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/B� 10 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 C1 � 1 � 0.5 �
25
From equation 5.37 C2 � 1 � 0.2lg � 1.48
0.1
Corrected settlement at 25 years � 0.62 � 1.48 � 66 � 61 mm say between
50 and 70 mm
214
� 0.62
278
Example 5.4
Nuclear reactors, their containment structures, and ancillary units weighing 900 MN are to
be constructed on a base 70�32 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/m2 . Design a suitable piled foundation and calculate the
settlement.
Overall loading on base
278 � 0.36 � 5.2 � 1000
40 � 1 000
278 � 0.46 � 2.5 � 1 000
50 � 1 000
278 � 0.4 � 10.3 � 1000
50 � 1000
� 13 mm
� 6 mm
� 23 mm
� 9.81[(2 � 1.9) � (10 � 0.9) � (10 � 0.9)] � 214 kN/m 2
� 900
70 � 32 � 0.402 MN/m 2
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