Retaining Walls - When to Use At-Rest Pressure

[metalchair]

Hello All,

I am working on a retaining wall structure that is essentially a basement without a floor over the top. So it is a square with strip footings underneath the perimeter, but the top is not restrained by a floor. From how I understand it, active and passive pressures develop when the wall is allowed to rotate slightly, and an at-rest pressure is used when rotation is prevented.

I may be overthinking this problem, but here we are. Essentially, in my mind, on any given wall, the top of the wall is being restrained by the two adjacent walls. I think it is fair to say that if the wall length on any given side was 2ft long, there is no way the top of the wall in question would be able to rotate, as it is so short and in such close proximity to the adjacent walls. Similarly, even with an extremely long wall, lets say 250ft, the very ends would likely not be able to rotate at where they intersect with the adjacent walls. For this latter case, I would think that active/passive pressures would be able to develop in the middle, and an at-rest condition would be created towards the ends.

Follow-up Questions:

1) Do you believe my assumption of an at-rest condition near the adjacent walls is fair, or would you treat it as an active pressure along the whole wall when a floor is absent to truly restrain the top?

2) If an at-rest condition is present, do you design your wall for this condition, or simply go with the active pressure condition?

3) When checking sliding of the wall of a structure like this, in an at-rest condition, is there no pressure on the air-side helping resist sliding?

Thank you all in advance.

 

[KootK]

I was taught to use at rest for this condition. The argument being that:

1) You need a fair bit of wall movement in order for the pressure to drop to the active condition and;

2) One of the primary goals of such a design is to prevent significant wall movement.

In a way , this becomes a poor man's version of designing for deflection. You're not really designing for deflection because you haven't calculated it but, at the least, you're improving your odds by using the larger load associated with at rest.

 

[metalchair]

Thanks, KootK. So this is an area where deflection would be noticeable. So essentially you are saying that if deflection (an the associated lower pressures) are to be avoided, designing to at-rest pressure is the best bet?

 

[KootK]

Precisely.

 

[metalchair]

Thank you once again. I think that is a missing piece of the puzzle that should help me understand this all more.

 

[jerseyshore]

I approach retaining walls the same way as Koot, if it's not a traditional basement wall, but I can't afford that movement, I design for at-rest.

 

[KootK]

I may be overthinking this problem, but here we are. Essentially, in my mind, on any given wall, the top of the wall is being restrained by the two adjacent walls. I think it is fair to say that if the wall length on any given side was 2ft long, there is no way the top of the wall in question would be able to rotate, as it is so short and in such close proximity to the adjacent walls. Similarly, even with an extremely long wall, lets say 250ft, the very ends would likely not be able to rotate at where they intersect with the adjacent walls. For this latter case, I would think that active/passive pressures would be able to develop in the middle, and an at-rest condition would be created towards the ends.

While I stand by my previous comment, I think that I failed to address an important aspect of your question.

Regardless of the concern for deflection -- or lack there of -- I feel that at rest pressure is appropriate if it is the case that the wall is unable to move meaningfully owing to its restraint condition.

 

[HTURKAK]

Addition to the previous posts ,

- When you design the retaining wall for AT REST condition, the pressure which may be considered at the toe Shall not be greater than at rest pressure ( the assumption of development of passive thrust literally means the soil side pressure will be active, and the wall should slide substantially .) . My suggestion would be ,use only base friction when checking sliding ..







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