. If you have the later study, I would appreciate a copy.

D2O