Pile Design and Construction Practice, Fifth edition

6 General principles and practices
resistance of the ground multiplied by the appropriate safety factor. The latter takes into
account the risks of excessive total and differential settlements of the structure as well as
allowing for uncertainties in the design method and in the values selected for the shearing
strength parameters.
The settlements of the foundations are then calculated, the loading adopted for these
calculations being not necessarily the same as that used to obtain the maximum working
load. It is the usual practice to take the actual dead load and the whole or some proportion
of the imposed load, depending on the type of loading, i.e. the full imposed load is taken for
structures such as grain silos, but the imposed wind loading may not be taken into account
when calculating long-term settlements.
There is no reason why this dual approach should not be adopted when designing structures
and their foundations, but it is important that the designer of the structure should make an
unambiguous statement of the loading conditions which are to be supported by the ground. If
he provides the foundation engineer with a factored ultimate load, and the foundation engineer
then uses this load with a safety factor of, say, 2.5 or 3 on the calculated shearing resistance of
the ground, the resulting design may be over-conservative. Similarly, if the ultimate load is
used to calculate settlements, the values obtained will be unrealistically large. The foundation
engineer must know the actual dead load of the superstructure and sub-structure and he must
have full details of the imposed loading, i.e. its type, distribution and duration.
Many of the conflicts between the design of structures and sub-structures to BS 8110 or similar
structural codes, and the design of piled foundations to BS 8004 have been dealt with in BS
EN 1997-1: 2004 Eurocode 7 (EC7), Geotechnical Design – Part 1 General rules (1.2) and BS EN
1992-1: 2004 Eurocode 2 (EC2), Design of Concrete Structures – Part 1-1 General rules and
rules for buildings (1.3) . These two Eurocodes will partially supersede BS 8004 and BS 8110 (and
other related geotechnical standards). However, until all the Eurocode packages for designing
the various parts of a structure are available together with the National Annexes, the British
Standards Institute advises that the current standards will remain valid for geotechnical investigation
and design and concrete design ‘until further notice’. This ‘coexistence period’for EC7
is likely to last for several years before the current standards are modified or withdrawn.
EC7 has to be read in conjunction with BS EN 1990 Eurocode 0: Basis of Structural
Design (1.4) and BS EN 1991-1 Eurocode 1: Part 1 Actions on Structures which ensure that
partial factors are considered in a logical and uniform manner avoiding the application of
global safety factors. In addition, reference has to be made to BSEN 1993-1: 2005 Eurocode 3
(EC3): Design of Steel Structures, Part 1-1 General Rules and Part 5: 2007 (EC3-5) Piling
and BS EN 1995-1: 2004 Eurocode 5 (EC5): Design of Timber Structures, Part 1-1 General
Rules. It should be noted that new European Standards have been published dealing with the
‘execution of special geotechnical works’ (bored piling, displacement piles, sheet piles,
micropiles, etc.) which have the status of current British Standards (designated as ‘BS EN’).
Clause 7 of EC7 deals with piled foundations from the aspects of actions on piles from
superimposed loading or ground movements, design methods for piles subjected to
compression, tension and lateral loading, pile-loading tests, structural design and supervision
of construction. In using Clause 7 of EC7 the designer is required to demonstrate that the
sum of the ultimate limit state components of bearing capacity of the pile or pile group
(resistances ‘R’) exceeds the ultimate limit state design loading (actions ‘F’) and that the
serviceability limit-state is not reached.
The EC7 loading scenarios, defined as ‘actions’ in the Eurocodes, are designed to cover
‘permanent unfavourable’, ‘permanent favourable’ and ‘variable’ situations and require the