Tall Cantilever Retaining Wall - Sliding Resistance

[BatMan]

Hi all,

Doing a 24-ft tall cantilever retaining wall with limited heel length (<5-ft due to prop. line). So this requires huge toe length (over 10-ft) and deep key at the bottom of the footing for sliding (4-ft). Now, this just seems a bit excessive and feel like I need to start looking at different types like walls on grade beams and piles. Has anyone done a similar retaining walls previously? Also, I've heard many people say, "add a key for additional sliding resistance", but this just seems like a chasing my own tail. Deeper you do down, you can count on passive pressure, but you are also adding more active as well. Any thoughts?

 

[KootK]

Has anyone done a similar retaining walls previously?

Sure, I've done this from time to time. Trickier detailing but altogether doable.

Any chance you've got a slab on grade etc that you can push into? If you didn't, I'm sure we wouldn't be having this conversation...

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.

 

[hokie66]

2/3 of the height for the width of the footing seems about right. In fact, that is my rule of thumb for preliminary design.

 

[bridgebuster]

Maybe my brain isn't working today, but if the wall will 24' tall at the property line (back of heel), aren't you're going to need support of excavation at the property line to build the wall? If that's the case build a soldier pile/lagging wall.

 

[msquared48]

Or soil nail.


Mike McCann, PE, SE (WA)

 

[dik]

Can you embed the wall in the soil with a bit of a flat surface to support the weight like a footing and resist lateral forces sort of like a 'continuous post'?

Dik

 

[XR250]

So how about this ..
Put your key directly under the stem. The 10 ft. toe will keep the soil confined in front of the key. The you should be able to use the bearing pressure value for your key resistance instead of the much smaller passive pressure. I am sure I will get some blowback about this idea!
Seems if your key is just an extension of your stem you could get some nice bar anchorage without
needing hooks. i.e. run your stem bars thru and your heel and toe bars thru. Again, I ain’t no concrete expert but intuitively it seems like it should work.

 

[BAretired]

The key should be located nearer the property line than the wall if the goal is to align the reinforcement from key to wall because the key reinforcement is on the front face whereas the wall reinforcement is on the back face. That exacerbates the problem that bridgebuster raised.



BA

 

[WARose]

[blue] (BatMan) [/blue]

Also, I've heard many people say, "add a key for additional sliding resistance", but this just seems like a chasing my own tail. Deeper you do down, you can count on passive pressure, but you are also adding more active as well. Any thoughts?

I'm not sure I've ever seen a situation like that. Considering the fact that the coefficient of passive pressure is sometimes on the order of 10 times higher than active pressure.....it's hard to imagine the situation getting worse (as far as sliding goes) as a key got deeper.

Hopefully you've got something like Enercalc where you can change the variables fast and get answers. Otherwise, it can get pretty time consuming.

I've done a few like you are describing and yep: the toe winds up being as long as the heel should be. Odd looking....but it happens.

 

[XR250]

The key should be located nearer the property line than the wall if the goal is to align the reinforcement from key to wall because the key reinforcement is on the front face whereas the wall reinforcement is on the back face. That exacerbates the problem that bridgebuster raised.

That makes sense. The main gist of my post, however, was can you use a higher value of "passive pressure" if the toe is sorta confining the soil?
If so, would there be a better place for the key to optimize the use of that?

I'm not sure I've ever seen a situation like that. Considering the fact that the coefficient of passive pressure is sometimes on the order of 10 times higher than active pressure.....it's hard to imagine the situation getting worse (as far as sliding goes) as a key got deeper.

I ran it in my program and found out the same thing as the OP.

 

[Settingsun]

24' is a big wall for this type of construction. 15' base width seems like a good result to me given only 5' of that is getting the benefit of the dead weight of the retained ground. How much is the 4' key actually doing for you? Quick numbers put it about the break-even point for me in terms of passive resistance vs additional active force, so not worth the money. A key usually adds cost out of proportion to the quantity of concrete in it. Similar for piles and ground beams compared to a simple slab. Base slab concrete is cheap if you've got the space: simple reinforcement layout and just pump concrete as long as it takes.

 

[WARose]

[blue](XR250)[/blue]

I ran it in my program and found out the same thing as the OP.

That's odd. Is that with nothing on the toe? Because with a 10' long toe he is describing.....more than likely that would be buried. (I can't imagine a footing that long being exposed.)

Say it was buried in 3' of ground......and (just guessing numbers here) Ca= 0.33, Cp= 3.0, and soil weight=110 pcf......by my numbers, the SF for sliding only gets better as the key gets deeper for any reasonable depth of a key. (Gets to 1.32 (from passive pressure alone) when the key gets to 8' in depth (not including wall footing depth).)

 

[JoelTXCive]

Your issue is a concern for me too. In fact I have been trying to find a documented solution for sometime.

What is the point of adding a key, when you have to extend the active pressure down onto the key? The active pressure is already at ~24+ feet of depth, so it is a huge number. The passive pressure, while accumulating much faster, is only at a depth of ~2ft (or whatever your footing thickness is). If you have 'disregard depth' on the passive side, then you are even further behind.

My own thought, is that if your key is located near the heel, then you should extend active pressure down onto it. But if you key is way up at the toe, then you should NOT apply active pressure to it. Only apply the passive pressure.

I saw a paper one time (which i wish I could locate again) that ran a 45-degree line off the corner of the heel. The author stated that if you kept your shear key out of this range (sort of like boussinesq line), then you did not need apply active pressure to the key.

 

[CConrad]

IBC 2018 clarifies the text that had everyone confused for two code cycles about having to run the active pressure to the bottom of the key. The intent of that section was not to make a blanket statement that active shall always extend to the bottom of the key. Instead, the intent was to require the designer to CONSIDER whether that would ever be a valid design condition, such as if there was a high potential that scour our other actions would remove all or most of the soil on the toe side, down to the level of the key.

See the new wording in IBC 2018, and I think it will give you much more confidence in removing that extra active pressure below the bottom of footing.

 

[WARose]

[blue](JoelTXCive)[/blue]

I saw a paper one time (which i wish I could locate again) that ran a 45-degree line off the corner of the heel. The author stated that if you kept your shear key out of this range (sort of like boussinesq line), then you did not need apply active pressure to the key.

And that's pretty logical. (I think I've seen that before myself.) Interestingly enough, Bowles (i.e. 'Foundation Analysis and Design', 5th Edition, p. 688-689), recommends placing the key at the end of the heel. (As opposed to directly under the stem. As was common practice for quite some time.) The reasoning given that there is a: "Potential sliding surface using the key [directly under the stem]." (It does not address it being placed at the tip of the toe.)

I assume this "sliding"/slip surface it refers to is some sort of shear failure in the soil (between the tip of the key and the bottom corner of the toe) that would happen before the full passive pressure could be mobilized. (At least that is what it looks like in the diagram.) Haven't given that much thought in the past.

Army Manual EM 1110-2-2502 ('Engineering and Design RETAINING AND FLOOD WALLS') also show the key at the back of the heel in some circumstances.

 

[BatMan]

Wow, alot of good inputs. Thank you all.

 

[hokie66]

I am sure you are welcome. But the most important advice, regardless of your design decision, is that given by bridgebuster. If you are excavating 24' (and more for the footing and key) on your side to build this wall, how do you support the excavation temporarily? Design is one thing, constructability is the other.

 

[JoelTXCive]

Here is a 4 page conference paper on the position of the shear key.

According to the Author (grad student), the heel is the best location based on finite element analysis. He or she does not seem to address what pressures are being applied to the key though.

However, it does look like the author has some good sources and references cited. That might provide further info if someone wants to make a research project.... lol

Link

https://gndec.ac.in/~igs/ldh/conf/2011/articles/Theme - K 2.pdf

 

[IDS]

JoelTXCive, interesting paper, but I think the conclusions should be taken with caution. At least some of the graphs appear to be inconsistent with the statement that the heel is the best place for the key.

Nonetheless, looking at global stability from a slip-circle perspective does suggest that either heel or toe are likely to be better than under the stem, and that the traditional approach may well be misleading.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JoelTXCive]

I forgot! There is an old engineering tips discussion on shear keys and the application of active pressures. The issue is not resolved, but there is good input....

This issue has been perplexing engineers since at least 2012......

Link

http://www.eng-tips.com/viewthread.cfm?qid=332224