Pile Design and Construction Practice, Fifth edition

296 Pile groups under compressive loading

The time factor for settlement at 30 years and static loading condition from equation 5.33 is

Therefore from equation 5.34, consolidation settlement 1.05 1 2.5 3.9

10.2 mm

The imposed loading would be intermittent in operation.
Checking from equation 5.25, for ds greater than 2B and L/B 1.1, Figure 5.25
givess 1.1.

Immediate settlement

Therefore the pile group would be expected to settle between 5 and 10 mm under the dead
and imposed loading from the bulk handling equipment.

Example 5.3

The driven and cast-in-place piles in Example 4.5 each carry a working load of 900 kN and
are arranged in a group of 20 rows of 15 piles spaced at 1.60 m centres in both directions.
Calculate the settlement of the pile group using the static cone resistance diagram in Figure
4.44. Length of pile group 19 1.6 30.4 m. Width of pile group 14 1.6 22.4 m.
The transfer of load from the piles to the soft clay in skin friction is relatively small, and
therefore the distribution of load shown in Figure 5.3b applies.
Depth to equivalent raft foundation below the surface of the sand
stratum or 22 m below ground level, as shown in Figure 5.44.

Length of equivalent raft L 30.4

Width of equivalent raft B 22.4

Pressure on soil beneath raft

The settlement can be calculated by the Schmertmann method. It is convenient to divide
the cone resistance diagram shown in Figure 4.44 into three layers between the base of the
equivalent raft and rock head. The sub-division of these layers and the superimposition of
the Schmertmann curves beneath the base of the raft are shown in Figure 5.44. The settlement
is calculated over a period of 25 years.

270 1 000

35.427.4

278 kN/m^2

(2 10 ^14 ) 27.4 m

(2 10 ^14 ) 35.4 m

23  15 10 m

1.1^63

15 0.87 1 0.44.67

9.1

5 mm

ft 1 0.30.2

log 30

3

2.5