soils friction factor

[MikeE55]

Do Geotechs include a factor of safety when stating soils friction factors in a soils report? We usually see a range of 0.3 to 0.5. Until recently, I have always thought a 1.5 factor of safety should be applied to this value. For example, weight x friction factor / 1.5 = allowable sliding force. Then a geotech told me that whenever I am given a friction factor of 0.30 or .35, the factor of safety has already been included. Are geotechs consistent in their reports, or do I need to clarify this with each one?

 

[Qshake]

I don't think it hurts to know the practice in your area and so if you actually work in several different locales then I would say ask. What does it hurt?!

Mostly, geotechs work with allowable values rather than ultimate. So you know that the information provided to you has been factored in some way.

 

[ishvaaag]

I think they must separate the test or laboratory data or results from the recommendations. So you may have both values in sight, and why the recommended value is proposed.

This you may accrue by stating specifications about the geotechnical report to be made. This the designer or some in accord with him can usually do.

 

[MikeE55]

Thanks for your response. If there are any geotechs out there, I would like to hear your response as to how you arrive at the concrete/soil friction factor? It seems to me I am just getting an educated guess in the report, with conflicting opinions as to whether it includes safety factor or not.

 

[Focht3]

This value is seldom determined by lab testing. It usually comes from experience coupled with the foundation loads and geometry. [blue]Qshake[/blue] is right - most of us provide allowable values. While the lab tests play an important role in my decision making process, the geologic conditions, engineering judgment and experience are frequently more important factors. Very few of the good geotechnical engineers in the U.S. do as [blue]ishvaaag[/blue] has suggested; after all, why would I want to invite second guessing of my results by someone with less education and experience in my area of expertise? Besides, that kind of report format is difficult to properly present.

The soil/concrete friction value at the base of a footing will vary based on the depth of the foundation, the groundwater levels, the type of soils at the interface, and the dead load bearing pressure. Expect this value to vary a lot.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

I still think the correct way is the one I propose. A designer can state the requirements for his or her wanted data. Furthermore, I have found lot of people that know far less than they accordingly to their entitlements should.

In any case, designers by now are usually to prove that what they propose are within the ranges of the accepted knowledge and codes; so must getoechnical engineers proposing what are just results of their work.

Furthermore, many times the reviewing designer is not just an individual, but very competent teams that understand all and every item the geotechnical party is delivering or able to deliver. So happens usually with public works. As well, many times, as a condition of insurance, or even good practice or specification of the professional chapter or town council, the report is to be included with the project. To assume that no one in all this process is to be able to have an educated look about the geotechnical consultancy looks to me extravagant.

 

[VAD]

Interesting debate. There are statements made by Ishvaaag and Focht3 which are useful. Indeed, this is one area which geotechnical engineers have often used handed down values. If you review "Foundation Design" By Wayne Teng which was a popular book, you will note values that were recommended in the AREA - Railroad Specifications in 1958.

Teng suggests values of one-third to two thirds the friction angle of the soil that bears against the footing. Yet Terzaghi, Peck and Mesri in Soil Mechanics in Engineering Practice provide values that relate to friction angle of cohesionless material and suggest that this is the soil that a footing should bear on. No one seems to suggest what material state the friction angle is derived from i.e loose, compact or dense state. Indeed, they also mention about the influence of the normal stress. This would be related to cohesionles soil but some have factors for clay soils.

As well,Tam et Al - LRFD design - Duncan's Work. However, it seems that one makes modifications to these values based on what is expected to take place during construction.

To the best of my knowledge I am not sure that a factor of safety is typically applied before providing such a value in the report. This number could be factored may times as there is uncertainity about values to be used especially if no laboratory testing is done.

AASHTO Standard Specifications for Highway Bridges Table 5.5.5B is of interest since it refers to values quoted as Ultimate. These values are also of the same order of magnitude reported by the AREA Specifications.

If I recall, many practitioners also use values derived by Potondny - See Geotechnique (Can't remember the year but may be around 1961) for friction between soils and concrete etc, at least for pile design.

What is important to this value for a shallow foundation is that it can be controlled by a weak layer below the footing. This brings my pet peeve that both structural engineer and geotechnical engineer need to discuss and work together on such issues. Most values that one sees in a report are or should be identified as "preliminary" so that it can invoke some discussion so that loads, depth of burial etc can be addressed.

In summary, it seems that there is not much information provided in any literature that explains clearly how a value of friction should be determined. It is expected that the keen professional would not just pick a value from a text but would study the characteristics of the proposed foundation, loads, construction, soil characteristics, etc before providing a suitable value.

Here I agree that experience and judgement come into the fore but one should explain how the recommended value is derived, regardless if is the best choice from several values in the literature, as we all have to do that regardless of the field we are in. This is just the issue with the scientific approach.

I think Dr. Michael O'Neill before passing on said that there is a lack of the "Foundation Engineer" in our practice, one that can appreciate both the soils and structural aspects. There is no doubt in my mind that in the absence of this person both structurals and geotechs have to work together on these issues.

I hope this helps.

Regards

[Cheers]

 

[BigH]

Another reference is the NAVFACs manual. I would say that the "friction value" given by the geotechnical engineer is usually that value to which the normal force is applied to get the "resisting" value. Then the factor of safety is determined on the whole of the force/moment equilibrium. In other words, while there may (most likely is) some factor of safety involved in the giving of the friction value, the resisting value is "given" as one of the force components to which Sum(resisting) over Sum(driving) > 1.5.

VAD points out a very good point. It is many times not the soil right at the interface of the footing to the soil but the governing stratum might be at a small depth below. This is why global stability is so important in doing retwall designs rather than just the overturning and sliding.

to all my colleagues!!!

 

[Focht3]

Good discussion.


[blue]VAD[/blue] is dead-on correct: the soil/concrete friction angle seldom controls the design; the actual failure surface (in most soils) is usually millimeters to centimeters away from the soil/concrete interface (in the absence of weak zones, that is.) This is true for footings, retaining walls, grouted tiebacks, driven piles and drilled piers. It is important to note that the friction value very seldom controls the design; if it does, then some detailed discussions between the geotechnical and structural engineers should take place.

[blue]VAD[/blue] also said, I think Dr. Michael O'Neill before passing on said that there is a lack of the "Foundation Engineer" in our practice, one that can appreciate both the soils and structural aspects. I think that Mike O'Neill was dead-on correct; his point of view - and influence - will be sorely missed. Very little peeves me more than a geotechnical engineer with little or no regard for the structural engineer's job, or the structural engineer with little or no regard for the geotechnical engineer's art. And I have the deepest respect for those very qualified engineers that recognize their own limitations, and seek assistance when the assignment is beyond their areas of expertise.

In my view it's more important to recognize what one doesn't know, than to simply be aware of one's own limited knowledge.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MikeE55]

I appreciate the good discussion. What prompted my question is that sliding is very often controlling my retaining wall designs. Typically soils reports give me a friction factor of 0.3 to 0.35. If the friction factor is this low, sliding can easily control the design, and the retaining wall footing will be as long as the wall is high. Shear keys also can end up being to deep to be practical to construct. When I look at design aids such as CRSI, the friction factor ranges from 0.55 to 0.45. This yields a reasonable looking design, and makes me think that the friction factor I am using must have too much factor of safety. I am in the Southeast, so there are many clay and silt type soils. It doesn't help that the IBC 2000 has a chart listing the active pressure for this type of soil as 60 psf, and the at-rest pressure as 100 psf (Table 1610.1). Of course, you geotech guys can specify otherwise in a soils investigation report - and we structural types appreciate it when you do. Otherwise, our designs look ridiculus.

 

[Focht3]

I'll bet the values that you are getting have already been factored. What does your geotech say?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

Not technical but illustrates this thread.

I wish that all of you could come to a conclusion so I could help my neighbor who is a general foreman doing some interchange(5) work on the Interstate. He says that due to arguments between his people, the state, and the geotechnical people they are driving a test piling nearly every day.
Their retaining walls are precast segments with some of the longest tie-back anchors in the world.

I printed out Dr. O'Neill's comments about a Foundation Engineer. His comment was that it would save a hell of a lot money on this job.

 

[Focht3]

[blue]unclesyd[/blue] opined,

I wish that all of you could come to a conclusion so I could help my neighbor who is a general foreman doing some interchange(5) work on the Interstate.

Huh?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MikeE55]

Focht3,

I have one geotech who says that any value below 0.5 is too low and must contain factor of safety. I have another who says that he is taking the value 0.3 directly from NAVFACS, and that 1.5 factor of safety must be applied for stability.

 

[Focht3]

I have another who says that he is taking the value 0.3 directly from NAVFACS, and that 1.5 factor of safety must be applied for stability.

THAT really aggravates me! You need to politely request the engineer to provide calculations to back up his recommendation...

And I'd look for another consultant next time -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[ishvaaag]

I think that one thing that prevents the use of a reliably established friction angle or friction factor is the number of tests required to determine the value for which there is a 5% probability of being exceeded. Were the values so established one could directly use the reliable friction factor and then care of the safety with the usual limit material, load (or watever) safety factors.

The statistical approach is still at hand if one is wanting the reliability. We are doing this every day with compression tests of concrete cylinders, and the works don't look disparate, so our knowledge of the even more variable soils would be much benefited by such approach.

 

[Focht3]

If you are interested in applying probabilistic methods to geotechnical designs, check out a paper titled Factor of Safety and Reliability in Geotechnical Engineering, by J. Michael Duncan, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 127 No 8, August 2001. And be sure to read all the discussions - and Duncan's closure - of the paper. This will give you a realistic - and usable - guide to selecting appropriate factors of safety.




Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.