Civil Engineering Reference
In-Depth Information
that a positive head of water is kept in the pile bore when boring through granu-
lar deposits to minimize the loosening effect and to prevent piping up the pile bore.
Water may have to be added in order to achieve this. Obstructions, either natural or
man-made, are cleared by the use of a heavy 'I' section chisel, with a hardened flat
point or blade.
3.4.3 Large-diameter percussion bored piles
Large-diameter percussion bored piling equipment is well suited to the penetration
of hard strata, which may include weak sedimentary rock. Where casing is advanced
as the drilling proceeds, unstable strata are supported and may be interspersed with
harder strata without compromising the boring action.
An early method of forming large diameter piles in difficult ground using a form of
percussion equipment was the 'Benoto' system. This employed a heavy rig to handle
large diameter casing that was pressed into the ground using an oscillatory semi-rotary
movement with an internal grab to remove the soil.
The casing was sectional, but because of the semi-rotary driving action, was not
screwed together, but employed a flush quick-action circumferential joint within the
twin-walled casing thickness. The boring tube had a hardened cutting edge and was
hydraulically clamped in a collar that transmitted the semi-rotary action and ver-
tical motion provided by hydraulic rams. In hard ground, rock jaws were fitted to
the hammer grab. The piles were reinforced and concreted in the usual way, the
casing being hydraulically jacked out of the ground during concreting. Typical work-
ing loads were 2000 to 5000 kN in suitable ground for pile diameters of between
670 and 1200mm respectively. Pile lengths of up to 40m were possible. Work-
ing on a similar principle, but with the oscillatory movement and the down-thrust
carried out as separate processes, are the Bade and Hochstrasser-Weise systems
which can give higher rates of progress. During the concreting stage of the latter
system, compressed air is applied to the air-space in the upper part of the casing
which is sealed off. This assists in lifting the casing that is oscillated as it is with-
drawn but the air supply needs to be carefully controlled to prevent an unstable
condition.
More recently, semi-rotary down-hole percussive hammer rigs powered by com-
pressed air, of the type used for well boring, have become available in sizes which
approach those of large-diameter piles. Large quantities of compressed air are required
to power these hammers and space is therefore needed on a site to house the com-
pressors although the set-up is adaptable in its arrangement. Generally speaking, a
down-hole hammer will require a separate reaming and casing operation from an
attendant auger rig. However, the plant will bore through weak rock with little diffi-
culty and in certain circumstances may be suitable for forming deep rock sockets (see
Figure 3.15). The Atlas Copco 'Symmetrix' system is a patented development of the
down-hole percussive method of drilling in which casing follows the bit and broadens
the scope of this type of hole boring. It is available for drilling piles into broken or hard
strata that may require temporary or permanent casing in diameters up to 914mm.
Further developments include the use of hydraulically powered down-hole hammers
that will work with drilling muds thereby combing the hole forming with a borehole
wall support system.
