Pile Design and Construction Practice, Fifth edition

238 Resistance of piles to compressive loads
skin friction will not be developed below the point where the clay settles by less than 15 mm
relative to site datum. After pile driving, the full thickness of the clay settles by 120 mm at
the surface of the layer. By simple proportion, a settlement of 5mm occurs at a point 12 �
15/120 � 1.5 m above the base of the layer. This assumes uniform compressibility in the
clay, but there is decreasing compressibility with increasing depth such that the settlement
decreases to less than 15 mm at a point not less than 2 m above the base of the layer. A closer
estimate could be obtained by a t–z analysis. However, the above approximate assessment
will be adequate for the present case.
Adopting Meyerhof’s factor from Figure 4.39 for the negative skin friction:
Unit negative skin friction 2 m above the base of clay layer
Unit negative skin friction at top of clay stratum
� 0.3 � 9.81 � 5 � 2 � 29 kN/m2 Unit negative skin friction 2 m below top of clay stratum (at groundwater level)
� 0.3 � 9.81 � [(5 � 2) � (2 � 1.9)] � 41 kN/m2 Total negative skin friction in clay
� � � 0.517[ 1
2(29 � 41)2 � 1
� 62 kN/m
2(41 � 62) � 8] � 783 kN
2
� 0.3��vo � 0.3 � 9.81[(5 � 2) � (2 � 1.9) � (8 � 0.9)]
Drainage of the fill will produce a medium-dense state of compaction for which Ko is 0.45
and Ks in equation 4.16 is 0.67 (Table 4.10) and � � 0.7 � 30� � 20�.
Therefore
Negative skin friction
�
� 102 kN
Total negative skin friction on pile � 102 � 783 � 885 kN
Total load on pile � 885 � 900 � 1785 kN
1
2 � 0.67 � 9.81 � 2 � 5 � tan 21� � � � 0.517 � 5
From Example 4.5 the average CPT resistance at a penetration of 28 m is about 8.75 MN/m
and the shaft frictional resistance from 12 to 28 m is 933 � 517/450 � 1072 kN
Ultimate base resistance at 28 m � �/4 � 0.517 2 � 8.75 � 10 3 � 1837 kN
Total pile resistance � 1072 � 1837 � 2909 kN
Safety factor on combined working load and downdrag � 2909/1785 � 1.6 (satisfactory).
The working stress on the pile shaft must be checked under the combined loading. For a
wall thickness of 4.47 mm the steel area is 7193 mm 2 .
Permissible load on steel at 30% yield stress
0.3 � 255 � 7193
� � 550 kN
1000
If the pile is filled with concrete with a working stress of 25% of the strength:
Allowable load on concrete �
� 1267 kN
Total allowed load � 1267 � 550 � 1817 kN.
1
4� � 0.5082 � 0.25 � 25 � 1000