Should a structural Engineer design MSE walls? 1

[Geotech2004]

Should structural Engineers design MSE walls and if so, is it there responsibility to check the Global stability?

 

[fndn]

The structural should perform the internal stability like location and the amount of steel, connection to footings etc, but a geotechnical should perform the external stability.

Usually the geotechnical will do the global stabilty including the equivalent fluid pressures, drainage requirements, bearing capacity, etc.

 

[Geotech2004]

thankyou very much fndn, for your response. If then the structural designs that wall, say a three tier wall, is required of them to inform their client to make sure the global is checked? Or does the responsibility for the structural end with his finishing the design and performing local stability.

 

[fndn]

Geotech2004;

In my opinion, you have two scenarios. Either the structural has designed such walls in the past and has an idea of geotechnical parameters to expect from the soils report or is new to mse walls. If he is experienced, he may take the values from the soils report or use more conservative parameters. Say if equivalent fluid pressure of 45 pcf is given, he may use 55 pcf.

If however, he is new to such wall designs, I think it is prudent to learn from literature or just pass up the project to others who have more experience.

If there was a failure and a suit was filed, experts from both sides will pinpoint as to the likely cause. Say it was lower than design earth pressures, then the structural will be out of harms way, although he has spent some money to get to that point.

 

[BigH]

Geotech2004 - I think a bit has to do with being the engineer of record. As the geotechnical consultant, you would, based on your site investigation, provide to the structural engineer the key factors on which to base his design - allowable bearing pressure, for instance. Usually retaining walls are the domain of the structural engineer - but if he has done these before, he will know that the global stability has to be checked. In your soils report, you would clearly state, given the stratigraphy, if global stability is a strong or moderate consideration. If he is founding a short wall on very good soil, you wouldn't likely (nor would he) actually carry out a "global" stability check unless there were other factors - geometry, previous deep excavation that might not have been backfilled properly, etc. But, if you have any hesitation that global stability is a concern, express it and ensure he knows it. But, again, he should know this as well - and ask you to verify.

 

[amanpreet]

I recommend personally to check for the global stability failure, thats one of the important checks and it may cause you trouble if you submit your design without the global stability failure as there may be slightest chance of failure then also. And you have to show that how much is the factor of safety against global stabilty falure.

regards
Amanpreet

 

[jdonville]

The DOTs that I have worked with have required the geotechnical to perform global stability analyses as part of the evaluation of external stability. They will generally require an analysis of drained and undrained conditions, as well as rapid drawdown if applicable.

Internal stability is usually the reponsibility of the specialty contractor/designer of the MSE wall (i.e., Tensar, Reinforced Earth Co., etc.)

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[Geotech2004]

I thank you all for your replys. The situation I am in involves a retaining wall with two and three tier sections. The structural designed the walls "far enough apart" to where he thought they acted independently and basically thought he wouldn't need to perform global. After I check the Global stability I discovered that the walls where not stable as designed. So at the moment, we are gooing through redesigning of the walls to ensure stability. It was a lesson for me to in future ask if global has been performed and not just take it for granted.

 

[jdonville]

Geotech2004,

FHWA publication NHI-00-043 gives formulas for determining if tiered/superimposed walls can be designed independently or not. Also included are guidelines for stability analyses for multiple-wall situations. The PDF can be dowloaded for free from FHWA at

http://www.fhwa.dot.gov/bridge/geopub.htm

It is, in my opinion, essential reading for MSE and RSS design and analysis. And an excellent example of US tax dollars at work.

Hope this helps,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[VAD]

Based on reviews of some MSE wall designs where structural and geotechnical engineers were involved, I am of the opinion that the design concepts are based on a thorough understanding of geotechnical issues especially since we are seeing varying MSE design concepts being promoted.

While the FHWA and AASHTO have coined approaches to design of these walls there are many more developments that are equally sound and have been proven through actual field monitoring.

Where a bridge structure is involved, there is need for both the geotech and structural engineer to be involved. For non-bridge structures, the geotechnical engineer should be sole designer.

The design of the MSE wall is not only passing on geotechnical parameters to the structural engineer especially if it a geosynthetic or wire mesh reinforced wall. One of the issues of the design is the facing elements which can be lead to overconservative designs if there is not a good understanding of lateral load transfer to the facing. This depends on the rigidity of the geosynthetic or wire msh inclusions amongst others.

 

[MSEMan]

As a practicing MSE specialist I'll add my two bits here. My own opinion is that both structural and geotechnical expertise is required. Here's why.

It's important to remember that the design rules for MSE walls (FHWA, AASHTO, BS8006, Clouterre) are simplified methods which have adapted the theory based on empirical observations. If you don't understand why, for example, the state of stress within the MSE mass varies (and differs depending on the type of reinforcement) then you shouldn't be designing a MSE wall. An engineer with at least a solid grounding in soil mechanics is best placed to make judgements about the behaviour of the mass when one strain rate is supressed. Also the design rules, because they are based on empircal observations fall apart when applied to novel cases. A geotech is likely to have more sophisticated tools to evaluate the overall stability of MSE structures in unusual applications (three or more tiers is a great example.)

On the other hand geotechnical engineers tend to be clueless about practical aspects of MSE wall design. Ask a geotech about the facing panel design and you are likely to get a blank stare. Structural engineers also tend to have a better feel for evaluating materials, loads and their application to structures such as parapets and anchor slabs and performance criteria when these criteria are driven by the supported structures (pavements or bridges).

I guess where I tend to end up on this issue is that a structural engineer is usually perfectly competent to handle MSE wall design or checking when he understands the basic theory of MSE walls, the MSE application is standard and incorporates standard materials, and the wall is founded in very competent soils.

In all other cases the structural engineer would be best advised to get specialist geotechnical input.

 

[wallguy]

A lot of great suggestions, may I follow up on MSEMan's comment. From my screen name you can tell my area is MSE segmental walls. Most steep slopes are a red flag for global. Wall designers will usually tell us that global has or has not been performed and then suggest that it be done if applicable. So my suggestion is get a Structural or Geotechnical whose expertise is in MSE and use them as your go to person and then when something is outside there scope of work they will be the first one to tell you who needs to review or stamp that section. Maybe a little oversimplification but seemed missed in earlier comments.

 

[layfieldman]

I was recently involved in the redesign of a 4 tier concrete panel wall system into a slope specifically because the global stability check had been missed. This was a design/build by the general contractor who used the design provided by the MSE wall supplier. The consultant thought the it was the contractors responsibility, the contractor thought it was the MSE wall suppliers responsibility and the MSE wall supplier thought it was the consultants responsibility. The owner was not prepared to pay any more than the bid amount as it was a design build project. In the end to prevent any lawsuits all parties agreed to re-design the wall system into a reinforced slope.

You must pay very close attention to the bid specifications as in many cases it will tell you who is responsible for the different aspects of design.

When in doubt contact a qualified geotechnical engineer who has experience in MSE walls.