Retaining wall quiz on keys

[YoungGunner]

This may be the wrong thread for this, but one of our employees is giving a training on retaining wall keys tomorrow and put out this quiz. I have some grievances against the answers and am curious for other's thoughts on the matter. I feel like some of the answers are "it depends" scenarios more than "one size fits all."

https://docs.google.com/forms/d/e/1...vsQUw6DVK3BpJ5e61TYabbsg/viewform?usp=sf_link

 

[dik]

What grievances do you have? Not a bad idea to see what the skill set of the crowd is....

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Aesur]

As dik said, what grievances do you have? When reading through those I have many questions I would ask for clarification on before I would even answer. For instance, the best and worst locations for the key.. depends, are we talking design wise or construction wise?

 

[Greenalleycat]

I just did it and failed a couple of the questions therefore it is dumb and your coworker sucks (/s)

Interesting quiz though, a couple of good things to think about in there
I flat out disagree with an answer or two and I didn't understand one of them, though I only gave myself a 6/10 for confidence as I hate retaining walls!

The question about adding a key to increase the bearing pressure...
I said that the key increases the overturning moment on the wall, which your coworker agrees with
However they also think that the key is adding weight which increases the bearing demand, and that the key is too close to the toe, it should be closer to the heel
I disagree with these...the weight of the key will be absolutely negligible in comparison to everything else going on, and that load will go straight through the key itself and into the soil - it won't affect the bearing demand under the toe of the wall
Also, moving the key further back increases the virtual height of the wall and this is what increases the bending demand on the wall and hence bearing on the soil...how does having it further forward make bearing demands higher?

The question about active/at-rest/lateral bearing/passive pressure I got wrong as I said passive pressure...I think this is a country terminology thing though as I've never heard of lateral bearing
It's definitely passive pressure here

The question about the client wanting a more economic footing just confused me

 

[JoelTXCive]

I've never heard of lateral bearing pressure either.

 

[BridgeSmith]

I was going to answer with basically all the same comments as greenalleycat, except I gave myself an 8/10 for my understanding of retaining walls.

Btw, the AASHTO LRFD Bridge Design spec now uses the more proper term of "passive resistance" rather than passive pressure.

Also, I'd be interested to see the calcs where adding a key would increase the bearing pressure from 1344 psf to 1512 psf. I think it would have to be a unnecessarily deep key to change the bearing pressure by that much. Every time I recalculated my bearing pressure for the increase due to a key, I found the difference to be negligible. I don't even bother to do it anymore, with the calculation of bearing capacity having such a large error factor.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[EireChch]

Joel - I agree with you re lateral bearing pressure!! We dont calculate lateral bearing pressure!

 

[Just Some Nerd]

I think the point being made by the question is that the passive pressure (400) is much higher than the bearing pressure of the soil (150), lateral or otherwise. I've never had anyone bring it up before, but I can see what is being said in that a passive earth pressure far beyond what you'd allow for bearing counts as a failure, and sliding capacity is governed by the bearing pressure instead. Not really qualified myself to argue which is correct.

 

[Settingsun]

All those questions and missed the big one: Does active pressure stop at the underside of base slab or continue to the bottom of the key?

I've seen lateral bearing pressure used in thrust block design for buried pipes. Always seems to be larger than traditional passive pressure for shallow pipes. I wonder though whether lateral bearing pressure here is meant to mean passive with a limit on movement. I've seen some geotechnical engineers on this forum use 'allowable bearing pressure' to refer to 25mm settlement limit, while 'allowable bearing capacity' is the ultimate capacity divided by FOS.

 

[Tomfh]

I found it a little prescriptive, one engineers view on “the right way”, when in reality a dozen engineers are going to do it all a little differently.

 

[BridgeSmith]

Steve49, is the lateral bearing pressure you've seen used more of an 'at-rest resistance' that is the pressure exerted before there's enough movement to engage full passive resistance? Maybe it's assumed hat the movement of the wall required to mobilize full passive pressure is not acceptable?

Anyway YoungGunner, please let us know what you learn about the quiz questions and answers in your training.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[skeletron]

I got a few wrong as noted above. But otherwise great idea to have this as part of a work presentation. I think it is useful to do a quick test with everyone and have a discussion about it, rather than just spitting knowledge from CRSI standards or whatever other reference they are using. It's also a painfully funny test to give structural engineers because nearly every answer could be..."it depends"...just the nature of our job to think of "the straw to break the camel's back" in any scenario.