pile group under its working load, any calculations of the transfer of load that are based on
elastic theory which do not take account of soil disturbance for several diameters around the
pile shaft and beneath the toe are unrealistic.
Therefore, while the authors base their approach to the calculation of pile carrying capacity
on soil mechanics methods, this approach is simply an empirical one which relates known
pile behaviour to simple soil properties such as relative density and undrained shearing
strength. These can be regarded as properties to which empirical coefficients can be applied
to arrive at unit values for the shaft friction and end-bearing resistances.
Observations made on full-scale instrumented piles have so far only served to reveal the
extreme complexities of the problems, and have shown that there is no simple fundamental
design method. The empirical or semi-empirical methods set out in this chapter have been
proved by experience to be reliable for practical design of light to moderately heavy loadings
on land-based or near-shore marine structures. Special consideration using more complex
design methods are required for heavily loaded marine structures in deep water. The engineer
is often presented with inadequate information on the soil properties. A decision then has to
be made whether to base designs on conservative values with an appropriate safety factor
without any check by load testing, or merely to use the design methods to give a preliminary
guide to pile diameter and length and then to base the final designs on an extensive field
testing programme with loading tests to failure. Such testing is always justified on a large-
scale piling project. Proof-load testing as a means of checking workmanship is a separate
consideration (see Section 11.4).
Where the effective overburden pressure is an important parameter for calculating the
ultimate bearing capacity of piles (as is the case for coarse-grained soils) account must
be taken of the unfavourable effects of a rise in groundwater levels. This may be local or
may be a general rise, due for example to seasonal flooding of a major river, or a long-
term effect such as the predicted large general rise in groundwater levels in Greater
London.
4.1.2 The behaviour of a pile under load
For practical design purposes engineers must base their calculations of carrying capacity on
the application of the load at a relatively short time after installation. The reliability of these
calculations is assessed by a loading test which is again made at a relatively short time after
installation. However, the effects of time on carrying capacity must be appreciated and these
are discussed in Sections 4.2.4 and 4.3.8.
When a pile is subjected to a progressively increasing compressive load at a rapid or
moderately rapid rate of application, the resulting load–settlement curve is as shown in
Figure 4.1. Initially the pile–soil system behaves elastically. There is a straight-line
relationship up to some point A on the curve and if the load is released at any stage up to
this point the pile head will rebound to its original level. When the load is increased
beyond point A there is yielding at, or close to, the pile–soil interface and slippage occurs
until point B is reached, when the maximum shaft friction on the pile shaft will have been
mobilized. If the load is released at this stage the pile head will rebound to point C, the
amount of ‘permanent set’being the distance OC. The movement required to mobilize the
maximum shaft friction is quite small and is only of the order of 0.3% to 1% of the pile
diameter. The base resistance of the pile requires a greater downward movement for its
full mobilization, and the amount of movement depends on the diameter of the pile. It may
140 Resistance of piles to compressive loads