Pile Design and Construction Practice, Fifth edition

192 Resistance of piles to compressive loads
(2) The pile was then required to be driven a further 75 mm without any reduction in the
driving resistance
(3) If the resistance was not maintained at 5 mm per blow, it was judged that the gravel layer
was thin at that point, and the pile was liable to break through to the clay. Therefore, the
pile had to be driven further to a total penetration of 20 m, which was about 3 to 4 m
below the base of the gravel, to obtain the required additional frictional resistance.
The effects of driving piles in groups onto a resistant layer underlain by a weaker
compressible layer must be considered in relation to the settlement of the group. This aspect
is discussed in Chapter 5.
4.6 The settlement of the single pile at the working
load for piles in soil
It is necessary to divide the calculated ultimate resistance of the pile (or the ultimate resistance
derived from load testing) by a safety factor to obtain the design working load on the pile.
A safety factor is required for the following reasons:
(1) To provide for natural variations in the strength and compressibility of the soil
(2) To provide for uncertainties in the calculation method used
(3) To ensure that the working stresses on the material forming the pile shaft are within the
safe limits
(4) To ensure that the total settlement(s) of the single isolated pile or the group of piles are
within tolerable limits
(5) To ensure that the differential settlements between adjacent piles or within groups of
piles are within tolerable limits.
The need for a safety factor or partial factors to cover the uncertainties in the calculation
methods will have been evident from the earlier part of this chapter, and in this respect they
are ‘factors of ignorance’ rather than absolute values. With regard to reason 4 above, the
load/settlement curves obtained from a very large number of loading tests in a variety of soil
types, both on displacement and non-displacement piles, have shown that for piles of small to
medium (up to 600 mm) diameter, the settlement under the working load will not exceed 10
mm if the safety factor is not lower than 2.5. This is reassuring and avoids the necessity of
attempting to calculate settlements on individual piles that are based on the compressibility of
the soils. A settlement at the working load not exceeding 10 mm is satisfactory for most
building and civil engineering structures provided that the group settlement is not excessive.
However, for piles larger than 600 mm in diameter the problem of the settlement of the
individual pile under the working load becomes increasingly severe with the increase in
diameter, requiring a separate evaluation of the shaft friction and base load. The question of
the correct safety factor then becomes entirely the consideration of the permissible settlement
or in EC7 terms compliance with serviceability limit-state. The load/settlement relationships
for the two components of shaft friction and base resistance and for the total
resistance of a large-diameter pile in a stiff clay are shown in Figure 4.27. The maximum
shaft resistance is mobilized at a settlement of only 10 mm but the base resistance requires
a settlement of nearly 150 mm for it to become fully mobilized. At this stage the pile has