The type of soils makes a big difference in such an analysis. For example, if your soil layers were exactly opposite (dense sand over firm clay), your result would be different. Granular soils tend to allow a larger load distribution, while cohesive soils tend to transfer load more directly in shear. Your condition would transfer a higher foundation load to the dense sand than if the other way around.

A lot would depend on the thickness of the "firm" clay layer compared to the footing size. If thin enough, you would not develop the traditional bearing capacity scenario (Prandtl . . . . or Terzaghi or others). You might be having a squeezing effect so that perhaps a more traditional "slope" style analysis might be appropriate. Just a thought . . .

Thanks for the comments.

BigH, may it be understood from your comments that there are not predefined design methods for this case. And do you mean a slope stability with a slope angle of zero under a sustained surcharge of equal to the foundation load?

When I said to check it by "traditional" slope methods - refer to oil tank analysis in Lambe and Whitman - then put in the hard layer and your "programme" would give a result - could consider to use wedge analysis if you wish - all predicated that the hard layer is "hard!!".

If the distance to the hard layer will not permit the full failure surface to develop, then to use traditional analysis is not correct.

Can also look up Section 4.8 of Fang's Foundation Engineering Handbook, 2nd edition. This gives some insight to the problem although they do not reference one of the original papers (by Button, 1957) on the issue. Hope this gives you some insight into the problem. But a thin layer, as I suggested might be better considered to be squeezing.

I will try the above references. Also is there a specific reference for design based on squeezing phenomenon? Thank you.