Retaining Wall Design - Vertical Shear Force Due to Delta

[CWEngineer]

In determining Vertical Shear (due to delta) for the Driving side, the CTWALL Program is using the following: Pv = (Po)tan(delta), where Po is the At-Rest Pressure. It does not include the effect due to surcharge when a surcharge load is placed behind the wall. I would assume when you have a surcharge load that the formula should be Pv = (Po+Psur)tan(delta), where the surcharge would contribute to the vertical shear force but that is not the case.

Appreciate if you guys can provide some guidance or explain why the surcharge load is not included in the calculation of the Vertical Shear Force due to delta.

Thanks