The uplift resistance Rt combines the self-weight of the soil overburden and footing + friction along the footing walls + fictitious (substitute) block of soil above the footing.
The soil properties are determined according to overburden type from the "Foundation" frame. If the chosen type is "input shape and soil" or "from geological profile" the angle of internal friction φ and cohesion c are taken from the assigned soils. In case of "input unit weigh" the "Bearing cap." frame in the "Verification on uplift" dialog window allows us to input the angle of internal friction φ and cohesion c of overburden.
Dialog window "Verification on uplift" - standard approach
The vertical bearing capacity check - spread footing in tension (uplift resistance) follows from:
earth pressure at rest due to overburden
design angle of internal friction of overburden
design cohesion of overburden
depth of the footing bottom
self-weight of foundation
Verification on uplift - standard approach