Pile Design and Construction Practice, Fifth edition

For a wall thickness of 19 mm in mild steel ( f y = 240 MN/m 2 )
Allowed load = 0.5 � 240 � 0.0626 = 7.5 MN
Resistance of piles to compressive loads 237
Therefore take a maximum working load of 7.2 MN.
Pile driving impact may increase the fracture frequency from 5 to 10, say, fractures
per metre giving a mass factor of 0.2. From Section 5.4 the modulus ratio of sandstone
is 300.
Deformation modulus = 0.2 � 300 � 20 � 1200 MN/m2 From equation 4.38:
7.2 � 15 � 1000 0.5 � 7.2 � 1000
Settlement of pile head �
�
5 0.0626 � 2 � 10 1.067 � 1200
� 8.6 + 2.8
� 11.4 mm (say 10 to 15 mm)
Example 4.9
A 5 m layer of hydraulic fill consisting of sand is pumped into place over the ground shown
in Figure 4.44. The calculated time/settlement curve for the surface of the hydraulic fill is
shown in Figure 4.46. Two years after the completion of filling a closed-end steel cased pile
with an outside diameter of 517 mm is driven to a penetration of 27 m to carry a working
load of 900 kN. Calculate the negative friction which is developed on the pile shaft and
assess whether or not any deeper penetration is required to carry the combined working load
and negative skin friction.
It can be seen from the time/settlement curve that about 120 mm of settlement will take
place from the time of driving the pile until the clay beneath the fill layer is fully consolidated.
This movement is considerably larger than the compression of the pile head under the
working load (about 10 mm of settlement would be expected under the working load of
900 kN). Therefore negative skin friction will be developed over the whole depth of the pile
within the hydraulic fill. Considering now the negative shaft friction within the soft clay, if
it is assumed that drag-down will not occur if the clay settles relatively to the pile by less
than 5 mm, then adding the settlement of the pile toe (10 mm at the working load) negative
Figure 4.46
Settlement in mm Load from fill
(kN/m2 )
0
Completion of
hydraulic fill
0.5
450 mm
670 mm
Commencement
of piling
2.0
120 mm
Time in years
Calculated ‘final’
settlement