Later earth pressures for anchored walls according to AASHTO

[DanielSgeo]

AASHTO LRFD bridge design specs show the AEPs we are all familiar with in chapter 3. But active pressure goes to 0 at the grade in front of the wall. It’s unclear to me, according to AASHTO, whether the passive in front of the wall should be reduced by the active pressure on the back of the piles. I think logic says that if the wall is moving laterally to engage passive, that there is an active (or at rest) pressure acting on the back side, much like we would do for a cantilevered wall. Thoughts?

 

[DaveAtkins]

Active pressure on the back side of the wall will be much more than zero. It will be the active coefficient times the depth of the soil plus surcharge above that level.

DaveAtkins

 

[EireChch]

I dont have AASHTO docs, but I understand that for stiff CLAYs, Ralph peck observed that at the grade level of a braced/anchored wall that the earth pressure does actually reduce to zero.

This was from his apparent earth pressure diagrams which were derived from monitoring retaining walls on Chicago subway.

 

[DaveAtkins]

Yes, that is true for a wall with multiple struts. I think that is also known as the "Terzaghi" load distribution.

DaveAtkins

 

[DanielSgeo]

Thanks all for the replies. I would note that the apparent EP diagrams for a braced wall with cohesionless soils also goes to zero at the mudline (Sabatini FHWA geotechnical enginering circular).

 

[PEinc]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1705951900/tips/AEP_Toe_Pressures_vpkuop.pdf[/url]Daniel, you did not indicate if you are using soldier beams or sheet piling. AASHTO's AEP diagrams assume a hinge support at subgrade. AASHTO does not detail well the design of the embedded portion of the soldier beam or sheet piling. For permanent, anchored, non-gravity walls, you need to consider the active, surcharge, and passive pressures acting on the embedded beam or sheet. Consider the retained soil above subgrade as an earth surcharge producing lateral, active pressure on the rear face of the soldier beam or sheet piling below subgrade. Resist this active with the passive resistance and the required safety factor. Using the AEP diagram and any surcharges, you will calculate the horizontal hinge reaction at subgrade. The passive resistance needs to resist the hinge reaction plus additional active earth below subgrade plus the overburden surcharge plus any surcharge pressures BELOW subgrade. I trust you know how to do this for both an individual soldier beam and a continuous SSP wall.

http://www.PeirceEngineering.com

 

[DanielSgeo]

Thanks PEInc. I was arriving at a the same conclusons myself. Figure 11.8.6.2-1 makes this pretty clear, but it's a bit convoluted to determine that this also applies to anchored walls. As an aside, I find it interesting that Pae is not used below excavation grade, even though we're considering the inertial effects on passive below grade. By the way, I'm looking at a soldier pile wall.

 

[PEinc]

AASHTO's main discussion on AEP wall design is for a uniform soil. You need to read more deeply if you have more than one soil stratum. Also, if multiple soil layers, chose the correct factor to convert total, theoretical earth pressure to total AEP. Also, AASHTO calls for use of an unsymmetrical trapezoidal shape for the AEP of an anchored, non-gravity wall.

http://www.PeirceEngineering.com