where F factor of safety. For seismic analyses, usually the minimum acceptable factor of safety is 5.
R shear resistance of the soil. For strip footings, R is the shear resistance per unit length of
the footing (lbft or kNm). For spread footings, R is the shear resistance beneath the
entire footing perimeter (lb or kN).
P footing load. For strip footings, P is the load per unit length of the footing (lbft or kNm). For spread footings, P is the total load of the footing (lb or kN). The footing load includes dead, live, and seismic loads acting on the footing as well as the weight of the footing itself. Typically the structural engineer would provide the value of P.
T vertical distance from the bottom of the footing to the top of the liquefied soil layer (ft or m)
pr shear strength of the unliquefiable soil layer (psf or kPa)
B width of the footing (ft or m)
L length of the footing (ft or m)