Jardine et al.(4.30)recommend safety factors of 1.3 to 1.6 for the shaft resistance in
compression for offshore foundations where settlements of the structures are not critical and
the design is based on permissible stress methods.
In contrast, the ICP method of design for tubular piles in sands is a simple one based on
the static cone penetration test. No other field work or special laboratory testing is required.
The method is wholly empirical and is justified by the assumption that the penetration of the
sleeved cone simulates the displacement of the soil by a closed-end or fully plugged pile.
The expression for the shaft resistance is calculated by the following sequence of equations:Unit shaft resistance = f= tan (^) f (4.27)
Radial effective stress at point of shaft failure = = (4.28)
Equalized radial effective stress = = 0.029qc(/Pa)0.13(h/R)–0.38 (4.29)
Dilatant increase in local radial effective stress = = 2G (^) f/R (4.30)
where
(^) f= (^) cr= interface angle of friction at failure
R= pile radius
G= operational shear modulus
In equation 4.27 (^) fcan be obtained either by constant volume shear box tests in the
laboratory or by relating it to the pile roughness and particle size of the sand (Figure 4.21).
The equalized radial stress in equation 4.29 implies that the elevated pore pressures around
the shaft caused by pile driving have dissipated. The term Pais the atmospheric pressure
which is taken as 100 kN/m^2. Because of the difficulty in calculating or measuring the high
radial stresses near the pile toe h/Ris limited to 8.
rd
rc vo
rf rc rd
rf
184 Resistance of piles to compressive loads
20
0.1
Values are for rough
steel interface
(CLA 6– 10 m)
1
Mean particle size, D 50 (mm)
1.0
22
Interface friction angle
cv
(°
)
24
26
28
30
32
34
36
38
Figure 4.21Relationship between interface friction angle and mean particle size of a silica sand (based
on Jardine et al.(4.30)).