are allowed to be used by Eurocode EC7 provided that their validity has been demonstrated
by experience in similar ground conditions or verified by static loading tests.
Steady progress has been made in developing analytical methods for calculating pile
capacity. With increasing experience of their use backed by research, the soil mechanics
approach can be applied to all forms of piling in all ground conditions, whereas even if a
reliable dynamic formula could be established its use would be limited to driven piles only.
However, dynamic formulae still have their uses in predicting the stresses within the
material forming the pile during driving and hence in assessing the risk of pile breakage, and
their relevance to this problem is discussed in Chapter 7.
1.5 Code of practice requirements
The uncertainties in the methods of predicting allowable or ultimate loads on piles are
reflected in the numerous ways of defining these loads in the many codes of practice which
cover piling. The British Standard Code of Practice BS8004: 1986 (Foundations) defines the
ultimate bearing capacity of a pile as ‘The load at which the resistance of the soil becomes
fully mobilized’and goes on to state that this is generally taken as the load causing the head
of the pile to settle a depth of 10% of the pile width or diameter. BS8004 does not define
ultimate loads for uplift or lateral loading. Specific design information is limited to stating the
working stresses on the pile material and the cover required to the reinforcement, the require-
ments for positional tolerance and verticality also being stated. No quantitative information is
given on shaft friction or end-bearing values in soils or rocks, but many countries place limits
on these values or on maximum pile loads in order to ensure that piles are not driven very heav-
ily so as to achieve the maximum working load that can be permitted by the allowable stress
on the cross-sectional area of the pile shaft.
A conflict can arise in British practice where structures, including foundation substructures,
are designed to the requirements of BS8110 and their foundations to those of BS8004. In the
former document partial safety factors are employed to increase the characteristic dead and
imposed loads to amounts which are defined as the ultimate load. The ultimate resistance of
the structure is calculated on the basis of the characteristic strength of the material used for
its construction which again is multiplied by a partial safety factor to take into account the
possibility of the strength of the material used being less than the designed characteristic
strength. Then, if the ultimate load on the structure does not exceed its ultimate resistance to
load, the ultimate orcollapse limit stateis not reached and the structure is safe. Deflections
of the structure are also calculated to ensure that these do not exceed the maximum values
that can be tolerated by the structure or user, and thus to ensure that the serviceability limit
stateis not reached.
When foundations are designed in accordance with BS8004, the maximum working load
is calculated. This is comparable to the characteristic loading specified in BS8110, i.e. the
most unfavourable combination of the dead and imposed loading. The resistance offered by
the ground to this loading is calculated. This is based on representativeshearing strength
parameters of the soils or rocks concerned. These are not necessarily minimum or average
values but are parameters selected by the engineer using his experience and judgement and
taking into account the variability in the geological conditions, the number of test results
available, the care used in taking samples and selecting them for test, and experience of
other site investigations and of the behaviour of existing structures in the locality. The
maximum load imposed by the sub-structure on the ground must not exceed the calculated
General principles and practices 5