Civil Engineering Reference
In-Depth Information
It is often not apparent at first sight how important the control of settlement and
differential settlement may be. For example, fine tolerances are sometimes specified
for items such as machinery bases and automatic stacking systems in warehouses.
In structures such as bridges, the sensitivity of the system may be very dependent on
the assumptions of deck design, whether it is a case of a simply supported span, spans
in a continuous beam arrangement or integral spans with continuity to a supporting
abutment.
It should be recognized that structural design and foundation design should not be
carried out in isolation. The loads from the structure will lead to deformations of the
ground which depend on soil types, and these deformations will in turn lead to internal
stress changes within the structure, thus changing the loads on the piles. Many cases
exist where such considerations may be unimportant, perhaps where the structure
rests on piles bearing onto hard rock, and it is therefore unnecessary to become too
involved in detailed analysis. However, cases also exist in plenty where soil/structure
interaction effects are important in terms of both the finished structure and adjacent
existing structures. These considerations may have a determining influence on the
selection of pile types.
10.3.1 The use of piles for vertical load bearing
The majority of piles are designed to carry vertical loads and any secondary lateral
loads to which they may be subjected may be small. Secondary loads derive in the main
from structures with a certain degree of redundancy and cannot by their nature be cal-
culated exactly. Derived loads usually represent combinations of dead loads, live loads
and wind loads, and the final structure is usually designed on the worst loading con-
dition, say for each column. It is important in considering real cases, and in particular
probable structural settlements, to take into account realistic conditions, although
each pile must of course still be capable of carrying safely the required maximum
structural load.
The starting point of most structural engineering designs is an assumption of a non-
deforming structure, although it is possible for simpler types of structure to incorporate
the concept of a settling foundation in order to assess how loads may change in such
a circumstance. This step in the procedure is then often followed by the acceptance
of some differential settlement of the foundations on a purely empirical basis. Such
empirical deformation criteria in relation to structural damage are discussed by a wide
variety of authors, e.g. Sowers (1962), Skempton and McDonald (1956), Meyerhof
(1947), Polshin and Tokar (1957), Bjerrum (1963) and Burland and Wroth (1975).
Summary information is given on these methods by Lamb and Whitman (1969) and
in Padfield and Sharrock (1983).
In order to satisfy the designer of a piled foundation who seeks the nearest possible
approximation to a non-deforming foundation, a great deal of attention is devoted to
ensuring that the deformation of each individual pile, and of all the supporting piles
acting as a group, is minimized. The choice of depth required for piles, particularly
where load is carried mainly by friction, is often based on this requirement, and in
general the deeper the piles are, the more likely is settlement restriction likely to be
achieved. This means that in many cases factors of safety and costs are raised in order
to achieve this result.
