I am also curious about this question.

In my opinion, strength of clay material depends also to water content. If the moisture of clay goes up, the strength parameters will be decreased. But definiton of this behavior is not seems to be possible. We can assume some layers to be affected by rainfall and can decrase the strenght parameters. But the main issue seems to be if there is any potential failure surface, we can not guess it. Water can be flow on some sand layers in material, can decrease the strenght parameters of clay significantly. I think only God can do what you saying :)

The procedure would imply a finite differences analysis with the input of permeability and other relevant parameters of all soils, the condition, type and permeability of the most superficial soil layers, showing at which rainfall intensity the specific layers are going to be saturated and at which depths and the subsequent effects on their geotechnical values. It does not sound exceedingly simple but arguably it might be done, maybe using some other geostudio permeability module as an input.

Alternative is to use a worst case scenario or credible worst case assuming some layers become saturated after rainfall and the subsequent effects on phi-c and Cu, if existing failure surfaces exist then on such surfaces softened values should be applied (which ones is also a critical and sometimes tricky choice).

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