Pile Design and Construction Practice, Fifth edition

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 =

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 = 1 400413 = 987 kN,

Unit pressure on base =

Unit ultimate base resistance =

In the absence of plate bearing test data, take Kin equation 4.36 as 0.02. Thus

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.

i 2 0.02^314

979

1 000 13 mm.

9 0.75 145979 kN/m^2.

987

1

4

 ^2

2314 kN/m

(^2) ,
(^13 3075) 4131438 kN.
12 (4133075) 1744 kN.
9 0.75 145 ^14  223075 kN.
0.3 73   1  6413 kN.
228 Resistance of piles to compressive loads