Geotechnical Engineering

DHARM

660 GEOTECHNICAL ENGINEERING

σh = average lateral pressure of soil against the pile surface; and

δ = angle of wall friction, which depends upon the material of the pile σh is given by

σh = Ks. q ...(Eq. 16.14)

where Ks = coefficient of earth pressure.

For loose sand (ID < 30%), Ks = 1 to 3, and

for dense sand (ID > 70%), Ks = 2 to 5

For piles in sands:

fs = σh tan δ, ca being zero.

The values of tan δ may be determined by direct shear tests in which one half of the

shear box is replaced by the same material as the pile surface.

Representative values of the coefficient of friction between sand and various pile mate-

rials are shown in Table 16.2.

Table 16.2 Coefficient of friction between sand and pile materials

(McCarthy, 1977)

S.No. Material Coefficient of friction tan δ

1. Wood 0.4


2. Concrete 0.45


3. Steel, smooth 0.2


4. Steel, rusted 0.4


5. Steel, corrugated tan φ

Values of ratio δ/φ as determined by Potyondy from shear box tests are shown in
Table 16.3.
Table 16.3 Values of δ/φ for pile materials in contact with dense
and dry sand (After Potyondy, 1961)

S. No. Material and surface condition δ/φ

1. Wood, parallel to grain 0.76


2. Wood, perpendicular to grain 0.88


3. Rough concrete, cast against soil 0.98


4. Smooth concrete, poured in form work 0.80


5. Steel, smooth 0.54


6. Steel, rusted 0.76

For piles in clays:

fs = ca, since tan δ is zero.

The adhesion ca may be expressed as

ca = α. c ...(Eq. 16.15)

where α is called the ‘adhesion factor’, which varies with the consistency of the clay.