Gravity Wall Design Question

[haloak5]

Hello. I am working on designing this gravity wall below. I wonder if I should ignore the lens of granular backfill (basically for drainage) and then do my active/passive force calculations on just the retained soil (clay)? or do I need to consider pressures for the granular backfill? It does not seem that significant (only 1 foot behind the wall)

Also, should I use Rankine or Coulomb? If I use, Coulomb should I use 2/3 of the clay's friction angle or the granular material? Using the clay for wall friction would be more conservative since the granular material will be stronger right? Technically the clay won't have contact with the wall though, so....

 

[DaveAtkins]

You should assume the lateral earth pressure is based on the clay, because as you said, the narrow granular layer will simply transfer the pressure from the clay to the wall. I would not assume there is any adhesion between the clay and the back of the wall, since again, you have that granular layer.

 

[geotechguy1]

I don't have a neat mathematical formula for you but yeah usually you ignore the gravel layer in this context and the forces are derived from long term properties of the soil behind it.

 

[haloak5]

So Rankine's theory would be appropriate for this wall situation?

 

[LOTE]

I always ignore the drainage stone layer unless it is specified for the entire active wedge.
I also typically use coulomb.

 

[EireChch]

The only difference between Rankine and Coulomb is friction on the back of the wall. In my experience, most walls like this would be designed with Rankine EPs but both are fine provided you use an appropriate wall friction coefficient.

 

[LOTE]

Coulomb can factor in the batter, which is a significant benefit in precast modular gravity walls.

 

[EireChch]

Fair point that I missed!

 

[PEinc]

Coulomb earth pressure coefficients can also address any sloping behind and in front of the wall. Rankine cannot.

 

[EireChch]

PEinc -Rankine can account for sloping ground above and below a wall

 

[PEinc]

PEinc -Rankine can account for sloping ground above and below a wall

Yes, if you use the Coulomb equations without wall friction.