Investigation before the design of the bridge - the analysis at the time of design because the design (including grading/topography) also has major impact on the global stability. DO NOT CARRY OUT THE GLOBAL STABILITY AFTER THE BID!!!!
No one is saying but it is my understanding that global
stability was not carried out by the designer or his geo firm. I believe it was there belief that the reinforced earth wall supplier they single sourced would carried this out.
It is know my understanding that on integral abutments the embankments are part of the structure therefore if the global stability was not carried out we have a big problem on our hands,the worst slope will be 1 to 1.3.
A '1 to 1.3' slope is pretty damn steep - how tall will it be? How long? Soil and groundwater conditions?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
I practice in the U.S. in Ohio. The way that the transportation department does MSE contracting is the have prequalified MSE designers perform the design function. The geotechnical engineer will make some reasonable assumptions up front in terms of dimensions of the wall and run settlement and stability analyses. Once the design is complete, there should be a check on the original assumptions. The MSE designer is responsible for the internal stability of the wall only.
When there is a stub abutment bearing in the MSE wall, there must be a greater degree of coordination between the MSE designer and the geotechnical engineer. Also, for this case a higher factor of safety is generally required.
I agree with the statements of the two above. I've been involved with MSE wall design for nearly 10 years in both the public and private sectors. Based on what I have observed it is rare to find a wall designer who will take responsibility for external (global) stability. I interpret the reason for this is that if global stability is adequate, the MSE wall designer doesn't spend anymore money having to "understand" the subsurface conditions. However, having said that, I am also aware of many MSE wall vendors who are capible of completing a turnkey package (total design from global to internal stability), but I typically see this relationship on the private side of the practice.
If this isn't convoluted enough, realize that proper scoping of services up front would have made things much clearer (i.e., if the vendor is to complete all design, that should have been stated under the original contract so the original engineering firm would not be carrying budget for it). Unfortunately, many geotechnical proposals are very vague in describing their scope of services and statements such as "provide recommendations relative to the geotechnical design of the retaining walls" are worse than saying nothing at all. Your problem isn't entirely an engineering problem, it's a project problem.
It is interesting, in what I've seen for a while (outside US/Canada) how many designers simply decide - we'll go for an MSE wall (or whatever other moniker you want to use) and they'll be responsible for everything. The project is bid and the MSE Wall group - as a subcontractor/supplier to a construction firm is faced with putting a 10m high MSE wall on 10m of very soft to soft clay - with NO thought of improving the ground, etc. And then . . . well - you are designing the wall, foundation is incidental???????? As one Willoughby-ite to another (Glen), I agree - the MSE wall designer can only be responsible for the internal stability - but, there are hiccups when designers/owners don't understant the basics.
AASHTO requires all retaining walls have adequate factors of safety for global stabilty for both static and seismic. Retaing walls that support abutments have higher factors of safety. The engineer of record (in my opinion) should be responsible for ensuring global stability.
Agreed. At the very least, the state DOT should assume responsibility. (That won't happen, of course.) This is definitely beyond what a contractor should be required to do in a traditional design/bid/build arrangement.
A design/build contract would have been more appropriate had the DOT wanted the contractor to assume design functions as well -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
BigH is correct at when global stability should be completed. However, I have come across projects where global stability is left to the contractor. This creats a problem if the contractor does not understand what they are to do before they bid. In the furture at any time you encounter projects where portions of the engineering are not completed or required by others (say a MSE firms), the contractor should contact a geo before biding to ensure all costs associated with project are in place. I had to deal with this problem after contractor bid, and they were not happy (not with me) after they realized that changes regarding base preparation had to be made. And yes no one checked global stability until it fall on my lap, after bid prices were sent in.
cdh61 - good points. Hopefully you didn't have a "major" problem. I have seen a situation where it was "discovered" after the contract started that the ground consisted of 8m of very soft clay onto which a 10m high MSE wall was to be constructed. Cost of the foundation preparation - e.g., stone columns, etc. - would that all be in the contractor's price? (He didn't decide to put this type of wall there - that was the designer who specified it. If it was a RCC wall, then the foundation would be the designer's). Would the "owner" pay for ground improvement? This type of passing the buck can have major ramifications - it isn't a bit of extra excavation and replace, but . . .
The owner has agreed that the slope stability wasn't completed and has provided a remedy of extending the strips of the RE wall. We are proceeding without delays and have submitted pricing subject to some adjustments.
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