Pile Design and Construction Practice, Fifth edition

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–zanalysis. 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

0.39.81 5  2 29 kN/m^2

Unit negative skin friction 2 m below top of clay stratum (at groundwater level)

0.39.81[(52)(21.9)] 41 kN/m^2

Total negative skin friction in clay

Drainage of the fill will produce a medium-dense state of compaction for which Kois 0.45
and Ksin equation 4.16 is 0.67 (Table 4.10) and. Therefore

Negative skin friction

102 kN

Total negative skin friction on pile 102  783 885 kN

Total load on pile 885  900 1785 kN

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  103 1 837 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

If the pile is filled with concrete with a working stress of 25% of the strength:

Allowable load on concrete

1 267 kN

Total allowed load 1 267 5501817 kN.

14

0.508^2 0.25 25  1000

0.3^255 ^7193

1000

550 kN

12 0.679.81 2  5  tan 21  0.517 5

0.7 30 20

0.517[^12 (2941)2^12 (4162)8] 783 kN

62 kN/m^2

0.3 vo 0.39.81[(52)(21.9)(80.9)]

238 Resistance of piles to compressive loads