Civil Engineering Reference
In-Depth Information
3 Where the upper part of a raking pile is embedded in a soil that is likely to suffer
time-dependent settlement, the pile will in due course be subject to bending stresses
unrelated to the structural design load conditions. This may require increase of
strength of the section, which is in turn reflected in costs.
4 Many machines used for pile installation carry the pile driving or forming equip-
ment on a long mast, so that they become intrinsically less stable, particularly
as the line of the pile gets further from the vertical position. In certain cases,
when working close to river banks or railway lines, for example, there is a
major limitation on how machinery can be positioned to produce the desired end
result.
5 Design of groups involving raking and vertical piles and with loads that are both
vertical and horizontal should have regard to the constancy of the relationship
between these. If, for example, the vertical load is near constant, but the horizontal
force varies greatly, then it is better to employ groupings with rakers balanced in
two opposed directions rather than to have an arrangement of vertical piles plus
piles raking in one direction only. This is simply to minimize the shears in the
heads of the piles when horizontal load falls to a minimum value (see Chapter 5,
section 5.4.2).
6 The use of raking piles to 'spread' load under vertically loaded foundations, where
the piles are fully embedded in the soil mass and where the whole foundation is
expected to undergo significant consolidation/creep settlement, must lead to large
bending stresses being developed in the piles. In certain cases this can lead to such
stress levels in the piles that the section will suffer damage, which may in turn lead
to severe problems in the supported structure.
It should, however, be said that where groups of raking piles derive their axial capac-
ity from strata that are hard and relatively non-deformable, they provide a stiffness
in terms of laterally applied forces which can be very desirable. The main issue in
design is to avoid large and unquantifiable secondary stresses, and provided this can
be achieved all will be well.
Where there are very heavy lateral loads to be carried and neither raking piles nor
single piles other than perhaps those of very large diameter are suitable, then diaphragm
piers or 'barrettes' have a useful potential application. They can be given high stiffness
in the direction of applied horizontal loading without fear of the problem of major
secondary stresses.
10.3.4 Tolerances in the installation of piles and their
structural effects
It is not possible to install piles consistently in the right position nor precisely
to the alignment, which is required by a design. The achievable tolerances are
strongly related to such factors as accurate setting up of the equipment, fixity in
machine parts, presence of obstructions in the ground and wide variations in the
properties of the soil especially near the point of entry of the pile into the sur-
face, inclination of strata, and operator error. Tolerances that can normally and
reasonably be achieved are quoted for example in the publication
Specification
for Piling and Embedded Retaining Walls
(ICE, 2007). This publication is quite
