wall/foundation soil interaction coefficient 1

[nsong]

I am reviewing a retaining wall design. In the calculations, two parameters caught my attention. They are called "Wall Soil Interaction Coefficient" and "Foundation Soil Interaction Coefficient". For both parameters, a value of 0.9 are asigned. To my knowledge, such coefficient is the tangent of DELTA, the friction angle between wall/foundation and soils, and the tangent of DElTA rangs normally between 50 to 75 percent of tangent of PHI, the internal friction angle of soil.

The soils used in the design are silty sands and sandy silts, the friction angles are around 28-30 degrees. I think the assigned coefficient of 0.9 is too high, however, the designers insist the value is appropriate and even get offended by my questioning.

Can anybody provide any opinion? Am I wrong? What value would be more appropriate? Thanks

 

[fndn]

You're 100% correct nsong. I routinely use (Tan phi)/1.5 for this friction coefficient, and in no time have I recommende above 0.39. The idea is to limit your soil/footing friction as well as your passive pressure so as to remain conservative.

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[geotechgal]

nsong,
Refer to Table 5.5.2B in the AASHTO Standard Specifications for Highway Bridges. They have a range of values for different types of soils and foundations. A max value of 0.7 may be used for concrete foundations on clean sound rock. I have never seen anyone use 0.9.

 

[BigH]

Could these be factors in LRFD analyses? The factors given by the others are our "normal" geo analyses but as we know, LRFD changes the tone a bit and there may be another "factor" that's not stated so that, in the end, it is down to our normal values. DM-7 has friction factors.

 

[Panars]

I think you are onto something, BigH. The AASHTO LRFD spec gives a resistance factor of 0.9 for sliding of foundations (represented by the symbol PHI). But you still need to multiply the normal force on the footing by the tangent of the friction angle (that is, tan(PHI sub f)). For a friction angle of 35 degrees, this would result in a nominal resistance of 0.63 times the normal force, a much more reasonable value for LRFD.

Perhaps the designer is getting confused by the use of PHI for both symbols. If they are using LRFD, then review section 10.6.3.3 with them.

 

[geotechgal]

BigH,
I checked the DM 7.2 after you said. Looks like AASHTO took the table from there. Its exactly the same one. Although the table id given in the ASD Analysis section, I think the values would be universal. Would'nt you agree?

 

[RCEJD]

We have seen the 0.9 floating around from some of the research labs that are being highered to study this condition, especially in the arena of segmented walls, and alternative wall systems. I have seen some studies that suggest that the NAVFAC data and equations discussed above are underestimating the values that are being achieved.

I agree with Panars and BigH as to the origin for vertical walls. I also agree that I have not seen the more experienced and knowledgeable engineers using values this high.

 

[sendy]

Hallo nsong,
If you mean the interaction between soil and reinforcement,
that coef of 0.9 really depends on the type of reinforcement and dimension of grid of reinforcement. It could be less than 0.9 , but (i think) not less than 0.5.

 

[Panars]

RCEJD- you might have misunderstood my posting. The "resistance" factors in LRFD that I am referring to (and I think may have been mistakenly used as "friction" factors by the designers nsong is referring to) are kind of like safety factors applied to the soil resistance (there are also "load" factors that increase the nominal load). In the AASHTO LRFD manual, the greek symbol PHI is used for both, thus causing some confusion.

If you did understand this already, then I sincerely apologize.