Soil Weight from Active Pressure Side of Retaining Wall - Does This Add to Resistances?

[AndBre44]

Hello all, looking for some thoughts regarding applied soil weight & pressures in a retaining wall design.

I began designing reinforced concrete retaining walls from "Design of Reinforced Concrete" by Jack McCormac, which I was recommended by a peer of mine, and I've been following a similar set of calculations to the ones in his book since. I've put an image below of a figure from one of the textbook examples below:

Recently, I had a client contact me who was looking to use a segmental block retaining wall from Recon (for those unfamiliar:

https://reconwalls.com/),

and their website included an analysis software that could be used when sizing their blocks. Looking through their documentation and running their software, I noticed that they were including the active soil pressure not just as a horizontal load, but also as a vertical force; thereby adding to their sliding and overturning resistance for the designed wall (see figure below from their websites documentation of their software).

I can see the potential merit in adding the weight of that soil as part of the wall system; it's just not something I ever gave consideration to and my prior design experiences (I don't believe I've seen EnerCALC account for something of the sort when I used it some time ago). It certainly seems more conservative to say the extra weight isn't there, but am I being too conservative in that assumption/leaving out a crucial part of my resisting force in this case? I mostly ask the question because this the wall in question is going to be roughly 12ft in uncovered height, and the Pa sliding/overturning resistance would make the difference between requiring a geogrid reinforcement or not. While it would be great to save my client the excavation cost, I'm also trying to be sure I'm not simply cherry-picking for extra resistance if its more common to be on the side of caution/conservativeness.

Any thoughts on the matter would be greatly appreciated.

 

[BridgeSmith]

For an MSE wall, or a strictly gravity wall (such as a segmental block wall w/o soil reinforcement or a gabion wall), it's common to use the vertical component of the retained soil pressure as a resisting force. We don't typically add a vertical force due to the soil behind the heel of a conventional semi-gravity wall, such as the one shown in your first diagram.

 

[LOTE]

For an MSE or segmental gravity wall (or other battered walls), the Coulomb theory of active earth pressure is typically used, which considers the vertical component of the retained soil. Conventional cantilever walls, especially those with long heals, typically use Rankine theory which does not apply a separate vertical component. You must use one theory throughout your full analysis to give accurate results.

 

[Smoulder]

For concrete cantilever walls (Rankine) you can take the active pressure parallel to the surface slope.

 

[EireChch]

If you have a heal with soil above it then yes, you take it as help in sliding and overturning. W4 in your first image is the what this is referring to.