Pile Design and Construction Practice, Fifth edition

298 Pile groups under compressive loading

Layer 2

Layer 3a

Layer 3b

Total 69 mm

Similarly, for L/B10 the uncorrected settlements are

Layer 1 19 mm

Layer 2 9 mm

Layer 3 21 mm

Total 49 mm

By interpolation the settlement for L/B 1.3 is 66 mm

Effective overburden pressure at base of raft

From equation 5.36

From equation 5.37

Corrected settlement at 25 years say between

50 and 70 mm

Example 5.4

Nuclear reactors, their containment structures, and ancillary units weighing 900 MN are to
be constructed on a base 7032 m sited on 8 m loose to medium-dense sand overlying a
moderately strong sandstone. Rotary cored boreholes showed that below a thin zone of weak
weathered rock the RQD value of the sandstone was 85% and the average unconfined
compression strength was 14 MN/m^2. Design a suitable piled foundation and calculate the
settlement.

Overall loading on base

Under this loading the settlement of the sand will be excessive. A piled foundation is
required and relatively few large-diameter piles will be more economical than a large
number of lightly loaded piles.
Adopt piles 1.5 m in diameter taken 2 m below weak weathered rock on to the moderately
strong sandstone. It will be possible to permit loading of the concrete forming the bored

900

70  32

0.402 MN/m^2

0.621.48 6661 mm

C 21 0.2lg 25

0.1

1.48

C 11 0.5^214

278

0.62

9.81[(21.9)(100.9)(100.9)] 214 kN/m^2

278 0.410.31 000

50 1 000

23 mm

278 0.462.51 000

50 1 000

6 mm

278 0.365.21 000

40 1 000

13 mm