Pile Design and Construction Practice, Fifth edition

292 Pile groups under compressive loading
Checking for compliance with EC7, the stability of the individual pile is calculated using
the procedure described in Section 4.2.3. The calculations are not included in this example.
The same procedure is used for checking the equivalent large-diameter pile for compliance.
The partial factors are as follows:
For actions (Table 4.1: Set A1, permanent unfavourable � 1.35, variable unfavourable � 1.5
Set A2, permanent unfavourable � 1.0, variable unfavourable � 1.3
Design value of actions Set A1, F d � (1.35 �250 �1 � 5 �110) �70 � 35175 kN
for Set A2 � (1.0 �250 �1.3 �110) �70 � 27510 kN
Soil parameter resistances for undrained shear strengths (Table 4.2): M1 � 1.0, M2 � 1.4
Base resistances for bored piles (Table 4.4), R1 � 1.25, R2 � 1.1, R3 � 1.0
Shaft resistances for bored piles (Table 4.4), R1 � 1.0, R2 � 1.1, R3 � 1.3
Combined resistances (Table 4.4), R1 � 1.15, R2 � 1.1, R3 � 1.0
The correlation factor � 3 in Table 4.6 for mean values of c u and three test profiles is divided
by 1.1 because the pile loads on the group will be redistributed by a stiff pile cap. Therefore
correlation factor for mean strengths � 3 � 1.33/1.1 � 1.21.
Calculating for design approach DA1, Combination1 (Sets A1 � M1 � R1), an adhesion
factor of 0.45 will be used for the concrete–soil interface of the equivalent pile.
Mean group resistance of pile � 9 �80 �65.34 �0.45 �85 �177.6
� 47045 �6793 � 53838 kN
Characteristic group resistance of pile � R ck � 53838/1.21 � 44 494 kN
Design value of combined resistance � R cd � 44494/1.0�1.15 � 38690 kN
which is greater than the design action of 35125 kN.
For design approach DA1, Combination 2 (Sets A2�M2�R1): R cd�44 494/1.4 �1.0
� 31781 kN
(�27510 kN)
Checking the equivalent pile for separate shaft and base resistances:
For DA1 Combination 1, R cd � 47045/1.21�1.0�1.25�6793/1.21�1.0�1.0
� 31104�5 614�36718 kN (�35175 kN)
For DA1 Combination 2, R cd � 47045/1.21 �1.4 �1.25�6793/1.21 �1.4 �1.0
�26227 kN (�27510 kN)
The lowest design resistance of the equivalent large-diameter pile from the above calculations
is 26227 kN; this can be compared with the ultimate bearing capacity of the block foundation
calculated by the method in Sections 5.1 and 5.21 of 98125 kN when divided by a nominal
safety factor of 2.5 gives an allowable bearing capacity of 39250 kN. This shows that for the
particular group dimensions of Example 5.1, the EC7 concept of an equivalent large-diameter
pile gives an over-conservative design.