Pile Design and Construction Practice, Fifth edition

358 Piles to resist uplift and lateral loading
the ultimate resistance in uplift of the pile for sustained loading conditions. In Figure 6.9 the
failure surface does not extend up to the ground surface.
For � � 25�, H/B � 3, giving H � 3B � 3 � 2 � 6 m. Also from Figure 6.9, with
� � 25�, K u � 0.88 and the value of the shape factor is the maximum of 1.3.
The weight W can be taken conservatively as the weight of a cylinder of soil 2 m in
diameter extending up to ground level (i.e. the weight of the soil displaced by the pile is
assumed to be equal to the weight of the concrete). Then from equation 6.5
Q u � (��40 � 2 � 6) � {1.3 � 1
2��2.1 � 9.81 � 2[(2 � 11) � 6]
�6 �0.88�tan 25�} � ( 1
4��2 2 �2.1�11�9.81) � 5534 kN
This value of the uplift resistance must not be greater than that given by the uplift resistance
Q ub of the projecting part of the enlarged base plus the skin friction Q us on the pile shaft.
The latter value is calculated on the overall depth minus the depth of the enlarged base
(i.e. 1.5 m) and a zone 1.5 m deep at the ground surface where the soil can shrink away from
the shaft. From the Brinch Hansen factors from Figure 5.6 for � � 25� are
N c � 21, N q � 10.5, s c � 1.2, d c � 1.6, d q � 1.6 � (1.6 � 1)/10.5 � approximate 1.6,
s q � 1.2 � (1.2 � 1)/10.5 � approximate 1.2, and i c � i q � 1.0.
The third term in equation 5.1 is small and can be neglected.
Qub � (22 � 0.62 �
) [40 � 21 � 1.2 � 1.6 � 1.0 � (2.1 � 9.81 � 11
4
� 10.5 � 1.2 � 1.6 � 1.0)] � 17 671 kN
From equation 4.7 the shaft friction within the stiff clay from the undrained shear strength
component only is
Q us � 0.3 � 40 � � � 0.6 [11�(2�1.5)] � 181 kN
Total uplift resistance � Q ub � Q us�17671 � 181 � 17832 kN
Although the length/diameter ratio is 5.5 it is advisable to take only one-half of this value as
the ultimate resistance; i.e. a value of 8926. Therefore the ultimate resistance is governed by
equation 6.5, i.e. has a value of 5534 kN.
Example 6.2
The floor of a shipbuilding dock covers an area of 210 � 60 m. The 0.8 mm floor is restrained
against uplift by precast concrete shell piles having an overall diameter of 450 mm which are
driven through 8 m of soft clay ( ) on to a strong shale (� � 2.3 Mg/m3 cu � 16 kN/m ). The
piles are spaced on a 3 m square grid and each pile carries an uplift load of 1100 kN. Design
a suitable anchorage system for the dock floor using stressed cable anchors.
From Figure 4.6, for cu/ �� vo � 16/9.81 � 0.8 � 8 � 0.25, � � 1.0 and length factor F, for
L/B � 8/0.45 � 18, of 1.0, Equation 4.8 gives:
2
Q s � 1 � 16 � � � 0.45 � 8 � 181 kN
For a safety factor of 2.5:
allowable uplift resistance of the pile in soft clay � 181 / 2.5 � 72 kN