When is Slope Stablity Analysis Required or Not Required
When is Slope Stablity Analysis Required or Not Required
(OP)
When is a slope stability analysis required on a project? I ask this from a few different perspectives...
Generally speaking this is in reference to a situation where you have a new retaining wall being installed on a new project. When is an "internal or internal compound stability" analysis (i.e. sliding/overturning/bearing checks) not enough? I mean if you have a short wall with flat top, flat bottom slopes. It doesn't seem very necessary to perform a slope (or global) stability analysis.
Does the "code" ever require it? By code I'd be referring to IBC, AASHTO or any other major codes.
Is it up to the project geotechnical engineer to decide?
If it is up to the Geotech, how do they decide?
Thanks in advance!
Generally speaking this is in reference to a situation where you have a new retaining wall being installed on a new project. When is an "internal or internal compound stability" analysis (i.e. sliding/overturning/bearing checks) not enough? I mean if you have a short wall with flat top, flat bottom slopes. It doesn't seem very necessary to perform a slope (or global) stability analysis.
Does the "code" ever require it? By code I'd be referring to IBC, AASHTO or any other major codes.
Is it up to the project geotechnical engineer to decide?
If it is up to the Geotech, how do they decide?
Thanks in advance!





RE: When is Slope Stablity Analysis Required or Not Required
As for how geotechs decide, one word: Experience. It all comes down the the site soils, slope geometry, and height of the wall. I have seen 3 foot high walls that I was worried about and 10 foot walls that I never gave a second thought to.
Mike Lambert
RE: When is Slope Stablity Analysis Required or Not Required
RE: When is Slope Stablity Analysis Required or Not Required
My question stems from the fact that (in my area) there still seems to be confusion over when a global/slope stability analysis is required and who is responsible.
A contractor might ask for some budget pricing for retaining wall design. Generally this is in regards to a segmental retaining wall design. The challenging part is that I believe the global/slope stability is the responsibility of the project geotechnical engineer, so I wouldn't include this as part of our scope. We would only do internal design checks. I make sure they are aware of this and this seems to be the norm from what I understand. However, I wanted to get a better feel to see if there was a different understanding of this situation. And also when a global/slope stability might be required (really any code requirements).
Anyway, thanks again!
EIT
www.HowToEngineer.com
RE: When is Slope Stablity Analysis Required or Not Required
In the private sector, the NCMA design manual has a section on global stability and does not say to not do it so they would seem to require it (same with compound stability which is a subset of global stability).
The AASHTO code is not silent on global stability either and discusses it in the retaining wall section. However, highway departments (DOT's) routinely do not address global stability if it does not appear to be a concern. On the other hand, if one is designing walls for a highway department project, the question "where is the global stability analysis?" seems to come up more often in my experience.
Opinion: The problem with global stability analysis of retaining walls is one of "false positives" which means the analysis will come up with "problems" when no problem exists and then requires a major effort to determine the actual soil profile at the wall location, the appropriate soil design strengths (long term, short term, granular, cohesive, rock), and then the appropriate analysis method (Bishop, Janbu, etc) and factor of safety (1.3 or 1.5). All of this can have economical consequence thus the need to be careful about responsibility for such items.
Observation: If there is a failure or performance problem with a retaining wall, all experts and bystanders will ask where the global stability analysis is and then blame everything on that if there is not one. The lawyers will remind everyone of the engineer's negligence to not have done a global stability analysis and so on. You just need to be careful and clear what you provide and what is the responsibility of others then pray that your walls never have problems.
RE: When is Slope Stablity Analysis Required or Not Required
RE: When is Slope Stablity Analysis Required or Not Required
A slope stability expert who doesn't know about slope stability? Good luck.
RE: When is Slope Stablity Analysis Required or Not Required
RE: When is Slope Stablity Analysis Required or Not Required
Or maybe this is the point of the ICS, so that you have some sort of an idea.
EIT
www.HowToEngineer.com
RE: When is Slope Stablity Analysis Required or Not Required
Now if the site geotechnical engineer is the retaining wall or slope designer, it can be argued that he/she has a responsibility to let the client know that additional work must be performed to acquire the information necessary to complete a complete wall/slope design (such as a boring and lab testing where the actual structure is located) and refuse to do such work without it. However, a wall designer, structural or civil, can only recommend that the appropriate additional work be performed by the owner's geotechnical engineer (or its client) and must qualify its design in the absence of complete geotechnical information necessary to perform all analysis possible with any degree of confidence.
FReund - My previous reply indicated that it is always required by lawyers and experts if there is a problem. I was asked to do a global stability analysis of a battered 5' gravity wall, level on top and bottom, no surcharges, when it was pretty clear that this failure mode was unlikely to say the least. It would tip over long before it would every experience a complex failure mode but that does not stop unreasonable requests nor experts from questioning everything.
ICS (Internal Compound Stability) is just a very limited subset of global stability thus it does not really help with the big stability picture. A wall on top of a steep slope or a set of tiered walls is completely missed by ICS thus it usefulness is very limited. If you are going to look a compound stability issues, there is no substitute for a complete evaluation but you need to be able to complete the soil-strength model from a site specific investigation to get results that are useful.
RE: When is Slope Stablity Analysis Required or Not Required
I guess the question is of "responsibility" - at what level does the geotechnical engineer's duty end and the engineer of record (the designer) begins. If I was doing a geotechnical report and, in my view, a global analysis was definitely required, I would do it - but I can see how some would go to the designer and tell him that it needs to be done and it will "cost" more money. But someone has to do it.
RE: When is Slope Stablity Analysis Required or Not Required
In public practice, it's a requirement of the contracts, which often cite AASHTO, FHWA or our own state-wide guidance.
We define critical structures (i.e., walls over 10 ft, structures whose failures would stop traffic, etc.) and require external and global stability to safety factor of 1.5. We define critical slopes also, as slopes over 25 ft (or slopes whose failures would stop traffic). For critical slopes and structures, we also require something more than N-values to assign strength. So, these features will necessitate, in-situ testing or laboratory strength testing.
f-d
ípapß gordo ainÆt no madre flaca!
RE: When is Slope Stablity Analysis Required or Not Required
In my area the majority of retaining walls are designed sans geotechnical report, or based on a general geotechnical report done for an entire subdivision or a larger site. The problem is that the 'interesting' areas with problems tend to be where owners want to not put buildings, such as right at the boundary of a swampy area. I frequently try explaining to my peers that every wall they design without a site-specific investigation is probably an automatic lawsuit loss, but the lawsuits never seem to happen, or get resolved otherwise. "Standard Local Practice" seems to win out.
RE: When is Slope Stablity Analysis Required or Not Required
On the legal front, experience tells me that notes have to be clear and responsibilities need to be defined to the point where you end up being responsible for a
RE: When is Slope Stablity Analysis Required or Not Required
geotechguy1 - I totally agree about lack of borings where needed and the owner's propensity to rearrange sites after the geotechnical investigation seeking out the swampy area to build a wall and drive lane.
On the legal front, experience tells me that notes have to be clear and responsibilities need to be defined to the point where you end up being responsible for a limited set of items that clearly rely on information provided by others or else you end up owning everything. Lawyers review all design drawings in great detail and will ask about every note assuming if you were not clear about something, it was your responsibility to be clear and what was the contractor to do, read your mind?