| Vibrating
The top of
the casing, generally with a wall thickness of 10 to 15 mm, is connected
to a vibrator by a set of hydraulic clamps. High frequency vibrations
produced by the vibrator reduce the friction between the casing and the
soil and thus enable the casing to penetrate under its own weight. As
the reduction of shaft friction is most pronounced in loose to medium
dense sands and gravels, and soft silts and clays, the use of vibrators
is predominant in these soil types. Furthermore, the soil volume affected
by vibrations and thus the depth of penetration of the casing depend on
the energy output of the vibrator. Upper limits for the use of vibrators
are casing diameters of 2000 mm and casing depths of 20 m.
Vibrators are more efficient in reducing the shaft resistance than the
point resistance. Therefore, large-diameter open tube piles and casings
can be installed efficiently by vibrators while it can be difficult to
install closed tubes.
The operating
frequency and vibration amplitude of modern hydraulic vibrators can be
varied continuously during drilling, which improves the drilling speed
and reduces the risk of ground vibrations.
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