Piling equipment and methods 93very fast rate of penetration in favourable ground. It is claimed that a rate of driving
averaging 18 m per minute may be achieved in loose to medium-dense granular soils. If the
electric generator used to power the exciter motors is mounted in a well-designed acoustic
chamber, the vibrators can be used in urban areas with far lower risk of complaints arising
due to noise and shock-wave disturbance than when impact hammers are used. However,
standard vibrators with constant eccentric moment have a critical frequency during starting
and stopping as they change to and from the operating frequency, which may resonate with
the natural frequency of nearby buildings. This can cause a short period of high amplitude
vibrations which are quite alarming to the occupants. The development of high frequency
(greater than 30 Hz), variable moment vibrators with automatic adjustment has virtually
eliminated this start-up and shut-down ‘shaking zone’, reducing peak particle velocity to
levels as low as 3 mm/s at 2 m (see Section 3.1.7). These are not as powerful as the standard
units. Although there are limitations in respect of the soil types in which they can be used
and notwithstanding the complexity of the machinery and its maintenance, the new range of
resonant-free vibrators will generate greater driving force and displacement amplitude to
overcome the toe resistance when driving longer and larger displacement piles(3.4).
Depth vibrators or ‘poker’vibrators are used extensively for improving the bearing capacity
and settlement characteristics of weak soils by vibro-compaction or vibro-replacement tech-
niques. For vibro-compaction (or ‘flotation’) the vibrator is either flushed to the required
depth using water jets or vibrated dry with air jets in partially saturated soil. As the poker is
withdrawn the horizontal vibrations cause a compact cylinder of soil to be formed at depth
as the soil particles are rearranged during densification, producing a depression at the surface
which has to be filled with granular material during the process. In the vibro-displacement
Figure 3.16Driving a pile casing with a PVE 200 m free hanging vibrator.