Wright and Reese (1978) ( McVay et al., 1994) gives an equation for calculating average skin friction on auger cast piles in sand. The equation reads:
fs = po'Ks tan (phi) =< 1.6 tsf
fs= average skin frictin on pile
Ks= 1.1
tan(phi)= tangent of the internal angle of friction of soil
po'=...