Geotechnical Engineering

DHARM

PILE FOUNDATIONS 659

300

280

240

200

160

120

80

40

0

Meyerhof

Hansen

Berezantzev

Bearing capacity factor, N /NNg Terzaghi

q

g

24 28 32 36 40 44 48

Angle of internal friction, °f

Note: N curve is after

Berezantzev et al. (1961)

g

Fig. 16.3 Bearing capacity factor Nq for piles in sand (Nordlund, 1963)

Vesic’s equation for qb is:

qb = 3q. Nq ...(Eq. 16.9)

where Nq = e3.8φ tan φ. Nφ ...(Eq. 16.10)

with the usual notation for Nφ [= tan^2 (45° + φ/2)].

Vesic’s values of Nq are given in Table 16.1:

Table 16.1 Vesic’s values of Nq for deep foundation

φ° 0 5 101520253035404550

Nq 1.0 1.2 1.6 2.2 3.3 5.3 9.5 18.7 42.5 115.4 4.22

For piles in clays, qb is given by:

qb = cNc + q ...(Eq. 16.11)

since Nq = 1 and Nγ = 0 for φ = 0°

Nc ranges from 6 to 10 depending upon the stiffness of the clay: a value of 9 is taken for

Nc conventionally.

It is also considered that q is not significant compared to cNc. Hence, for all practical

purposes

qb = 9c ...(Eq. 16.12)

(for piles in clay)

The general form for the unit skin friction resistance, fs, is given by

fs = ca + σh tan δ ...(Eq. 16.13)

where ca = adhesion, which is independent of the normal pressure on the contact area. Cohe-

sion c is used if the shearing is between soil and soil;