Design of pile groups - Piling Engineering - page 217

and also the loads that would be likely to be applied. Limit state design codes are
in effect endeavouring to quantify this process, with separate consideration of each
component.
5.4.2 Factors of safety for working stress design
The question of what factor of safety should be applied in pile design deserves parti-
cular consideration. Traditionally, it has been customary to design piles with ultimate
axial capacities between 2 and 3 times the required working load. For example, for
bored piles, Burland and Cooke (1974) recommend the use of a factor of safety of 2.0
on the combined shaft and base capacity, or, for underreamed piles, the use of partial
factors of unity on the shaft capacity and 3.0 on the base capacity. ∗
In many areas of design, the size of the factor of safety reflects the confidence with
which the ultimate capacity may be estimated. However, in many circumstances the
allowable settlement of a pile foundation will be the overriding criterion in design,
and the factor of safety against collapse will not be relevant. In general, it will
be found that a tight settlement criterion will lead to factors of safety that are in
excess of 3.
In situations where the settlement is not a key issue, for example where piles are used
to support storage tanks or other structures that may undergo substantial settlement
without damage, there are strong arguments for adopting relatively low factors of
safety. Maintained load tests on piles generally show that the creep or consolidation
settlement of the pile starts to increase significantly once the load reaches about 70%of
the ultimate capacity. This finding may be used to advantage to control more precisely
the load distribution within a group of piles. Thus, by designing piles with a factor of
safety of 1.5, higher loads in piles at the edge or corners of a group may be avoided,
since the stiffness of such piles will start to decrease at higher load levels. This approach
relies on relatively uniform ground conditions.
In Sweden, there has been a move towards what is termed 'creep piling' (Hansbo
and Jenderby, 1983). In essence, piles are designed at the load at which they will start
to 'creep' significantly, which corresponds to a factor of safety of about 1.5. It has been
found that the load distribution among the piles in a foundation may be controlled
more precisely using this approach, giving resulting savings in the structural design of
the pile cap.
In summary, then, it may be argued that the factor of safety should be determined
by the required settlement characteristics of the structure. Low factors of safety may
be used where large settlements may be tolerated, even to the extreme of a 'factor
of safety' of close to unity, where piles are used as settlement reducers beneath a
primarily raft foundation (see section 5.3). For large groups of piles, the settlement
criterion will almost always dominate the design. Thus, provided there is a mini-
mum factor of safety, which may be as low as 1.5, the design should be approached
in terms of satisfying the settlement criterion. For another possible approach, see
section 10.3.
∗ A factor of safety of 2.0 is often deemed sufficient where test piles have been loaded to failure. However,
a higher factor of safety, 2.5, is recommended where only proof loads are applied to working piles.