Pile Design and Construction Practice, Fifth edition

Resistance of piles to compressive loads 143
and clays, has shown that if safety factor of 2.5 is taken on the ultimate resistance then the
settlement of the pile head at the allowable load is unlikely to exceed 10 mm. For piles of
diameters up to about 1000 mm, failure or ultimate loads as determined by loading tests are
usually assumed to be the loads causing a pile head settlement of 10% of the diameter.
When using permissible stress methods for piles in groups it is accepted that a structure
can suffer excessive distortion caused by group settlement long before an individual pile in
the group has failed in bearing resistance. Hence a separate calculation is made of group
settlement based on a realistic assessment of dead load and the most favourable or
unfavourable combinations of imposed loading, using unfactored values of the compressibility
of the ground in the zone influenced by the group loading (see Chapter 5).
Where piles are end bearing on a strong intact rock the concept of a safety factor against
ultimate failure does not apply, since it is likely that the pile itself will fail as a structural
unit before shearing failure of the rock beneath the pile toe occurs. The allowable loads are
then governed by the safe working stress in compression and bending on the pile shaft (or
the Eurocode regulations for the characteristic strength of the pile divided by the appropriate
material factor) and the settlement of the pile due to elastic deformation and creep in the
rock beneath the base of the pile, together with the elastic compression of the pile shaft.
4.1.4 Determining allowable loads in compression using the
procedure in the Eurocode British Standard EN 1997-1:2004
This account of the procedure adopted in the above Eurocode (referred to in this and
following chapters as Eurocode 7 or EC7) (1.2) is only a brief review of a lengthy document
containing many provisos, exceptions, and cross-references to other Eurocodes. The background
to the scope and purpose of EC7 is outlined in Chapter 1. If the engineer proposes
to undertake the design of a foundation complying in all respects with the code requirements
it is essential for the whole document to be studied together with the other relevant codes
including BS EN 1990 (1.3) and 1992 (1.4) . The commentary by Frank et al. (1.5) is also helpful
to a thorough understanding of the code. The main purpose of this section is to describe the
use of partial factors and the associated ‘design approach’ for determining allowable pile
loads (referred to in EC7). EC7 requires a structure, including the foundations, not to fail to
satisfy its design performance criteria because of exceeding various limit states. The
ultimate limit state can occur under the following conditions:
(a) Loss of equilibrium of the structure and the ground considered as a rigid body in which
the strengths of the structural materials and the ground are insignificant in providing
resistance (State EQU)
(b) Internal failure or excessive deformation of a structure and its foundation (State STR)
(c) Failure or excessive deformation of the ground in which the strengths of the soil or rock
are significant in providing resistance (State GEO)
(d) Loss of equilibrium of a structure due to uplift by water pressure or other vertical
actions (State UPL) and
(e) Hydraulic heave, internal erosion, and piping caused by hydraulic gradients (State HYD).
State EQU could occur when a structure collapses due to a landslide or earthquake. This
state is not considered further in this chapter. Design against occurrence of the other
states listed above involves applying partial factors to the applied loads (actions) and to
the ground resistance to ensure that reaching these states is highly improbable.