Chapter 4
Calculating the resistance of piles
to compressive loads
4.1 General considerations
4.1.1 The basic approach to the calculation of pile resistance
The numerous types of pile and the diversity in their methods of installation have been
described in Chapters 2 and 3. Each different type and installation method disturbs the
ground surrounding the pile in a different way. The influence of this disturbance on the shaft
friction and end-bearing resistance of piles has been briefly mentioned (see Section 1.3).
This influence can improve or reduce the bearing capacity of the piles, and thus a thorough
understanding of how the piles are constructed is essential to the formulation of a practical
method of calculating loading capacity.
The basic approach used in this chapter to calculate the resistance of piles to compressive
loads is the ‘static’or soil mechanics approach. Over the years much attention has been
given by research workers to calculation methods based on ‘pure’soil mechanics theory.
They postulate that the interface friction on a pile shaft can be determined by a simple
relationship between the coefficient of earth pressure ‘at rest’, the effective over-burden
pressure and the drained angle of shearing resistance of the soil, but they recognize that
the coefficient of earth pressure must be modified by a factor which takes into account the
method of pile installation. Similarly, they believe that the end-bearing resistance of a pile
can be calculated by classical soil mechanics theory based on the undisturbed shearing
resistance of the soil surrounding the pile toe. The importance of the settlement of the pile
or pile groups at the working load is recognized and methods have been evolved to calculate
this settlement, based on elastic theory and taking into account the transfer of load in shaft
friction from the pile to the soil.
The concepts of this research work commenced on quite simple lines, the two main
groups, namely driven piles and bored piles, only being differentiated when considering pile
behaviour. However, as the work progressed from the laboratory to the field, particularly in
the study of the behaviour of instrumented full-scale piles, it was observed that there were
very fundamental departures from classical soil mechanics theory, and the all-important
effects of installation procedures on pile behaviour were realized. The installation of piles
results in highly complex conditions developing at the pile–soil interface which are often
quite unrelated to the original undisturbed state of the soil, or even to the fully remoulded
state. The pore water pressures surrounding the pile can vary widely over periods of hours,
days, months or years after installation, such that the simple relationships of shaft friction to
effective overburden pressure are unrealistic. Similarly, when considering deformations of a