Drilled Shaft Clay TZ (Reese and O'Neill) [O’Neil et al., 1999]

t_u = ultimate skin friction
c_uz = undrained shear strength at depth z
α = unitless shaft friction factor, 0 from the ground surface to a depth of 5 ft and also 1 diameter of drilled shaft, otherwise 0.55

where
t = skin friction
Z = normalized settlement
D = pile diameter