Pile Design and Construction Practice, Fifth edition

226 Resistance of piles to compressive loads
Depth below sea bed (m)
Figure 4.42
Undrained shear strength, kN/M2 Ultimate pile capacity, MN
0
0
100 200 300 0 10 20 30 40 50
20
40
60
80
100
120
140
Average c u
cu (gsub = 0.65Mg/m3 s9 = 0.26
vo
)
160 Undrained shear strength
versus depth
Base resistance
Penetration for working load
of 16MN (safety factor = 2)
Total pile capacity
Shaft friction
Ultimate resistance versus depth for 1220 m OD
tubular steel pile with closed end
factor of 2. At a depth of 113 m the shaft friction is 30 MN. Therefore, the safety factor on
uplift is 30/8, that is, nearly 4.
Checking the working stress on the steel,
For a wall thickness of 25 mm max. compressive stress =
= 170 Nmm2 16 � 10 6
1
4�(12702 � 12342 )
This is 50% of the yield stress for high-tensile steel which is satisfactory for the easy
driving conditions. The thickness can be decreased to 16 mm in the lower 50 m of the pile.
Example 4.3
A building column carrying a dead load of 1100 kN and an imposed load of 300 kN is to be
supported by a single bored pile installed in firm to stiff fissured London Clay (Figure 4.43).
Select suitable dimensions and penetration depth to obtain a safety factor of 2 in total pile
resistance, or safety factors of 3 in end bearing and unity in shaft friction. Calculate the
immediate settlement at the working load.
A pile of 1 m diameter is suitable. A penetration depth of 13 m (12 m below cut-off level)
will be tried.