# Postion of overturning moment for retaining wall

1. I sometimes design my retaining walls with longer toes and heels, this is because in my FEA program, when I pull my footing under my wall towards the toe, I get lower pressures on my soil (springs) that when I pull it towards the heel (even with the added benefit of the earth on top). This tends to happen when I don't have too much counter balancing soil on my heel. Anyone else get these results? I've never seen any long toe retaining walls in the literature.

2. When calulating over turning moment by hand, all of the literature I've seen takes the point of rotation at the toe of the wall. If I look at my FEA design, the wall seems to be rotating much more around the heel. Is it justifiable to calculate the overturning moment at the toe? This seems to be the point that's most favorable for design, but is it realistic?

• It is difficult to speak to either of your questions without more information about your modeling techniques. #2 suggests that you might be having some boundary condition issues in your FEA or that I am misunderstanding what you mean when you say "rotating...around the heel." – William S. Godfrey- S.E. Nov 21 '16 at 14:15
• No need for modeling techniques, looking for people who #1.Have designed long toe retaining walls #2. Calculate the overturning moment by hand. When I say rotating around the heel, I mean that the overturning moment is calculated about the leftmost point of the footing. – user2817017 Nov 24 '16 at 12:27
• You speak explicitly to FEA modeling in both of your questions. The assumptions you use when doing that FEA modeling impact the observations you discuss in both questions. Your first question appears to be asking if anyone else has observed similar FEA results. It's impossible to answer that question without knowing more about your model(s). Further, #2 is even harder to answer because you indicate that your model is not behaving as expected. It's impossible to help you figure out why that might be happening with no information about the specifics of the model. – William S. Godfrey- S.E. Nov 28 '16 at 1:23
• If you are attempting to ask a question about a fundamental behavior and how it relates to classical hand methodologies, I suggest that you reword your question, or better yet, resubmit a better worded question. – William S. Godfrey- S.E. Nov 28 '16 at 1:26
• I have designed plenty of retaining walls with long toes in situations with low allowable bearing capacities. Usually designed with RetainPro (which is not FEA), but also using "by hand" calcs as well. – Rick supports Monica Jan 24 '17 at 22:27

## 1 Answer

I have designed plenty of (probably several dozen) retaining walls with long toes in situations with low allowable bearing capacities. Usually designed with RetainPro (which is not FEA), but also using "by hand" calcs as well. So your results are not unusual.

With regards to rotation about the toe or heel: overturning and rotation are two different things. You actually want the wall to rotate about the heel just a bit so you can transition from at-rest to active soil conditions, and relieve the locked-in horizontal soil load (from compaction behind the wall).

However, if the wall is going to catastrophically fail, which is what an OTM check is looking at, it WILL rotate about the toe eventually. At the point in time it fails, such a wall will probably have initially rotated around the heel, but the overturning failure will be about the toe. Given the presence of the soil underneath constraining movement into the soil mass, how could it not? But the walls you are analyzing don't seem to be in danger of catastrophically failing (or your FEA would not converge).

Think of it this way: in order for the wall to fail due to the applied overturning moment about the heel, the footing toe would have to rotate INTO the soil. This cannot happen unless there is a slope failure, but that is a separate discussion topic. And that failure mode is not being analyzed in your FEM- that is, unless it's a geotechnical analysis package and you have some kind of decent soil modeling design parameters, which nobody ever has.

Try this experiment in your FEM: put an X fixity at the footing toe (so it cannot slide). Then incrementally ramp up the horizontal load only (no corresponding increase in vertical load). The structure will eventually rotate about the toe. Checking this kind of load condition- high horizontal loading (usually due to live load surcharge BEHIND the wall heel) with high resistance to sliding- is the purpose of the OTM check. Under such a loading condition, it would make no sense to check the OTM about the heel. The wall simply will not fail that way unless there is a localized bearing capacity failure.

• "How could it not?" If I take my FEA model and incrementally up the load, it will eventually tip over, but not rotating about the toe : The rotation point depends on many of the problem variables. I guess I'm trying to see why calculating the moment about the toe is justified as it is the most non-conservative point you can take on your wall. How usefull is OTM if it doesn't represent something that would actually happen? – user2817017 Jul 11 '17 at 17:00
• @user2817017 I understand but the boundary assumptions of your FEA model are not valid after a certain amount of deflection. What I'm saying is that overturning about the toe IS what will happen under certain conditions that the FEA model is not considering. – Rick supports Monica Jul 11 '17 at 18:41
• @user2817017 Think of it this way: in order for the wall to fail due to the applied OTM about the heel, the footing toe would have to rotate INTO the soil. This just isn't going to happen (unless there is a slope failure, which is a separate discussion and is not being analyzed in your FEM unless it's a geotechnical analysis package and you have some kind of decent soil modeling design parameters, which nobody ever has). – Rick supports Monica Jul 11 '17 at 18:44
• @user2817017 Try this experiment in your FEM: put an X fixity at the footing toe (so it cannot slide). Then incrementally ramp up the horizontal load only (no corresponding increase in vertical load). The structure will eventually rotate about the toe. Checking this kind of load condition- high horizontal loading with high resistance to sliding- is the purpose of the OTM check. – Rick supports Monica Jul 11 '17 at 18:47
• I think I get what you are saying. In my FEA model, the wall DOES rotate into the soil, because the soil stiffness is way lower than the footing flexure stiffness, but this kind of failure should be limited by my soil max pressure or wall max deflection. – user2817017 Jul 11 '17 at 23:58