Page 152 - Numerical Analysis and Modelling in Geomechanics

MODELLING OF GROUND WAVES 133

Figure 5.1 Schematic of ground waves caused by pile driving.
blow. They are noisy, which limits their use in residential areas. Diesel hammers
are economical and effective but they are not controllable and they cause air
pollution, so they are now rarely used. The hydraulic hammer, single or double
acting, is efficient, controllable and environment-friendly, and is now the
standard choice of impact hammer, whether for small pile elements on land or
for very large tubes offshore.
All the above hammers are effective in installing piles into most soil types,
provided that the correct hammer is selected for the work and that the pile drive
is guided effectively, e.g. panel driving should be used for sheet pile walls, pile
guide frames for long piles. The presence of a dolly, i.e. packing material
between hammer and pile head to cushion the impact, changes the shape of the
force-time impulse, reducing the peak and extending the time duration.
Many records of ground vibrations have been made in the last 30 years (e.g.
Attewell and Farmer, 1973; Uromeihy, 1990; Hiller, 2000), so that the general
pattern of vibrations is well known. At any surface measurement station within
some 2 m to 50 m from the pile, there are three components of disturbance in the
radial, transverse and vertical directions. The shape of the vibration as a function
of time is typically a half-wave with pre- and post-cursor minor fluctuations, see
Figure 5.2.
The magnitude of vibration of a soil particle can be defined in terms of
maximum acceleration, velocity or displacement. The most commonly used
parameter is maximum velocity, v max , partly because it is easily measured, and
also because ground strain is defined approximately by

(5.1)

where c is the wave transmission velocity.