The state of soil under tail slab is uncertain, thus it is usually conservatively ignored. Note that the critical shear plane is at the interface of base slab and soil, and the design shear force of the key equal to the sum of horizontal forces minus the sum of vertical forces times friction coefficient. Both forces (H & V) are calculated with respect to the critical shear plane.

The passive force will be far in excess of the active, and would reduce the overall design load if you were to account for it. So in my opinion, if you are designing the key as a cantilevered beam with only the passive pressure, then you're being conservative but correct. That is how I would design it as well.

@retired13

The design shear of the key should not be equal to the cyan area mentioned in the below photo?

I think the sketch was not drawn to-scale, the cyan area seems to indicate that, if the development length exceeds the preliminary key length (selected to satisfy force equilibrium), than you have to extend the key to ensure adequate bar development length. By doing so, you will need to re-calculate the passive force with reference to the new key base (bottom of the cyan block). I don't think it is a good way to handle this situation, rather, you shall try select a smaller bar with closer spacing, thus A_s remains the same with shorter Ld requirement. Make sense?

@retired13:

it is very clear now, thank you all a lot.

You are welcome.