Wall Friction Angle (delta) for passive pressure calculation for soldier pile walls

[galsenstruct]

In using Caquot and Kerisel method to estimate passive earth pressure coefficient Kp (AASHTO Fig 3.11.5.4-2), for the special case of soldier pile and lagging wall (drilled shaft / H-pile), is it more prudent to ignore wall friction?
I would like the opinions of the more experience engineers here.
Per FHWA-NHI-10-016 Drilled Shafts manual (page 12-50):
"For passive earth pressure, large values of wall friction with a wedge-type
solution derived from Coulomb theory can be unconservative and therefore it is recommended that wall friction be taken as zero."
Per NAVFAC DM7-02 (page 7.2-112):
"For granular soils, determine K+p, without wall friction".
Per civil tech shoring suite manual:
"The wall friction in the passive zone is insignificant should be ignored because the area within the soldier pile is too small"


In your experience in designing soldier pile walls what value of delta do you typically use? What range of Kp values have you use before? Does Kp values >20 sound unreasonable?
Thanks

 

[PEinc]

I usually use Coulomb's equations with wall friction equal to 0.5Phi for active and passive coefficients. My Kp is usually less than 7 or 8 for good soils. I would never use anything near 20. CivilTech recommends using no more than 15 degrees for wall friction. Nearly every soils book discusses earth pressure coefficients with and without wall friction considered. However, the references you mentioned recommend not using wall friction on passive pressures. Why have wall friction in the books if it should not be used? The key is to use wall friction moderately, not blindly or overly aggressively (such as Kp = 20).

http://www.PeirceEngineering.com

 

[fattdad]

kp=tan^2(45+phi/2)*phi/10

In this instance, I'm approximating Cp as phi/10. You can do a chart lookup if you want to.

So for a soil acting on a 12-in pile that has at least 3-fold pile-to-pile spacing and penetrating soil with a friction angle of 30 degrees, I'd get a Kp value of 9.

f-d

¡papá gordo ain’t no madre flaca!

 

[galsenstruct]

PEinc thanks for your response.
Have you ever use the AASHTO fig 3.11.5.4-2.
For the special case of vertical wall with sloping backfill, what value of beta should one use.
For example:
phi=34, beta=+32, horizontal bottom of excavation,

I am under the impression that even with a sloping backfill, AASHTO fig 3.11.5.4-1 (assuming beta=0) should be used in the case of horizontal bottom of excavation, is that right?

Thanks

 

[PEinc]

I do not use the AASHTO figures for earth pressure coefficients, even for highway projects. I usually use Coulomb equations. However, PADOT requires use of Rankine earth pressure coefficients (apparently because PADOT has projects that are more important than every other state).
For a soldier beam with level soil in front of the beam embedment, beta is 0 degrees. Sometimes, the ground surface slopes down (negative beta) and the Kp will be reduced.
If the ground is level in front of the soldier beam wall, beta/phi = 0.
I may be wrong, but you seem to be a little bit confused about the graphic for 3.11.5.4-2 which shows a vertical wall being pushed back into the soil. For a soldier beam, the passive soil is in front of the beam and is not normally referred to as backfill.

http://www.PeirceEngineering.com

 

[galsenstruct]

PEinc, thanks again for your contribution.
And yes, the aashto fig 3.11.5.4-2 was a bit confusing.

you said "PADOT requires use of Rankine earth pressure coefficients". Is this in DM4 or in specification? Or is it just your experience dealing with their design review unit?

Thanks

 

[FixedEarth]

You have asked a question that took Dr. Azizi, 36 pages to explain ("Engineering Design in Geotechnics", 2nd Ed, pp 350-386). In general, we limit wall friction to 0.67*Phi on the active side and 0.50*Phi on the passive side when using Coulomb's theory. However Coulomb's Kp equations do not include cohesion and are on the unconservative side for soils with Phi > 30 Deg.

http://www.soilstructure.com/

 

[PEinc]

For non-gravity walls, PADOT's DM-4 says to use Rankine unless the ground surface is sloped. However, I recently did a permanent anchored wall design where the ground surface behind the wall was slightly sloped and they (Central office in Harrisburg) would not let me use anything but Rankine earth pressure coefficients. No mater what DM-4 says, they don't read it the same way I do.

http://www.PeirceEngineering.com