Pile Design and Construction Practice, Fifth edition

Pile groups under compressive loading 301

Unit negative skin friction at top of layer

Unit negative skin friction at groundwater level (see Figure 5.45)

Unit negative skin friction at bottom of layer

Therefore total negative skin friction in soft clay:

Because the pile will settle due to yielding of the stiff clay when the full working load is
applied, the pile will move downwards relative to the lower part of the soft clay. Thus negative
skin friction will be developed only over about 80% of the length within the soft clay. Thus

approximate total negative skin friction in soft clay 0.8 746 597 kN

The negative skin friction in the sand can be calculated using the coefficients for Ks in
Table 4.10. Although the compacted sand fill is dense it will be loosened by pile boring to
give a coefficient Ksof 1 and a value of of 30. From equation 4.16:

negative skin friction on pile in sand fill

Total negative skin friction on pile 359  597 956 kN

Total applied load on piles in centre rows 956  2200 3156 kN

The required pile penetration depth is calculated on the basis of the building loading, with
a check being made to ensure that the safety factor on the combined building load and
negative skin friction is adequate.

Required ultimate pile resistance for overall safety factor of 2 (Section 4.6)
2  2200 4 400 kN

Take a trial penetration depth of 10 m into the stiff clay stratum. At the pile base level
cub 190 kN/m^2 and the average value of cuon the shaft is 140 kN/m^2. Thus

Ultimate base resistance

Load to be carried in skin friction

The adhesion factor for a straight-sided pile can be taken as 0.45. Therefore from
equation 4.7:

total load 2 465 0.45 140  1.2l

4 4001 935 2465 kN

14  1.2^2  9  1901935 kN

12  1 9.812.1 4  tan 30    1.2 4359 kN

1.2[^12 (4.78.8) 3 ^12 (8.843.3)2.5] 746 kN

0.309.81[(2.14)(1.63)(0.62.5)] 43.3 kN/m^2

0.309.81[(2.14)(1.63)] 8.8 kN/m^2

0.30 vo 0.309.812.1 4 4.7 kN/m^2