Introduction - Piling Engineering - page 5

and construct a pier from the bottom of the excavation upwards, since concrete could
be mixed at the surface and poured in. This meant that for relatively light loads, small-
diameter piles could be constructed by percussion boring equipment as used for well
sinking, and so the art of bored piling developed.
Bored piling offered certain advantages over driven types for at least part of the
market, since a small tripod machine could be used to suspend the boring tool and
could at the same time provide enough effort to enable relatively short lengths of
steel tube, which were necessary to retain soft and unstable upper ground strata, to
be removed after concrete placing was complete. When compared with the relatively
large machines of the same period needed to insert driven piles, the equipment was
small and could readily operate on sites to which the larger machines could not gain
easy access.
In its early days, the method was probably a good deal less reliable than the methods
used to install driven piles, but as companies built up experience, reliability improved.
Nevertheless it was still difficult to cope with water inflows at depth, and in order
to overcome this problem the 'pressure pile' was invented. In this system an airlock
was screwed on to the top of the steel lining tube which was used to stabilize the
hole. This was done at the end of the excavation stage and, using appropriate air
pressures within the tube, water was expelled from the boring. Concrete was then
placed in batches through the airlock and thus the segregation which might have
occurred had concrete been poured directly into water, or by means of an unreliable
bottom-opening skip, was avoided. The method proved rather cumbersome and has
now fallen into disuse. Instead concrete is now poured into water-filled boreholes using
tremie pipes.
Today, although small tripod or similar percussion boring equipment remains, the
majority of bored piles are formed using rotary augering machines. This results from
rapid development of this type of plant since about 1950. Machines are now available
to bore a wide range of pile diameters from about 150mm to over 2m, while yet
other equipment is designed to enable concrete or grout to be injected to form a pile
through the hollow stem of a continuously flighted boring auger as the laden auger is
withdrawn from the soil.
At the same time driven piles have developed considerably and two main forms exist
for land-based work, namely the pre-cast jointed pile and the cast-in-place pile. The
former type of pile replaces to a large extent the long precast piles which were common
in the early part of this century and requires much smaller and lighter handling and
driving plant, while the latter offers a cheap and reliable pile where the pile penetration
required is not more than about 18m.
Steel H piles and tubular steel piles are used for limited applications. The H section
pile causes minimal soil displacement and can stand up to fairly heavy driving con-
ditions while the tubular pile finds application in many shoreline works where both
direct load and bending capacity are often needed. Very large and long tubular piles
are used extensively in major offshore works such as the founding of oil platforms at
sea. The cost of steel is however such that these types of pile find only rather occasional
application for inland jobs.
The design of piles and pile groups has advanced steadily in recent times with much
of this work being carried out by engineers who specialize particularly in foundation
engineering. Effective stress methods are being developed for individual pile designs,