Civil Engineering Reference
In-Depth Information
5.4 Discussion of design principles
Recent years have seen major advances in understanding the manner in which pile
foundations interact with the surrounding soil. This and the preceding chapter have
presented some of the modern analytical methods available, concentrating on those
that may be used directly (that is without excessive computational effort) in design
calculations.
The choice of piles of sufficient axial capacity is still generally the key issue in pile
design. However, interpretation of the word 'sufficient' must be made in the light of
the foundation requirements. In particular, settlement considerations now play a more
dominant role in pile design. As will be discussed further below, the choice of what
'factor of safety' is appropriate for pile foundations will depend to a large extent on
how much settlement may be tolerated.
Practical considerations, such as those discussed in Chapters 2 and 3, will play an
important part in pile design, particularly as regards choice of pile type, installation
method and even the overall layout of the pile group. The role of analysis is then
to optimize the foundation and to assess, through parametric studies, the effect of
uncertainties in assumed soil and other design parameters. It may be helpful to review
here some of the main advances in analytical methods that have been made over the
last decade or so.
1 In calculating the axial capacity of a pile simple 'total stress' methods have been
augmented by a better understanding of the effective stress changes that occur
during and after pile installation. No complex pile design would now be complete
without an assessment of the effective stress state implied by a total stress calcu-
lation of capacity. Quantifying the gradual degradation in shaft friction along the
length of piles driven into sand is a significant conceptual change from previous
design methods based on a critical depth.
2 The concept of 'group efficiency', based on a comparison of the capacity of the
individual piles in a group with that of the surrounding block, is generally consid-
ered inappropriate. Individual piles may be considered more or less efficient when
part of a group, due to differences in the stress state around the pile. However, this
is a separate consideration. The two modes, of individual pile and block failure,
should be appraised independently.
3 The concept of 'efficiency' of a group is more appropriately used in respect of the
stiffness of the foundation. Conventionally designed large pile groups will have
efficiencies as low as a few percent, and this has highlighted the need to improve
design methods for such groups, optimizing the quantity and location of the piles.
4 The lateral response of piles may be estimated simply using a consistent approach,
treating the soil as a continuum. This approach avoids uncertainty in the choice of
coefficient of sub-grade reaction, and enables group effects to be assessed within
the same framework.
5 Computer methods of pile group analysis under general loading conditions are
now widely available. The methods take proper account of the lateral support
offered by soil around the piles and enable more realistic estimates to be made
of the load distribution in the piles under working conditions. In particular,
unnecessary use of raking piles to take any lateral component of load may be
avoided.
