In practice one has to make a choice
between the use of vertical piles used singly or in groups to carry
such loads or of groups incorporating at least some piles installed
to an angle of rake. The capacity of a pile as a structural unit to
carry shear loads at its head depends on the strength of the section,
and when the forces become high, one is impelled to find some
structurally acceptable solution which keeps stresses within
reasonable limits.
However, in choosing the possible
option of raking piles one should be aware of the problems and
limitations that may be involved. Some of the factors involved are as
follows:
1 Raking piles are usually more
expensive than vertical piles. This is partly involved with extra
time taken to set up and maintain the equipment in position, the less
efficient use of hammers in the case of driven piles, and the
difficulties of concrete placing in bored piles.
2 The standards of tolerance that can
be maintained in the installation of raking piles are not as good as
for vertical piles. Most analyses of pile groups of this kind ignore
the effect of tolerances, but if tolerances are properly taken into
account they can have a significant effect on calculated pile loads,
depending on pile grouping and numbers, with small groups being
usually most sensitive.
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 equipment 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.
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 capacity 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.
0 comments:
Post a Comment