where QD = downdrag load acting on the pile or pier (lb or kN)
R = radius of the pile or pier (m or ft)
L1 = vertical distance over which the pile or pier is subjected to the downdrag load (ft or m)
s′ v = average vertical effective stress over the portion of the pile or pier subjected to the
downdrag load (psf or kPa)
k = dimensionless parameter equal to s′h divided by s′v (i.e., similar to Eq. 4.18). Equation
4.19 can be used to estimate the value of k for loose sand deposits. Because of the
densification of the cohesionless soil associated with driven displacement piles, values
of k between 1 and 2 are often assumed
fw = friction angle between the cohesionless soil and the perimeter of the pile or pier
(degrees). Commonly used friction angles are fw = 3/4 f for wood and concrete piles
and fw = 20° for steel piles
cA = adhesion between the cohesive soil and pile or pier perimeter (psf or kPa). Figure 6.13
can be used to determine the value of the adhesion (cA) for different types of piles and
cohesive soil conditions