Ka for wall retaining layered soil profile with inclined surface

[tstjohn]

Hi Eng-tips

A vertical-back gravity wall retains two distinctly different soil materials, an upper layer and a lower layer, with the boundary between the two horizontal. The upper layer has a slope angle beta > 0, which forms the ground surface. When computing Ka for the materials, Ka for the upper layer should of course take into account beta >0 (Coulomb approach). However for Ka for the lower layer, would you also use beta as the ground surface inclination or take beta as 0, since there is a horizontal boundary between the two materials?

I'm interested in the theory of this problem - it is theoretical only and I appreciate layers are not horizontal and there are other simplifications that could be adopted to simplify the problem.

P.s my first post and I did search but couldn't find this scenario discussed.
Thanks

 

[Sjotroll]

Hi, I would still use the surface angle. When you have a homogeneous soil layer behind the wall, you calculate one value of Ka which includes the surface angle, and you apply it to all depths. Imagine if you had the same homogeneous layer but you divided it into several "sub-layers" of the same material, you would still use the same Ka even for the lower "sub-layers" which have a horizontal boundary with the previous layer. Now just change one or more sub-layer with a material of different properties - by analogy, the surface angle still applies.