Where piles
are driven wholly in soft soils no shoe need be provided. The ends of
the piles are usually cast in the shape of a blunt point. A sharper point
is preferred for driving into hard clays or compact sands and gravels.
The metal drive shoe commonly seen on concrete piles whether driven in
soft or hard conditions is based on a design used to stop timber piles
from splitting or brooming, and in soft conditions no metal shoe of any
kind is required. Where the piles are to be driven into soil containing
large cobbles or boulders, a shoe is needed to split the boulders or to
prevent breaking of the toe when the pile pushes large cobbles or boulders
to one side. The area of the top of the metal shoe in contact with the
concrete of the pile should be large enough to ensure that the compressive
stress on the concrete is within the safe limits.
Where piles
are required to penetrate rock, to obtain lateral resistance for example,
a special rock point is used. This design is particularly suited to driving
on to a sloping rock surface when, under careful blows of a heavy hammer
with a short drop, the sharp edge of the hollow ground point will bite
into the rock so preventing the point from slipping down the rock surface.
The point is seated into rock with very light blows of the hammer until
it is evident that the point is wholly within rock; the hammer drop can
then be increased to ensure a satisfactory penetration of the point.
Precast concrete
piles cannot be expected to split large boulders when the boulders are
in contact with one another or are embedded in hard or compact soil. In
these cases, special measures must be taken driving the piles.
