Pile Design and Construction Practice, Fifth edition

144 Resistance of piles to compressive loads
Serviceability limit states are concerned with ensuring that the deformations of a structure
due to ground movements below the foundations do not damage the appearance or reduce
the useful life of the structure, or cause damage to finishes, non-structural elements,
machinery or other installations in the structure.
EC7 requires structures and their foundations to have sufficient durability to resist
weakening from attack by substances in the ground or the environment.
The design methods using EC7 can be advantageous compared to permissible stress methods
when designing structures using BS 8110 (Code of practice structural concrete) or BS 5950
(structural steelwork in buildings), or BS 5400 (bridges), which are all limit state codes, in that
difficulty is avoided in achieving compatibility between them and the foundation code BS 8004
which is based on permissible stresses. When the use of BS 8004 for the foundations in conjunction
with limit state codes for the superstructure cannot be avoided, the superstructure
designer should specify clearly whether or not the loads applied to the foundations are factored
or unfactored total loads, or are dead loads combined with factored imposed loading.
In addition to EC7, Eurocodes relevant to pile foundation design are the following:
● BS EN 1990 Basis of structural design
● BS EN 1991 Action on structures
● BS EN 1992 Design of concrete structures
● BS EN 1993 Design of steel structures
● BS EN 1994 Design of composite concrete and steel structures
● BS EN 1995 Design of timber structures
● BS EN 1996 Design of masonry structures
● BS EN 1998 Design of structures for earthquake resistance
● BS EN 1999 Design of aluminium structures
As a preliminary EC7 requires the structure to be considered in three categories of risk
from the foundation aspect. Geotechnical Category 1 covers structures having negligible
risk of failure or damage due to ground movements, or where enough is known about the
ground conditions to adopt a routine method of design, provided that there are no risk problems
associated with excavation below groundwater level.
Category 2 includes conventional structures and their foundations with no exceptional
risk or difficult ground or loading conditions. Structures requiring piling come into this
category provided that there is adequate geotechnical data based on routine methods of
ground investigation.
Category 3 applies to all categories not coming within the scope of 1 and 2. It includes very
large or unusual structures and those involving abnormal risks or exceptionally difficult
ground or loading conditions, also structures in highly seismic areas and areas of site
instability. EC7 (Clause 2.2) lists 15 geological and environmental features which need to be
considered generally in foundation design. All of these are relevant to piled foundations for
which the code prescribes three basic approaches to design. These are the following:
● Static load tests
● Empirical or analytical calculations
● Dynamic load tests.
Design by prescription and by the observational method are also referred to in the general
part of the code. The prescriptive method applies to the tables of allowable bearing pressures