Retaining Wall - Using Passive (Resisting) Pressure in Capacity Analysis of Stem Wall 1

[CWEngineer]

I am seeking guidance regarding the use of passive (resisting) pressures to assist in the analysis of the structural design of a stem wall. EM 1110-2-2502 allows the use of passive resistance for structural design (please see attached document). Would it be acceptable to use the net factored pressure to check the capacity of a 2 ft thick stem wall (as shown in the attached document). For example if the factored active pressure is 900 psf and the factored passive pressure is 200 psf, can 700 psf be used to analyze the wall?

I have not really seen information regarding this topic. Not sure if you can just subtract for example 200 psf from 900 psf in a 2 ft thick wall. Also, if the use of passive (resistance) pressures is allowed, should it be factored?

Thanks

 

[3Fan]

We design our cantilevered retaining walls following the AASHTO LRFD Specifications. Per their guidance in section 11.6.3.5, any contribution of the passive earth pressure in front of the wall is ignored unless the wall extends below the depth in which the soil was excavated or is disturbed by items such as scour, freeze/thaw, etc.

 

[Teguci]

I suggest summing the retained side force and passive pressure side force separately. This way you can appropriately apply the safety factor. Your excerpt from EM 1110-2-2502 calls for a safety factor of 2. I usually design to an SF of 1.5 for sliding.

As 3Fan notes, it is appropriate to "sacrifice" a certain amount of resisting pressure considering the environment you are working in. I've never bought in to the trench problem though. Anyone who wantonly digs a trench in front of a retaining wall is being grossly negligent.

The retained side pressure and the resisting side pressures extend all the down to the bottom of the concrete. They don't magically disappear at the bottom of the stem.

 

[WARose]

It depends what we are talking about. For the sliding SF check, I've considered them (active & passive) happening together. But for moment calculation in the wall or overturning SF check: I've always considered active pressure alone (at least in one scenario for wall moment). The wall has to move a bit before “engaging” the passive pressure......therefore the wall could see active alone. (And that likely will control.) For the moment in a key (extending below the footing; if necessary) I've always just considered the passive pressure alone. (Min. steel almost always controls there anyway.)

 

[CWEngineer]

My question was in terms of the structural design (flexural and shear capacity check of stem)instead of the stability (sliding and overturning) of the wall).

 

[WARose]

My question was in terms of the structural design (flexural and shear capacity check of stem)instead of the stability (sliding and overturning) of the wall).

I addressed that.

 

[CWEngineer]

Sorry about that WARose, think we posted about the same time. If the passive pressure is engaged, still try to figure out if the active and passive would be additive. Also if a load factor would also be added to the passive pressure?

 

[Teguci]

Per ASCE LRFD factors, the retained soil pressure gets a factor of 1.6, the resisting soil pressure gets a factor of 0.9 or, if the load is not permanent 0. With the use of these factors, I'd have no problem considering the passive pressure in order to remove that last bit of shear stress so that you don't have to bump the bottom of the wall up to a 26" wall.

 

[Settingsun]

I'd say that you're pushing the design about as hard as you can if you use the passive pressure as a benefit to structural design. Have a think about the risk you're willing to take on in the design of your particular wall, not just whether you can find a set of numbers that add up and perhaps slip through some loopholes in the codes.

Some more recent design guides suggest using at-rest pressure for structural design instead of active. The reason for this is that we design for a factor of safety on sliding & bearing failure/settlement to prevent them from happening. If the wall doesn't move, then the pressure doesn't reduce to active. Normal (pre-failure) structural deflection of the stem isn't sufficient to mobilise the active pressure, so the stem would have to undergo hinge rotation to achieve the design assumption.

So there's a wide range in the guidance, with at-rest being at the conservative end, and (Ka minus Kp) being at the other end.

 

[Guest262653]

As steveh49, I also use a larger pressure than the active for the stem structural design, depending on the possibility of limit state mobilization. It comes down to engineering judgment based on the type of foundation (rock socket or soil base), wall height and necessary displacement/rotation for the mobilization of the corresponding limit states (Eurocode 7 provides some guidance on Annex C.3).

That said, for the structural design of a stem wall on a soil foundation, I completely disregard passive pressure and use the average value between the at rest and active pressure: k=(ka+k0)/2.

 

[CWEngineer]

Thank you for your responses. They were very helpful in clarifying what I was trying to figure out.