Bottom Heave of Cut in Clay

[ELT10]

I am designing a 20 ft deep trench with 10 ft of granular fill on top of 10 ft of clay. Using the factor of safety equation for bottom heave in Das Section 10.5, the cohesion c is multiplied by H and divided by B'. My question is, should I multiply c by 20 ft (where H is the full depth of the cut) or multiply c by 10 ft (where H is the depth of the clay). My thought is that the benefit of the cohesion only applies to the height of the clay and not the height of the fill. Cohesion is not depended on the depth of embedment. So I should use cohesion times 10 ft.
Here are the equations and diagram from Das followed by calculation. Thank you!

 

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By 20 feet.

Mike Lambert

 

[dik]

Is there a similar formula for a 'pit' as opposed to a trench?

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-Dik