Civil Engineering Reference
In-Depth Information
Typical ratios of diameter (
D
) to wall thickness (
t
) are in the range 30 to 50 and such
piles may be top-driven in the conventional way or bottom driven using an internal
hammer and some means of closing the pile base. Short sections of pile may be welded
together or alternatively if they comprise thicker-walled cylindrical piles they can be
screwed together to achieve longer pile lengths. Once installed any internal soil plug
can be removed and the pile filled with concrete although it is more usual to leave
the soil plug in place. Where thin-walled piles are bottom driven they are unlikely to
be used as load-bearing piles without concrete infill as discussed under driven cast-
in-place piles. Spirally welded tubular steel piles in diameters from around 200 to
3000mm are available with wall thicknesses from 10mm up to 25mm. At the other
end of the scale, small diameter steel piles (100 to 200mm) with thin walls (typically
3mm) can be driven with a down-hole air hammer ('Grundomat piles'). These may
have working capacities of 20 to about 50 kN and are described in the section dealing
with micro-piles.
3.2.5.2 Steel piles as retaining walls
In retaining wall applications, various combinations of box and plain 'U' or 'Z' section
piles may be employed to greatly increase the bending resistance of the stand-alone
sections. For additional stiffness, 'H' sections can be combined with sheet pile sections.
The connections can be achieved with 'ball and socket' adaptors that can be welded
to the edge of the 'H' section flanges that will link up with the clutch of the sheet
pile. At the same time, within limits, a degree of rotation can be introduced into the
horizontal wall alignment. Special interlock connectors permit corners to be formed
as required, not necessarily at 90 degrees. Up to around 5 degrees of angular change
can be achieved with standard pile sections allowing circular cofferdams to be formed
to diameters of around 15m without special sections. When sheet steel piles are used
to form cofferdams in marine applications, water penetration through the pile joints
may be significantly reduced or eliminated using sealants applied to the pile interlocks
preferably prior to delivery to site. For sheet piled basements, permanent sealing against
groundwater may be achieved by welding, using either with a bead of weld along the
interlock or with a separate steel plate over the exposed edge of the pile interlock.
Horizontal sealing into a basement may be achieved by welding a horizontal steel rib
onto the piles following installation which is then cast-into the basement floor slab.
3.2.6 Timber piles
For modest loads and piling depths of about 12m, timber piles are quite suitable, pro-
vided certain precautions are taken. They are infrequently used in the United Kingdom,
although they are still in common use in Scandinavian countries and the United States.
In Australia, treated hardwood piles are produced and installed by Koppers Timber
Piles P/L who specialize in this type of pile and have developed a strong market for
it. In general, working loads are unlikely to exceed 500 kN, partly because of diffi-
culties in driving piles to give sets appropriate to greater working loads and partly
because of the generally smaller cross-sections of timber piles and lower compressive
strengths. Stresses should be checked according to CP112 for compressive loads par-
allel to the grain. Pile cross-sections are usually square although circular sections can
