Say, there are four layers below a square footing. Up to the depth of footing the soil is clay.

First layer: clay soil, thickness H1, coefficient of consolidation: Cv1
Second layer: clay soil, thickness H2, coefficient of consolidation: Cv2
Third layer: sandy soil
Fourth layer: clay soil, thickness H4, coefficient of consolidation: Cv4
The fourth layer is followed by impermeable rock.

How would you apply Terzaghi's classical formula , t = Tv/Cv*hd^2 to this problem? (hd = drainage path)
To be more specific, how can we calculate time required for 50% consolidation of entire soil system? What will be the values of Cv and hd for the first layer?