Pile Design and Construction Practice, Fifth edition

under-reamed piles and adopting a safety factor of 2 on combined end bearing and shaft
friction is given by

Therefore straight-shafted piles can be used for the eight inner piles as shown in Figure 6.46.
The maximum working load on these is one-half or less than one-half of the outer piles.
The overall depth to the base of the pile group of 13.7 m is only a little greater than the
overall width of the group, i.e 13 m to the outsides of the pile shafts. Therefore it is
necessary to check that block failure will not occur due to eccentric loading.

Eccentricity of loading with respect to base of pile group

From equation 5.3a, the width of an equivalent block foundation for winds in a north-south
direction 10.40(22.43) 5.54 m. The overall dimensions of this block foundation
are thus 135.54 m. Tangent of the angle of inclination of the resultant force tan
1 500/30 000 0.05, and thus 2.87.
From Figure 5.6, for  0 , Nc 5.2; from Figure 5.7 for B'/L' 5.54/13.0 0.43,
sc 1.1; from Figure 5.9 for D/B 7.7/5.54 1.4, dc 1.2. The horizontal force of 1500 kN
in Figure 6.46 is less than cuB'L' 190 5.5413.0 13 684 kN. Therefore equation 5.11
can be used to obtain the inclination factor ic 1 1500/2 190 5.5413.0 0.95.
From Figure 5.6, N 1.0. From Figure 5.7, s 0.95; d 1.0. From equation 5.12,
iq 1 1500/30 000 0.92, therefore from equation 5.13, i 0.92^2 0.85.
The second term in equation 5.1 is zero, therefore

qub 190 5.21.11.20.951.00.59.811.85.541.01.0

0.950.85

1238  39 1277 kN/m^2

Qab 1277 5.5413.0 92 005 kN

Factor of safety against base failure 92 005/30 000 3.1 which is satisfactory.
Checking for compliance with the EC7 recommendations, for the ultimate limit-state
GEO and using the three design approaches (Section 4.1.4), for Design Approach DA1
combination (sets A1M1R1), the vertical load on the pile group in respect of overturn-
ing is a permanent stabilising action. Table 4.1 gives G 1.0, V'dis 1.030 000
30 000 kN. The horizontal wind load is a variable unfavourable action, thus H'd 1.5
1500 2 250 kN

Eccentricity of loading in respect of base of pile group 2250(3513.7)/30000 3.65

For winds in a north–south direction, width of equivalent block foundation 10.4
(23.65) 3.10 m.
The overall dimensions of the transformed block foundation are 13.010.47.7 m deep.
The material factor, cu, for set M1 in Table 4.1 is 1.0, giving characteristic cu 190 kN/m^2.

2.43 m

1500  (3513.7)

30 000

Qa

(9  190  14   12 )(0.5  190    1  7.7)

2

1820 kN

370 Piles to resist uplift and lateral loading