Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 295

factor on the ultimate resistance of a single pile of 3.1. For uniformity in design and
construction it is desirable to adopt a pile of the same dimensions to carry the bulk handling
plant. A group of forty-two piles arranged in seven rows of six piles should be satisfactory.
Spacing the piles at centres equal to three times the width the dimensions of the group are
6 1.35 8.10 m by 51.35 6.75 m. A suitable pile cap in the form of a thick slab
would be 10.59.01.25 m deep. Take a depth of water of 12 m and a height of 4 m from
water level to the underside of the pile cap.

Weight of pile group above sea-bed level

9.81([10.59.01.252.5]{420.45^2 [(121.5)(42.5)]}) 5233kN

Working load on each pile

Safety factor on ultimate load of 777 kN (Example 4.4) 777/267 2.9, which is
satisfactory.

Because the piles are driven into a uniform sand carrying their load partly in skin friction
and partly in end bearing the distribution of load shown in Figure 5.3a applies.

Depth below sea bed to equivalent raft

Thus the dimensions of the equivalent raft are

In calculating settlements it is only necessary to consider the dead and imposed loading
from the bulk handling plant. The piles and pile cap settle immediately as they are con-
structed and the pile cap is finished to a level surface.

Pressure on sand below raft due to weight of plant

At level of raft, effective overburden pressure

From Figure 5.26 for a standard penetration test N-value of 15 blows/300 mm, Icis 4 10 -2.
Assume for the purposes of illustration that the previous overburden pressure was
75 kN/m^2. Then from equation 5.31b the immediate settlement for an effective pressure
increase of 63 kN/m^2 is:

From Figure 5.27 the depth of influence zIfor Bof 9.1 m is 5 m. This is less than the thick-
ness of the compressible layer. Hence the thickness factor, fs, is unity. From equation 5.32a:

Shape factor,fs (^) 

1.2510.49.1

10.40.10.25

2

1.05

i 63 9.10.7^4 ^10

 2

3

3.9 mm

1.29.814.67 55 kN/m^2

6  1000

10.49.1

63 kN/m^2

B 6.75(^14  2 4.67) 9.1 m

L 8.1(^14  2 4.67) 10.4 m

2

3 ^7 4.67 m

( 6 5.233)

42

1 000 267 kN