Retaining wall design - one behind the other

[oldandgrey]

I am designing a stepped retaining wall system where the upper wall is within the zone of influence of the lower wall. Many available design programs allow for the vertical load to be included in the analysis (location, width, load, excentricity, etc). Non, that I can find, allow for the lateral load developed by the friction between the soil and the footer of the upper wall to be applied to the lower wall in any rational manner - except in some rule of thumb manner. Does anyone have an idea of a program that will do this or an idea of how to apply the Boussinesq equation to a analysis of both a lateral and vertical load. Also, any suggestions of reasonable rules of thumb would also be appreciated.

Thanks much - the hair is getting greyer.

 

[JAE]

Have you tried RetainPro? I think it allows for an adjacent "footing" load above the wall you design.

 

[csd72]

Apply the vertical reaction of the upper wall as a line load for the lower wall. Apply this at the reaction center.

Apply the horizontal reaction as a line load to the lower wall at the same height to get the additional overturning. At half the height of the footing.

 

[jdonville]

oldandgrey,

Or apply the foundation load as a strip load/uniform surcharge above the lower wall. Don't forget to check both walls (upper, upper+lower) for global stablility!

FHWA has specific design guidance for superimposed MSE walls.

Jeff

 

[ChipB]

For the most part, I agree with csd72.
The only thing I would do different, is apply the horizontal forces at their locations:
Passive pressure: 1/3 up from B.O. Ftg.
Friction force: @ B.O. Ftg.

You need to use a live load (1.6) factor not the actual forces for the horizontal forces on the lower wall as the F.S. against sliding for the upper wall is still 1.5.

FYI, depending on how close the upper wall is in relation to the lower wall, the reinforcing and footing geometry for the lower wall should be pretty close to what it would be if it were full height.

 

[skier1578]

JAE is correct - retainpro does have the function you are referring to. It is a very simple program to use, but does have some quirks that will give you some very bad designs so be careful.

 

[oldandgrey]

Thanks for all the imput from all of you. Interestingly JAE and skier1578 both mention RetainPro - which is the program I am using. I agree it has the capability to enter the upper wall footing load, however that is not the end of the story if the upper load is a retaining wall with a horizontal thrust. Both the publication "Basics of Retaining wall Design" by Brooks, which came with the program,(see page 44 in the Fifth Edition) and the User's Manual (page 45 of the Manual for Retainpro 6, under "Adjacent Footing Load") warn about "piggyback" walls and carefully warn about the added lateral load generated by the upper wall. This is a significant load. The "Basics" manual has two suggestions for ways to handle this load, as have several of you who have so kindly offered advice. Considerable difference of opinion. Perhaps best to be conservative. I was hoping someone had the definitive answer, but it probably doesn't exist. Again thanks much to all of you.

 

[lsmfse]

oldandgrey, I've got a diagram provided by one of our soils engineer which gives some guidance for load application on stacked retaining walls. They take it from Peck, Hanson & Thornburg, "Foundation Engineering" Second edition. I could possibly post a link to it if I knew how to do it as I am old and grey as well.

 

[oldandgrey]

lsmfse,

Thanks for the input. I can probably find the book in a not so local college engineering library (I guess we are both old and grey since I have at least one of Peck's texts, but not that one).

Thanks