Pile Design and Construction Practice, Fifth edition

232 Resistance of piles to compressive loads
q c�1 � (7 � 15) � 0.5 � 11 MN/m 2 , and over 8 diameters above the toe q c�2 � (5 � 8) �
0.5 � 6.5 MN/m 2 as equation 4.19, giving average q c at 28m of 8.75 MN/m 2 . Therefore
Base resistance � 8.75 � 0.45 2 �10 3 � 1392 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 m2 .
Gross base area � 0.656 m2 .
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/m2 At 14.5 m ultimate unit shaft friction � 0.008 � 9 � 0.072 MN/m2 At 17.0 m ultimate unit shaft friction � 0.008 � 9.9 � 0.079 MN/m2 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 MN
From 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 MN
Figure 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 2
From Figure 4.11, for q c � 9.9 MN/m 2 , � � 37�. From Figure 4.13, N q � 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.