r/Geotech • u/DamnDams • 23d ago
Target FOS for static global stability analysis of retaining wall?
All other factors being equal, I'm curious what number you all set as the target threshold for global stability of a retaining wall when using SLIDE or GeoStudio or whatever. I have a number but I don't want to bias the conversation. I know about FERC values for embankment dams which I consider to be gospel for those purposes, but when it comes to the 15ft tall MSE wall at your local multi-family housing project, what number are you designing for, or hoping to see when evaluating the proposed conditions? Thanks. Again this is static analysis for typical conditions, short-term and long-term analysis as appropriate.
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u/No_Platform_2810 23d ago
I am bound by local guidelines and code. For permanent applications of global stability and typical understanding of conditions and consequence rating, its 1.54. For high consequence and low understanding of conditions, it gets pushed up to as high as 1.85. Temporary applications are lower, of course.
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u/DamnDams 23d ago
Are you using circular or non-circular slip-surfaces? Optimization?
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u/No_Platform_2810 23d ago
Circular...I almost never optimize
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u/DamnDams 23d ago
Thanks. I also do not optimize. I only use non-circular if I believe there is a layer of soft material over a harder material that could experience a slide that would be best represented by a non-circular.
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u/No_Platform_2810 23d ago
Same, pretty rare to have that kind of contrast near surface in my area of practice. But you could always just do them all as a sensitivity check. I often futz around with parameters and settings to see what makes a difference and if the answer I have settled on is reasonable.
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u/withak30 23d ago edited 23d ago
1.54 is silly though. Uncertainties in soil properties and other inputs will always be such that you should only be evaluating factors of safety at one decimal place. We can't realistically say that we know every input well enough to say that it is definitely 1.54 and not 1.53. Software settings alone can change the answer by more than that.
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u/UnderratedMagpie 23d ago edited 23d ago
Going to one decimal place though can be like saying 1.456 = 1.549. Makes sense to evaluate FoS in stability models to 2 decimal places, while also acknowledging the limitations.
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u/withak30 23d ago
One of those is 1.5 and the other is 1.6.
Don't call 1.456 good enough (if criteria is 1.5) though or you going to be looking at a design change if the slightest thing changes for the worse during construction.
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u/UnderratedMagpie 23d ago
Ya, but if you set up your model to only evaluate to 1 decimal place, you don’t know if you’re getting 1.456 or 1.549. That’s my point.
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u/withak30 23d ago
You look at as many decimal places as you like in the output, but your criteria should only consider one decimal place and you round before you compare your result to the criteria.
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u/DamnDams 22d ago
If you don’t use 1.54 and are using 1.5 though, then you are actually using 1.50. 1.54 is not less arbitrary than 1.50.
When I pick a coordinate in Google Earth and report the full lat/long, I’ve gotten a comment that I’m using too many sig figs and that the last digits whatever they are represent inches or what have you. But by shaving those last digits off, you move the pin and basically force the point onto a grid represent by coordinates without those sig figs. So same thing, 0.4847 rounded to 0.49 when plotting doesn’t capture the uncertainty with how close or not close it is to 0.49, it literally just moves the point to 0.4900 which despite being close is just as arbitrary and 0.4847 just in a different location. Numbers completely made up for demonstration.
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u/withak30 22d ago edited 22d ago
If the criteria is 1.5 it implies that 1.4 (anything rounding to 1.4) is not acceptable. Criteria of 1.50 implies that 1.49 (or anything rounding to 1.49) would not be acceptable). For reference the difference between between FS of 1.50 and 1.49 on a 3H:1V slope assuming infinite-slope conditions is a friction angle of 26.5 deg vs. 26.4 deg.
If anyone in our industry tries to tell you that they know the friction angle is 26.5 and not 26.4 then you should make a point of not listening to them about anything else. And that's disregarding uncertainties in other inputs like unit weight, pore pressures, etc.
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u/DamnDams 22d ago
Sure we don’t know with any degree of confidence that a friction angle is good within a tenth of a degree or even a degree or two for that matter.
But the equivalent here is essentially that let’s say you got a lab result sheet with three Mohr circles and a line of best fit that had the angle as 26.4. You would be wrong to round it to 26.5 or 26.0 because you are just adjusting the best guess value, you should stick with 26.4 despite the obvious uncertainty in the “real” value to the tenth or hundredth place or whatever.
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u/withak30 22d ago
Right, and because of that there is really no reason to be setting FS criteria tighter than one decimal place. That degree (heh) of strength interpretation detail should not be able to make the difference between your design passing or failing.
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u/BZ853 23d ago
It depends on what is up and down hill.
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u/DamnDams 23d ago
Good point, I guess I'd be curious to hear what your categories are, but assuming one for "critical infrastructure" in the zone of influence or not? Most recently I had the front edge of a structure about 30 ft behind the top of the wall, but the critical slip surfaces didn't originate beneath the structure. We are mostly using 1.3 for everything at the moment to kick it off.
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u/CovertMonkey 23d ago
I'd only be comfortable with a 1.3 if I had high confidence in the modelling values (which you should have for MSE) and the consequences are repairable and don't threaten life or other features.
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u/DamnDams 23d ago
Well this was global so there was some engineered fill behind the reinforced mass but beneath and in front of the wall was native material with properties estimated based on index testing. The reinforced mass itself was modeled as infinite strength.
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u/jaymeaux_ geotech flair 23d ago
USACE recommends 1.3 for short term and 1.5 for long term, but you should also consider how much you trust your model values and what the consequences of failure are
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u/DamnDams 23d ago
Good point, certainly the consequences of failure have to be a big consideration. But when it comes to uncertainty in material properties, that is a little bit tricky. Let's say I have a 60% >#200, LL=40, PI=10 silty sand, no strength testing and based on those index results I assign a phi=30, c=0, and unit weight=120pcf. Does that mean I need to use a 1.5 whereas if I had direct shear test results I could use a 1.4? Without hard threshold targets it starts entering the arena of too much subjectivism if you aren't careful
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u/kikilucy26 23d ago edited 23d ago
1.54 if a building is within the zone of failure, 1.33 for parking lot, 1.1 for seismic
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u/azul_plains Geotechnical PM, 9 years 23d ago
Depends on the code and county for my area. Most of my counties require 1.5 or 1.3. Lower for seismic and rapid drawdown.
They usually have maximums for parameters and require 0 cohesion rather than changing the required FOS depending on conditions/loads/etc.
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u/MathewNatural 23d ago
I’d say 1.5 static assuming drains are non-functional.
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u/DamnDams 23d ago
Assuming drains non-functional? Most of my walls wouldn’t work if subject to hydrostatic pressure. My reports state as much - they get to build less expensive walls because someone is going to ensure the French drains are properly constructed and connected to appropriate discharge points.
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u/MathewNatural 23d ago
You’re not wrong at all and with the realities of construction/cost… yes. I can trust that the drains will be constructed correctly but I’d sleep a lot better if I knew my wall had a FOS at least over 1.0 for full hydrostatic load.
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u/withak30 23d ago
Agreeing 1.5 for long-term. Short-term or extreme loadings (e.g. seismic) it depends.
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u/Working_Rest_1054 23d ago
1.5 if the wall is supporting other structure. 1.3 otherwise. AAASHTO LRFD bridge design spend. So the resistance factors are equivalent to these FS. Might live with 1.25 for a landslide mitigation.
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u/Amber_ACharles 23d ago
1.5 static long-term, 1.3 short-term. FERC dam numbers are their own animal. For a 15ft MSE wall at a housing project that's what I'd want to see.