232 Resistance of piles to compressive loads
qc 1 (7 15) 0.5 11 MN/m^2 , and over 8 diameters above the toe qc 2 (5 8)
0.5 6.5 MN/m^2 as equation 4.19, giving average qcat 28m of 8.75 MN/m^2. Therefore
Base resistance 8.75 0.45^2 103 1 392 kN
Total pile resistance 933 1392 2225 kN.
Factor of safety 2225 / 900 2.5 (satisfactory).Example 4.6
Calculate the ultimate bearing capacity of a 914 mm OD 19 mm wall thickness tubular
steel pile driven with a closed end to a depth of 17 m below ground level in the soil
conditions shown in Figure 4.18a and compare the capacity with that of an open-end pile
driven to the same depth.
Pile characteristics are: External perimeter 2.87 m
Internal perimeter 2.75 m
For 38 mm shoe, net cross-sectional area at toe 0.1046 m^2.
Gross base area 0.656 m^2.
The shaft friction on the pile shaft is calculated from the characteristic curve shown in
Figure 4.18a. Ignore any shaft friction in the soft clay. The coefficient from Table 4.12
is 0.008.
At 6.5 m ultimate unit shaft friction 0.008 4 0.032 MN/m^2
At 14.5 m ultimate unit shaft friction 0.008 9 0.072 MN/m^2
At 17.0 m ultimate unit shaft friction 0.0089.9 0.079 MN/m^2
Total external shaft friction 6.5 to 17.0 m
[(0.032 0.072) 8 (0.072 0.079) 2.5] 0.5 2.87 1.74 MNFrom average curve in Figure 4.18a, base resistance 9.9 0.656 6.49 MN
Total ultimate pile resistance 1.74 6.49 8.23 MNFigure 4.19 shows that the limiting value of about 10 MN/m^2 for base resistance in fine
to coarse sand is not exceeded.
The total pile resistance can be checked by converting the cone resistance to the equivalent
angle of shearing resistance of the sand. Assume ground water level at ground level.
Effective overburden pressure at 17 m 8 6.5 10 10.5 157 kN/m^2From Figure 4.11, for qc 9.9 MN/m^2 , 37 . From Figure 4.13, Nq 70.
Therefore, ultimate unit bearing pressure 70 157 10 –^3 11 MN/m^2 , which is close
to the value taken directly from Figure 4.18a.
Adopting a factor of safety of 2.5, the allowable load by permissible stress methods is:
8.23/2.5 3.29 MN.
The allowable load can be checked for compliance with the EC7 recommendations. For an
allowable load of 3.29 MN, the design actions are for set A1, 1.35 3.29 4.44 MN, and
for set A2, 1.0 3.29 3.29 MN.