Base shear depth of influence

[geotechguy1]

We're all familiar with the typical 'depth of influence' type calculations for vertical compression loads, eg. bousinesq stress distribution bulbs (take the depth of influence as the depth at 0.1 to 0.2Q), or the eurocode / british standard esque technique of setting the depth of influence as the depth at which the increase in effective stress is 20% of the in-situ effective stress.

So, what about a base-shear type case? Suppose you have a tank sitting on a concrete slab. The slab is sitting on 1m of gravel (although it could also not be, doesn't really matter) - below that is an arbitrary depth of normally consolidated fine grained soil. For seismic cases, base shear is generated and resisted by friction along the bottom of the slab. Suppose you need 100% of the friction to resist the base shear.

I've been queried (have never been asked this before): what happens to the stress state in the soil and what's the depth of influence? Does the base shear cause an increase in shear stress that decreases with depth with a zone of influence? (eg. at 0.25B depth you have a shear stress increase of 50%? Or do you essentially have a very confined shearband outside of which the stress state is unaffected.