When is Slope Stablity Analysis Required or Not Required

[RFreund]

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!

EIT

http://www.HowToEngineer.com

 

[GeoPaveTraffic]

No code requirements that I've ever seen, but I'm sure they exist in some areas.

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

 

[oldestguy]

In my case usually in regard to your last question, it is similar to my veterinarian and his methods. I took my "sick" do to see the vet yesterday and the result of his examination was to give a pain shot and send me home with pain pills for a week. His diagnosis was likely a strained back muscle. However, his equipment includes MRI, etc. (which he mentioned), but I got the implication that the determination was not exactly 100 percent, but likely to be "good enough". So goes it with my geotech experience as to what is likely the risk, etc. and is it worth the expence for a lot of tests, etc. Experience means a lot.

 

[RFreund]

Thanks for the responses.
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

http://www.HowToEngineer.com

 

[Doctormo]

Building codes (IBC, IRC) tend to be silent on civil and geotechnical issues as they are for buildings (thus the name). They really only address foundation walls or walls adjacent to buildings which would rarely involve a global stability analysis. The IBC also says wall under 4' do not require a permit so architects and site engineers routinely stack numerous 4' walls on top of each other trying to sidestep the code much less a require a global stability analysis.

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.

 

[Okiryu]

This is a good question. Nice to hear the above opinions and looks like all are in the same page. By sure, I am not as experienced as Mike, OG or Doctormo, and considering that I normally work in building projects, the few times I have done slope stability analysis for retaining walls, was when we were analyzing for wall footings in slopes. The wall footings served as retaining walls as well. As Doctormo indicated, FOS were typically less than 1.5 so we used piles for these projects.

 

[geotechguy1]

Most engineers in my area argue that they are only responsible for the stability of the wall, not for the global stability of a slope. It's an argument that works 99% of the time on contractors and clients, until someone decides to go to court, whereupon you will probably get laughed out of the court room for making that claim.

A slope stability expert who doesn't know about slope stability? Good luck.

 

[BigH]

I am sorry but arguing that the geotechnical engineer is not responsible for global stability - but only the wall stability is total BS - and I don't mean Bachelor of Science. You need to look after the whole picture, not just a small part of it.

 

[RFreund]

BigH - Is there any situation where you say OK by inspection? I mean going through a global stability analysis might not be economically feasible in some situations, no? Say a 5' tall wall on flat ground? Any parameters where you could sensibly say "OK, by inspection"?

Or maybe this is the point of the ICS, so that you have some sort of an idea.

EIT

http://www.HowToEngineer.com

 

[Doctormo]

BigH - Most site geotechnical reports that I see do not address retaining walls other than maybe an equivalent fluid pressure for a basement wall and do not include any type of slope/global stability analysis nor do they provide sufficient information (i.e.: any soil strength testing) for anyone to assume such responsibility for such analysis from their reports. It is very common for geotechnical engineers to limit their scope of work as usually defined in the report for economic reasons. Why no piezometers, why no shear testing, why no environmental checking and so on? It is equally reasonable for a wall designer, if not the geotechnical engineer, to limit their scope of analysis based on similar qualifications but they need to be equally clear about it. We all know that concrete retaining wall designs rarely include any sort of global stability analysis but if the structural engineer was asked, he/she would say that of course it was not checked and that the geotechnical engineer should do that if necessary and so on.

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.

 

[BigH]

The lawyer/legal issue aside, when a geotechnical engineer determines the characteristics of the subsurface conditions and then appreciates the wall design and location - it will be fairly obvious, in my view, as to whether an overall global stability analysis is required. For instance, the 5 ft retaining wall on flat ground. Unless the soil is horribly "bad" - an analysis wouldn't be required - same with excavtions. However, if the 5 ft retaining wall was sitting on a clay slope where the retaining wall and the ground behind were added, then a global stability analysis would be appropriate. BTW, I never gave "equivalent fluid pressures" - always gave the appropriate coefficient of active and or passive or at rest values.

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.

 

[fattdad]

In private practice, it's a requirement of self preservation.

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!

 

[geotechguy1]

>The lawyer/legal issue aside, when a geotechnical engineer determines the characteristics of the subsurface conditions and then appreciates the wall design and location - it will be fairly obvious, in my view, as to whether an overall global stability analysis is 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.

 

[Doctormo]

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

 

[Doctormo]

The reply sent itself somehow...

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?