Pile Design and Construction Practice, Fifth edition

228 Resistance of piles to compressive loads
An adhesion factor of 0.3 should be used to allow for delays and softening effects while
drilling the enlarged base. Thus
total shaft friction on pile shaft = 0.3 � 73 � � � 1 � 6 � 413 kN.
As before a reduction factor of 0.75 should be used on the shearing strength of 145 kN/m 2
at a depth of 10 m. Thus from equation 4.4,
end-bearing resistance =
For a safety factor of 2 on the total pile resistance,
allowable working load
For a safety factor of 3 on base resistance and 1 on skin friction,
allowable working load
Therefore a 1 m diameter bored pile with an enlarged base of 2 m diameter at a depth of
10 m below ground level is satisfactory for a working load of 1400 kN.
Considering the settlement of the straight-sided pile, the ultimate shaft friction of 1841 kN
exceeds the working load, and therefore the settlement of the pile will be no more than that
required to mobilize the ultimate resistance; i.e. a settlement of about 10 mm may be expected.
For the full mobilization of shaft resistance at the working load,
load on base = 1400 � 413 = 987 kN,
Unit pressure on base =
Unit ultimate base resistance = 9 � 0.75 � 145 � 979 kN/m2 987
1
4� � 2
.
2 � 314 kN/m2 ,
In the absence of plate bearing test data, take K in equation 4.36 as 0.02. Thus
� i � 2 � 0.02 � 314
979 � 1000 � 13 mm.
9 � 0.75 � 145 � 1
4� � 2 2 � 3075 kN.
� 1
2(413 � 3075) � 1744 kN.
� ( 1
3 � 3075) � 413 � 1438 kN.
This is an acceptable value.
However, in view of the efficiency of modern pile drilling equipment it is likely that the
cost of under-reaming and concreting to form an enlarged base at 10 m will exceed the cost
drilling the extra 3 m for a straight-sided pile.
Checking the design of the straight-sided pile for compliance with the EC7 requirements,
from Table 4.9:
Factored pile diameter � 0.95 � 1.0 � 0.95 m, giving area of base � 0.71 m 2 and
area of shaft for 12 m penetration � 35.81 m 2 .