soils friction factor
soils friction factor
(OP)
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?






RE: soils friction factor
Mostly, geotechs work with allowable values rather than ultimate. So you know that the information provided to you has been factored in some way.
RE: soils friction factor
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.
RE: soils friction factor
RE: soils friction factor
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 VPL for tips on how to make the best use of Eng-Tips Fora.
RE: soils friction factor
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.
RE: soils friction factor
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
RE: soils friction factor
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.
RE: soils friction factor
VAD 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.
VAD 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 VPL for tips on how to make the best use of Eng-Tips Fora.
RE: soils friction factor
RE: soils friction factor
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: soils friction factor
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.
RE: soils friction factor
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 VPL for tips on how to make the best use of Eng-Tips Fora.
RE: soils friction factor
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.
RE: soils friction factor
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 VPL for tips on how to make the best use of Eng-Tips Fora.
RE: soils friction factor
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.
RE: soils friction factor
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.