where Pp = passive resultant force

where Pp = passive resultant force (lb per linear ft or kN per linear m of wall length)
kp = passive earth pressure coefficient (dimensionless)
gt = total unit weight of the soil located in front of the wall footing (pcf or kN/m3)
D = depth of the wall footing, i.e., vertical distance from the ground surface in front of the
retaining wall to the bottom of the footing (ft or m)