Upper bond strength for interface elements

we use interface elements to model interaction between structure and surrounding soil. we typically use R_int  for strength and elasticity of interface. 
my question is that is  there any upper bond for interface element stiffness and strength?

for example when we have layers of soil and one layer rock beside structure, we will have significant difference in signma_n of interface elements in rock layer. of course its a stiffer layer and so there will be higher interface stiffness and higher normal force.

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